the physics of the accretion process in the formation and evolution

The physicsof the accretion process

in the formation and evolutionof Young Stellar Objects

Carlo Felice Manara

München 2014

The physicsof the accretion process

in the formation and evolutionof Young Stellar Objects

Carlo Felice Manara

DissertationanderFakultätfürPhysik

derLudwig--Maximilians--UniversitätMünchen

vorgelegtvonCarloFeliceManaraausMailand, Italien

München, den22. Maj2014

Erstgutachter: Prof. Dr. BarbaraErcolanoZweitgutachter: Prof. Dr. AndreasBurkertTagdermündlichenPrüfung: 8. Juli2014

Tomytwogirls

ThisworkhasbeencarriedoutattheEuropeanSouthernObservatory(ESO) underthesu-pervisionofLeonardoTestiandwithintheESO/InternationalMaxPlanckResearchSchool(IMPRS) studentfellowshipprogramme.ThemembersoftheThesisCommitteewere: BarbaraErcolano, LeonardoTesti, ThomasPreibisch, andAntonellaNatta.

vi

Contents

Abstract xvii

Zusammenfassung xvii

ListofAcronyms xxi

1 Introduction 11.1 Settingthescene: theevolutionarypathfrommolecularcloudcorestostars

andplanets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Star-diskinteraction: accretion . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.2.1 Magnetosphericaccretion . . . . . . . . . . . . . . . . . . . . . . . 51.3 Accretionasatracerofprotoplanetarydiskevolution . . . . . . . . . . . . 7

1.3.1 Evolutionofaccretionrateswithtimeinaviscousdisk . . . . . . . 71.3.2 Evolutionofaccretionrateswithtimeduetophotoevaporation . . . 131.3.3 Thedependenceofaccretionrateswiththemassofthecentralstar . 15

1.4 TheroleofthisThesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201.4.1 ThestateoftheartatthebeginningofthisThesis . . . . . . . . . . . 201.4.2 OpenissuesatthebeginningofthisThesis . . . . . . . . . . . . . . 241.4.3 ThestructureofthisThesis . . . . . . . . . . . . . . . . . . . . . . . 25

2 Modelingtheaccretionspectrum 292.1 Accretionspectrummodelsintheliterature . . . . . . . . . . . . . . . . . . 292.2 Theslabmodel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

2.2.1 Thehydrogenemission . . . . . . . . . . . . . . . . . . . . . . . . . 322.2.2 TheH− emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352.2.3 Thetotalemissionoftheslabmodel . . . . . . . . . . . . . . . . . . 37

2.3 Dependenceofselectedfeaturesontheinputparameters . . . . . . . . . . 372.3.1 ContributionofH andH− tothetotalemission . . . . . . . . . . . . 382.3.2 Balmerjump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 392.3.3 BalmerandPaschencontinua . . . . . . . . . . . . . . . . . . . . . 40

3 Photospherictemplatesofyoungstellarobjectsandtheimpactofchromosphericemissiononaccretionrateestimates 433.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 443.2 Sample, observations, anddatareduction . . . . . . . . . . . . . . . . . . . 453.3 Spectraltypeclassification . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

3.3.1 Spectraltypingfromdepthofmolecularbands . . . . . . . . . . . . 483.3.2 SpectralindicesforM3-M8stars . . . . . . . . . . . . . . . . . . . . 49

3.4 Stellarparameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 563.4.1 Stellarluminosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . 563.4.2 Stellarmassandage . . . . . . . . . . . . . . . . . . . . . . . . . . 58

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CONTENTS

3.5 Lineclassification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 583.5.1 Emissionlinesidentification . . . . . . . . . . . . . . . . . . . . . . 583.5.2 Hα equivalentwidthand10%width . . . . . . . . . . . . . . . . . 603.5.3 Lineluminosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

3.6 Implicationsformassaccretionratesdetermination . . . . . . . . . . . . . 623.6.1 Accretionluminositynoise . . . . . . . . . . . . . . . . . . . . . . . 663.6.2 Massaccretionratenoise . . . . . . . . . . . . . . . . . . . . . . . . 69

3.7 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 723.A Commentsonindividualobjects . . . . . . . . . . . . . . . . . . . . . . . . 733.B NIR spectralindices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 743.C On-linematerial . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

4 Accuratedeterminationofaccretionandphotosphericparametersinyoungstellarobjects 854.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 864.2 Sample, observations, anddatareduction . . . . . . . . . . . . . . . . . . . 87

4.2.1 Targetsselectionanddescription . . . . . . . . . . . . . . . . . . . 874.2.2 Observationsanddatareduction . . . . . . . . . . . . . . . . . . . 88

4.3 Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 894.3.1 Parametersofthemulticomponentfit . . . . . . . . . . . . . . . . . 904.3.2 Determinationofthebestfit . . . . . . . . . . . . . . . . . . . . . . 914.3.3 Comparisontosyntheticspectra . . . . . . . . . . . . . . . . . . . . 934.3.4 Stellarparameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

4.4 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 954.4.1 OM1186 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 954.4.2 OM3125 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

4.5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 974.5.1 Agerelatedparameters . . . . . . . . . . . . . . . . . . . . . . . . . 1004.5.2 Sourcesoferrorinthepreviousclassifications . . . . . . . . . . . . 1004.5.3 Implicationsofourfindings . . . . . . . . . . . . . . . . . . . . . . 102

4.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

5 Onthegascontentoftransitionaldisks 1055.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1065.2 Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108

5.2.1 Sampledescription . . . . . . . . . . . . . . . . . . . . . . . . . . . 1085.2.2 Observationalstrategy . . . . . . . . . . . . . . . . . . . . . . . . . 1105.2.3 Datareduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

5.3 Accretionandphotosphericparameters . . . . . . . . . . . . . . . . . . . . 1145.3.1 Methoddescription . . . . . . . . . . . . . . . . . . . . . . . . . . . 1145.3.2 Infraredcolorexcess . . . . . . . . . . . . . . . . . . . . . . . . . . 123

5.4 Windsignatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1245.5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

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5.5.1 Accretionproperties . . . . . . . . . . . . . . . . . . . . . . . . . . 1265.5.2 Windproperties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1285.5.3 Accretionandwindpropertiesinobjectswithinnerdiskemission . 1325.5.4 Constraintonthegascontentoftheinnerdisk . . . . . . . . . . . . 1335.5.5 Discussiononthegascontentoftheinnerdisk . . . . . . . . . . . . 137

5.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1395.A Commentsonindividualobjects . . . . . . . . . . . . . . . . . . . . . . . . 140

5.A.1 Sampleproperties . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1405.B Additionalliteraturedata . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141

6 Accretionasafunctionofstellarpropertiesinnearbystarformingregions 1436.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1436.2 AccretionintheLupusclouds . . . . . . . . . . . . . . . . . . . . . . . . . 144

6.2.1 Sample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1456.2.2 Stellarandsubstellarproperties . . . . . . . . . . . . . . . . . . . . 1466.2.3 Accretionratedetermination . . . . . . . . . . . . . . . . . . . . . . 1506.2.4 AccretionpropertiesofstellarandsubstellarobjectsinLupus . . . . 157

6.3 Accretioninthe σ-Orionisregion . . . . . . . . . . . . . . . . . . . . . . . 1596.4 Newdatainthe ρ-Ophiucusembeddedcomplex . . . . . . . . . . . . . . . 160

6.4.1 Sample, observations, anddatareduction . . . . . . . . . . . . . . . 1626.4.2 Stellarandsubstellarproperties . . . . . . . . . . . . . . . . . . . . 1636.4.3 Accretionpropertiesof ρ-Ophiucusyoungstellarobjects . . . . . . 174

6.5 NewdataintheUpperScorpiusassociation . . . . . . . . . . . . . . . . . 1786.6 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180

6.6.1 Accretionluminosityandstellarluminosityrelation . . . . . . . . . 1816.6.2 Accretionasafunctionofstellarmass . . . . . . . . . . . . . . . . . 1846.6.3 Accretionasafunctionofage . . . . . . . . . . . . . . . . . . . . . 187

6.7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191

7 Conclusions 193

Acknowledgments 210

ix

Contents

x

ListofFigures

1.1 Evolutionarysequenceofyoungstellarobjects . . . . . . . . . . . . . . . . 31.2 Fractionofyoungstellarobjectssurroundedbyprotoplanetarydisksasa

functionoftime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.3 Schemeofthemagnetosphericaccretionscenario . . . . . . . . . . . . . . 61.4 Evolutionofthedisksurfacedensityinthespreadingringcase . . . . . . . 91.5 Evolutionofthedisksurfacedensityaccordingtosimilaritysolutionsinthe

casewhere ν ∝ R. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111.6 Evolutionofmassaccretionrateswithtimeaccordingtosimilaritysolution

viscousmodels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121.7 Evolutionofmassaccretionratesaccordingtodifferentphotoevaporation

models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141.8 Firstobservationsofthemassaccretionrateasafunctionofage . . . . . . 211.9 ObservationandmodelingoftheUV-excessofyoungstellarobjectsdueto

accretion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221.10 Observationsofthemassaccretionratesasafunctionofageinvariousregions 221.11 Observationsofthemassaccretionratesasafunctionofageandmassin

theOrionNebulaCluster . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

2.1 Schemeoftheangularvelocityasafunctionoftheradiusintheinnermostregionofadiskextendingdowntothestellarsurface . . . . . . . . . . . . 30

2.2 Exampleoftheemissionofaslabmodel . . . . . . . . . . . . . . . . . . . 382.3 RatiooftheemissionduetoH tothatduetoH− intheslabmodel . . . . . 392.4 Balmerjumpobtainedwiththeslabmodelasafunctionoftheinputpa-

rameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 402.5 RatiooftheemissionfromtheBalmerandPaschencontinuaobtainedwith

theslabmodelasafunctionoftheinputparameters . . . . . . . . . . . . . 412.6 Balmercontinuumslopeobtainedwiththeslabmodelasafunctionofthe

inputparameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 412.7 Paschencontinuumslopeobtainedwiththeslabmodelasafunctionofthe

inputparameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

3.1 SpectraofClassIII YSOswithSpT earlierthanM3 . . . . . . . . . . . . . . 503.2 SpectraofClassIII YSOswithSpT betweenM3andM5 . . . . . . . . . . . 513.3 SpectraofClassIII YSOswithSpT laterthanM5 . . . . . . . . . . . . . . . 523.4 DistributionofspectraltypesoftheClassIII YSOsdiscussedinthiswork . . 553.5 Exampleof thecombinationofaflux-calibratedand telluric removedX-

Shooterspectrumwithmodelspectrawiththesameeffectivetemperature . 563.6 Hertzsprung-RusselldiagramoftheClassIII YSOsofthiswork . . . . . . . 573.7 Portionofthespectrum, showingemissioninallBalmerlinesfromHβ up

toH12 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 593.8 Hα equivalentwidthasafunctionofspectraltype . . . . . . . . . . . . . . 61

xi

LIST OF FIGURES

3.9 Hα equivalentwidthasafunctionofthe10%Hα width . . . . . . . . . . . 613.10 log(Lacc,noise/L⊙) obtainedusingdifferentaccretiontracers . . . . . . . . . 623.11 log(Lacc,noise/L⊙) obtainedusingdifferentaccretiontracersasFig. 3.10. . . 633.12 Meanvaluesof log(Lacc,noise/L⊙) obtainedwithdifferentaccretiondiagnos-

ticsasafunctionofTeff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 673.13 Meanvaluesof log(Lacc,noise/L⊙) obtainedwithdifferentaccretiondiagnos-

ticsasafunctionoflogTeff . . . . . . . . . . . . . . . . . . . . . . . . . . . 683.14 logMacc,noise asafunctionof logM⋆ . . . . . . . . . . . . . . . . . . . . . . 693.15 logMacc,noise asafunctionof logM⋆ forClassII objectslocatedindifferent

starformingregions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 703.16 Spectraltypeoftheobjectsasafunctionofthespectralindexvaluesob-

tainedwithdifferentNIR indices . . . . . . . . . . . . . . . . . . . . . . . . 753.17 SpectraofClassIII YSOswithspectraltypeearlierthanM3intheNIR arm . 783.18 SpectraofClassIII YSOswithspectraltypesbetweenM3andM5intheNIR

arm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 793.19 SpectraofClassIII YSOswithspectraltypelaterthanM5intheNIR arm . . 803.20 SpectraofClassIII YSOswithspectraltypeearlierthanM2intheUVB arm 813.21 SpectraofClass III YSOswithspectral typesbetweenM2andM4in the

UVB arm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 823.22 SpectraofClassIII YSOswithspectraltypelaterthanM4intheUVB arm . 83

4.1 Hertzsprung-RusselldiagramdiagramoftheONC fromDaRioetal.(2012)withgreenstarsshowingthepositionsofthetwotargetsofthisstudy . . . . 89

4.2 BestfitfortheobjectOM1186 . . . . . . . . . . . . . . . . . . . . . . . . . 954.3 Comparisonoftheextinction-andveiling-correctedspectrumofOM1186

withasyntheticspectrumwithTeff =4350K and log g=4.5. . . . . . . . . 964.4 BestfitfortheobjectOM3125 . . . . . . . . . . . . . . . . . . . . . . . . . 984.5 Comparisonoftheextinction-andveiling-correctedspectrumofOM3125

withasyntheticspectrumwithTeff =4000K and log g=4.0 . . . . . . . . . 994.6 Hertzsprung-RusselldiagramdiagramoftheONC fromDaRioetal.(2012)

withcoloredstarsshowingthenewpositionsofthetwotargetsofthisstudy 101

5.1 BestfitforM-typetransitionaldisks . . . . . . . . . . . . . . . . . . . . . . 1175.2 Bestfitforlate-K typetransitionaldisks . . . . . . . . . . . . . . . . . . . . 1195.3 Bestfitforearly-K typetransitionaldisks . . . . . . . . . . . . . . . . . . . 1205.4 BestfitforG-typetransitionaldisks . . . . . . . . . . . . . . . . . . . . . . 1215.5 (J −K)colorvseffectivetemperatureforthetargets . . . . . . . . . . . . . 1235.6 (J−[3.6])colorvseffectivetemperatureforthetargets . . . . . . . . . . . . 1245.7 Normalized[OI] 630nmlineforthetransitionaldisksinoursample . . . . 1255.8 Logarithmofthemassaccretionratevsinnerholesizeforoursample . . . 1285.9 Logarithmofthemassaccretionratevslogarithmofthestellarmassforour

sampleandforasampleofclassicalTTauristars . . . . . . . . . . . . . . . 129

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List of Figures

5.10 Logarithmicluminosityofthelow-velocitycomponentofthe[OI] 630nmlinevsinnerholesizeforoursample . . . . . . . . . . . . . . . . . . . . . 130

5.11 Logarithmicluminosityofthelow-velocitycomponentofthe[OI] 630nmlinevsthelogarithmoftheaccretionluminosity . . . . . . . . . . . . . . . 131

5.12 Logarithmofthemassaccretionratevslogarithmofthestellarmassofoursample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133

5.13 Logarithmicluminosityofthelow-velocitycomponentofthe[OI] 630nmlinevsthelogarithmoftheaccretionluminosityofourobjects . . . . . . . 134

6.1 HistogramcomparingthenumberofobjectsintheLupusI andIII cloudsanalyzedherewiththetotalClass II YSO populationoftheregion . . . . . 146

6.2 Hertzsprung-RusselldiagramfortheLupussample . . . . . . . . . . . . . . 1506.3 ExamplesofthebestfitofX-ShooterspectraofClass II YSOsinLupus . . . 1526.4 MassaccretionrateasafunctionofmassfortheLupussample . . . . . . . 1576.5 BestfitoftheUV-excessforthe σ-OriClass II YSOs . . . . . . . . . . . . . 1606.6 Massaccretionrate Macc asafunctionofmassforthe σ-Orisample . . . . 1616.7 Hertzsprung-Russelldiagramforthe ρ-OphClass II YSOsanalyzedhere . . 1636.8 Spectraof ρ-Ophtargetsfrom600to2450nm . . . . . . . . . . . . . . . . 1696.9 Spectraof ρ-Ophtargetsfrom600to2450nm . . . . . . . . . . . . . . . . 1706.10 Spectraof ρ-Ophtargetsfrom600to2450nm . . . . . . . . . . . . . . . . 1716.11 Spectraof ρ-Ophtargetsfrom600to2450nm . . . . . . . . . . . . . . . . 1726.12 Spectraof ρ-Ophtargetsfrom600to2450nm . . . . . . . . . . . . . . . . 1736.13 Accretionluminosityderivedfromvariousemissionlinesluminosityforthe

ρ-Ophtargets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1756.14 Accretionluminosityderivedfromvariousemissionlinesluminosityforthe

ρ-Ophtargets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1766.15 Massaccretionrateasafunctionofmassforthe ρ-Ophsample . . . . . . . 1776.16 Comparisonofmassaccretionrateasafunctionofmassforthe ρ-Ophsam-

plewiththeliteraturedata . . . . . . . . . . . . . . . . . . . . . . . . . . . 1786.17 BestfitoftheUV-excessforthetargetslocatedinUpperScorpius . . . . . . 1806.18 Accretionluminosityvsstellarluminosityforthewholesample . . . . . . . 1826.19 Massaccretionrateasafunctionofmassforthewholesample . . . . . . . 1856.20 Massaccretionrateasafunctionofmassforthewholesample . . . . . . . 1866.21 Massaccretionrateasafunctionofageforthewholesample . . . . . . . . 1886.22 Massaccretionrateasafunctionofagefortwosubsampleswithasmaller

stellarmassrange . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1896.23 Massaccretionratenormalizedtothestellarmassasafunctionofagefor

thewholesample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191

xiii

List of Figures

xiv

ListofTables

2.1 Parametersforgeneratingphoto-detachmentcrosssectionsforH− . . . . . 362.2 Parametersforgeneratingfree-freeabsorptioncoefficientforH− inthewave-

lengthrange0.182 µm < λ < 0.3645 µm . . . . . . . . . . . . . . . . . . . 362.3 Parameters forgenerating free-freeabsorptioncoefficient forH− for λ ≥

0.3645 µm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

3.1 Knownparametersfromtheliterature . . . . . . . . . . . . . . . . . . . . . 473.2 Detailsoftheobservations . . . . . . . . . . . . . . . . . . . . . . . . . . . 483.3 Stellarparametersderivedfortheobjectsinoursample . . . . . . . . . . . 533.4 Spectraltypesobtainedusingthemethodbasedonthespectralindicesde-

scribedinSect. 3.3andinAppendix 3.B . . . . . . . . . . . . . . . . . . . 543.5 SpectralindicesfromRiddicketal.(2007, etreferencetherein)adoptedin

ouranalysisforspectraltypeclassification . . . . . . . . . . . . . . . . . . 553.6 FluxesandequivalentwidthsofBalmerlines . . . . . . . . . . . . . . . . . 643.7 Fluxesandequivalentwidthsofheliumandcalciumlines . . . . . . . . . . 653.8 Valuesof logMacc,noise atdifferent M⋆ andages . . . . . . . . . . . . . . . . 713.9 NIR spectralindicesanalyzedinAppendix 3.B . . . . . . . . . . . . . . . . 77

4.1 Parametersavailableintheliteraturefortheobjectsanalyzedinthiswork . 884.2 Spectral featuresused to calculate thebest-fit inourmulticomponentfit

procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 924.3 Newlyderivedparametersfromthemulticomponentfit . . . . . . . . . . . 984.4 Parametersderivedfromevolutionarymodelsusingthenewlyderivedpho-

tosphericparameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

5.1 Transitionaldisksobservinglog . . . . . . . . . . . . . . . . . . . . . . . . 1135.2 Class III YSOspropertiesandobservinglog . . . . . . . . . . . . . . . . . . 1135.3 Stellar, disk, andaccretionparametersofthetargets . . . . . . . . . . . . . 1185.4 Derivedpropertiesofanalyzedlines . . . . . . . . . . . . . . . . . . . . . . 1225.5 Derivedpropertiesof[OI] lineat λ 630nm . . . . . . . . . . . . . . . . . . 1265.6 Propertiesof[NeII] λ12.8µmlinefromtheliterature . . . . . . . . . . . . . 1325.7 PropertiesofCO fundamentaltransitionfromtheliterature . . . . . . . . . 1355.8 Derivedpropertiesofthegas . . . . . . . . . . . . . . . . . . . . . . . . . . 1375.9 Stellaranddiskparametersavailableintheliterature . . . . . . . . . . . . . 142

6.1 Dataincludedinthisstudy . . . . . . . . . . . . . . . . . . . . . . . . . . . 1446.2 SelectedYSOsintheLupusregion . . . . . . . . . . . . . . . . . . . . . . . 1476.3 Spectraltypes, extinction, andphysicalparametersoftheLupusClass II YSOs1516.4 AccretionpropertiesofLupusYSOs . . . . . . . . . . . . . . . . . . . . . . 1546.5 Coordinates, spectraltypes, physicalandaccretionparametersofthe σ-Ori

Class II YSOs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158

xv

List of Tables

6.6 SelectedYSOsinthe ρ-Ophiucusregionandobservinglog . . . . . . . . . 1626.7 Spectraltypes, extinction, andphysicalparametersofthe ρ-OphClass II YSOs1656.8 Accretionluminosityandmassaccretionratesofthe ρ-OphClass II YSOs . 1746.9 YSOs in theUpperScorpiusassociationanalyzedhere: coordinatesand

observinglog . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1796.10 StellarandaccretionparametersfortheUpperScorpiusYSOs . . . . . . . 180

xvi

Abstract

TheformationofplanetsisthoughttohappeninprotoplanetarydiskssurroundingyoungstarsduringthefirstfewMyrsoftheirpre-main-sequenceevolution. Inordertounderstandplanetformationadetailedknowledgeofthediskevolutionprocessisneeded. Bystudyingthe interactionof thediskwith thecentralstar, whichincludesaccretionofmatterduetoviscousprocesses in thedisk, wecanconstrain thephysicalconditionsof the innergaseousdiskinwhichplanetformationtakesplace. WiththerecentadventoftheX-Shooterspectrograph, asecondgenerationinstrumentoftheESO/VLT,theexcessemissionduetoaccretionintheultravioletcanbestudiedsimultaneouslywiththeaccretionsignaturesinthevisibleandinthenear-infrared, finallygivingacompleteviewofthisphenomenon.

InthisThesisI havestudiedvariousX-Shooterdatasetsofyoungstarstodeterminetheintensityandthepropertiesoftheaccretionprocessatvariousphasesofdiskevolutionandasafunctionofthecentralstarmassandage. TofullyexploitthepotentialoftheX-Shooterspectra, I havedevelopedaninnovativemethodofanalysistoderiveaccretionandstellarparameterswithanautomaticalgorithm. Thisisbasedonasetofmodels, composedofasetofphotospherictemplatesofyoungstarsthatI gatheredandcharacterized, asetofslabmodels, thatI havecoded, toreproducetheemissionduetotheaccretionshock, andareddeninglawtotakeintoaccountextinctioneffects. Thismethodallowstoaccuratelyde-termineforthefirsttimethestellarandaccretionparametersofthetargetsself-consistentlyandwithnopriorassumptions, asignificantimprovementwithrespecttopreviousstudies.I haveappliedthismethodologytodeterminethecorrectstellarparametersoftwoobjectsintheOrionNebulaClusterthatwerereportedintheliteraturetohaveananomalousoldage. Myanalysishasshownwhypreviousinvestigationscouldnotresolvethedegeneracybetweenvariousparameters, whilethemethodologydevelopedinthisThesiscould.

I haveappliedmymethodologytoarelativelylargesampleoftransitionaldisks, whicharethoughttobeevolveddiskswithalargegapinthedustydiskbetweentheouterdiskandthecentralstar. I showedthat, whenaccretionispresent, theirpropertiesaresimilartothoseoflessevolveddisks. Understeady-stateassumptionsthisimpliesthepresenceofanefficientmechanismtotransportgasfromtheouterdisktotheinnerregionsofthesystemthroughthedustdepletedgap. Inordertoinvestigatetheevolutionofaccretion, I havethenusedacombinedsampleofallthe ∼90X-ShooterspectraI havestudiedofyoungstarswithdisks. Mysamplecoversarangeofenvironmentsandstellarmasses, andmyaccurateanalysismethodallowsforamuchbetterdeterminationoftheaccretionversusstellarmassrelation. TheslopeofthisrelationisingoodagreementwiththepredictionsofX-rayphotoevaporationmodels. Ontheotherhand, thesignificantlysmallerspreadinvaluesthatI findcomparedtopreviousworkscanbeexplainedasasmallspreadofinitialconditions, suchasinitialcorerotationrates. ByremovingthedependenceoftheaccretionrateswiththestellarmassI havebeenabletosearchforapurelyevolutionarytrendofaccretion. Ingeneral, viscousevolutionmodelscanreproducetheobservedtrendwithsmallvariationsofthefiducialdiskparameters.

TofollowthepathmarkedbythisThesis, futureaccretionstudiesshouldfocusoncom-pletesamplesinvariousstarformingregions. Thesewillbethencoupledwithon-goingsurveyswithotherobservationaltools, suchasALMA inthesub-mmwavelengthrange,targetingotherpropertiesofprotoplanetarydisks, forexamplediskmasses.

xvii

Zusammenfassung

xviii

Zusammenfassung

Eswird angenommen, dass Planeten in protoplanetaren Scheiben von jungen Sternenentstehen. DiesejungenSternebefindensichindenerstenMillionenJahrenihrerEntwick-lungbevorsiedieHauptreiheerreichen. UmPlanetenentstehungzuverstehen, mussderEntwicklungsprozessderScheibeimDetailverstandenwerden. IndemmandieWechsel-wirkungenzwischenderScheibeunddemzentralenStern, zudenenzumBeispielMasse-nakkretionaufdenSterndurchReibungsprozesseinderScheibegehören, untersucht, kannmandiephysikalischenEigenschaftenderinnerenGasscheibe, inderdiePlanetenentste-hen, abschätzen. MitHilfedeskürzlich installiertenX-Shooter Spektrographen, einemESO/VLT InstrumentderzweitenGeneration, kanndiezusätzlicheEmissionimultraviolet-tenBereich, diedurchdieAkkretionentsteht, mitdenAkkretionsmerkmalenimoptischenundnahinfrarotenBereichverglichenwerden. DadurcherhältmaneinumfassenderesBilddiesesProzessesalszuvor.

In dieser Doktorarbeit habe ich verschiedene X-Shooter Daten von jungen Sternenuntersucht, umdie Intensität und die Eigenschaften desAkkretionsprozesses, abhängigvonderMasseunddesAltersdeszentralenSternsundvondenverschiedenenPhasenderScheibenentwicklung, herauszufinden. UmdasvollePotentialderX-ShooterSpek-trenauszunutzen, habeicheineinnovativeMethodezurAnalyseentwickelt, umdarausAkkretions-undSterneigenschaftenmiteinemautomatischenAlgorithmusabzuleiten. DieseMethodebasiertaufeinerReihevonModellen, dieichausphotosphärischenVorlagenfürjungeSterneausgewrtethabe, aus``slab''Modellen, dieichprogrammierthabeumdieEmissiondesAkrretionsschockszureproduzieren, undeinerAnnahmefürdieRötungdesLichtsdurchExtinktion. DieseMethodeermöglichteszumerstenMaldiestellarenundAkkretions-parameterderObjekteeinheitlichundohnevorigeAnnahmenzubestimmen.DiesermöglichteinesignifikanteVerbesserung, verglichenmitvorangegangenenStudien.IchhabedieseMethodeangewandt, umdiekorrektenstellarenParametervonzweiOb-jektenimOrionNebelzubestimmen, dieinderLiteraturmitstarkabweichendemhohenAlterangegebenwurden. MeineAnalysehatgezeigt, warumvorherigeUntersuchungendieseWidersprüchenichtauflösenkonnten, aberwarummeineMethode, dieichindieserArbeitentwickelthabe, diesermöglicht.

IchhabemeineMethodeaufeinrelativgroßeAuswahlvonScheibenimÜbergangssta-diumangewandt, fürdiemanannimmt, dasssieweitentwickelteScheibensind, weilderenStaubscheibegroßeLückenzwischenderäußererScheibeunddemZentralsternaufweist.Ichhabegezeigt, dass, inAnwesenheitvonAkkretion, dieEigenschaftenderweitentwick-eltenÜbergangsscheibengleichdererinwenigerentwickeltenScheibensind.

Unter derAnnahmevon steady-stateBedingungen, impliziert dies eineneffizientenMechanismus, derGasvonderäußerenScheibedurchdiestaubarmeLückeindieinnereRegionderScheibetransportiert. UmdieEntwicklungderAkkretionzuuntersuchen, habeich alle 90 aufgenommenenX-Shooter Spektren von jungen Sternenmit Scheiben ver-wendet. DieseSpektrenumfassenverschiedenestellareUmgebungenundSternmassenundmeine genaueMethode zurAnalyse ermöglicht eine deutlich verbesserte Bestim-mungderAkkretions-Sternenmasse-Relation. DievonmirbestimmteSteigungdieserRela-tionstimmtgutmitdenVorhersagenderRöntgenstrahlungs-Evaporationsmodelleüberein.AußerdemkanndiesignifikantkleinereStreuungderWerteimVergleichmitfrühergefun-

xix

Zusammenfassung

denengrößerenStreuungen, miteinerkleinenStreuungderAnfangsbedingungen, wiezumBeispielanfänglicheKernrotation, erklärtwerden. DurchdasEntfernenderAbhängigkeitderAkkretionsratevonderSternmasse, waresmirmöglichnachreinevolutionärenTrendsinderAkkretionzusuchen. AllgemeinkönnenEntwicklungsmodelle, diedieViskositätberücksichtigen, denbeobachtetenTrendderScheibenparametermitkleinenAbweichun-gengutreproduzieren.

UmdieindieserArbeitaufgezeigteMethodefortzusetzen, solltenzukünftigeAkkre-tionsstudiensichmöglichstaufeinevollständigeAuswahlanObjektenausverschiedenenSternentstehungsgebietenfokussieren. DiesekönnendannmitDaten, dieandereEigen-schaftenvonprotoplanetarischenScheibenuntersuchen, vonanderenaktuellenBeobach-tungen, zumBeispielimALMA sub-mmWellenlängenbereich, kombiniertwerden.

xx

ListofAcronyms

YSO YoungstellarobjectPMS Pre-Main-SequencecTTs ClassicalTTauristarwTTs WeaklinedTTauristarTD TransitionaldiskHRD Hertzsprung-RusselldiagramSpT SpectraltypeTeff EffectivetemperatureL⋆ Stellar(bolometric)luminosityM⋆ StellarmassR⋆ StellarradiusAV Extinctionin V -bandLline LineluminosityLacc AccretionluminosityMacc MassaccretionrateLacc,noise NoiseontheaccretionluminosityduetochromosphericactivityMacc,noise NoiseonthemassaccretionrateduetochromosphericactivityONC OrionNebulaClusterUV UltravioletUVB UltravioletarmoftheX-Shooterspectrograph(λλ 300-559.5nm)VIS VisiblearmoftheX-Shooterspectrograph(λλ 559.5-1024nm)NIR Near-infraredarmoftheX-Shooterspectrograph(λλ 1024-2480nm)VLT VeryLargeTelescopeGTO Guaranteedtimeobservations

xxi

List of Acronyms

xxii

1Introduction

Whenlookingatthenightskyandstaringatthemultitudeofstarsbigquestionshaveal-waysaroseinmankind, fromthequestionsofwhatstarsareandhowtheyhaveformed,toultimatequestionssuchas: ``ArewealoneintheUniverse?'' or ``A chetantefacelle?''(``Whereforesomanylights?'', G.Leopardi). Astronomyisthesciencethataimsatanswer-ingthescientificquestionsthatcometoourmindwhenlookingatthenightsky. Butwedonothaveonlyquestions, wealsohaveevidence. Inparticular, ifwehaveoneevidence,thisisthatplanetsform. Weliveonaplanet, theonlyplanetknown, sofar, thatishost-inglife. Somehow ∼4.5billionyearsagosomethinghappenedandthisbeautifulplanetwasformed. Wealsoknowthatotherplanetsexist, intheSolarSystembutalsobeyond.Inrecentyears, wearediscoveringthatextraSolarSystemplanets(exoplanets)notonlyexist, butarecommonaroundMainSequencestars(e.g., Mayor&Queloz, 2012). Weknowthatplanetsforminthediskssurroundingformingstars, theso-called``protoplane-tarydisk'', buthowthiswholeprocessofstarandplanetformationinfactworksisstilltobeunderstood.

ThisThesisaimsatunderstanding theprocesses that lead to the formationofa star,its interactionwith thesurroundingprotoplanetarydisk, andultimatelyhowthisaffectshowplanetsareformed. Inparticular, I willfocusonwhatwecanlearnfromthestudyoftheaccretionofmatterfromtheprotoplanetarydisktothecentralformingstar. HereIintroducethecontextinwhichthisThesissets, startingfromageneralintroductionoftheprocessofstarandplanetformation, thendiscussingindetailtheprocessofaccretion, andfinallyexplainingthepathofthisThesis, whichwillbediscussedinthenextchapters.

1.1 Settingthescene: theevolutionarypathfrommolecularcloudcorestostarsandplanets

Theprocessofstarformationstartsfromthegravitationallyinducedcollapseofadensemolecularcloudcore(e.g., Shuetal., 1987, toppanelofFig. 1.1). Eachcollapsingcoreresults intheformationofoneormoreprotostarsstillembeddedintheparentalcloud.Duringthisstage, thejustformedprotostarisreferredtoasaClass 0object. Anysmallinitialrotationalvelocityofthecloudismagnifiedbyitsrapidcollapse, andtoconservetheangularmomentumthesystemevolves formingadiskaround thecentralprotostar.

1

1. Introduction

Theprotostar-disksystemisstilltooembeddedtobeobservedatopticalwavelength, butthediskisdetectedatmm-wavelengthusinghighresolutioninterferometry(e.g., Tobinetal., 2013). As theenvelope fallsonto theprotostarandonto thedisk itbecomesmoreopticallythinandthesystementersintheso-calledClass I stage. DuringtheClass 0andClass I phasesthesystemoriginatespowerfuljetsthatdispersealargeamountofangularmomentum(e.g., Franketal., 2014). Indeed, atthebeginningofthecollapsetheangularmomentumof the cloudcore is ordersofmagnitude larger than the total angularmo-mentumofthestar(s)thatwillbetheoutcomeoftheprocess. Thisexcessmomentumisdispersedbythesystempossiblybyjetlaunchingordiskwinds, andbyviscousprocesseswhichdriveaccretionofmaterialfromthediskontothestar. Thisaccretionprocessstartsalreadyinthesefirstphasesatahighpace, anddecreaseswithtimeasI willdescribeinSect. 1.2.

As theenvelope isdissipated theyoungstellarobject (YSO) isobservableatvisiblewavelengthsand is referred toasaClass II object, or classicalTTauri star (cTTs). Dur-ingthisphasetheprotoplanetarydisk, composedofdustandgas, isopticallythickandevolvesviscously(cf. Sect. 1.3). Thisisanimportantphaseforaccretionstudies, asallthesignaturesofaccretionandthespectralfeaturesusefultodeterminethestellarpropertiesofthecentralobjectareobservable. Duringthisstagetheluminosity(L⋆)andtemperature(Teff)ofthecentralprotostar1 aresuchthatthelocationoftheseobjectsisabovetheMainSequenceontheHertzsprung-Russeldiagram(HRD),i.e. L⋆ islargerthantypicalMainSequencestarsofthesame Teff. Thisisduetothelargerradiioftheseobjects, whichstillcontractduetogravitationalcollapseuntiltheyreachtheMainSequence. Duetothislo-cationontheHRD,YSOsinthisandlaterphasesareusuallycalledPre-Main-Sequence(PMS) objects. ThetimeusuallyneededbythesePMS starstoreachtheMainSequencecanbeevenlongerthan ∼100Myr, dependingonthe Teff oftheobject. Ontheotherhand, theClass II phase, i.e. thestageinwhichtheprotoplanetarydiskisstillopticallythick, isfoundobservationallytolasttypically∼3-5Myr(e.g., Haischetal., 2001; Hernándezetal., 2007;Fedeleetal., 2010, seeFig. 1.2). Thisfinding, obtainedwithdifferentproxiesfordetectingdisks(e.g., IR-excess, accretionsignatures), hasbeenrecentlychallengedbythediscoveryofopticallythickdisksaroundolderobjects(e.g., Beccarietal., 2010). Also, Belletal.(2013)showedthatthetypicaltimescaleoftheopticallythickdiskphasecouldbelongerduetoincorrectestimateoftheagesofsomestarformingregions. ThiswillbediscussedinmoredetailinChapter 4. However, itistruethat ingeneral thefractionofobjectsstillsurroundedbyanopticallythickdiskinagivenregiondiminisheswithtimefollowinganexponentialdecrease. Thisisastringentconstraintontheavailabletimetoformaplanetinaprotoplanetarydisk, andabetterunderstandingofthistimescaleiscrucialforplanetformationtheories.

Beforethediskisdissipated, (atleast)afractionoftheobjectsundergoaphasewheretheypresentanopticallythinregion, i.e. significantlydustdepleted, thuscalled hole orgap, while theouterpart is stilloptically thick. TheYSOs in thisphaseare referred to

1DuringthecourseofthisThesisI couldalsorefertothisclassofobjectsas stars. However, itisworthmentioningthatprotostars, unlikemainsequencestars, arenotyetburninghydrogen.

2

1.1 Setting the scene: the evolutionary path from molecular cloud cores to stars and planets

Figure 1.1: Currentpictureoftheevolutionarysequenceofyoungstellarobjects. Thethickarrowshowsincreasingtime. Cartoonsarenotonscale. Adaptedfrom André (2002).

3

1. Introduction

Figure 1.2: Fractionofyoungstellarobjects surroundedbyprotoplanetarydisks indifferent star formingregionsasafunctionoftheageoftheregions. Thebluelinereferstothefractionofobjectswithopticallythick innerdiskdetected frommid-infraredobservations, that is fromdustemission. The red line is thefractionofaccretingobjects, thuswithasignificantlydenseinnergaseousdisk. From Fedeleetal. (2010).

astransitionaldisks(TDs, e.g., Espaillatetal., 2014), butwhetheralltheobjectsundergothisphaseisnotclear, aswellasthepropertiesofthedisksinthisclassofobjects. I refertoChapter 5 ofthisThesisforadetailedanalysisoftheseobjects. I justreportherethatapossibleexplanationforthedepletionofdustintheinnerpartofthesedisksistheformationofaplanet. However, it is stillunclearwhether these gaps in thedisksarecreatedbyplanetsalreadyformedoractuallypromoteplanetformation. Thefinalphaseofevolution,theClass III stage, referstoYSOswhosecircumstellardiskhasbecomeopticallythinduetoitsevolution. Inthisphasetheinteractionwiththecentralstarthroughaccretionisnotoccurringanymore. Theiremissionlines, whicharerelatedtotheaccretionprocessasIdiscussinSect. 1.2, arethereforeweakeranddueonlytochromosphericemissionatthisstage. Non-accreting(Class III) YSOsarealsoreferredtoasweaklinedTTauristars(wTTs).TheseYSOsareofgreatinterestforseveralreasons, inparticularbecausetheyaregreatphotospherictemplatesofYSOs, asI willdiscussinChapter 3.

Duringthesephasesplanetformationtakesplace. Itisstillnotclearwhenthishappens,eitherinthesefirstfewMyrsofevolutionoreveninthehypotheticalcaseofaseconddiskformationevent(Sciclunaetal., submitted)atlaterstages. Inanycase, thecurrentideaisthatnoplanetscanformwithoutthepresenceofagasand/ordustrichdisk. Indeed, planetformationisthoughttohappeneitherbygrowthofsolidparticlesinthediskswhichaccreteotherparticlesuntiltheybecomeaplanet(coreaccretion scenario), orbyfragmentationofmassiveandgravitationallyunstabledisks(diskinstability scenario). Inthiscontext, abetterunderstandingontheevolutionaryprocessesandofthepropertiesoftheprotoplanetarydiskateveryphaseofevolutionisofhelpforplanetformationtheories. InthenextSection

4

1.2 Star-disk interaction: accretion

I discussindetailwhytheprocessofaccretionisofparticularinterestinthiscontext.

1.2 Star-diskinteraction: accretion

AsintroducedinthepreviousSection, protoplanetarydisksareoneofthemostimportantmeansbywhichYSOsdissipateangularmomentum. Thishappensthroughturbulentpro-cessesinthediskwhichresultinviscousaccretion. Theseminalworkof Lynden-Bell&Pringle (1974)describesthisprocesswiththeideathatinfinitesimalmassparticlescarryalltheangularmomentumonacircularorbitataninfinitelylargeradius, whiletheremain-ingmassdriftsinwardsaftertransferringallitsangularmomentumtotheaforementionedparticles. Thematerialdriftinginwardsisaccretedonthecentralstar. Inthispicturetheparticlesinthediskneedtohavesomekindofturbulentinteractionthroughwhichan-gularmomentumisexchanged. Thisisthoughttobeaconsequenceofthedevelopmentofmagneto-hydro-dynamic(MHD) instabilities, andinparticularofmagneto-rotationalin-stabilities(MRI).Otherpossibilitiesfordrivingturbulencesaregravitationalinstabilityorbaroclinicvortices. However, atpresenttimenoneoftheseprocessesalonecanproducestrongandstableturbulencesinthewholedisk(e.g., Turneretal., 2014). It isusualtomodelviscositythroughthesimpleandsuccessfulparametrizationintermsofanunknowndimensionlessparameter, theShakura-Sunyaev α parameter(Shakura&Sunyaev, 1973).Fromdimensionalanalysisitispossibletoscalethekinematicviscosityintermsofachar-acteristiclengthandaturbulentvelocity(Hartmann, 2009). Itisalsogenerallyassumedthatthelengthscaleoftheturbulenceinadiskislessthanthescaleheight(H), andthattheeddyvelocityislessthanthesoundspeed(cs), sinceotherwisethesewouldbedissi-patedthroughshocks. Accordingtothisprescription, thekinematicviscosity(ν)hasthefollowingform: ν = αcsH. Thevalueof α isconstrainedtobe α ≤1forthetwopreviouslystatedupperlimits. I stressthatthisformisjustaparametrizationbasedondimensionalanalysis, notatheoryofviscosity.

Infact, accretionisknowntohappenandviscousevolutionofdisksiscommonlyob-served (see Sect. 1.4.1). Themainobservational signaturesof accretionpresent in thespectraofcTTsarethestrongexcesscontinuumemissionintheultraviolet(UV) partofthespectrumandthewealthofstrongemissionlinesatallwavelengths. Thosearemostlyhydrogenserieslines, suchasBalmer(Hα, Hβ, etc.), Paschen(e.g., Paβ), orBrackett(e.g.,Brγ)serieslines, butalsohelium(e.g., He I lineat λ 587.6nm)andcalciumlines(e.g.,Ca II infraredtripletat λλ 849.8-854.2-866.2nm). Thesesignaturesareoriginatedintheregionwherethereisinteractionofthediskwiththecentralformingstar. Thecurrentandwidelyacceptedparadigmdescribinghowthematerialisaccretedfromthediskontothestaristhe magnetosphericaccretion scenario, whichI introduceinthenextsubsection.

1.2.1 Magnetosphericaccretion

Toexplaintheobservationalsignaturesofaccretiondifferentmodelshavebeendeveloped.In thedescriptionof Lynden-Bell&Pringle (1974) thediskextendsdown to the stellar

5

1. Introduction

Figure 1.3: Schemeofthemagnetosphericaccretionscenario. Thematerialischanneledfromthediskontothestarbythestrongmagneticfieldsoftheyoungstellarobjects. From Hartmann (2009).

surfaceandtheinnermostpartofthedisk-theso-called boundarylayer -emitsaradiationwhichisstrongintheUV partofthespectrum. Thispicture, whichI discussinmoredetailinSect. 2.1, isknownnottobevalidgiventhatthestrongmagneticfieldsoftheseYSOs(e.g.,Johns-Krull, 2007; Johnstoneetal., 2014)disruptthediskatfewstellarradii. Moreover,thestrongemissionlinesobservedinthespectraofcTTshavealsoverybroadlineprofiles(≳200km/s)whichare thought tobeconnectedwithmaterialmovingathighvelocity(e.g., Muzerolle et al., 2003). Finally, this process of accretion is known to behighlyvariable(e.g., Costiganetal., 2012, 2014)andthesurfaceofthesestarspresentvariousnon-axisymmterichot spots (e.g., Bouvier et al., 2007). Themagnetosphericaccretionmodelis, sofar, theonlymodelthatcanexplainalltheseobservablesinaqualitativeway.

InFig. 1.3 I showaschematicviewoftheinnerdiskinthecontextofmagnetosphericaccretion(e.g., Hartmann, 2009). Insidethedustsublimationradiusthediskiscomposedonlyofgas, whichismostlyionizedbythestrongradiationofthecentralstar. Thisionizedmaterialischanneledbytheinteractionwiththestellarmagneticfieldstofallonthecentralobjectonhotspots, whicharethoughttobelocatedawayfromthestellarequatorinthecaseinwhichthemagneticfieldissimplydipolar. Thedistancefromwhichthematerialisaccretedon thestar iscommonlyassumed tobe ∼5stellar radii (R⋆), but thisvaluedependsonthestrengthofthemagneticfieldand, tolesserextent, tothemassaccretionrate(Macc)andthemass(M⋆)ofthestar, asI discussinChapter 5. Thematerialchanneledalongthemagneticfieldlinesisacceleratedandreachesalmostfree-fallvelocity, givingraisetothebroademissionlinesobservedinthespectra. Whenthematerialimpactsthestellarsurface, thekineticenergyisdissipatedlocallycreatingahotspot(T ≳ 8000-10000K),thatisresponsibleofthecontinuumexcessemissionintheUV partofthecTTsspectra.Dependingonthegeometryofthesystem, thismodelcouldexplainthevariabilityoftheaccretiontracers(lines, continuum), eveninasteadystateaccretionconfiguration, asa

6

1.3 Accretion as a tracer of protoplanetary disk evolution

changeofthelineofsightoftheobject, whereinsomecasestheaccretionshocksandfunnelsarevisible, whiletheyarenotinotherrotationalperiodphases(e.g., Costiganetal., 2014).

In this context, the gravitationalpotential energy released is converted in accretionluminosity(Lacc)accordingtothefollowingrelation:

Lacc =GM⋆Macc

R⋆

(1− R⋆

Rm

), (1.1)

where Rm is theradiusatwhichthedisk is truncatedbythemagneticfield. AssumingRm = 5R⋆ (e.g., Gullbringetal., 1998), theamountofmaterialaccretedonthestaris:

Macc =LaccR⋆

0.8 GM⋆

. (1.2)

1.3 Accretionasatracerofprotoplanetarydiskevolution

AsI discussedsofar, accretionisaprocesspresentatalltheearlystagesofstarandplanetformation. Itstartsasaccretionontothestar-disksystemfromtheparentalenvelope, andthenproceedasaccretionfromthediskontothestardrivenbyviscousprocesses. Thisprocessisknowntohavestrongimpactonthepropertiesofthedisk. Inparticular, itisimportant to stress thatplanetsare forming in evolving disks, where theamountofgasanddust, thechemicalcomposition, thetotalmass, andmanyotherparametersarenotconstantwithtime. Theprocessofaccretiongovernsandkeepstrackofthisevolution, andforthisreasonitiscrucialtounderstandhowitworks. A clearconnectionbetweenthestrengthoftheaccretionprocessandthediskcompositionisgivenbytheamountofgaspresentintheinnermostregionofthedisk. AsI alsodiscussinChapter 5, thedensityofthegasintheinnerdiskdependsdirectlyon Macc. Moreover, recentstudiesareshowingthataccretionburstschangethechemicalcompositionofthedisk, forexamplebyinfluencingthechemistryof thegaseousphaseof ices inprotoplanetarydisks (e.g., Banzattietal.,2012, 2014).

Tobetterunderstandthisprocessanditsdependencewithdiskevolutionitisimportanttostudyhowaccretionevolveswithtime, i.e. withtheageofthecentralobject, andhowitdependsonthepropertiesofthecentralYSO,suchasitsmass. ThiswillbethepathofthisThesis, asI describeinSect. 1.4, whileI discussthecurrentmodelsadoptedtoexplainsomeofthesepropertiesofaccretioninthefollowingtwosubsections.

1.3.1 Evolutionofaccretionrateswithtimeinaviscousdisk

TheevolutionofaninfinitesimallythinviscuousKepleriandiskwithtimeisdescribedwiththebasicsequationsofviscousfluiddynamics: thecontinuityequationandtheNavier-Stokesequations. Beingthediskassumedtobeinfinitesimallythin, theseequationscan

7

1. Introduction

bewrittenincylindricalpolarcoordinatescenteredonthestar. Withthefurtherassumptionthatthediskisaxisymmetric, the continuityequation hasthefollowingexpression:

∂Σ

∂t+

1

R

∂

∂R(RΣvR) = 0, (1.3)

where Σ isthesurfacedensity, R theradiusofthedisk, and vR theradialvelocity.Asshownby, e.g., Lodato (2008), thegeneralequationdescribingtheevolutionofthe

surfacedensityinaKepleriandiskisderivedbysolvingthemomentumequationinclud-ingviscous forces, knownasNavier-Stokesequation, forageometrical thindisk in theazimuthaldirection. Theexpressionoftheconservationofangularmomentumequationisderivedtobethefollowing:

∂

∂t(ΣΩR2) +

1

R

∂

∂R(ΣvRR

3Ω) =1

R

∂

∂R

(νΣR3dΩ

dr

), (1.4)

where Ω istheorbitalfrequency. BycombiningthelatterequationwithEq. (1.3), thetermvR iseliminatedand, using Ω =

√GM⋆/R3, the diskevolutionequation isobtainedinthe

form:∂Σ

∂t=

3

R

∂

∂R

[R1/2 ∂

∂R(νΣR1/2)

]. (1.5)

Thisisadiffusivepartialdifferentialequationwhichis, ingeneral, notlinear. However,iftheviscosity ν isnotafunctionof Σ, thenthisequationislinearandsolutionscanbeobtained. Therefore, theevolutionofaviscousdiskdependsupontheformoftheviscosityν, which in turncouldhaveacomplicateddependenceon thediskproperties. In thefollowingI derivesomesimplesolutionsundersomegeneralassumptionsonthekinematicviscosity.

Steadystatesolution

Firstofall, it isusefultoderiveasolutionofEq. (1.5)whichisindependenton ν. Thiscanbedoneassumingthattheflowinthediskissteady. AsI showbelow, alsosimilaritysolutionssuggestthatthemassflowisnearlyconstantwiththediskradiusatagiventime,sothiscanbeareasonableassumption.

Consideringthatthemassinaninfinitesimaldiskannulusis dM = 2πRΣdR, themassflowinthedisk(M )is:

M =dMdt

= −2πRΣvR, (1.6)

wherethemassfluxisassumedtobeinward(i.e. anegative vR impliesapositive M ). Inthecaseofsteadysurfacedensityevolution, thecontinuityequation(Eq. (1.3))implies:

RΣvR = const, (1.7)

since ∂Σ/∂t vanishesbyhypothesis. This implies that M is constantwith radius. ThesolutionofEq. (1.5)isthenobtainedfromEq. (1.4), assumingthatthe ∂/∂t termiszero,

8


Figure 1.4: Evolutionofthedisksurfacedensity(Σ)inthecasewhere ν isaconstantandtheinitialsurfacedensitydistributionisathinringcenteredat R = R1 (spreadingring). VariousplotsshowthedistributionofΣ fordifferentvaluesof τ , startingfromthemostpeakeddistributionat τ =0andspreadingoutwithtime.From Hartmann (2009).

that M = −2πRΣvR (Eq. (1.6)), integratingover R, assumingthatat R = Rin thereisnotorque(i.e., dΩ/dR|Rin = 0), andsubstituting Ω =

√GM⋆/R3. Thissolutionhastheform:

Σ(R) =M

3πν

(1−

√Rin

R

). (1.8)

Atradii R ≫ Rin, thisequationimpliesthat 3πνΣ ∼ M thatmeans, since M isconstant,that Σ and ν areinverselyproportional. In thisworkI willmeasure M onthestar, i.e.Macc= M(R → 0). Giventhatitisnotyetpossibletomeasure M(R) alongthedisk, itisnotyetpossibletoconstrainwhetherthissteadystatecaseisplausible.

Spreadingring

Theeasiestassumptionontheviscosityisthat ν isconstantwithtimeandindependentofRandΣ. Inthiscase, thesolutionofEq. (1.5)isknownasthe spreadingring (e.g., Lynden-Bell&Pringle, 1974), asthediskisexpandingbothinwards(accretingonthestar)andoutwards(transferringangularmomentum)undertheactionofviscousforces. Thisspreadingpermitstheconservationofangularmomentumwhiledissipatingthisquantityfromthecentralstartoouterdiskradii. Thissolutionisobtainedassumingasinitialconditionaninfinitesimallythinringofmass m, whoseshapeisdescribedwitha δ functioncenteredattheradius R1,thatis:

Σ(R, t = 0) =m

2πR1

δ(R−R1). (1.9)

9

1. Introduction

The solutionof Eq. (1.5)with this initial conditionhasbeenderivedby Lynden-Bell&Pringle (1974)tobe:

Σ(x, τ) =m

πR21

x−1/4

τe−

(1+x2)τ I1/4

(2x

τ

), (1.10)

where I1/4 isamodifiedBesselfunction, x = R/R1, τ = 12νt/R21 = t/tν , withtheviscous

timescale(tν )definedas:

tν ∼ R21

ν. (1.11)

Theevolutionof Σ describedbyEq. (1.10)isshowninFig. 1.4. AsI introducedabove,itisclearthatthediskisspreadingwithtime. Thematerialmovinginwardslosesangu-larmomentumandaccretesonthestar, whilematerialsmovingoutwardsgettheexcessangularmomentumandbringitfarfromthecentralstar. Theradiusatwhichthereisthetransitionbetweenmaterialmovinginwardsandoutwardsis Rtr ∼ tν/R1 ∼ R1 (t/tν). Thismovesoutwardswithtime, thusimplyingthattheangularmomentumwillbetransferredtoaninfiniteradiusbyanegligiblefractionofmass.

Similarity-solution

Themostinterestingassumptionontheviscosity, however, istoassumethatthisdependsonthediskradiusasapower-law, i.e. ν ∝ Rγ. WhensolvingEq. (1.5)withthisassump-tionontheviscosity, thesolutionsareusuallyreferredas self-similarsolutions. Theinitialconditionsforthissolutionaredescribedas:

Σ(R, t = 0) =C

3πν1rγexp

(−r(2−γ)

)(1.12)

where r ≡ R/R1 and ν1 ≡ ν(R1), being R1 thecut-offradius, and C isascalingconstant.Thesesolutionshavebeenderivedinthegeneralcaseby Lynden-Bell&Pringle (1974)tobe:

Σ(R, t) =C

3πν1rγT−(5/2−γ)/(2−γ) exp

(−r(2−γ)

T

), (1.13)

wherethedimensionlesstime T is:

T = t/tν + 1, (1.14)

withtheviscoustimescalebeing, inthiscase:

tν =1

3(2− γ)2R2

1

ν1. (1.15)

FromEq. (1.13)itispossibletoderivetheequationforthemassfluxthroughthedisk:

M(R, t) = −2πRvRΣ = CT−(5/2−γ)/(2−γ) exp(−r(2−γ)

T

)[1− 2(2− γ)r(2−γ)

T

]. (1.16)

10


Figure 1.5: Evolutionofthedisksurfacedensityaccordingtosimilaritysolutionsinthecasewhere ν ∝ R.Thelinesshow, fromtoptobottom, theself-similarsolutionatincreasinglylargetimes. (From Lodato (2008))

Inthecase T ≫ 1, thatiswhenthetimeismuchlongerthantheviscoustimescale, wherethediskhasevolvedandthusisindependentfromtheinitialconditions, themassaccretionrateonthestar(Macc), i.e. thelimit r → 0, hasthisform:

Macc(t) ∝ t−(5/2−γ)/(2−γ) ∝ t−η, (1.17)

andthisequationisvalidonlywhen γ < 2 forexistenceconditions. Inthisframework Macc

decreaseswithtimeasapower-law, wherethenewparameter η = (5/2− γ)/(2− γ) istheexponent. Thisvaluecanbedeterminedfromobservations, asI willdescribeinSect. 1.4.1.Thiscaninturnconstrainthedependenceofviscosityontheradius, i.e. γ, inthecontextofsimilaritysolutions, fromthefollowingrelation:

γ =2η − 5/2

η − 1. (1.18)

Itiseasilyunderstoodthatthemassofthediskshoulddecreasewithtime, i.e. Mdisk ∝ t−α

with α > 0. Thisdecreaseofmassofthediskisduetoaccretionontothecentralstar, i.e.Macc= dMdisk/dt ∝ t−(α+1). Thisimpliesthat η = α+1 whichresultsinthecondition η > 1if α > 0.

I showinFig. 1.5 theevolutionofthedisksurfacedensitypredictedbythesimilaritysolutioninthesimple(butnottoounrealistic)casewhere γ = 1, i.e. ν ∝ R. Thiswouldimplythat Macc decreaseswithtimeasapower-lawwithexponent η=1.5. Thesolutionsin thiscase, derivedbye.g., Hartmann (2009), lead to somesimpleexpressionsusefultodescribetheevolutionofatypicalTTauristardisk. Assumingthatthemainsourceof

11

1. Introduction

104 105 106 107 108

Age [yr]

10-11

10-10

10-9

10-8

10-7

10-6

Macc [

M¯/y

r]

Figure 1.6: Evolutionofmassaccretionrateswithtimeaccordingtosimilaritysolutionviscousmodels. ThisiscalculatedusingtheexpressionsofEq. (1.23), R = 5R⋆, andthefiducialparametersof Hartmann (2009).

irradiationof thedisk is thecentralstar, that thedisk is ``flared'' insuchawaythat itstemperaturecanbedescribedas:

T (R) ∼ 10

(100AU

R

)1/2

K, (1.19)

andthatthediskhadan``initial''radius R02, theevolutionofthesurfacedensityis:

Σ ∼ 1.4× 103e−R/(R0td)

(R/R0)t3/2d

(Md(0)

0.1M⊙

)(R0

10AU

)−2

g cm−2, (1.20)

with Md(0) beingtheinitialdiskmass, and td isrelatedtotheageofthesource t by:

td = 1 +t

ts, (1.21)

wherethescalingtime ts is:

ts ∼ 8× 104(

R0

10AU

)( α

10−2

)−1(

M⋆

0.5M⊙

)1/2(T100

10K

)−1

yr. (1.22)

2R0 isthelengthscaleoftheinitialdensitydistributionofthedisk, i.e. theradiusatwhich60%ofthemassresidesinitially.

12


Also M canbeexpressedinthefollowingsimpletermundertheaforementionedassump-tions:

M ∼ 6× 10−7 e−R/(R0td)

t3/2d

(1− 2R

R0td

)(Md(0)

0.1M⊙

)(R0

10AU

)−1

×( α

10−2

)( M⋆

0.5M⊙

)−1/2(T100

10K

)M⊙ yr−1,

(1.23)

with T100 beingthedisktemperatureat100AU.Theevolutionof Macc, i.e. M calculatedat R = 5R⋆, with timeaccording to this

descriptionisshowninFig. 1.6 usingthefiducialparametersof Hartmann (2009).Accordingtothisdescriptionoftheevolutionofviscousdisks, observationalconstraints

ontheslopeoftherelationbetween Macc andtheageoftheobjectscanbetranslatedingeneralpropertiesof theprotoplanetarydisk. Inparticular, theevolutionof thesurfacedensityinthedisk, thatmeansoftheamountofmaterialavailabletoformplanetsatvar-iousradii, isderivedfromthisobservation. AsI describeinSect. 1.4.1, determiningtheexponentofthispower-lawdecayhasbeenoneofthefirsttasksofobserversinthelastfifteenyears. I willthendiscussinChapter 6 theconstraintsI canputonthismodelandonthephysicalconditionsofthedisksfromthedataanalyzedinthisThesis.

1.3.2 Evolutionofaccretionrateswithtimeduetophotoevaporation

Anotherimportantprocessgoverningtheevolutionofprotoplanetarydisksisphotoevap-oration. Thisprocess, inparticular, is thought tobeoneof themaindriverof thefinaldispersalofdisks. Inparticular, internalphotoevaporationfromthecentralstarisprobablypresentinmost, ifnotall, theYSOs, atleastatlaterstagesofPMS evolution, i.e. intheClass II phaseand inTDs. Thisisdrivenbythehigh-energyradiationcomingfromthecentralstar, whichcanbeUV and/orX-rays, heatingthegasonthesurfaceofthedisktosufficientlyhightemperaturestounbounditfromthegravitationalwellofthestar. Thishappensatsufficientlylargeradiiwherethethermalenergyoftheheatedlayerexceedsthegravitationalbindingenergy. Underthisconditiontheheatedgasescapesthediskinaphotoevaporativewind (e.g., Armitage, 2010; Alexanderetal., 2014). Here I donotdiscussthedetailsofthevariousphotoevaporationmodels, butI stresswhatisthegeneralqualitativepredictionofallthemodels, whichisthatthediskisclearedviaaninside-outprocess. Thequantitativedifferencesbetweenthemodelsarediscussed, e.g., by Alexanderetal. (2014, andreferencestherein).

In general, the evolutionof a viscousdiskunder the effects of photoevaporation isdescribedbytheevolutionequationEq. (1.5)withtheadditionofatermdescribingtheremovalofmassduetothisprocess:

∂Σ

∂t=

3

R

∂

∂R

[R1/2 ∂

∂R(νΣR1/2)

]+ Σwind(R, t), (1.24)

wherethismasslossterm(Σwind)variesaccordingtothedifferentmodelsofphotoevapo-ration. Thereasonforwhichitispossibletoincludethistermintheevolutionequation

13

1. Introduction

Figure 1.7: Evolutionofmassaccretionratesaccordingtodifferentphotoevaporationmodels. Thesolidlinesrefertovaluesof Macc reportedontheleftaxis, whiledashedlinestovaluesofthemasslossrate, thatarereportedontherightaxis. Thesequantitiesareshownasa functionof time. The black lines are for theEUV photoevaporationmodel, the bluelines fortheFUV photoevaporationone, whilethe redlinse forX-raymodel. From Alexanderetal. (2014).

isthattheflowleavingthediskhasthesamespecificangularmomentumasthediskatthelaunchpoint(e.g., Armitage, 2010). Accordingtophotoevaporationmodelstherearethreephasesofthediskevolution. Inthefirstphasetheinwardmassflowinthediskduetoviscousevolutionismuchlargerthanthemasslossinthewindduetophotoevaporation(Macc≫ Mwind). Inthisphasetheeffectsofphotoevaporationarenegligible. Eventually,Macc, whichdecreaseswithtimeasexpectedbyEq. (1.17), becomescomparabletothewindmasslossrate. Atthisstage, thematerialdriftinginwardsfromtheouterradiusisde-tachedfromthediskatthegravitationalradius Rg = GM⋆/cs, whichisthelocationwherethesoundspeed(cs)oftheionizedgasisequaltotheorbitalvelocityinthedisk. Atthisdiskradiusthereisthedevelopmentofanannulargapinthegassurfacedensity, asallthematerialinflowingfromtheouterdiskisdivertedintothephotoevaporatedflow. Fromthispoint, theinnerdiskiscutofffromitsouterdiskmassreservoirandrapidlyaccretesontothecentralstar. Thetimescaleonwhichthisdrainingoftheinnerdiskhappensistheviscoustimescale, which, forthisradius, is ∼ 105 yr. Finally, theouterdiskisdirectlyirradiatedbyhigh-energyradiation. Thisresultsinanincreasedrateofmasslossduetophotoevaporation, andinafastdispersaloftheouterdiskonsimilartimescales(e.g., Owenetal., 2012). Following Alexanderetal. (2014), I showinFig. 1.7 theexpectedevolutionof Macc and Mwind accordingtothethreemainmodelsofphotoevaporation(FUV,EUV,X-rays). Inthisplotisclearthattheevolutionof Macc isthesameastheoneexpectedfromviscousevolutionmodelsuntil Macc∼ Mwind, whenarapiddecreaseof Macc, duetotherapiddiskdispersal, happens.

14


This inside-outmechanism isoneof theclaimedmechanism toexplain transitionaldisks(seeSect. 1.1 andChapter 5), butstillcannotexplainalltheobservations. Surely,thisprocessisknowntohappenbutalsotocoexistwithothermechanisms, andtheinter-playbetweenthesevariousprocessesisstillunderdebate(e.g., Rosottietal., 2013). Forexample, I discussinChapter 5 thatthetransitionaldisksanalyzedinthisThesisarestillaccretingataratesimilartocTTs, thushavestillagaseousinnerdisk. Possiblysomeofthesediskshavenotevolvedduetophotoevaporation.

1.3.3 Thedependenceofaccretionrateswiththemassofthecentralstar

Thedependenceof Macc onM⋆ wasinitiallynotexploredindetailfromatheoreticalpointofview. Thebasic reasonwas that inaviscousdiskwhereviscosity isdrivenbyMRIturbulencethereisnodependencebetweenthetwoquantitiesaccordingtotheory. Forexample, usingthelayeredaccretiondisktheory, Gammie (1996)showedthat Macc intheinnerdiskis:

Macc = 1.8× 10−8( α

10−2

)2( Σa

100 g · cm−2

)3

M⊙ yr−1, (1.25)

where Σa is the surfacedensity in theactive layerof thedisk, and α theusualparam-eter todescribe thedisk viscosity. In this result there is no explicit dependenceuponM⋆, andnofurther theoreticalargumentswereavailableat that timetopredictanyde-pendencebetweenthesetwoobservables. AsI describeinSect. 1.4.1 indetail, thefirstobservationalworksaimedatstudyingaccretionincTTswereallfocusedonobjectswithM⋆ ∼ 0.5 − 1 M⊙ (e.g., Valentietal., 1993; Kenyon&Hartmann, 1995; Hartmannetal., 1998). Inthenextyearsmoreeffortsweredonetostudylowermassobjects, and, inparticular, itbecameclearthatbrowndwarfs(BDs)possescircumstellardiskslikecTTs,withinfraredexcess(e.g., Natta&Testi, 2001; Jayawardhanaetal., 2003a)andaccretionsignatures(e.g., Jayawardhanaetal., 2003b; Nattaetal., 2004; Mohantyetal., 2005). Theinitialideawasthat, ifBD formationisjustascaled-downversionofstarformation, Macc

shouldscaleproportionallyto M⋆ downtotheBD regime. However, thankstothesuffi-cientlylargeintervalofM⋆ accessibletoobservations(roughly2ordersofmagnitudewhenincludingBDs)the Macc-M⋆ relationcouldbeobservationallyinvestigated, and, strikingly,thisdependencewasfoundtobe Macc∝ M2

⋆ , withaconsiderablylargespreadinthevaluesof Macc atany M⋆ (e.g., Muzerolleetal., 2003; Nattaetal., 2004; Mohantyetal., 2005;Nattaetal., 2006). Theseresultscalledforanexplanation, anddifferentpossibilitieshavebeenexploredinthenextyears. Inparticular, itbecameclearthat, giventhat M⋆ changesnegligiblyduringtheClass II phase, the Macc-M⋆ planeisadiagnostictoolfortheevolutionofaccretionduringthisphase, yieldinginformationontherelativeamountoftimespentinvariousregionsoftheplanebytheobjects(Clarke&Pringle, 2006). Sofar, nofinalsolutionofthisproblemhasbeenfound. HereI discussthepossiblescenariosintroducedtoexplainthe Macc-M⋆ dependence, whileI discusstheobservationalresultsandthestillopenissuesonthisrelationatthebeginningofthisThesisinSect. 1.4.1.

15

1. Introduction

Bondi-Hoyleaccretion

Anearlyexplanationfor theobserved Macc-M⋆ relationwassuggestedby Padoanetal.(2005), whoexplainedthisasaconsequenceofBondi-Hoyleaccretion. Theysuggestedthat, giventhattheoriginofdiskturbulencedrivingtheviscousevolutionisnotyetclear, theonlypossibilitytocompareviscousevolutionmodelswithobservationistoassume``adhoc''valuesfortheviscosity. Moreover, allYSOsarestillassociatedwiththeirparentalcloud, sotheassumptionthatdisksevolveinisolationcouldbeincorrect. TheirproposedevolutionofYSOsisdividedintwophases. InthefirstonethecorecollapsesandthereissphericalaccretionwhichendswiththeformationofacentralPMS objectsurroundedbyadisk. Thisverymassivediskisgravitationallyunstableandrapidlyaccretesontothecentralobjectinlessthan1Myr. Inthesecondphasethesystemaccretesmaterialfromthelarge-scalegasdistributionintheparentcloud, andthediskactsasashort-termbufferbetweenthelarge-scalestructureandthecentralPMS star. TheaccretionofmaterialfromtheparentcloudisdrivenbyBondi-Hoyleaccretion, whoserateis:

MBH =4πG2ρ∞

(c2∞ + v2∞)3/2M2

⋆ , (1.26)

where ρ∞ isthegasdensityoftheparentalcloud, c∞ itssoundspeed, v∞ thegasvelocityrelativetothestar, andallthesequantitiesdependontheenvironment, thereforearein-dependentonthepropertiesofeachindividualYSO.Ifthevaluesof Macc areassumedtobethesameasexpectedfromBondi-Hoyleaccretion, thantheorder-of-magnitudeoftheobservationismatchedandthedependenceof Macc withM⋆ isapower-lawwithexponent2, asexpected.

However, thereareseveralcaveatstothisdescription. First, theassumptionthat Macc∼MBH means thatall themasswhichisaccretedonthestar+disksystemsteadyflowsinthedisktothecentralPMS star. Infact, theactualrateofaccretiononthestarcouldbelower(e.g., Mohantyetal., 2005). Then, thisdescriptionimpliesthat Macc itselfshoulddepend stronglyon theenvironment inwhich theYSO is located. Thiswas, however,neverobserved, asdescribedby, e.g., Mohantyetal. (2005), Hartmannetal. (2006), andAlexander&Armitage (2006). Finally, explaining theevolutionofprotoplanetarydiskswithoutangularmomentumtransportisaquestionableassumptionthatcouldnotexplain,forexample, thefactthatTTauristarsposseslargedisks(e.g., Hartmannetal., 2006)andthat disks in the early phases are very small (e.g., Miotello et al., 2014), so theyhavetoexpandafterwards. Therefore, thisexplanationisnotthoughttobedescriptiveofthephenomenon.

Initialconditionsandbasicviscousevolution

Alexander&Armitage (2006)firstlyproposedanexplanationforthe Macc∝ M2⋆ relation

asaconsequenceofpropertiesof thedisksat formation, followedbyclassicalviscousevolution. Fromthesolutionofthesurfacedensityevolutioninthecontextofsimilarity

16


solution(ν ∝ Rγ, Eq. 1.13), byexpressingtheconstant C, thatis:

C =3 Md(0)(2− γ)ν1

2R21

, (1.27)

theyderive:

Σ(R, t) =Md(0)(2− γ)

2πR21r

γT−(5/2−γ)/(2−γ) exp

[−r(2−γ)

T

], (1.28)

where Md(0)istheinitialdiskmass, r = R/R1 isadimensionlessradius, R1 isthescaleradiusthatsetstheinitialdisksize, and T = t/tν + 1 isthedimensionlesstime. Astheviscoustimescaleis:

tν =R2

1

3(2− γ)2ν1, (1.29)

theyderivethat Macc, i.e. theaccretionrateontothestar(r → 0)is:

Macc =Md(0)

2(2− γ)tνT−(5/2−γ)/(2−γ), (1.30)

whichisthesameasinEq. (1.17), butwiththeproportionalityconstantexpressed. Theirassumptionis that the Macc∝ M2

⋆ relationisvalidalreadyfor theinitialaccretionrates.GiventhatobservationsshowthatinBDsthedisk-to-starmassratioiscomparabletothatofcTTs, theyassumethat Md(0) ∝ M⋆, thustheirassumptionis fullfilledif theviscoustimescaledependson M⋆ inthefollowingway:

tν ∝ 1

M⋆

. (1.31)

Thishasstrongimplicationsonpossibleobservables. Firstofall, Eq. (1.31)impliesthattheviscoustimescaleislongerforBDsthanforTTauristars, withtypically tν ∼ 106 yrforBDsif tν ∼ 104 − 105 yrforcTTs(e.g., Hartmannetal., 1998). Lowermassobjectswillthusevolvemoreslowlywithtimethanhighermassones. Asaconsequence, thespreadinthevaluesof Macc onthe Macc-M⋆ planeshouldbesmalleratlowerM⋆. Then, fromtheexpressionoftheviscoustimescaleandtheviscosityitselftheyderivethat, typically, theinitialdiskradiusforBDsarelargerthanforcTTs, withscaleradii R0 forBDs ∼ 50-100AU.However, thefactthattheviscoustimescaleisshorterforhighermassstarsimpliesthatdisksaroundtheseobjectswillspreadforviscousevolutionfasterthanthosearoundBDs,sotherewouldbenoobservabledifferencesat laterstages. Implicit intheirdiscussionthereisthesuggestionthattheobservedspreadofvaluesof Macc atany M⋆ isduetoagedifferences.

A similarapproachofsuggestingthattheobservedspreadisduetotheimprintoftheinitialconditionsatformationhasbeenusedby Dullemondetal. (2006)toexplaintheMacc-M⋆ relation. Theymodelthecollapseofvariousrotatingmolecularcloudcoresandthesubsequentviscousevolutionofthediskwhichforms. Byassumingthatcoreswithsignificantlydifferentmasseshavesimilar rotation rates tobreakup rate ratios theyfind

17

1. Introduction

atrendof Macc∝ M1.8±0.2⋆ . Theobservedspreadcanbetheninterpretedasaspreadin

theinitialcorerotationrates. Thissimplebutdescriptivesolutionhassomeobservableconsequences, aswell. First, theprocessofstarformationissimilarforhighermassstarsandBDs. Then, themassofthediskshoulddependstronglyon M⋆, roughly Md ∝ M2

⋆ ,and, inparticular, shouldbeatightlycorrelatedwith Macc.

Assumptionsondiskviscosity

A possible explanation couldbe that the viscosity of thedisk, whichdrives accretion,dependson M⋆. IfMRI isdrivingthediskturbulence, Mohantyetal. (2005)suggestthattheamountofionization, andthustheturbulence, islowerforcooler, i.e. lower-mass,objects. Thisqualitativeargumenthasbeenlateronsomehowdevelopedby Hartmannetal. (2006), who suggest that Macc dependson M⋆ if irradiationby thecentral star istakenintoaccountinalayeraccretionmodel(Gammie, 1996). Basically, Macc isgivenbytheaccretionrateofthelayeredmodelatacriticalradius(RC )wherethetemperatureishighenoughtoenhanceMRI inthedisk. Theyassumethiscriticalradiustobetheinneredgeofthedustydisk, whichisfrontallyilluminatedbythecentralstar. Byfixingthedustdestructiontemperaturetheyderivethat RC ∝ L

1/2⋆ . Assumingaclassicalalpha-viscosity

descriptionofthedisk, i.e. ν = αcs/Ω, theyobtainthat:

M ∝ νΣ ∝ αc2sΩ−1Σa ∝ αΣaL

3/4⋆ M−1/2

⋆ , (1.32)

wherealsointhiscase Σa isthesurfacedensityoftheactivelayerofthedisk. InPMS stars,atleastforM⋆ ≲ 2M⊙, thedependenceofthestellarluminositywiththemassis L⋆ ∝ M2

⋆ ,so, ifthediskispurelyinternallyirradiated, theaccretionrateat RC wouldbe:

M ∝ αΣaM⋆. (1.33)

Withthisdescriptionthemassaccretionratedependsonthestellarmass, buttheslopeofthisdependenceisnotassteepasobserved. However, thisdescriptioncouldrepresenttheupperenvelopeofaccretionratesobserved(Hartmannetal., 2006). Thesuggestionwouldbethattheupperenvelopeiscomposedbyobjectswhosediskisdescribedbythelayeredaccretiondiskmodelwithirradiationfromthecentralstar, whiletheotherobjectsfallingwellbelowthisupperenvelopeinthe Macc-M⋆ plane(mostlyBDsandlow-massTTauristars)haveasmallandmagneticallyactivediskthatsubstantiallyevolvedviscouslyandhasnowalowerrateofaccretion. Thisimpliesthatlower-massobjectsevolvefasterthansolar-masscTTs, sotheevolutionof Macc withtimeshouldbedifferentforthetwoclassesofobjects, with Macc slowlydecreasingforhighermasscTTswithrespecttoBDsandlow-massstars.

Self-gravitation

Someeffortshavebeenmadeintheliteraturetoexplaintheevolutionofprotoplanetarydisksunderself-regulateddiskswhichexperiencegravitationalinstabilitiesdrivingthedisk

18


turbulenceandthesubsequentaccretionprocess. Inparticular, Vorobyov&Basu (2008)discusshow Macc woulddependon M⋆ insystemswheregravitationalinstabilitiesaretheonlydriverofdiskturbulence, andhenceaccretion. Bycarryingoutseveralsimulationsonthefirst3Myrofevolutionoflow-andintermediate-massTTauristars(0.25M⊙ < M⋆ <3.0M⊙), theyderivea Macc-M⋆ relationwithapower-lawofexponent1.7. Atthesametime,theypredictthat Macc shouldcorrelatealmostlinearlywiththediskmass. Finally, theydiscussthattherecouldbeabimodalityintheobserved Macc-M⋆ relation, withdifferentbehaviorsinthevery-low-massendandinthe M⋆ > 0.2M⊙ range. Followingthiswork,Vorobyov&Basu (2009)developedasetofmodelstodescribetheevolutionofbothobjectswith M⋆ > 0.2M⊙ andoflower M⋆. Inordertobeabletodescribebothclassesofobjectstheyincludeintheirdescriptionalsoaneffectiveturbulent α-viscositywith α=0.01toallowforaccretion in the lowermassobjects, where thedisk isnotmassiveenough todrivesubstantialgravitationalinstabilities. Alsointhiscasethevaluesof Macc predictedbytheirmodelsaresimilartothoseobserved. The Macc-M⋆ relationisclearlyreproduced, withanexponentofthepower-lawwhichis1.8whenincludingallthetime-averagedmodelsatall M⋆. Theyalsosuggestthatthelargespreadof Macc atanygiven M⋆ ispartlyduetointrinsicvariabilityduringtheevolutionandpartlytoobject-to-objectvariationsduetodifferentinitialconditions. Finally, theypredictwiththeirmodelsastrongbimodalityinthe Macc-M⋆ relation, withvaluesoftheexponentofthepower-lawvaryingfrom2.9forYSOswith M⋆ < 0.2M⊙ to1.5forobjectswith 0.2M⊙ ≤ M⋆ < 3.0M⊙.

Photoevaporation

AsI discussedinSect. 1.3.2, protoplanetarydisksaresubjecttophotoevaporationfromthecentralstar. Underthisdescription, Clarke&Pringle (2006)suggestedthatthelowestaccretionratemeasurableatagivenmassissetbytheonsetofphotoevaporation. Asthedecreaseof Macc withtimeisapower-lawdecay, youngstarsspendmostoftheirtimeatthelowestpossibleaccretionrates, andthisisthesituationatwhichitismoreplausibletoobservethem. Therefore, the Macc-M⋆ relationcouldbedriveninlargesamplesbytheselow-accretors. Followingonthispath, ifthelowestaccretionissetbyphotoevaporation(seeSect. 1.3.2), thenthemostplausibleaccretionrateofastaris Macc=Mwind, with Mwind

beingthemass-lossrateduetophotoevaporation. Clarke&Pringle (2006)computetheex-pected Macc-M⋆ relationinthecaseinwhichUV radiationdominatesthephotoevaporativeflow, whichresultsinthefollowingmass-lossratedependenceof M⋆:

Mwind ∝ (M⋆ϕ)1/2, (1.34)

where ϕ istheionizingflux. IntheUV photoevaporationcase ϕ simplyscaleswithstellarluminosity, implying that Mwind ∝ M1.35

⋆ . Thispower-lawexponent, however, ismuchlowerthanwhatisobserved.

Ontheotherhand, steeperdependenceonM⋆ isfoundwhenconsideringX-raydrivenphotoevaporation, asrecentlyshownby Ercolanoetal. (2014). InthecaseofX-rayphoto-

19

1. Introduction

evaporation, Owenetal. (2012)derivedthatthemass-lossrateisdescribedas:

Mwind = 6.25× 10−9

(LX

1030 ergs−1

)1.14

M⊙ yr−1, (1.35)

where LX istheX-rayluminosity, andtheveryweak(∼ M−0.068⋆ )dependenceonthestel-

larmass is ignored. With thisdescriptionthe Macc-M⋆ relationisdrivenbythe M⋆-LX

one, whichcanbeconstrainedobservationally. ObservationsofvariousobjectsinOrion(Preibischetal., 2005)andinTaurus(Güdeletal., 2007)suggestthat LX ∼ M1.5−1.7

⋆ forobjectswithM⋆ < 1M⊙, whichresultsinadependence Macc∼ M1.5−1.7

⋆ inagreementwithmostoftheobservations, asI discussinthenextsection.

1.4 TheroleofthisThesis

Themainaimof thisThesis is tounderstand thepropertiesofprotoplanetarydisksandtheirevolutionbystudyingobservationallytheinteractionwiththecentralformingstarsthroughaccretion. AsI introducedintheprevioussections, accretiononTTauristarshasbeenasubjectofstudysincemanyyearsbutmanyquestionsremainopen. Morestringentconstrainton theevolutionofcircumstellardiskscanbeobtainedonlyanalyzing largesamplesofobjectswithdifferentintrinsicproperties, suchasage, mass, orevolutionarystage, andwithaccuratemethodologies.

1.4.1 ThestateoftheartatthebeginningofthisThesis

Thefirstobservationalworksinthisfield(e.g. Valentietal., 1993; Gullbringetal., 1998;Calvet&Gullbring, 1998; Hartmannetal., 1998)havemostlybeenfocusedononenearbylow-massstar-formingregion, Taurus, whichwaseasilyaccessibleinthenorthernskyandincludedmanystrongaccretors. Thankstotheseworksthegeneralpictureoftheevolutionofdiskswasobtained: typical Macc areoftheorderof ∼ 10−8M⊙/yrforobjects1-3Myroldanddecreasewithageasapower-law(Macc∝ t−η)withexponent η ∼1.5(Hartmannetal., 1998, seeFig. 1.8). Thisresultwouldimplythat, accordingtosimilaritysolutionmodels(seeSection 1.3.1, inparticulartheequations 1.17 and 1.18), thediskviscosityscaleslinearlywiththediskradius(ν ∝ R). Therefore, viscousevolutionmodelhavebeenshownwiththeseworktobeabletoreproducethegeneralevolutionofdisks.

TheseearlyworkswereallbasedontheobservationoftheUV-excessduetoaccre-tionbymeansoflow-resolutionspectroscopyor U -bandphotometryandthesubsequentmodelingofthisexcesstoderive Lacc (seeFig. 1.9). Measurementsofthisexcesscouldbecarriedoutonlywithinstrumentsandtelescopeswithhigh-sensitivityat λ ≲ 4000Å,suchaslargeground-basedtelescopes(morethan4m, e.g. Lick, Palomar, MMT atthattime)or theHubbleSpaceTelescope. AsbothUV-excessandstrongemissionlinesareoriginatedintheaccretionflow(seeSection 1.2.1), itbecameclearthatalesstelescope-timedemandingmethodtoderiveaccretionratesforlargesampleofobjectswouldbeto

20

1.4 The role of this Thesis

Figure 1.8: Firstobservationsofthemassaccretionrateasafunctionofagefrom Hartmannetal. (1998). Theoverplottedlinesarethelinearbestfitassumingdifferentuncertaintiesonthetwoquantities. Thedashedlinerepresentsafitwithslope1.5.

studytheluminosityofemissionlines(Lline)atopticalwavelengths. Therelationsbetweenthe Lacc measureddirectlyfromtheUV-excessand Lline havethenbeencalibratedinvar-iousworks(e.g., Gullbringetal., 1998; Calvetetal., 2004; Nattaetal., 2006; Herczeg&Hillenbrand, 2008; Dahm, 2010). Thisopenedthepossibilitytostudylargersamplesanddifferentstarformingregions, suchasOrion(Robbertoetal., 2004; Fangetal., 2009;Rigliacoetal., 2011a; Manaraetal., 2012), ρ-Ophiucus(e.g., Nattaetal., 2004, 2006;Gattietal., 2006), theCepheusOB2region(e.g. Sicilia-Aguilaretal., 2005, 2010), theChameleonclouds(e.g., Biazzoetal., 2012), andevenstarformingregionsintheMag-ellanicClouds(e.g., Spezzietal., 2012). Theanalysisoftheseregionsconfirmedinmostofthecasesthegeneralbehaviorpredictedbyviscousevolutionmodels(e.g., Robbertoetal., 2004; Sicilia-Aguilaretal., 2010; Manaraetal., 2012, seeFig. 1.10 and 1.11), butstartedtoshowthelimitsofthisdescription, whichcannotexplainsomeoftheobservedeffects, suchasthepresenceofmanynon-accretingobjectsatanyage(Sicilia-Aguilaretal., 2010), thelargespreadofvaluesof Macc atanyage, orthelowvaluesof η derivedwhenanalyzinglargesamples, whichhavebeenfoundtobeevenbelowthephysicallimitη > 1 (seediscussionafterEq. 1.18)insomecases(Manaraetal., 2012).

Atthesametime, theawarenessthatstellaragesdeterminedwithpre-main-sequenceevolutionarymodelsarehighlyuncertaingrew(e.g. Hartmann, 2003; Soderblom, 2010;Baraffe&Chabrier, 2010; Preibisch, 2012)andresearcherstartedtoputmoreeffortinex-ploringotherrelations, suchastherelationbetween Macc and M⋆, claimedfirstlytobeMacc∼ M2

⋆ in Muzerolleetal. (2003). Manyfollowingworks(e.g. Nattaetal., 2006; Fangetal., 2009; Rigliacoetal., 2011a; Manaraetal., 2012)confirmedobservationallytheex-istenceofthisrelationandderivedvaluesoftheexponent ∼1.5-2(see, e.g., Fig. 1.11). ThelastworkscarriedoutonsignificantlylargeandhomogeneoussamplesofobjectsbeforethisThesis, however, notedthatthisexponentisnotthesameatallmasses. Thediffer-

21

1. Introduction

Figure 1.9: Observation andmodelingof theUV-excess of young stellar objects due to accretion. Theobservedspectrumismoreintenseinthisrangethanapurephotospherespectrum. Theexcessemissionismodeledwithashockmodel. From Gullbringetal. (2000).

Figure 1.10: Observationsofthemassaccretionratesasafunctionofageinvariousregionsfrom Sicilia-Aguilaretal. (2010). Theoverplottedbluelinesrepresentvarioussimilaritysolutionmodelsthatreproducetheobservedtrend. Atanyagevariousobjectswithundetectableaccretionarefoundandareshownasopendownwardstriangles.

22


Figure 1.11: Observationsof themassaccretionrates (Macc)asa functionofageandmass in theOrionNebulaClusterobtainedfrom U -bandandHα photometryby Manaraetal. (2012). Ontheleft-handsideMacc isreportedasafunctionoftheageoftheobjects, whileontherighthandsideasafunctionof M⋆.Differentpanelsrefertodifferentmassoragerangesreportedontheplot. Theredlineisthebestfit.

23

1. Introduction

entslopesindifferentmassbinsimplythatintermediate-massstars(M ∼ 1M⊙)evolveonlongertimescalesthanlower-massones(Rigliacoetal., 2011a; Manaraetal., 2012), asitcanbeseenfromthesteepeningofthe Macc-M⋆ relationinFig. 1.11. Thiswassomehowpredictedbysometheoreticalworks(seeSect. 1.3.3)butthesmallsamplesandhighuncer-taintiesofpreviousworkspreventedtostudythiseffectindetail. Inalltheworks, regardlessthemethodologyusedortheregionanalyzed, alargescatterofthevaluesof Macc atanyM⋆ (∼ 2-3dex, e.g., Nattaetal. 2006; Manaraetal. 2012)wasalwaysobserved. Itwasnotclear, however, whetherthisspreadwasduetoobservationaluncertainties, todifferentmethodologies, tovariabilityofaccretion, orreal. Moreover, thelackofcompletesamplessuggestedthatthisrelationcouldbedrivenbyselectioneffects(Clarke&Pringle, 2006),giventhatlowaccretionratesarehardlymeasuredandthespectrumofstrongaccretorsistoostronglyveiledtodeterminetheirstellarproperties. Finally, alltheseresultsdependstronglyontheevolutionarymodelsusedtoderivethestellarproperties, inparticular M⋆,andalltheseuncertaintieshadbeenanobstaclefortheoreticianstobetterconstrainthephysicalmechanismdrivingthisrelation.

1.4.2 OpenissuesatthebeginningofthisThesis

Alltheworkspresentedintheprevioussectionsuggestedthatthemaindriveroftheevo-lutionofprotoplanetarydiskisthediskviscosity, butthiscannotbetheonlydriverandothereffects, suchasphotoevaporationorenvironmentaleffects, havetobeincluded. Ob-staclesagainstabetterunderstandingoftheimportanceoftheseeffectshadbeenthehighuncertaintiesinthemeasurementsof Macc availableintheliteratureandthelackoflargeandhomogeneoussamples. Inparticular, themainissuesthatI addressinthisThesisarereportedhere.

Whataretheobservationaluncertaintiesontheestimateof Macc duetochromo-sphericactivity?Youngstarsareknowntobestronglychromosphericallyactive, buttheeffectofthisemis-sionontheestimatesofaccretioninYSO wasnotclear. Inparticular, thequestionwastodeterminewhatistheminimumobservableaccretionrateinaccretingYSO andwhetherthisthresholddependsonthemassoftheYSO.TheinitialpartofmyThesisisdedicatedtoestablishingthisintrinsiclimitduetochromosphericactivity(cf. Chapter 3).

Whatistheeffectofveilingduetoaccretionandofextinctionontheestimateofstellarparametersofyoungstellarobjects?Whendetermining stellar parameters ofYSOs various effects thatmodify the observedspectrumshouldbetakenintoaccount, inparticularveilingduetoaccretionandextinc-tion. Inthepast theseeffectswereusuallyconsideredindependently, andthequestionwastoestimatehowmuchthiswouldhaveaffectedthestellarparametersdetermination,andhowtobetterconstrainthesepropertiesaltogether. AspartofthisThesisI developaselfconsistentmethodtoaccuratelyderivethemassaccretionratesandthephotospheric

24


parametersfrombroadbandintermediateresolutionspectroscopicdata(cf. Chapter 4).

Isaccretionsimilaratallevolutionarystagesofprotoplanetarydisks?ThepropertiesofaccretionintheClass II phaseofYSO evolutionhasbeenstudiedinmanycases, butfewstudieshavebeenfocusedonmoreevolvedstagesofevolution, suchasthetransitionaldiskphase. Thequestionwasthenwhetherthepropertiesofaccretioninthesedisksarethesameoflessevolvedobjectswhenstudiedindetail. Ifthelaterphasesofdiskevolutionarephotoevaporationdominatedwhiletheearlystagesareviscositydominated,thenachangeinpropertiesisexpectedasphotoevaporationseversthelinkbetweentheinnerdiskandtheoutermassreservoire. Aspartofmywork, I applytheoriginalmethod-ologydevelopedinthefirstpartofthisThesistoasampleoftransitiondiskstoaddressthisevolutionaryaspect(cf. Chapter 5).

Isthelargeobservedspreadofaccretionratesreal?Alltheaforementionedstudiesalwaysfoundalargespreadofvaluesof Macc atany M⋆

andage, butitwasnotclearwhetherthiswasarealeffectoronlyanobservationalbias.Strongerconstraintontheextentofthisspreadisneededtounderstandthe Macc-M⋆ re-lation, andthusthepropertiesofprotoplanetarydisksingeneral. TheaccuratemethoddevelopedinthisThesistoderiveaccretionandphotosphericparametersisthenappliedtoalargesampleofaccretingyoungstellarobjectsinordertoevaluatewhetherthelargespreadisintrinsicornot(cf. Chapter 6).

Whatisthedependenceofaccretionwiththemassofthecentralobjectsandwithitsage?Asdiscussedintheprevioussections, differentmodelspredictdifferentbehaviorsof Macc

asafunctionofthestellarmassandage. Theobservationaldatawereaffectedbylargeuncertaintyandscatterinthemeasurementsof Macc andthephotosphericproperties. OneofthegoalsofthisThesisistoimprovesignificantlyonthemeasurementsandattempttoderivemorestringentconstraintsforthemodels(cf. Chapter 6).

1.4.3 ThestructureofthisThesis

InthecontextpresentedaboveI startedtheworkofmyThesisaimingatputtingstrongerob-servationalconstraintsonourunderstandingofprotoplanetarydiskevolution. Forthisrea-sonI havedevelopedamodelfortheaccretionspectrum, collectedreliablephotospherictemplates, anddesignedananalysismethodtoobtainrobustresultsonlargesamplesofobjects. ThismethodisappliedinthisThesistospectraobtainedwithasecondgenerationESO/VLT instrument, theX-Shooterspectrograph. Thisinstrumentcoverssimultaneouslythespectralrangefrom λ ∼ 300nmto λ ∼ 2500nmwithhigh-sensitivityandmediumresolution(uptoR∼9900intheUV partofthespectrum, uptomorethan18000intheopticalpart, Vernetetal. 2011). Thankstothelargewavelengthcoverageofthisinstrumenttheexcessemissioninthenear-UV regioncanbestudiedsimultaneouslywiththeeffects

25

1. Introduction

ofveilingofthephotosphericfeaturesatallwavelengths, givinganunprecedentedviewoftheaccretionphenomenontogetamuchbetterunderstandingoftheinnerdiskevolution.I analyzeinthefollowingsamplesofobjectswithdifferentstellarproperties(L⋆, M⋆, age),atdifferentevolutionarystages, andlocatedindifferentregions. ThestructureoftheThesisisthefollowing.

In Chapter 2 I describethemodelfortheaccretionspectrumthatI havedevelopedandthatI use inmyanalysisofYSO spectra. Then, in Chapter 3 I presenttheanalysisofasampleof24spectraofnon-accreting(Class III) YSOs. Inparticular, I discusshowI havederivedwithdifferentmethodstheirstellarpropertiesinordertobeabletousetheminmyanalysisasphotospherictemplates. I alsoexplainhowI usethesespectratodeterminethebiasintroducedbythestrongchromosphericemissionoftheseyoungobjectsinthedeterminationofaccretionratesbasedsolelyonlineluminosity.

Themodelfortheaccretionspectrumandthephotospherictemplatesarethenusedin the restof theThesis to study thepropertiesof accretionasa functionof the stellarpropertiesoftheyoungstellarobjectsandasafunctionoftheevolutionarystageoftheprotoplanetarydisk. Inparticular, theabovedescribedmodelandtemplatesarethemainingredientoftheautomaticfittingprocedureI havedevelopedtodeterminesimultaneouslystellarandaccretionpropertiesoftheobservedYSOsfrombroad-bandspectra. Thisproce-dureisdescribedindetailin Chapter 4, whereI applyittotwoapparentlyoldobjectsstillsurroundedbyprotoplanetarydisksintheOrionNebulaCluster. Myanalysisshowshowtheseobjectshavebeenmisclassifiedintheliteraturebecausetheeffectsofveilingandextinctionhavenotbeenconsideredproperly. ThestellarparametersI determineprovethattheseobjectshaveanagecompatiblewiththerestofthepopulationofthecluster,thusshowingthattheyaretypicalcTTsandnotageoutliers.

WiththemethodI havedevelopedI alsostudythepropertiesofmoreevolvedstagesofdiskevolution. Inparticular, in Chapter 5 I presenttheanalysisofspectraforasampleof22transitionaldisks(seeSection 1.1). InthisChapterI showthatthetypicalaccretionratesforthetransitionaldisksinmysamplearecompatiblewiththoseoflessevolvedaccretingYSOs(cTTs)withsimilarstellarparameters. WiththeknowledgeoftheiraccretionpropertiesI amabletodeterminethepropertiesoftheirinnerdisk, inparticulartodeterminetheamountofgaspresentintheinnerdiskoftheseobjects. Thisestimateisneededfordiskevolutionmodelstodeterminewhatmechanismisdrivingtheevolutionoftransitionaldisks.

ThemethodologyandtheknowledgeacquiredinthisThesisallowtostudyobjectsinvariousregionsandwithdifferentstellarproperties. WithmymethodI havecontributedtothestudyofalargenumberofcTTslocatedintheLupusclouds(36Class II YSOs Alcaláetal., 2014)and8objectsinthe σ-Orionisregion(Rigliacoetal., 2012). In Chapter 6 Ianalyzeasampleof17objectsintheyoung ρ-Ophiucusclustertogetherwith3moretargetsintheolderUpperScorpiusregion. InthisChapterI thenanalyzealltheaforementionedtargetsandtheotheronesstudiedinthisThesistoconstrainthepropertiesofaccretionasafunctionofthestellarproperties(L⋆,M⋆)andoftheirages. I showhowmymethodologyallowstoachieveasignificantlysmallerspreadofaccretionrateatanyvalueofthestellarproperties, andhowviscousevolutionmodelscanreproducethegeneralpropertiesofthetargetsanalyzedhere.

26


Finally, in Chapter 7 I summarizemyfindingsanddiscussthefutureperspectiveofthiswork.

27

1. Introduction

28

2Modelingtheaccretionspectrum

Thediskaccretionprocessgivesrisetoastrongcontinuumemissionatallwavelengths, inparticularintheultravioletpartoftheelectromagneticspectrum, butalsoatopticalwave-lengths. Asoutlinedin the Introduction, in thisThesis I constrain theevolutionofpro-toplanetarydisksthroughobservationsofspectraofyoungstellarobjectsbydeterminingtheintensityandpropertiesoftheaccretionprocess. InthisChapterI presentthephysicalbackgroundof theslabmodel I use toderive thecontinuumemissionduetoaccretionfromtheobservedspectra. I firststartinSection 2.1 withashortdescriptionofthemodelsadoptedintheliteratureandtoevidencetheirsimilaritiesanddifferences. Then, inSec-tion 2.2 I reporttheequationsneededtoproperlymodelanaccretionspectrumwithaslabmodel. Finally, I describeinSection 2.3 thedependenceofthesomeimportantfeaturesinthemodelontheinputparameters.

2.1 Accretionspectrummodelsintheliterature

In thedescriptionofviscousaccretiondisksby Lynden-Bell&Pringle (1974), which isdiscussedinSection 1.2, thediskextendsdowntothestellarsurface. Intheinnermostpartofsuchdiskthereisa boundarylayer ofinfinitesimalradius. Thisisaregionwheretheangularvelocityoftheparticlesinthedisk(Ω)hasacontinuoustransitiontotheangularvelocityofthestar(Ω⋆), asitisshowninFigure 2.1. Thisregionisverynarrow, ascanbederivedfromtheaxisymmetricmomentumequation:

vrdvrdr

− v2Φr

= −1

ρ

dPdr

− GM

r2, (2.1)

where P isthepressure. Insidetheboundarylayergravityisnotbalancedbytherotationalsupportandtheradialvelocityissmallinmostcases. Therequirementto, instead, havingtheradialpressureforcestobalancetheforceofgravity, is:

1

ρ

dPdr

∼ c2s∆R

∼ GM

r2∼ Ω2

Kr, (2.2)

29

2. Modeling the accretion spectrum

Figure 2.1: Schemeof theangularvelocityasa functionof the radius in the innermost regionofadiskextendingdowntothestellarsurface. TheangularvelocityvariesfromtheKeplerianvelocitytotheangularvelocityofthestar(Ω⋆)overanarrowregion. Thisisknownastheboundarylayer. From Hartmann (2009)

where cs isthesoundspeedintheboundarylayer,∆R itsradialextent, andΩK theKeplerianangularvelocity. Sincetheverticalscale-heigtis H = cs/ΩK, I obtainthat:

∆R

R∼(H

R

)2

, (2.3)

thatimplies ∆R ≪ R fortheassumptionthatthediskisthin, i.e. H ≪ R.Theenergyreleasedbyparticlesmovingfromonepointintheboundarylayertorest

onthestellarsurfaceisthesameastheonereleasedbyaparticlegettingtoacircularorbitclosetothestellarsurfacefromanorbitfarfromthecentralstar, i.e. GM⋆/2R⋆. Forthisreason, theboundarylayeremitsaluminositycomparabletothatreleasedbythewholedisk. Thishigher luminosityonaverynarrowregions leads to theconclusion that thetemperatureoftheboundarylayerismuchhotterthanthemaximumtemperatureofthedisk, soitemitsradiationatwavelengthsshorterthanthecentralstar. ForthisreasontheexcessemissionintheUV observedinyoungstarssurroundedbyadiskwasthoughttobeoriginatedinthisboundarylayer(e.g., Lynden-Bell&Pringle, 1974). Withinthispicturetheemissionresponsiblefortheexcessduetoaccretioniswellreproducedwitha slabofhydrogen athightemperatures(∼ 104 K).Therefore, thefirstmodelsusedtoreproducetheaccretionspectrumofTTauristarswereslabmodels. Inparticular, Valentietal. (1993, andreferencetherein)reproducedtheobservedUV-excessinthespectraofvariousaccretingYSOswiththismodel.

AsI discussedinSection 1.2.1, instead, thecurrentparadigmadoptedtodescribethegeometry of the inner disk, and thus of the regionwhere accretion takes place, is themagnetosphericaccretion model. In thisdescriptionthedisk is truncatedat fewstellarradiibythestrongmagneticfields (e.g., Hartmann, 2009), sotheboundarylayer isnotpresent. Todescribeindetailtheemissionarisingfromthismagnetosphericregionmore

30

2.2 The slab model

detailedmodeling thana simple slab isneeded. Calvet&Gullbring (1998)developedamodelwhichtakesintoaccounttheshocksoriginatedbythematerialfallingontothestellarsurfaceatfree-fallvelocities. InthismodeltheheatedphotospherebelowtheshockisopticallythickandemitsmostlyinthePaschenandBrackettcontinua. Ontheotherhand,theemissioncomingfromthepreshockandattenuatedpostshockregionsisopticallythinanddominatesthespectrumintheBalmerjumpregionandatshorterwavelengths. ThismodelhasbeenshowntoreproducewithgoodaccuracyboththeUV-excess(e.g. Calvet&Gullbring, 1998; Calvetetal., 2004; Inglebyetal., 2013)andtheshapeoftheemissionlines(e.g. Muzerolleetal., 1998a)ofaccretingTTauristars.

Nevertheless, inthepresentdaysbothmodelsarestillused. Ononehand, theshockmodelshaveamorerobustphysicalrelevanceinthesensethattheyaimatreproducingtheshockonthestellarsurfaceproducedbytheaccretionflow. However, thegeometryoftheinnerdiskregionisnotcompletelyunderstoodandalsothesemodelslackinmatchingtheobservationoftheveilingduetoaccretionatlongerwavelengthsinspectraofaccret-ingTTauri stars (e.g., Inglebyetal., 2013). Ontheotherhand, slabmodels, whicharebasedontheoldassumptiononthegeometryofthestar-diskboundary, arefastandeasytocomputeandcanreproducewithgoodagreementstheobservations(e.g., Valentietal.,1993; Herczeg&Hillenbrand, 2008). Moreover, itshouldbenotedthatthemostimpor-tantreasonforwhichthesemodelsareneededistoestimatethebolometriccorrectiontoderivethetotalexcessemissionduetoaccretion. Mostofthisexcessemissionarisesintheregionscoveredbytheoptical-UV spectra, butanonnegligiblefractionisemittedatwavelengths ≲300nm, inaspectralregionthatisnotaccessiblewithground-basedtele-scopes. Comparisonofthebolometriccorrectionfactorsondifferentobjectsshowsthatthederivedvaluesaresimilarwiththetwomodels(e.g., Calvet&Gullbring, 1998; Herczeg&Hillenbrand, 2008). AlsoresultsobtainedinthisThesisusingtheslabmodel, forexamplethevaluesof Macc obtainedfor5TDsinChapter 5 (TW Hya, LkCa15, CS Cha, GM Aur,DM Tau), arecompatiblewiththosederivedforthesameobjectsby Inglebyetal. (2013)usingshockmodels. Thisconfirmsthatbothmodelsdescribethetypicalemissionduetoaccretionwithsimilaraccuracy. Theslabmodelisthusagoodenoughdescriptionoftheprocess, andI willusethissimplerdescriptioninthisThesistomodeltheobservedspectraofaccretingYSOs. However, beingthegeometricalassumptionsatthebaseoftheslabmodelinaccurate, I decidetouseonlythefluxoftheslabtomeasure Lacc, whileI neverreportinthefollowingthevaluesofthephysicalquantitiesadoptedinthemodel, suchasthetemperatureoftheslab.

2.2 Theslabmodel

HereI modelthecontinuumemissionarisingfromaplane-parallelslabcomposedofpurehydrogenandassumedinLTE.Theinputparametersneededtodescribethesystemarethetemperatureoftheslab(Tslab), theelectrondensity(ne), andtheslablength(Lslab). Thelattercanbesubstitutedwiththeopticaldepthatagivenwavelength(τλ0 ), asthesetwo

31


quantitiesarerelatedbythefollowingrelation:

Lslab =τλ0 ·Bλ0(Tslab)

jH,λ0

, (2.4)

where Bλ0(Tslab) isthevalueoftheblackbodyoftemperatureequaltotheslabtemperatureatthegivenwavelength, and jH,λ0 theemissivityofhydrogenatthatwavelength. Giventhattheopticaldepthisoneofthephysicalquantitiesthatgovernstheslabemissivity, Iconsiderasasetofinputparametersthetemperature, theelectrondensity, andtheopticaldepthofapurehydrogenslab(oneofthetwocomponentsofmyslab, theotherbeingtheH−)at300nm(τ300). ThisparticularwavelengthischosentobeclosetotheBalmerjump,sothisparameterisstrictlyrelatedtotheopticaldepthofthehydrogencomponentoftheslabatthejump.

InthefollowingI derivetheemissivityoftheslab(jslab,ν )includingboththeemissivityfromhydrogen(jH,ν )andthatfromH− (jH−,ν ), whichisanimportantsourceofopacityatdensitieshigherthan∼ 1013 cm−3. FromtheemissivityI thenderivetheintensityoftheslab(Islab,ν ), whichisthenconvertedinobservedflux(Fslab,ν )assumingtheemissiontoariseatadistance D fromaslabwitharea Areaslab usingthefollowingrelation:

Fslab,ν = Islab,ν ×Areaslab

D2. (2.5)

2.2.1 Thehydrogenemission

Thecontinuumspectrumofhydrogeniscomposedbythefollowingtwoemissions: thefree-boundemission, whichoriginatesfromradiativerecombination, andthefree-freera-diation, whicharisesfromelectrontransitionsintheencounterswithprotons. I willdiscusstheequationsgoverningtheemissionfromthesetwocomponentsinthenexttwosubsec-tions.

Free-boundemissivity

Free-boundemissionarisesfromphotonsemittedwhenafreeelectroniscapturedbyanion. Thisprocess, alsoknownasradiativerecombination, isthereverseofthephotoion-izationprocess. Thisisdescribedbythefollowingreaction:

H+ + 2e− ↔ H(n) + e−. (2.6)

Inthisdescriptionthedetailbalancelaw(Kirchoff'slaw)canbeusedtorelatetheemissivitytothecross-sectionforphotoionization:

jH,ν,fb = κν,fbBν(T )

= [nb]LTE · σb,pi(ν)[1− e−hν/kBT

]Bν(T )

(2.7)

where κν,fb istheabsorptioncoefficient, [nb]LTE isthenumberdensityofatomsinboundstatewiththeassumptionofLTE,and σb,pi(ν) thecross-sectionofphotoionization. The

32

2.2 The slab model

numberdensityof atoms inbound state, i.e. thepopulationof the n-th level, canbederivedconsideringthephotoionizationreaction(Eq. 2.6)andthelawofmassaction, andis:

[nb]LTE =h3

(2πkBT )3/2

(mH

mpme

)3/2g[H(n)]

g(e−)g(H+)ehν0/n

2kBTnen(H+), (2.8)

where ν0 = 3.28795 ·1015 Hzistheionizationfrequencyforhydrogen. Giventhatelectronsandprotonshavespin1/2, thevalueof g(e−) and g(H+) isinbothcases2, whileforthehydrogenatom g[H(n)] = 4n2. I thusderivethat:

[nb]LTE =h3

(2πmekB)3/2n2T−3/2ehν0/n

2kBTneni. (2.9)

According to Draine (2011), thecross-sectionofphotoionizationcanbeapproximatedreasonablyas:

σb,pi(ν) ∼ σ0

(hν

Z2i IH

)−3

, (2.10)

where IH istheionizationpotentialforhydrogen, and σ0 isthecrosssectionatthreshold,givenby:

σ0 ≡29π

3e4Z−2

i απa20 = 6.304× 10−18Z−2i cm2. (2.11)

InsertingtheseexpressionsinEq. (2.7)I obtain:

jH,ν,fb =8

3

(2π

3

)1/2Z2

i e6

m3/2e c3(kBT )1/2

nenie−hν/kBT · gfb

= 5.44 · 10−39 Z2i

T 1/2nenie

−hν/kBTgfb erg cm−3 s−1 Hz−1 sr−1,

(2.12)

where gfb isthefree-boundGaundfactor, whoseexpressionis:

gfb(ν, T ) =2hν0Z

2i

kBT

∞∑n=m

n−3ehν0/n2kBTgn(ν), (2.13)

with m beingthequantumnumberof thelowerfinalstateof thefree-boundtransition,whoseexpressionis:

m = integer

(ν0Z

2i

ν

)1/2

+ 1. (2.14)

Accordingto Seaton (1960), gn(ν) canbeapproximatedas:

gn(ν) = 1 + 0.1728 ·

[(ν

ν0Z2i

)1/3

− 2

n2

(ν

ν0Z2i

)−2/3]+

−0.0496 ·

[(ν

ν0Z2i

)2/3

− 2

3n2

(ν

ν0Z2i

)−1/3

+2

3n4

(ν

ν0Z2i

)−4/3].

(2.15)

33


Free-freeemissivity

Whenelectronsinteractwithanionwithoutbeingcapturedtheyemitfree-freeradiation,alsoknownasbremsstrahlungradiation. Inoursystemtherearefreeelectronswhoseveloc-itydistributionisdescribedwithaMaxwell-Boltzmandistribution. Withinthisassumptionthefree-freeemission, accordingtoclassicaltheory, isnearlyindependentoffrequencyexcept for theexponentialcutoff resulting from theMaxwelliandistributionofelectronvelocities. Following, e.g., Spitzer (1978), theemissivityforthisfree-freetransitionis:

jH,ν,ff =8

3

(2π

3

)1/2Z2

i e6

m3/2e c3(kBT )1/2

nenie−hν/kBT · gff

= 5.44 · 10−39 Z2i

T 1/2nenie

−hν/kBTgff erg cm−3 s−1 Hz−1 sr−1,

(2.16)

where Zie isthechargeoftheprotonsinteractingwiththeelectronsand ni theirdensity;gff istheGauntfactorforfree-freetransition. Itisinterestingtonotethatalsointhiscase,asinthefree-boundemission, thereisadependenceoftheemissivitywith ne ·ni, whichistypicalofbinaryinteractions. Accordingto Seaton (1960), gff canbeexpressedwiththefollowingasymptoticexpansion:

gff (ν, T ) = 1 + 0.1728 ·(

ν

ν0Z2i

)1/3(1 +

2kBT

hν

)+

−0.0496 ·(

ν

ν0Z2i

)2/3[1 +

2

3

kBT

hν+

4

3

(kBT

hν

)2].

(2.17)

Totalemission

Giventhatthemodeledslabiscomposedsolelyofhydrogen, I assume Zi=1and ni = np

inthepreviousformulas. Moreover, I assumethatthemediumisneutral, so ne = np . ThetotalemissivityofhydrogenisthusobtainedaddingtogetherEq. (2.12)andEq. (2.16). Theresultisthefollowing:

jH,ν = 5.44 · 10−39e−hν/kBTT−1/2[gff (ν, T ) + gfb(ν, T )]n2e erg cm

−3 s−1 Hz−1 sr−1 (2.18)

FromthetotalemissivityofHydrogenI thenderivetheintensity:

IH,ν = jH,νLslabβH,ν , (2.19)

where βH,ν istheescapeprobabilityatthefrequency ν:

βH,ν =1− e−τH,ν

τH,ν

, (2.20)

and τH,ν istheopticaldepth. Thisisobtainedfromthefollowingequation:

τH,ν = κH,νLslab =∞∑

n=m

σn,pi(ν)nbLslab, (2.21)

34

2.2 The slab model

with m calculatedwiththeexpressionofEq. (2.14)and σn,pi(ν) beingthephotoionizationcrosssectionfromlevel n. UndertheassumptionofLTE I cancompute κν usingdetailbalanceas:

κH,ν = jH,ν/Bν(T ), (2.22)

andthustheopticaldepthatanyfrequencycanbecalculatedas:

τH,ν = jH,νLslab/Bν(T ). (2.23)

2.2.2 TheH− emission

Thenegativehydrogenion(H−)isamajorsourceofcontinuousabsorptionindensere-gions, suchasstellaratmospheres. ThemainreactionswhereH− isinvolvedarethephoto-detachmentmechanism:

hν +H− → H+ e−, (2.24)

whichisabound-freetransitionthatcontributesuptotheionizationthreshold λ0 =1.6419µm, andthefree-freemechanism:

hν + e− +H → H+ e−. (2.25)

AbsorptioncoefficientsforH−

ThecalculationoftheabsorptioncoefficientsforthesetransitionshasbeenpresentedinJohn (1988), andthetotalabsorptioncoefficient(κH−,λ,tot)perunitelectronpressureperhydrogenatomisthesumoftheabsorptioncoefficientsofthetwotransitions. Inparticular,thecoefficientofabsorptionfromH− photo-detachmentis:

κH−,λ,fb(λ, T ) = 0.750 T−5/2eα/λ0T (1− e−α/λT )σλ cm4 dyne−1, (2.26)

where λ0 = 1.6419 µmis thephoto-detachment thresholdwavelength forH− and α =1.439 · 108 Å·K,wavelengthsareassumedin µm, temperaturesinK,and σλ isthephoto-detachmentcross-section, whoseexpressionis:

σλ = 10−18λ3

(1

λ− 1

λ0

)3/2

f(λ) cm2, (2.27)

validfor λ < λ0. Inthelatter f(λ) isafunctionwiththefollowingform:

f(λ) =6∑

n=1

Cn

(1

λ− 1

λ0

)(n−1)/2

cm2. (2.28)

Theparameters Cn arereportedinTable 2.1 andarevalidintherange0.125≤ λ(µm) ≤1.6419foranytemperature.

35


Table 2.1: Parametersforgeneratingphoto-detachmentcrosssectionsforH−

n Cn

1 152.5192 49.5343 −118.8584 92.5365 −34.1946 4.982

Notes. From John (1988).

Table 2.2: Parametersforgeneratingfree-freeabsorptioncoefficientforH− inthewavelengthrange0.182µm < λ < 0.3645 µm

n An Bn Cn Dn En Fn

1 518.1021 −734.8667 1021.1775 −479.0721 93.1373 −6.42852 473.2636 1443.4137 −1977.3395 922.3575 −178.9275 12.36003 −482.2089 −737.1616 1096.8827 −521.1341 101.7963 −7.05714 115.5291 169.6374 −245.6490 114.2430 −21.9972 1.50975 0.0000 0.0000 0.0000 0.0000 0.0000 0.00006 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000


Thefree-freeabsorptioncoefficientforH−, instead, isfittedbythefollowingformula:

κH−,λ,ff(λ, T ) = 10−29

6∑n=1

(5040

T

)(n+1)/2

×

×λ2An +Bn + Cn/λ+Dn/λ

2 + En/λ3 + Fn/λ

4cm4 dyne−1.

(2.29)

Alsointhiscasewavelengthsareassumedin µmandtemperaturesinK.ThecoefficientsofthisexpressionarereportedinTable 2.2 forthewavelengthrange0.182 µm < λ < 0.3645µmandinTable 2.3 forwavelengthsgreaterthan0.3645 µm, andarevalidintherange0.5≤ (5040/T [K]) ≤3.6, whichistherangeinwhichI calculatethemodels.

Table 2.3: Parametersforgeneratingfree-freeabsorptioncoefficientforH− for λ ≥ 0.3645 µm

n An Bn Cn Dn En Fn

1 0.0000 0.0000 0.0000 0.0000 0.0000 0.00002 2483.3460 285.8270 −2054.2910 2827.7760 −1341.5370 208.95203 −3449.8890 −1158.3820 8746.5230 −11485.6320 5303.6090 −812.93904 2200.0400 2427.7190 −13651.1050 16755.5240 −7510.4940 1132.73805 −696.2710 −1841.4000 8624.9700 −10051.5300 4400.0670 −655.02006 88.2830 444.5170 −1863.8640 2095.2880 −901.7880 132.9850


36

2.3 Dependence of selected features on the input parameters

ThetotalabsorptioncoefficientforH− isthenobtainedinthefollowingway:

κH−,λ,tot = [κH−,λ,bf + κH−,λ,ff] · nenHkBT cm−1, (2.30)

wherethefactor nenHkBT takesintoaccounttheunitsofthecalculationsabove. Thevalueof nH isderivedinthesamewayas [nb]LTE (seeEq. (2.9)), while ne and T = Tslab areinputparameters.

TotalemissionofH−

TheemissivityoftheH− emissionisderivedusingthelawofdetailbalance:

jH−,ν = κH−,ν,totBν(T ). (2.31)

Toderivetheintensityoftheemission, whoseexpressionis:

IH−,ν = jH−,νLslabβH−,ν , (2.32)

I needtocalculatetheopticaldepthas:

τH−,ν = κH−,ν,totLslab, (2.33)

andI insertthisexpressioninEq. (2.20)replacing τH,ν toderive βH−,ν .

2.2.3 Thetotalemissionoftheslabmodel

ThetotalemissionfromtheslabcanbederivedinLTE bycomputingthetotalopticaldepth(τslab,ν )asthesumoftheopticaldepthsofthetwocomponents, H andH−, simplyaddingtheresultsofEq. (2.23)and(2.33)toderive τslab,ν = τH,ν + τH−,ν . Then, thetotalintensityoftheslabemissionisobtainedas:

Islab,ν = τslab,νBν(Tslab)βslab,ν , (2.34)

where βslab,ν isobtainedusingEq. (2.20)andthetotalopticaldepthoftheslab τslab,ν .Finally, thefluxemittedbytheslabisobtainedusingEq. (2.5), wherethefactor Areaslab

isderivedwhenfittingthespectrumofanobject. I showinFig. 2.2 anexampleoftheoutputspectrumofthemodel.

2.3 Dependenceofselectedfeaturesontheinputparame-ters

Intheemissionspectrumoftheslabmodel(seeFig. 2.2 foranexample)thereareseveralfeaturesthatareofgreatinterestforouranalysis. Thoseare, inparticular, thevalueoftheBalmerjump, theratiooftheemissionintheBalmercontinuumandPaschencontinuum

37


102 103

Wavelength [nm]

106

107

108

109

1010

1011λF λ

Input parameters: T=7000 K, Ne=1e+13 cm−3 , τλ=300nm=1

H emissionH− emission

Total

Figure 2.2: Exampleoftheemissionofaslabmodelwithinputparameters Tslab=7000K, ne = 1013 cm−3,and τλ=300nm =1. Thetotalemissionisrepresentedbythesolidline, whilethedashedlineistheemissionfromtheH componentandthedottedlinetheonefromtheH−. TheH− emissionismodeledonlyfrom λ >125nmasexplainedinSect. 2.2.2.

regions, andtheslopesoftheBalmerandPaschencontinua. HereI discussthedependenceofthesefeaturesontheinputparameters. I willalsoconsidertheseparatecontributionofH andH− emissiontothesefeatures. Inthiscontextitisinterestingtounderstandwhataretheratioofthesetwocomponentsasafunctionofthemodelparameters, andI willstartbydiscussingthisaspect.

2.3.1 ContributionofH andH− tothetotalemission

InFig. 2.3 the ratioof theH toH− emission is shownasa functionof the three inputparametersof the slabmodel. The fourpanels show thevalueof this ratiovs Tslab fordifferentdensities, whereeachpanelreferstoavalueoftheopticaldepthoftheH slabatλ=300nm. FromtheseplotsitisclearthatthecontributionofH isdominantinmostoftheparameterspacedescribedbyourmodel. TheH− emissionhasthelargestcontributiontothetotalslabemissionatanydensitywhen Tslab ≲ 6000K,andforhigherslabtemperaturesforincreasingdensities. At ne ∼ 1016 cm−3 H− isthedominantsourceofemissionalmostforallvalueof Tslab. TherelativecontributionofH− withrespecttotheoneofH decreaseswithincreasingopticaldepthofthehydrogencomponent. Thesefindingsarecompatiblewithwhat Valentietal. (1993)reported, astheystatethatthecontributionofH− intheir

38


2

1

0

1

2

3

4

5

6

τλ=300nm = 0.1 τλ=300nm = 0.5

5000 6000 7000 8000 9000 10000

Tslab [K]

2

1

0

1

2

3

4

5

6

τλ=300nm = 16000 7000 8000 9000 10000

Tslab [K]

τλ=300nm = 5

log R

ati

o e

mis

sion H

/H−

ne = 1e+11

ne = 1e+12

ne = 1e+13

ne = 1e+14

ne = 1e+15

ne = 1e+16

Figure 2.3: RatiooftheemissionduetoH tothatduetoH− intheslabmodelasafunctionoftheinputparameters. Thefourpanelsrepresentmodelswithincreasinginputhydrogenopticaldepthat λ=300nm.Ineachpanel, wherethelogarithmicvalueoftheratiooftheH overH− emissionisshowedvstheslabtemperature, thecoloredlinescorrespondtoincreasingelectrondensitiesasreportedinthelegend.

slabmodelsisbasicallynegligibleatdensities≲ 1013cm−3, whilethisbecomesadominantsourceofemissionatdensities ≳ 1015cm−3.

2.3.2 Balmerjump

I definethevalueoftheBalmerjumpastheratiobetweenthefluxat λ ∼360nmoverthefluxat λ ∼ 400nm. ThisisthesamedefinitionthatI willuseinthenextchapters, suchasinChapter 4. ThelogarithmofthisvalueisshownasafunctionofthevariousinputslabparametersinFig. 2.4. InthoseplotsweseethatinapureH slabmodel(dashedlines intheplots)thevalueoftheBalmerjumpwoulddecreasemonotonicallywith Tslab, wouldbeindependentontheelectrondensity, andwoulddecreasewithincreasingopticaldepth.Thereasonforthemonotonicallydecreaseofthejumpwith Tslab isthatthepopulationofthesecondlevelinthehydrogenatomdecreasesmoreslowlythaninthehigherlevels(e.g.,Valentietal., 1993). Ontheotherhand, theH− emissiondoesnothaveanyBalmerjump,thereforeitscontribution(dottedlines inFig. 2.4)isalmostconstantandindependentonthedensityandtheopticaldepth. Theseeffects, togetherwiththerelativecontributionofHandH− tothetotalemissiondiscussedabove, resultinthefactthattheBalmerjumpoftheoutputslab, whenconsideringbothcomponentstogether(solidlines inFig. 2.4), follows

39


0.0

0.5

1.0

1.5

2.0lo

g Fλ

=36

0nm/Fλ

=40

0nm τλ=300nm = 0.1 τλ=300nm = 0.5

5000 6000 7000 8000 9000 10000

Tslab [K]

0.0

0.5

1.0

1.5

2.0

log Fλ

=36

0nm/Fλ

=40

0nm τλ=300nm = 1

6000 7000 8000 9000 10000

Tslab [K]

τλ=300nm = 5 ne = 1e+11

ne = 1e+12

ne = 1e+13

ne = 1e+14

ne = 1e+15

ne = 1e+16

Figure 2.4: Balmerjumpobtainedwiththeslabmodelasafunctionoftheinputparameters. Thefourpanelsrepresentmodelswithincreasinginputhydrogenopticaldepthat λ=300nm. Ineachpanel, wherethevalueoftheBalmerjumpisshowedvstheslabtemperature, thecoloredlinescorrespondtoincreasingelectrondensities. The solid lines represent thevalueobtained from the totalemission, while thedashed line isobtainedfromthehydrogenemissionalone, andthedottedlinesfromtheH− emission.

thebehaviorofthepureH emissionatlowdensitiesandhighertemperatures, whileitisdifferentinregionsoftheparameterspacewheretheH− contributionisstronger. ThefactthattheBalmerjumpoftheslabissmallerthanthepureH oneduetotheH− emissionisinagreementwiththeresultsreportedby Herczeg&Hillenbrand (2008).

2.3.3 BalmerandPaschencontinua

TheratioofthecontributionoftheemissionfromtheBalmercontinuumwithrespecttothatarisingfromthePaschencontinuumisshowninFig. 2.5. ItisclearthattheemissionisdominatedbytheBalmercontinuumregionforlowopticaldepth, whilethetwoemissionsbecomecomparableathigheropticaldepth. Theshapeof theH− contribution, whichpeaksinthePaschencontinuumregion, anditsrelativeimportancetotheH emission(seeabove)resultsinastrongerBalmertoPaschencontinuumratiowithincreasing Tslab andatlowerdensities.

TheBalmercontinuumslopeisstrictlyconnectedwithderivingthecontributiontothetotalexcessemissionduetoaccretionatthewavelengthsnotcoveredbyground-basedUV spectra, i.e. at λ ≲ 310nm. A largefraction(∼ 30-40%)oftheexcessemissiondue

40


0.0

0.5

1.0

1.5

2.0

2.5 τλ=300nm = 0.1 τλ=300nm = 0.5

5000 6000 7000 8000 9000 10000

Tslab [K]

0.0

0.5

1.0

1.5

2.0

2.5 τλ=300nm = 1

6000 7000 8000 9000 10000

Tslab [K]

τλ=300nm = 5

log R

ati

o B

alm

er/

Pasc

hen c

onti

nuum

em

issi

on

ne = 1e+11

ne = 1e+12

ne = 1e+13

ne = 1e+14

ne = 1e+15

ne = 1e+16

Figure 2.5: RatiooftheemissionfromtheBalmerandPaschencontinuaobtainedwiththeslabmodelasafunctionoftheinputparameters. SymbolsasinFig. 2.4

0.000

0.005

0.010

0.015

τλ=300nm = 0.1 τλ=300nm = 0.5

5000 6000 7000 8000 9000 10000

Tslab [K]

0.000

0.005

0.010

0.015

τλ=300nm = 1

6000 7000 8000 9000 10000

Tslab [K]

τλ=300nm = 5

Slo

pe B

alm

er

conti

nuum

ne = 1e+11

ne = 1e+12

ne = 1e+13

ne = 1e+14

ne = 1e+15

ne = 1e+16

Figure 2.6: Balmercontinuumslopeobtainedwiththeslabmodelasafunctionoftheinputparameters.SymbolsasinFig. 2.4

41


0.002

0.000

0.002

0.004

0.006

0.008 τλ=300nm = 0.1 τλ=300nm = 0.5

5000 6000 7000 8000 9000 10000

Tslab [K]

0.002

0.000

0.002

0.004

0.006

0.008 τλ=300nm = 1

6000 7000 8000 9000 10000

Tslab [K]

τλ=300nm = 5

Slo

pe P

asc

hen c

onti

nuum

ne = 1e+11

ne = 1e+12

ne = 1e+13

ne = 1e+14

ne = 1e+15

ne = 1e+16

Figure 2.7: Paschencontinuumslopeobtainedwiththeslabmodelasafunctionoftheinputparameters.SymbolsasinFig. 2.4

toaccretionarisesshortwardtheBalmerjump, andtheexactamountofthisemissionisrelatedwiththisslope, whichisdefinedhereastheslopeofthespectrumbetween ∼330nmand ∼ 360nm. InFig. 2.6 thisquantityisshownasafunctionofthevariousinputparameters. SimilarlytotheBalmerjump, alsotheBalmercontinuumslopeofthepureHemissionisindependentonthedensityandbecomeslesssteep, i.e. bluer, withincreasingtemperatureandopticaldepth, similarlytowhatfoundby Herczeg&Hillenbrand (2008).ThecontributionofH− makestheslopeofthiscontinuumbluerinallcases, beingmoreimportantintheregionsoftheparametersspacediscussedabove.

SimilartrendsarefoundalsoforthePaschencontinuumslope, definedastheslopebe-tween ∼400nmand ∼ 477nm, whichisshowninFig. 2.7, asalso Herczeg&Hillenbrand(2008)noted. In thiscasetheslopeis important for theeffectsofveilingofabsorptionfeaturesintheredderpartofthespectrum. Itisimportanttonotethattheslopeofthiscon-tinuumisclosetozero, whichiswhatisassumedinvariousstudiestomodeltheveilingintheredpartoftheopticalspectrum(e.g. Inglebyetal., 2013; Herczeg&Hillenbrand,2014). Nevertheless, thisslopeisnotzeroinmanypartoftheparameterspace, andthiswillbeshowntobeagoodmodelfortheveilingofthephotosphericfeaturesacrossthespectrum(see, e.g., Chapter 4). IncomparisontotheBalmercontinuumslopethePaschenslopedependsslightlyontheopticaldepthofH at λ=300nm.

42

3Photospherictemplatesofyoungstellar

objectsandtheimpactofchromosphericemissiononaccretionrateestimates

PublishedasManara, Testi, Rigliaco, Alcalà, Natta, Stelzeretal., A&A,2013, 551, 107;``X-Shooterspectroscopyofyoungstellarobjects:

II.Impactofchromosphericemissiononaccretionrateestimates''∗

Abstract

Context: Thelackofknowledgeofphotosphericparametersandthelevelofchromosphericactivityinyounglow-masspre-mainsequencestarsintroducesuncertaintieswhenmeasur-ingmassaccretionratesinaccreting(ClassII) youngstellarobjects. A detailedinvestigationoftheeffectofchromosphericemissionontheestimatesofmassaccretionrateinyounglow-massstars isstillmissing. Thiscanbeundertakenusingsamplesofyoungdiskless(ClassIII) K andM-typestars.Aims: OurgoalistomeasurethechromosphericactivityofClassIII premainsequencestarstodetermineitseffectontheestimatesoftheaccretionluminosity(Lacc)andmassaccretionrate(Macc)inyoungstellarobjectswithdisks.Methods: UsingVLT/X-Shooterspectra, weanalyzedasampleof24nonaccretingyoungstellarobjectsofspectraltypebetweenK5andM9.5. WeidentifiedthemainemissionlinesnormallyusedastracersofaccretioninClassII objects, andwedeterminedtheirfluxesinordertoestimatethecontributionofthechromosphericactivitytothelineluminosity.Results: WehaveusedtherelationshipsbetweenlineluminosityandaccretionluminosityderivedintheliteratureforClassII objectstoevaluatetheimpactofchromosphericactivityontheaccretionratemeasurements. Wefindthatthetypicalchromosphericactivitywouldbiasthederivedaccretionluminosityby Lacc,noise< 10−3L⊙, withastrongdependenceontheTeff oftheobjects. Thenoiseon Macc dependsonstellarmassandage, andthetypical

∗ThischapterdependsoncollaborativeworkwhichI hadtheopportunitytotaketheleadof. InparticularI wasresponsibleforpartofthedatareduction, inparticularforthefluxcalibrationandthetelluriccorrectionofthespectra, theinterpretation, andthediscussion. Finally, I wasresponsibleofthecompositionofthemanuscript.

43

3. Photospheric templates of young stellar objects and the impact of chromospheric emissionon accretion rate estimatesvaluesoflog(Macc,noise)rangebetween ∼ −9.2 to −11.6M⊙/yr.Conclusions: Valuesof Lacc≲ 10−3L⊙ obtainedinaccretinglow-masspremainsequencestarsthroughlineluminosityshouldbetreatedwithcautionbecausethelineemissionmaybedominatedbythecontributionofchromosphericactivity.

3.1 Introduction

Circumstellardisksareformedasanaturalconsequenceofangularmomentumconserva-tionduringthegravitationalcollapseofcloudcores(e.g. Shuetal., 1987). Intheearlyphasesofstarformation, thediskallowsforthedissipationofangularmomentumchan-nelingtheaccretionofmaterialfromtheinfallingenvelopeontothecentralyoungstellarobject(YSO).Atlaterstages, whentheenvelopeisdissipated, planetarysystemsforminthedisk, whilethestar-diskinteractioncontinuesthroughtheinnerdiskandthestellarmag-netosphere. Thisphenomenonconstrainsthefinalstellarmassbuild-up(e.g. Hartmannetal., 1998), anditstypicaltimescalesareconnectedwiththetimescalesonwhichdisksdissipateandplanetformationoccurs(e.g. Hernándezetal., 2007; Fedeleetal., 2010;Williams&Cieza, 2011).

AccretioncanbeobservedusingtypicalsignaturesinthespectraofYSOs, suchasthecontinuumexcessinthebluepartofthevisiblespectrum(e.g. Gullbringetal., 1998)andtheprominentopticalandinfraredemissionlines(e.g. Muzerolleetal., 1998a; Nattaetal., 2004; Herczeg&Hillenbrand, 2008; Rigliacoetal., 2012). Themeasurementsusedtodeterminethebolometricaccretionluminosity(Lacc)areeitherdirectorindirect. Directmeasurementsareobtainedbymeasuringtheemissioninexcessofthephotosphericonein theBalmerandPaschencontinuaandadoptingamodel tocorrect for theemissionat thewavelengthsnotcoveredby theobservations, whichoriginatesmostlybelowtheU-band threshold (e.g. Valentietal., 1993; Gullbringetal., 1998; Calvet&Gullbring,1998; Herczeg&Hillenbrand, 2008; Rigliacoetal., 2012)orbylineprofilemodeling(e.g.Muzerolleetal., 1998a). Indirectmeasurementsareobtainedusingempiricalcorrelationsbetweenemissionlineluminosity(Lline)and Lacc (e.g. Muzerolleetal., 1998b; Nattaetal.,2004, 2006; Mohantyetal., 2005; Rigliacoetal., 2011a).

Measurementsofmassaccretionrate(Macc)aresubjectedtomanyuncertainties, be-causethisquantitydependson Lacc andonthemass-to-radius(M⋆/R⋆)ratio. ThisratioisnormallydeterminedfromthepositionoftheobjectontheHR diagramandasetofevolu-tionarymodels. Uncertaintiesonspectraltype(SpT) oftheobjectsaffectthedeterminationofeffectivetemperature(Teff), whilethoseontheextinction(AV )andthedistancemainlyaffecttheestimateofstellarluminosity(L⋆). It isnottrivialtodeterminethoseparame-tersinaccretingstarsbecauseofaccretionshocksonthestellarsurfaceproducingveilinginthephotosphericlines(e.g. Calvet&Gullbring, 1998)andmodifyingthephotometriccolors (e.g. DaRioetal., 2010a). Moreover, thederivationof Lline, fromwhich Lacc isdetermined, isaffectedbyanotherstellarproperty, namelythechromosphericactivityoftheYSOs(Houdebineetal., 1996; Franchinietal., 1998). Chromosphericlineemissionisusuallysmallerthantheaccretion-poweredemission, butitcanbecomeimportantwhen

44

3.2 Sample, observations, and data reduction

accretiondecreasesatlaterevolutionarystages(Inglebyetal., 2011)andinlowermassstars, whereaccretionratesarelower(Rigliacoetal., 2012). Therefore, thisisanimportantsourceofuncertaintythathasnotbeeninvestigatedindetailsofar.

Partof the INAF consortium'sGuaranteedTimeObservations (GTO) ofX-Shooter, abroad-band, medium-resolution, high-sensitivityspectrographmountedontheESO/VLT,hasbeenallocatedforstar formationstudies, inparticular to investigateaccretion, out-flows, andchromosphericemissioninlow-massClassII youngstellarandsubstellarob-jects(Alcaláetal., 2011). ThetargetsobservedduringtheGTO werechoseninnearby(d< 500pc)starformingregionswithlowextinctionandwithmanyverylow-mass(VLM)YSO (M⋆ < 0.2M⊙). Generally, thoseYSOsforwhichmeasurementsinmanyphotometricbandswereavailable, bothintheIR (e.g. Hernándezetal., 2007; Merínetal., 2008)andinthevisiblepartofthespectrum(e.g. Merínetal., 2008; Rigliacoetal., 2011a), wereselected. Toderive Lacc ofagivenClassII YSO,aClassIII templateofthesameSpT astheClassII isneeded. Therefore, duringthisGTO survey24ClassIII targetsintherangeK5-M9.5wereobserved, providingthefirstbroad-bandgridoftemplatespectraforlow-massstarsandbrowndwarfs(BDs). Sincethesespectrahaveaverywidewavelengthrange(∼350-2500nm)coveringpartoftheUV spectrum(UVB),thewholevisible(VIS),andthenearinfrared(NIR), thissampleallowsustodeterminethestellarparametersof thetargets, derivethechromosphericemissionlinefluxesandluminosities, hencedeterminetheimplicationsofchromosphericemissionontheindirectaccretionestimatesinClassIIobjects.

Thepaperisstructuredasfollows. InSect. 3.2 wediscussthesampleselection, theobservation strategy, and thedata reductionprocedure. In Sect. 3.3 wedescribehowSpTsofourtargetshavebeendetermined, whileinSect. 3.4 wederivetheirmainstellarparameters. InSect. 3.5 weidentifythemainlinespresentinthespectraandderivetheirintensities. InSect. 3.6 wediscusstheimplicationsofthelineluminosityfoundforstudiesof Macc inClassII YSOs. Finally, inSect. 3.7 wesummarizeourconclusions.

3.2 Sample, observations, anddatareduction

AmongtheobjectsobservedintheGTO survey, weselectedonlythosethathavebeenclassifiedasClassIII objectsusing Spitzer photometricdata. ThesamplecomprisesClassIII YSOsinthe σ-Orionis, LupusIII,andTW Hyaassociations. Intheend, thenumberoftargetsis24: 13objectsaremembersoftheTW Hyaassociation, 6oftheLupusIII cloud,and5of the σ-Orionis region. TheirSpTsrangebetweenK5andM9.5 (seeSect. 3.3).ThreeBDs, namelyPar-Lup3-1, TWA26, andTWA29, areincludedinoursample, withSpT M6.5, M9, andM9.5, respectively. DataavailablefromtheliteraturefortheseobjectsarereportedinTable 3.1. SixYSOsinoursamplearecomponentsofthreeknownwidevisualbinarysystems. Inallcaseswewereabletoresolvethem, giventhattheirseparationsarealwayslargerthan6′′.

Alltheobservationsweremadeintheslit-noddingmode, inordertoachieveagoodskysubtraction. Differentexposuretimesandslitdimensionswereusedfordifferenttargetsto

45

3. Photospheric templates of young stellar objects and the impact of chromospheric emissionon accretion rate estimateshaveenoughS/N andtoavoidsaturation. Thereadoutmodeusedinalltheobservationswas``100,1x1, hg", whiletheresolutionofourspectraisR =9100, 5100, and3300intheUVB armforslits0.5′′, 1.0′′, and1.6′′, respectively; R =17400, 8800, and5400intheVIS armforslits0.4′′, 0.9′′, and1.5′′, respectively; R =11300, 5600, and3500intheNIR armforslits0.4′′, 0.9′′, and1.5′′, respectively. WereportinTable 3.2 thedetailsofallobservationsforthiswork.

ThedatawerereducedusingtwoversionsoftheX-Shooterpipeline(Modiglianietal.,2010), runthroughthe EsoRex tool, accordingtotheperiodinwhichthedatawereac-quired. Version1.0.0wasused for thedataofDecember2009andMay2010, whileversion1.3.7wasusedfordatagatheredinJanuary2011, April2011, andApril2012. Thetwoversionsledtoresultsthatareverysimilar. Thereductionwasdoneindependentlyforeachspectrographarm. Thisalsotakestheflexurecompensationandtheinstrumen-talprofileintoaccount. Weusedthepipelinerecipe xsh_scired_slit_nod, whichincludesbiasandflat-fieldcorrection, wavelengthcalibration, order-tracingandmerging, andfluxcalibration. Regarding the lastpoint, bycomparing the response functionsofdifferentfluxstandardsobservedduringthesamenight, weestimatedanintrinsicerroronthefluxcalibrationoflessthan5%. Giventhatsomeobservationsweredonewithpoorweatherconditions(seeing∼3.5′′)orwithnarrowslits, wethencheckedthefluxcalibrationofeachobjectusingtheavailablephotometricdata, usuallyinthe U,B, V,R, I, J,H,K bands, asreportedinTable 3.1. Weverifiedthatallthespectramatchthephotometricspectralenergydistribution(SED) wellandadjustedtheflux-calibratedspectratomatchthephotometricflux. Binarieswerereducedinstaremode. Telluricremovalwasdoneusingstandardtel-luricspectraobtainedinsimilarconditionsofairmassandinstrumentalset-upofthetargetobservations. ThiscorrectionwasaccomplishedwiththeIRAF1 task telluric, usingspectraoftelluricstandardsfromwhichphotosphericlineswereremovedusingamultigaussianfitting. Thecorrectionisverygoodatallwavelengths, withonlytworegionsintheNIRarm(λλ 1330-1550nm, λλ 1780-2080nm)wherethetelluricabsorptionbandssaturate.Moredetailaboutthereductionwillbereportedin Alcaláetal. (2014).

1IRAF isdistributedbyNationalOpticalAstronomyObservatories, whichisoperatedbytheAssociationofUniversities forResearch inAstronomy, Inc., under cooperativeagreementwith theNational ScienceFoundation.

46


Table3.1:

Knownparametersfrom

theliterature

Nam

eOthernam

esRA(J2

000)

DEC

(J200

0)Region

DSpT

UB

VR

IJ

HK

Ref

TWA9A

CD

−36

742

9A11

4824

.22

−37

2849

.15

TWHya

68K5

...12

.52

11.26

...6.94

8.68

8.03

7.85

1,2,3

SO87

9...

053905

.42

−02

3230

.34

σ-O

ri36

0K5

17.09

14.70

14.44

13.53

12.83

11.55

10.86

10.67

3,4,5,6,7,8,9

TWA6

Tyc71

83147

71

101828

.70

−31

5002

.85

TWHya

51M0

...13

.39

11.81

...9.94

8.87

8.18

8.04

1,3,10

TWA25

Tyc77

60283

112

1530

.71

−39

4842

.56

TWHya

54M0

...12

.85

11.44

...9.50

8.17

7.50

7.31

1,3,11

,12

TWA14

UCAC212

4275

5311

1326

.22

−45

2342

.74

TWHya

96M0.5

......

13.80

...10

.95

9.15

8.73

8.50

2,3,13

TWA13

BRXJ112

1.3−

3447

S11

2117

.24

−34

4645

.50

TWHya

59M1

...12

.88

11.46

...9.57

8.43

7.73

7.49

1,3,11

,12

TWA13

ARXJ112

1.3−

3447

N11

2117

.40

−34

4650

.00

TWHya

59M1

...13

.43

11.96

...9.88

8.43

7.68

7.46

1,3,11

,12

TWA2A

CD

−29

888

7A11

0913

.81

−30

0139

.80

TWHya

47M2

...12

.55

11.07

...9.20

7.63

6.93

6.71

1,2,3

Sz12

2...

161016

.42

−39

0805

.07

LupIII

200

M2

...15

.33

13.73

13.28

12.12

10.89

10.12

9.93

3,14

,15,16

TWA9B

CD

−36

742

9B11

4823

.73

−37

2848

.50

TWHya

68M1

...15

.43

14.00

...11

.45

9.98

9.38

9.15

1,2,3

TWA15

B...

123420

.47

−48

1519

.50

TWHya

111

M2

...13

.30

12.20

13.41

11.80

10.49

9.83

9.56

3,17

,18,19

TWA7

Tyc71

90211

11

104230

.06

−33

4016

.62

TWHya

28M2

...12

.21

10.91

...9.10

7.79

7.12

6.90

1,3,10

TWA15

A...

123420

.65

−48

1513

.50

TWHya

111

M1.5

...13

.30

12.20

13.51

11.90

10.56

9.93

9.67

3,18

,19

Sz12

1...

161012

.19

−39

2118

.11

LupIII

200

M3

...15

.70

14.06

13.88

11.84

10.08

9.31

9.03

3,15

,16

Sz94

...16

0749

.59

−39

0428

.79

LupIII

200

M4

...16

.31

16.00

14.76

12.90

11.45

10.81

10.56

3,15

,16

SO79

7...

053854

.92

−02

2858

.35

σ-O

ri36

0M4

21.05

19.84

18.61

17.26

15.50

13.80

13.20

12.87

3,5,20

,21

SO64

1...

053838

.58

−02

4155

.86

σ-O

ri36

0M5

21.65

......

18.28

16.36

14.56

13.97

13.65

3,5,22

,23

Par−

Lup3

−2

...16

0835

.78

−39

0347

.91

LupIII

200

M6

...16

.88

15.49

15.02

13.09

11.24

10.73

10.34

3,15

,24

SO92

5...

053911

.39

−02

3332

.78

σ-O

ri36

0M5.5

22.57

......

...16

.54

14.45

13.93

13.57

3,5,9,22

SO99

9...

053920

.23

−02

3825

.87

σ-O

ri36

0M5.5

21.41

21.12

18.97

17.57

15.56

13.61

13.04

12.78

3,5,21

,22

Sz10

7...

160841

.79

−39

0137

.02

LupIII

200

M5.5

...17

.31

16.06

15.42

13.20

11.25

10.62

10.31

3,15

,16,25

Par−

Lup3

−1

...16

0816

.03

−39

0304

.29

LupIII

200

M7.5

...17

.74

19.92

18.15

15.28

12.52

11.75

11.26

3,15

,24

TWA26

2MJ113

9511

−31

5921

113951

.14

−31

5921

.50

TWHya

42M9

...20

.10

...18

.10

15.83

12.69

12.00

11.50

3,26

,27

TWA29

DEN

IS−PJ124

514.1−

4429

0712

4514

.16

−44

2907

.70

TWHya

79M9.5

......

......

18.00

14.52

13.80

13.37

28,29

References.(1)Torresetal.(200

6);(2)

Barrado

YNavascués

(200

6);(3)

Cutrietal.(200

3);(4)

Caballeroetal.(201

0);(5)

Rigliacoetal.(201

1a);

(6)Morrisonetal.(200

1);(7)Hernánd

ezetal.(200

7);(8)Saccoetal.(200

8);(9)Caballero

(200

8);(10)

Høgetal.(200

0);(11)

Zuckerm

an&

Song

(200

4);(12

)Messinaetal.(201

0);(13

)Riazetal.(200

6);(14

)Góm

ez&M

ardo

nes(200

3);(15

)Merínetal.(200

8);(16

)Ciezaetal.(200

7);

(17)Shkolniketal.(201

1);(18

)Sam

us'etal.(200

3);(19

)Zuckerm

anetal.(200

1);(20

)Oliveiraetal.(200

6);(21

)Sherryetal.(200

4);(22

)Rigliaco

etal.(201

2);(23

)Burninghametal.(200

5);(24

)Com

erón

etal.(200

3);(25

)Hughesetal.(199

4);(26

)Reidetal.(200

8);(27

)DEN

ISCon

sortium

(200

5);(28

)Kirkpatricketal.(200

8);(29

)Loo

peretal.(200

7);

Notes.D

istancesto

TW

Hyaobjectsareobtainedby

Weinbergeretal.(201

3),byTorresetal.(200

8),andbyMam

ajek

(200

5),toσ-O

ribyBrown

etal.(199

4),and

toLup

usIII

byCom

erón

(200

8).

47

3. Photospheric templates of young stellar objects and the impact of chromospheric emissionon accretion rate estimates

Table 3.2: Detailsoftheobservations

Name SLITS texp ObservationUVB VIS NIR date

TWA9A 0.5′′ 0.4′′ 0.4′′ 150s 16-17May2010SO879 1.0′′ 0.9′′ 0.9′′ 3600s 11-12Jan2011TWA6 0.5′′ 0.4′′ 0.4′′ 100s 12Jan2011TWA25 0.5′′ 0.4′′ 0.4′′ 120s 16-17May2010TWA14 0.5′′ 0.4′′ 0.4′′ 400s 12Jan2011TWA13B 0.5′′ 0.4′′ 0.4′′ 150s 16-17May2010TWA13A 0.5′′ 0.4′′ 0.4′′ 150s 16-17May2010TWA2A 0.5′′ 0.4′′ 0.4′′ 100s 16-17May2010Sz122 1.0′′ 0.9′′ 0.9′′ 600s 18Apr2012TWA9B 0.5′′ 0.4′′ 0.4′′ 800s 16-17May2010TWA15B 0.5′′ 0.4′′ 0.4′′ 600s 16-17May2010TWA7 0.5′′ 0.4′′ 0.4′′ 100s 12Jan2011TWA15A 0.5′′ 0.4′′ 0.4′′ 600s 16-17May2010Sz121 1.0′′ 0.9′′ 0.9′′ 500s 18Apr2012Sz94 1.0′′ 0.9′′ 0.9′′ 600s 16-17May2010SO797 1.0′′ 0.9′′ 0.9′′ 2400s 23-24Dec2009SO641 1.0′′ 0.9′′ 0.9′′ 3600s 23-24Dec2009Par−Lup3−2 1.0′′ 0.9′′ 0.9′′ 1200s 16-17May2010SO925 1.0′′ 0.9′′ 0.9′′ 3600s 21-22Dec2009SO999 1.0′′ 0.9′′ 0.9′′ 2400s 24-25Dec2009Sz107 1.0′′ 0.9′′ 0.9′′ 600s 22Apr2011Par−Lup3−1 1.0′′ 0.9′′ 0.9′′ 600s 16-17May2010TWA26 1.0′′ 0.9′′ 0.9′′ 3600s 22Mar2010TWA29 1.6′′ 1.5′′ 1.5′′ 3600s 22Mar2010

Thefullyreduced, flux-, andwavelength-calibratedspectraareavailableonVizier2.

3.3 Spectraltypeclassification

A carefulSpT classificationofthesampleisimportantinordertoprovidecorrecttemplatesforaccretionestimatesofClassII YSOs. Moreover, theprocedureusedtoderivetheSpTofClassII andClassIII YSOsshouldbeashomogeneousaspossible. InthissectionwedescribetwodifferentmethodsofderivingSpT fortheseobjects. First, weusethedepthofvariousmolecularbandsintheVIS partofthespectrum. Then, wedescribethesecondmethod, whichconsistsofusingspectralindicesintheVIS andintheNIR partofthespec-trum. Theseprovideuswithareliable, fast, andreddening-freemethodfordeterminingSpT forlargesamplesofYSOs.

3.3.1 Spectraltypingfromdepthofmolecularbands

FortheSpT classificationoftheobjects, weusedtheanalysisofthedepthofseveralmolec-ularbandsinthespectralregionbetween580nmand900nm(Luhman, 2004; Allen&Strom, 1995; Henryetal., 1994). ThisregionincludesvariousTiO (λλ 584.7-605.8, 608-639, 655.1-685.2, 705.3-727, 765-785, 820.6-856.9, 885.9-895nm), VO (λλ 735-755,

2http://vizier.u-strasbg.fr/viz-bin/VizieR-3?-source=J/A%2bA/551/A107/spectra

48

3.3 Spectral type classification

785-795, 850-865nm), andCaH (λλ 675-705nm)absorptionbands, andafewphoto-sphericlines(theCaII IR tripletat λλ 849.8, 854.2, 866.2nm, theNaI doubletat λ 589.0and589.6nm, theCaI at λ 616.2nm, ablendofseverallinesofBaII,FeI andCaI at λ649.7nm, theMgI at λ 880.7nm, andtheNaI andKI doubletsat λλ 818.3nmand819.5and λλ 766.5nmand769.9, respectively.

InFigs. 3.1, 3.2, and 3.3 weshowtheVIS spectraoftheobjectsinthewavelengthrangebetween580and900nm. Allthespectraarenormalizedat750nmand, forthesakeofclarity, smoothedtoaresolutionofR∼2500at750nmandverticallyshifted. ThedepthofthemolecularfeaturesincreaseswithSpT almostmonotonically, andbycomparingthespectraofthetargetsusingtogetherdifferentwavelengthsubrangesanddifferentmolecularbands, wecanrobustlyassignaSpT toourobjectsthankstothedifferencesinthedepthofthebands, withuncertaintiesestimatedtobe0.5subclasses.

Withthisspectraltypingprocedure, weclassifiedalltheobjectsinoursamplewithSpTearlierthanM8. TheagreementbetweentheSpTsderivedhereandthoseintheliteratureisgood, withonlythreecaseswherethedifferenceistwospectralsubclasses. WeassumetheSpT availablefromtheliteratureforthetwoYSOswithSpT laterthanM8(Reidetal., 2008;Kirkpatricketal., 2008), becausetheclassificationbycomparingmolecularbandsdepthwithotherspectrainthesampleisnotpossiblebecauseoursamplehasagapbetweenM6.5andM9. Wecanonlyconfirmthat theseobjectshaveaSpT later thantheothertargetsinthesampleandthattheirSpT differby0.5subclass. TheSpT obtainedherearelisted inTable 3.3 andinthefirstcolumnofTable 3.4, andtheyareusedfortherestoftheanalysis. ThedistributionofSpT forthesampleisshowninFig. 3.4. TherangeoftheM-typeisalmostentirelycovered, providingagoodsampleforthegoalsofthispaperandasolidlibraryoftemplatesthatcanbeusedfor Lacc estimatesofClassII YSOs.

InAppendix 3.C theNIR andUVB spectraofallthesourcesareshown. AlsointhesecasesatrendwithSpT canbeseen.

3.3.2 SpectralindicesforM3-M8stars

Spectral indicesprovidea fastmethodofdeterminingSpT for largesamplesofobjects.HerewetestsomeoftheseindicesforM-typeobjects. Riddicketal. (2007)testedandcalibratedvariousspectralindicesintheVIS partofthespectrumforpremainsequence(PMS) starswithSpT fromM0.5toM9. Infact, theysuggestusingsomereliablespectralindicesthatarevalidintherangeM3-M8. WefindthatthebestSpT classificationcanbeachievedbycombiningresultsobtainedusingthesetofindicesthatwereportinTable 3.5.Weproceededasfollows. ForeachobjectwecalculatedtheSpT withalltheseindices,andthenassignedthemeanSpT usingthoseresultsthatareinthenominalrangeofvalidityofeachindex. TypicaldispersionsoftheSpT derivedwitheachindexarelessthanhalfasubclass. Wereportthefinalresultsobtainedwiththeseindicesinthesecondcolumn(VIS_ind)ofTable 3.4. Comparingtheseresultswiththosederivedintheprevioussection,wereportanagreementwithinonesubclass forall theobjects intherangeM3-M8, asexpected. However, therearefourYSOsclassifiedM1-M2fromthedepthofmolecular

49


Figure 3.1: SpectraofClassIII YSOswithSpT earlierthanM3inthewavelengthregionwherethespectralclassificationhasbeencarriedout(seetextfordetails). Allthespectraarenormalizedat750nmandoffsetintheverticaldirectionby0.5forclarity. Thespectraarealsosmoothedtotheresolutionof2500at750nmtomaketheidentificationofthemolecularfeatureseasier.

50


Figure 3.2: SameasFig. 3.1, butforSpTsbetweenM3andM5.

51


Figure 3.3: SameasFig. 3.1, butforSpTslaterthanM5.

52


Table 3.3: Stellarparametersderivedfortheobjectsinoursample

Name SpT Teff [K] log(L⋆/L⊙) M⋆[M⊙] <log(Lacc,noise/L⊙)>TWA9A K5 4350 −0.61 0.81 −3.38SO879 K7 4060 −0.29 1.07 −2.88TWA6 K7 4060 −0.96 0.66 −3.37TWA25 M0 3850 −0.61 0.84 −3.04TWA14 M0.5 3780 −0.83 0.73 −3.23TWA13B M1 3705 −0.70 0.68 −3.32TWA13A M1 3705 −0.61 0.70 −2.85TWA2A M2 3560 −0.48 0.55 −3.36Sz122 M2 3560 −0.60 0.54 −2.57TWA9B M3 3415 −1.17 0.37 −3.93TWA15B M3 3415 −0.96 0.37 −3.45TWA7 M3 3415 −1.14 0.37 −3.84TWA15A M3.5 3340 −0.95 0.30 −3.23Sz121 M4 3270 −0.34 0.37 −3.25Sz94 M4 3270 −0.76 0.28 −3.38SO797 M4.5 3200 −1.26 0.19 −4.27SO641 M5 3125 −1.53 0.12 −4.51Par−Lup3−2 M5 3125 −0.75 0.18 −3.89SO925 M5.5 3060 −1.59 0.10 −4.65SO999 M5.5 3060 −1.28 0.13 −4.30Sz107 M5.5 3060 −0.79 0.16 −3.69Par−Lup3−1 M6.5 2935 −1.18 0.10 −4.74TWA26 M9 2400 −2.70 0.02 −6.54TWA29 M9.5 2330 −2.81 0.02 −6.64

Notes. The spectral type-Teff relation is adopted from Luhmanetal. (2003) forM-typeobjectsand fromKenyon&Hartmann (1995)forK-typeobjects.

53


Table 3.4: SpectraltypesobtainedusingthemethodbasedonthespectralindicesdescribedinSect. 3.3 andinAppendix 3.B

Name SpTa VIS_indb H2O_K2 H2O sH2OJ sH2OK sH2OH1 IJ IH HPTWA9A K5 ... ... ... ... ... ... ... ... ...SO879 K7 ... ... ... ... ... ... ... ... ...TWA6 K7 ... ... ... ... ... ... ... ... ...TWA25 M0 ... ... ... ... ... ... ... ... ...TWA14 M0.5 ... ... ... ... ... ... ... ... ...TWA13B M1 M3.3 ... ... ... M1.0 M1.2 ... M0.5 ...TWA13A M1 M3.3 ... ... ... M1.0 M0.7 ... M0.7 ...TWA2A M2 M3.3 M2.0 ... M2.6 M1.5 M0.1 M1.4 M0.5 ...Sz122 M2 M3.4 M2.2 ... M2.6 M2.7 M2.4 M2.5 M2.0 ...TWA9B M3 M3.7 M4.0 ... M5.1 M3.1 M3.4 M4.4 M3.4 ...TWA15B M3 M3.8 M4.5 ... M6.7 M3.8 M3.3 M5.0 M2.1 ...TWA7 M3 M3.8 M2.6 ... M3.5 M4.3 M3.3 M3.3 M3.7 ...TWA15A M3.5 M3.8 M3.8 ... M7.2 M4.1 M4.0 M5.3 M2.5 ...Sz121 M4 M4.3 M3.0 ... M3.6 M1.2 M5.8 M2.9 M2.1 ...Sz94 M4 M3.7 M4.3 ... M4.9 M3.7 M3.0 M4.9 M4.0 ...SO797 M4.5 M4.7 M2.3 ... M2.1 M6.0 M5.0 M3.6 M5.9 ...SO641 M5 M5.2 M5.3 M5.8 M4.7 M4.2 M5.1 M5.9 M6.6 ...Par−Lup3−2 M5 M5.0 M5.7 M5.7 M5.8 M4.0 M4.5 M6.1 M5.0 ...SO925 M5.5 M5.5 M7.3 M6.3 M3.9 M1.5 M5.0 M6.2 M7.3 ...SO999 M5.5 M5.4 M3.6 M5.8 M3.9 M5.7 M5.4 M6.1 M6.5 ...Sz107 M5.5 M5.5 M5.7 M5.9 M6.1 M6.1 M5.6 M6.4 M4.5 ...Par−Lup3−1 M6.5 M6.4 M6.6 M7.1 L0.8 M5.7 M8.6 L0.0 M5.5 ...TWA26 M9 ... M8.4 M8.3 L0.0 M7.7 M9.9 L0.4 M6.8 M9.1TWA29 M9.5 ... M9.4 M8.7 M9.3 M8.2 L0.8 L0.2 M7.2 M9.5

Notes. (a) SpT derivedinthisworkasexplainedinSect. 3.3.1. (b) ResultsobtainedusingthespectralindicesintheVIS partofthespectrum, asexplainedinSect. 3.3.2. AlltheothercolumnsrefertotheresultsobtainedusingNIR spectralindices, asexplainedinAppendix 3.B.SpT arereportedonlyintherangeofvalidityofeachindex.

54


K3 K5 K7 M1 M3 M5 M7 M9 SpT

0

1

2

3

4

5

Number

Figure 3.4: DistributionofspectraltypesoftheClassIII YSOsdiscussedinthiswork. Eachbincorrespondstoonespectralsubclass.

Table 3.5: Spectralindicesfrom Riddicketal. (2007, etreferencetherein)adoptedinouranalysisforspectraltypeclassification

Index Rangeofvalidity Numerator[nm] Denominator[nm]VO 7445 M5-M8 0.5625(735.0-740.0)+0.4375(751.0−756.0) 742.0−747.0VO 2 M3-M8 792.0−796.0 813.0−815.0c81 M2.5-M8 811.5−816.5 (786.5−791.5)+(849.0−854.0)R1 M2.5-M8 802.5−813.0 801.5−802.5R2 M3-M8 814.5−846.0 846.0−847.0R3 M2.5-M8 (802.5−813.0)+(841.5−846.0) (801.5−802.5)+(846.0−847.0)TiO 8465 M3-M8 840.5−842.5 845.5−847.5PC3 M3-M8 823.5−826.5 754.0−758.0

bandsthatwouldbeclassifiedM3fromthevaluesoftheRiddick'sindices. ThissuggeststhatspectralclassificationM3obtainedwithspectralindicescould, infact, beearlierbymorethanonesubclass.

TherearealsoindicesbasedonfeaturesintheNIR partofthespectrum(e.g Testietal., 2001; Testi, 2009; Allersetal., 2007; Rojas-Ayalaetal., 2012). InAppendix 3.B wecomparetheresultsobtainedwiththeseNIR indiceswiththeSpT derivedintheprevioussection toconfirm thevalidityof someof these indices for theclassificationofM-typeYSOs.

55


Figure 3.5: Exampleofthecombinationofaflux-calibratedandtelluricremovedX-Shooterspectrum(black)withmodelspectrawiththesameeffectivetemperature(green), whichisnormalizedattheredandblueedgesoftheX-ShooterspectrumandonlyshownoutsidetheX-Shooterrange. ThetelluricbandsintheNIRarereplacedwithalinearinterpolation(red). InthisfigureweshowtheexampleofTWA14.

3.4 Stellarparameters

Toestimatethemass(M⋆), radius(R⋆)andageforeachtargetwecomparethephotosphericparameters(L⋆,Teff)withthetheoreticalpredictionsofthe Baraffeetal. (1998)PMS evolu-tionarytracks. WederivetheTeff ofeachstarfromitsSpT usingthe Luhmanetal. (2003)SpT-Teff scale, whiletheprocedureforestimating L⋆ isdescribedinthenextparagraph.Wethenuse(L⋆,Teff)toplacethestarsontheHR diagramandestimate(M⋆, age)byin-terpolatingthetheoreticalevolutionarytracks. TheseparametersarereportedinTable 3.3.Finally, weobtain R⋆ from L⋆ andTeff.

3.4.1 Stellarluminosity

Giventhebroadwavelengthcoverage(∼300-2500nm)oftheX-Shooterspectra, forobjectswith2300 < Teff < 4400K onlyalowpercentageofthestellarflux(≲ 10-30%)arisesfromspectral regionsoutside theX-Shooter spectral range. Weuse, therefore, the followingproceduretoestimatethetotalfluxofourobjects: first, weintegratethewholeX-Shooterspectrumfrom350nmto2450nm, excluding the last50nmofspectraoneachside,whichareverynoisy, andtheregionsintheNIR betweenthe J , H andK-bands(λλ 1330-1550nm, λλ 1780-2080nm, seeSect. 3.2), wherewelinearlyinterpolateacrossthetelluricabsorptionregions. WethenusetheBT-Settlsyntheticspectrafrom Allardetal. (2011)withthesameTeff asourtargets(seeTable 3.3), assuminglogg=4.0, whichistypicaloflow-mass

56

3.4 Stellar parameters

0.02 M sun

0.05 M sun

0.10 M sun

0.20 M sun

0.40 M sun

0.60 M sun

0.80 M sun

1.00 M sun

1.20 M sun

3.353.403.453.503.553.603.65log T eff [K]

−3

−2

−1

0

1

log

(L

*/L

sun)

TWALupIIIσOri

Figure 3.6: Hertzsprung-RussellDiagramoftheClassIII YSOsofthiswork. Datapointsarecomparedwiththeevolutionarytracks(dottedlines)by Baraffeetal. (1998). Isochrones(solidlines)correspondto2, 10, 30,and100Myr.

YSOs, andnormalizedwithourspectraat350and2450nmtoestimatethecontributiontothetotalfluxemittedoutsidetheobservedrange. Thematchofthenormalizationfactorsatthetwoendsisverygoodinallcases. WeshowinFig. 3.5 anexampleofthisprocedure.

Themainsourceofuncertaintyin L⋆ comesfromtheuncertaintyinthespectroscopicflux. Fortheobjectsobservedinexcellentweatherconditions, thismatchesthefotometricfluxestobetterthanafactor ≲1.5. Itisthenreasonabletoassumesuchanuncertaintyforalltheobjectsinoursample, afternormalizingthespectroscopicfluxtothephotometry(seeSect. 3.2). Thiswouldleadtoanuncertaintyoflessthan0.2dexinlogL⋆.

ToconvertthebolometricfluxesobtainedinthiswayinL⋆ weadoptthesedistances: weassumethatYSOsinthe σ-Oriregionhaveadistanceof360pc(Brownetal., 1994), thoseinTW Hyathedistanceslistedin Weinbergeretal. (2013), in Torresetal. (2008), andinMamajek (2005), andthoseinLupusIII of200pc(Comerón, 2008), asreportedinTable 3.1.ThederivedstellarluminositiesarelistedinTable 3.3. Thesevaluesarecomparabletothoseobtainedwithphotometricdataintheliterature, withatypicaldifference ≲ 0.2dex. Asaresult, theyareconsistentwithourdeterminations, withintheerrors.

57

3. Photospheric templates of young stellar objects and the impact of chromospheric emissionon accretion rate estimates3.4.2 Stellarmassandage

InFig. 3.6 weshowtheHertzsprung-Russelldiagram(HRD) ofourPMS stars, builtusingTeff asreportedinTable 3.3 and L⋆ derivedinSec 3.4.1. WeassignM⋆ andagetoourPMSstarsbyinterpolatingevolutionarytracksfrom Baraffeetal. (1998)intheHRD.TheresultingM⋆ arereportedinTable 3.3. Forfourobjects(Sz107, Sz121, TWA26, andTWA29) thepositionintheHRD impliesanage <1Myr, wheretheoreticalmodelsareknowntobeveryuncertainand, infact, aretypicallynottabulated(Baraffeetal., 1998). Weestimatethemassoftheseobjectsbyextrapolatingfromtheclosesttabulatedpoints, butwewarnthereaderthatthevaluesareaffectedbyhighuncertainty.

OurYSOsaredistributedalongdifferentisochrones; LupusYSOsappeartobeyoungerthantheothers(age≲2Myr), while σ-OriYSOsaredistributedinisochronalagesintherange2≲age≲10Myr; finally, TWHyatargetsappeargenerallyclosetothe10Myrisochrone.Thisisingeneralagreementwithwhatisfoundintheliterature; indeed, Lupushasanes-timatedageof ∼1-1.5Myr(Hughesetal., 1994; Comerón, 2008), whilethe σ-Oriregionisusuallyconsideredtobeslightlyolder(∼3Myrinaverage), andrangesfrom ≲1MyrtoseveralMyr(ZapateroOsorioetal., 2002; Oliveiraetal., 2004). For theTW Hyaas-sociation, theageestimatesare ≳10Myr(Mamajek, 2005; BarradoY Navascués, 2006;Weinbergeretal., 2013).

3.5 Lineclassification

ThespectraofourobjectsarecharacterizedbyphotosphericabsorptionlinesthatdependontheSpT and, insomecases, ontheage. ToassessthePMS statusoftheobjectsinoursample, wecheckthatthelithiumabsorptionfeatureat λ 670.8nm, whichisrelatedtotheageoftheYSOs(e.g. Mentuchetal., 2008), isdetectedinallbutone(Sz94)oftheobjects.WediscussinmoredetailtheimplicationsofthenondetectioninSz94inAppendix 3.A,andweexplainwhythisobjectcouldbeconsideredinouranalysisasYSO anyway. Thevaluesofthelithiumequivalentwidth(EWLiI)fortheotherobjectsinthesampleare ∼ 0.5Å.A detailedanalysisofthislineandtheotherphotosphericabsorptionlinesoftheobjectsinoursamplewillbecarriedoutby Stelzeretal. (2013).

Inadditiontothese, wedetectmanyemissionlines, typicallyH,He, andCalines, thatoriginateinthechromosphereofthesestars. Inthisworkweconcentrateontheemissionlinescharacterization, sinceweareinterestedinthechromosphericactivity.

3.5.1 Emissionlinesidentification

To understand the contribution of the chromospheric emission to the estimate of Lacc

throughtheluminosityofaccretion-relatedemissionlines, wefirstidentifiedinourspectrathelinestypicallyrelatedtoaccretionprocessesinClassII YSOs. Here, wedescribewhichlineswedetectedandreporttheirfluxesandequivalentwidthsinTables 3.6 and 3.7.

58

3.5 Line classification

HβHγ

Hδ

Hε

H8H9H10H11

H12

Ca H K

380 400 420 440 460 480Wavelength [nm]

0.0

0.5

1.0

1.5

2.0

log (F

λ/F436)

Figure 3.7: Portionofthespectrum, showingemissioninallBalmerlinesfromHβ uptoH12, aswellastheCaII H andK lines, oftheYSO TWA13B.ThespectrumhasbeensmoothedtoaresolutionR =3750at375nm.

ThemostcommonlinedetectedinYSOsistheHα lineat656.28nm, whichispresentinthespectraofallourobjects. EmissioninthislinehasbeenusedasaproxyforYSOidentificationandhasbeenrelatedtoaccretionprocesses(e.g. Muzerolleetal., 1998a;Natta et al., 2004). This line is also generated in chromospherically activeYSOs (e.g.White&Basri, 2003). Similarly, theotherhydrogenrecombinationlinesof theBalmerseriesareeasilydetectedinalmostallofourClassIII objectsuptotheH12line(λ 374.9nm). Itisnoteasy, nevertheless, todeterminethecontinuumaroundBalmerlinesbeyondtheH9line(λ 383.5nm), andtheHϵ line(λ 397nm)isblendedwiththeCaII-K line. AnexampleofaportionofthespectrumfromHβ toH12isshowninFig. 3.7.

ThehydrogenrecombinationlinesofthePaschenandBrackettseries, inparticularthePaβ (λ 1281.8nm)andBrγ (λ 2166nm)lines, havebeenshowntoberelatedtoaccretionbyMuzerolleetal. (1998a). Theselineshavesubsequentlybeenusedtosurveystarformingregionswithhighextinction(Nattaetal., 2004, 2006) inorder toobtainaccretionrateestimatesforverylow-massobjects. WedonotdetectanyoftheselinesinourClassIIIspectra, confirmingthatchromosphericactivityisnotnormallydetectablewiththeselines.

ThecalciumII emissionlinesat λλ 393.4, 396.9nm(CaHK) andat λλ 849.8, 854.2,866.2nm(CaIRT) arerelatedtoaccretionprocesses(e.g. Mohantyetal., 2005; Herczeg&Hillenbrand, 2008; Rigliacoetal., 2012), butalsotochromosphericactivity(e.g. Montes,1998). TheCaII H andK linesaredetectedin90%ofourobjects. TheCaII IRT linesaredetectedin11outof13objectswithSpT earlierthanM4. Theseemissionlinesappearasareversalinthecoreofthephotosphericabsorptionlines. Forall11objectswithSpT M4orlater, theCaII IRT linesarenotdetected.

TheHeI lineat λ 587.6nmisalsoknowntobeassociatedwithaccretionprocesses(Muzerolleetal., 1998a; Herczeg&Hillenbrand, 2008), butinClassIII YSOsitisknowntobeofchromosphericorigin(e.g. Edwardsetal., 2006). Thelineisindeeddetectedin

59

3. Photospheric templates of young stellar objects and the impact of chromospheric emissionon accretion rate estimates22(92%)objects. OtherHeI linesat λλ 667.8, 706.5, and1083nmareusuallyassociatedwithaccretionprocesses(Muzerolleetal., 1998a; Herczeg&Hillenbrand, 2008; Edwardsetal., 2006). WedetectonlyinSz122theHeI linesat λλ 667.8, 706.5nm, whilewedetectin8(33%)oftheobjectstheHeI lineat λ 1083nm.

Finally, thereisnotraceofforbiddenemissionlinesinanyofourX-Shooterspectra,consistentwiththeexpectedabsenceofcircumstellarmaterialinClassIII YSOs.

3.5.2 Hα equivalentwidthand10%width

A commonlyusedestimator for theactivity inPMS stars is theEW of theHα line (e.g.White&Basri, 2003). Thisisusefulespeciallywhendealingwithspectrathatarenotflux-calibratedorwithnarrow-bandphotometricdata. TheabsolutevaluesofthisquantityasafunctionoftheSpT oftheobjectsareplottedinFig. 3.8, andthevaluesarereportedinTa-ble 3.6. Weobserveawell-knowndependenceofEWHα withSpT thatisduetodecreasingcontinuumfluxforcooleratmospheres. Withrespecttothethresholdtodistinguishbe-tweenaccretingandnonaccretingYSOsproposedby White&Basri (2003), allourtargetssatisfythecriteriaof White&Basri (2003)forbeingnonaccretors.

AnotherdiagnostictodistinguishbetweenaccretingandnonaccretingYSOsisthefullwidthoftheHα lineat10%ofthelinepeak(White&Basri, 2003). Thisdiagnostichasbeenshowntobecorrelatedwith Macc, butwithalargedispersion(Nattaetal., 2004).Figure 3.9 showstheEWHα absolutevaluesversusthe10%Hα width. Weseethatformostofourobjectsthe10%Hα widthisinthe ∼100-270km/srange, andforonlythreeobjectsthisvalueissignificantlyabovethethresholdsuggestedby White&Basri (2003)of270km/s(TWA6, Sz122, andSz121). InAppendix 3.A wediscusstheseobjects, andweexplainwhyTWA6andSz121canbeconsideredinouranalysis, whileSz122shouldbeexcludedbecauseitisprobablyanunresolvedbinary. Thisisprobablydueeithertohighvaluesof vsini fortheseobjects, whichbroadenthelineprofile, ortounresolvedbinarity.ForBDs, thethresholdfordistinguishingaccretorsfromnonaccretorsissetatvaluesofthe10%Hα widthof200km/s(Jayawardhanaetal., 2003a). ThisissatisfiedforalltheBDsinthesample. Thevaluesof10%Hα widtharelistedinTable 3.6.

3.5.3 Lineluminosity

Wemeasure thefluxofeach linebyestimating thecontinuum in theproximityof thelinewith the IDL astroliboutlier-resistantmean task resistant_mean. We then subtractthecontinuumfromtheobservedfluxandcalculatetheintegral, checkingthatthewholeline, includingthewings, isincludedinthecomputation. Tocompute Lline weadoptthedistancesreportedinTable 3.3.

FortheCaII IRT lines, wheretheemissionappearsinthecoreoftheabsorptionfea-ture, wesubtract thecontinuumfromourspectrumfollowing theprescriptiongivenbySoderblometal. (1993). UsingaBT-Settlsyntheticspectrum(Allardetal., 2011)ofthesame Teff smoothedatthesameresolutionofourobservedspectrum, weobtainanesti-

60

3.5 Line classification

K4 K5 K6 K7 M0 M1 M2 M3M4 M5 M6 M7 M8 M9 Spectral Type

1

10

|EW

Hα|

[Å]

TWALupIIIσOri

Figure 3.8: Hα equivalentwidthasafunctionofspectraltype. Thedashedlinesrepresenttheboundarybetweenaccretorsandnonaccretorsproposedby White&Basri (2003)fordifferentSpT.

100 100010% Hα Width [km/s]

1

10

|EW

Hα|

[Å]

TWALupIIIσOri

Figure 3.9: Hα equivalentwidthasafunctionofthe10%Hα width. TheverticaldashedlinererpesentstheWhite&Basri (2003)criterionfortheboundarybetweenaccretorsandnonaccretors. Theobjectswith10%Hα widthbiggerthan270km/sare, fromrighttoleft: Sz122, Sz121, TWA6, andTWA13A.

61


TWA9A (K5)

−5

−4

−3

−2 SO879 (K7) TWA6 (K7)

TWA25 (M0)

−5

−4

−3

−2 TWA14 (M0.5) TWA13B (M1)

TWA13A (M1)

−5

−4

−3

−2 TWA2A (M2) Sz122 (M2)

Hα Hβ Hγ Hδ H8

H9

H10

H11

HeI

587

HeI

1083

CaII

393

CaII

8498

CaII

854

CaII

866

Paβ

Br γ

10%Hα

TWA9B (M3)

−5

−4

−3

−2

Hα Hβ Hγ Hδ H8

H9

H10

H11

HeI

587

HeI

1083

CaII

393

CaII

8498

CaII

854

CaII

866

Paβ

Br γ

10%Hα

TWA15B (M3)

Hα Hβ Hγ Hδ H8

H9

H10

H11

HeI

587

HeI

1083

CaII

393

CaII

8498

CaII

854

CaII

866

Paβ

Br γ

10%Hα

TWA7 (M3)

Accretion tracers

log(

Lac

c,no

ise

/Lsu

n)

Figure 3.10: log(Lacc,noise/L⊙) obtainedusingdifferentaccretiontracersandtherelationsbetween Lline andLacc from Alcaláetal. (2014). ThemeanvaluesobtainedusingtheBalmerandHeIλ587.6 linesareshownwiththebluesolidlines, andthe1σ dispersionisreportedwiththebluedashedlines. Upperlimitsarereportedwithredemptytriangles. The10%Hα widthisreportedwithabluefilledcircle.

mateofthelineabsorptionfeaturethatisthensubtractedinordertoisolatetheemissioncoreoftheline; finally, weintegrateoverthecontinuumsubtractedspectrum. WereportinTables 3.6 and 3.7 thevaluesobtainedforthefluxesandthelineEWs.

WeincludeinTable 3.6 thevaluesoftheobservedBalmerjump, definedastheratiobetweenthefluxat ∼360nmandat ∼400nm. TypicalvaluesfoundintheliteratureforClassIII YSOsrangebetween ∼0.3and0.5(Herczeg&Hillenbrand, 2008; Rigliacoetal.,2012). InClassII YSOs, instead, theobservedBalmerjumpvaluesareusuallyhigher, upto ∼6(Hartiganetal., 1991; Herczeg&Hillenbrand, 2008; Rigliacoetal., 2012). Fortheobjects inoursample, thisquantityrangesbetween ∼0.35and ∼ 0.55 (seeTable 3.6),withtheexceptionofSz122. ThevaluesoftheBalmerjumpratioforthethreeBDsinthesamplearenotreported, becausetheSNR oftheUVB spectrumofthesesourcesistoolowtoestimatethisquantity.

3.6 Implicationsformassaccretionratesdetermination

ThephysicalparameterthatisusedtoestimatetheaccretionactivityinClassII YSOsisMacc, whichisderivedfrom Lacc andthestellarparametersusingthefollowingrelation

62

3.6 Implications for mass accretion rates determination

TWA15A (M3.5)

−5

−4

−3

−2 Sz121 (M4) Sz94 (M4)

SO797 (M4.5)

−5

−4

−3

−2 SO641 (M5) Par − Lup3 − 2 (M5)

SO925 (M5.5)

−7

−6

−5

−4

−3 SO999 (M5.5) Sz107 (M5.5)

Hα Hβ Hγ Hδ H8

H9

H10

H11

HeI

587

HeI

1083

CaII

393

CaII

8498

CaII

854

CaII

866

Paβ

Br γ

10%Hα

Par − Lup3 − 1 (M6.5)

−7

−6

−5

−4

−3

Hα Hβ Hγ Hδ H8

H9

H10

H11

HeI

587

HeI

1083

CaII

393

CaII

8498

CaII

854

CaII

866

Paβ

Br γ

10%Hα

TWA26 (M9)

Hα Hβ Hγ Hδ H8

H9

H10

H11

HeI

587

HeI

1083

CaII

393

CaII

8498

CaII

854

CaII

866

Paβ

Br γ

10%Hα

TWA29 (M9.5)

Accretion tracers

log(

Lac

c,no

ise

/Lsu

n)

Figure 3.11: log(Lacc,noise/L⊙) obtainedusingdifferentaccretiontracersasFig. 3.10.

(Hartmannetal., 1998):

Macc =

(1− R⋆

Rm

)−1LaccR⋆

GM⋆

=LaccR⋆

0.8GM⋆

, (3.1)

wherethefactor0.8isduetotheassumptionthattheaccretionflowsarisefromamagne-tosphericradius Rm ∼ 5R⋆ (Shuetal., 1994).

Whenestimating Lacc inClassII YSOswiththedirectmethodofUV-excessfitting(e.g.Valentietal., 1993), thecontributiontothecontinuumexcessemissionduetochromo-sphericactivityisprobablynegligible, anditisnormallytakenintoaccountusingaClassIII YSO ofthesameSpT asatemplatefortheanalysis(e.g. Herczeg&Hillenbrand, 2008;Rigliacoetal., 2011b, 2012). However, Lacc isoftenderivedusingspectral lines lumi-nosityandappropriateempiricalrelations(seee.g. Muzerolleetal., 1998a; Nattaetal.,2004; Herczeg&Hillenbrand, 2008, andreferencestherein). Severalofthelinesnormallyusedforthesestudiesareinfluencedbychromosphericactivity(Sect. 3.5), whichshouldbeestimatedandsubtractedfromthelineemissionbeforecomputing Lacc. Thisprocedureisnottrivial, sinceitisdifficulttoproperlydisentanglethetwoemissionprocessesineachobjectandeachline. Allthe Lacc − Llines relationsarealwaysbasedontheuncorrectedvaluesof Lline. Thiscorrection, asshownin Inglebyetal. (2011)and Rigliacoetal. (2012),canbeimportantinobjectswithlow Lacc and, inparticular, inVLM stars. Therefore, thechromosphericemissionactsasasystematic``noise"thataffectsthe Lacc measurements.

63


Table3.6:

Fluxesand

equ

ivalentw

idthsofBalmerlines

Nam

eHα

Hβ

Hγ

Hδ

H8

H9

H10

H11

10%Hα

F 360/F

400

TWA9A

14.0

±1.1

4.7±

1.8

2.7±

1.5

2.2±

1.0

1.4±

0.2

1.0±

0.1

0.82

±0.07

0.91

±0.47

135.0

0.38

−1.48

−0.68

−0.62

−0.59

−0.80

−0.83

−0.43

−0.65

SO87

91.8±

0.2

0.52

±0.15

0.23

±0.08

0.20

±0.06

0.15

±0.01

0.11

±0.01

0.10

±0.00

0.07

8±

0.02

224

2.0

0.38

−2.08

−1.17

−0.70

−1.12

−1.62

−1.57

−1.74

−0.77

TWA6

36.4

±4.0

8.4±

2.1

5.1±

3.9

3.3±

0.7

2.0±

0.1

1.5±

0.1

1.4±

0.1

1.3±

0.7

362.0

0.41

−4.17

−1.68

−1.02

−1.43

−1.47

−1.54

−1.67

−1.36

TWA25

47.3

±3.7

18.8

±3.4

10.4

±2.6

7.5±

1.4

4.8±

0.2

3.4±

0.2

2.7±

0.2

2.2±

0.3

165.4

0.42

−2.69

−1.94

−1.86

−1.81

−2.26

−1.97

−1.14

−1.15

TWA14

15.4

±1.1

3.6±

0.5

2.1±

0.5

1.4±

0.2

0.87

±0.02

0.63

±0.02

0.50

±0.02

0.41

±0.07

262.2

0.45

−5.77

−3.18

−3.03

−2.91

−3.30

−2.73

−1.28

−1.87

TWA13

B23

.8±

1.9

8.8±

1.4

4.8±

1.1

3.3±

0.5

1.9±

0.1

1.3±

0.1

0.95

±0.08

0.81

±0.12

121.9

0.39

−2.37

−1.94

−1.88

−1.75

−2.07

−1.65

−0.51

−0.94

TWA13

A85

.7±

5.5

30.2

±4.0

17.0

±3.4

10.6

±1.3

4.8±

0.1

3.0±

0.1

2.3±

0.1

1.7±

0.2

287.8

0.45

−7.58

−6.07

−5.48

−5.11

−4.30

−3.41

−1.28

−1.68

TWA2A

42.0

±5.0

13.9

±2.1

6.6±

1.5

4.4±

0.8

2.8±

0.1

2.0±

0.1

1.4±

0.1

1.3±

0.2

154.7

0.38

−2.25

−1.88

−1.56

−1.36

−1.64

−1.38

−0.43

−0.84

Sz12

26.4±

0.8

3.2±

0.3

2.3±

0.6

1.5±

0.4

1.2±

0.1

1.0±

0.1

0.86

±0.05

0.59

±0.08

660.3

0.74

−6.76

−8.67

−10

.00

−8.02

−11

.57

−11

.69

−10

.02

−6.27

TWA9B

5.9±

0.6

2.0±

0.1

1.0±

0.1

0.71

±0.06

0.37

±0.01

0.24

±0.01

0.18

±0.01

0.16

±0.01

137.6

0.39

−4.81

−4.83

−4.20

−3.50

−3.09

−2.49

−0.71

−1.54

TWA15

B6.8±

0.3

2.0±

0.1

1.1±

0.1

0.72

±0.06

0.38

±0.01

0.27

±0.02

0.19

±0.01

0.16

±0.01

157.7

0.45

−8.96

−7.94

−7.19

−6.48

−6.20

−5.18

−1.60

−3.11

TWA7

46.7

±3.9

13.3

±0.9

6.8±

0.6

4.5±

0.4

2.6±

0.1

1.8±

0.1

1.4±

0.1

1.1±

0.1

111.6

0.41

−4.73

−4.53

−3.82

−3.36

−2.97

−2.36

−0.48

−1.68

TWA15

A10

.4±

0.6

3.0±

0.2

1.8±

0.1

1.3±

0.1

0.62

±0.02

0.47

±0.02

0.33

±0.02

0.25

±0.03

250.3

0.55

−12

.43

−10

.34

−10

.80

−9.68

−6.33

−8.07

−3.36

−4.06

Sz12

15.9±

0.8

1.1±

0.1

0.50

±0.10

0.28

±0.06

0.15

±0.01

0.12

±0.01

0.10

±0.01

0.08

7±

0.02

037

7.9

0.38

−7.01

−5.06

−3.79

−2.90

−2.33

−2.39

−2.32

−2.02

Sz94

2.3±

0.2

0.75

±0.05

0.42

±0.04

0.27

±0.02

0.14

±0.01

0.08

7±

0.00

60.06

4±

0.00

60.04

7±

0.00

818

8.9

0.39

−5.67

−5.79

−5.49

−4.43

−3.91

−2.73

−2.66

−1.18

SO79

70.16

±0.02

0.03

5±

0.00

30.01

6±

0.00

20.00

99±

0.00

150.00

57±

0.00

060.00

37±

0.00

110.00

30±

0.00

110.00

22±

0.00

0714

9.3

0.43

−6.85

−6.83

−5.23

−3.89

−5.95

−3.71

−3.47

−1.48

SO64

10.09

0±

0.00

90.01

9±

0.00

10.00

97±

0.00

100.00

53±

0.00

060.00

33±

0.00

030.00

25±

0.00

030.00

18±

0.00

030.00

12±

0.00

0213

1.2

0.38

−8.04

−9.08

−8.38

−5.42

−6.91

−5.98

−5.97

−1.74

Par−

Lup3

−2

1.1±

0.2

0.26

±0.03

0.14

±0.02

0.08

0±

0.01

10.04

1±

0.00

30.02

6±

0.00

30.01

9±

0.00

50.01

7±

0.00

513

8.7

0.36

−4.97

−4.88

−4.07

−2.67

−1.90

−1.38

−0.65

−1.39

SO92

50.08

1±

0.00

90.01

2±

0.00

10.00

50±

0.00

110.00

30±

0.00

13<0.00

12<0.00

11<0.00

0975

<0.00

0877

142.2

0.68

−8.55

−6.81

−5.59

−4.29

......

......

SO99

90.20

±0.02

0.03

1±

0.00

30.01

6±

0.00

20.00

91±

0.00

200.00

50±

0.00

110.00

36±

0.00

060.00

24±

0.00

050.00

21±

0.00

0714

7.1

0.45

−8.70

−7.99

−7.80

−5.31

−4.23

−4.18

−2.43

−1.35

Sz10

71.8±

0.2

0.40

±0.03

0.21

±0.02

0.13

±0.02

0.06

4±

0.01

00.04

9±

0.00

70.03

6±

0.00

60.02

5±

0.00

825

4.9

0.38

−11

.72

−11

.77

−9.88

−6.91

−4.28

−4.23

−2.57

−2.07

Par−

Lup3

−1

0.22

±0.02

0.02

4±

0.00

70.01

6±

0.00

70.00

73±

0.00

60<0.00

82<0.00

66<0.00

72<0.00

5514

0.0

1.43

−15

.78

−16

.82

−29

.99

−9.01

......

......

TWA26

0.05

2±

0.00

80.01

2±

0.00

20.00

85±

0.00

160.00

68±

0.00

170.00

38±

0.00

07<0.00

20<0.00

22<0.00

1813

8.4

1.55

−9.91

−27

.07

−66

.62

−35

.39

−97

.42

......

...TW

A29

0.01

9±

0.00

3<0.00

16<0.00

14<0.00

18<0.00

21<0.00

17<0.00

28<0.00

2811

4.9

1.56

−14

.24

......

......

......

...

Notes.Fluxes(10−

14ergs−

1cm

−2)arereportedinth

efirstrow

sforeachobjectw

ithth

eirerrors,w

ithequ

ivalentw

idhts(Å)inthesecond

row

s.TheHα10

%widthisinkm/s.Thelastcolum

nreferstotheBalmerjum

pratio

,intenedastheratiobetweentheflu

xat

∼36

0nm

totheflu

xat

∼40

0nm

.Upp

erlimitsarereportedwith

<.

64

3.6 Implications for mass accretion rates determinationTable3.7:

Fluxesand

equ

ivalentw

idthsofheliumand

calcium

lines

Nam

eHeI

λ587.6

HeI

λ1083

CaII λ

393.4

CaII λ

849.9

CaII λ

854.2

CaII λ

866.2

TWA9A

5.71

e-15

±3.31

e-15

<1.10

e-14

9.59

e-14

±7.44

e-16

4.53

e-13

±3.21

e-15

2.27

e-13

±4.47

e-15

2.59

e-13

±4.36

e-15

−0.08

...−13

.46

0.16

0.38

0.46

SO87

91.08

e-15

±6.52

e-16

<5.63

e-16

9.82

e-15

±1.57

e-16

4.04

e-14

±3.13

e-16

1.78

e-14

±3.74

e-16

2.26

e-14

±3.65

e-16

−0.15

...−18

.79

-0.04

0.28

0.37

TWA6

1.87

e-14

±1.47

e-14

2.99

e-14

±9.97

e-15

9.66

e-14

±4.87

e-15

<3.54

e-15

<4.89

e-15

<2.85

e-15

−0.25

−0.39

−12

.29

......

...TW

A25

2.39

e-14

±1.14

e-14

3.80

e-14

±3.85

e-14

2.46

e-13

±1.60

e-15

8.53

e-13

±6.41

e-15

3.86

e-13

±9.47

e-15

3.73

e-13

±7.90

e-15

−0.19

−0.16

−20

.04

0.01

0.23

0.35

TWA14

6.27

e-15

±5.04

e-15

1.61

e-14

±5.51

e-15

3.92

e-14

±1.13

e-15

1.54

e-13

±1.29

e-15

7.35

e-14

±2.48

e-15

6.42

e-14

±1.05

e-15

−0.31

−0.44

−19

.84

0.02

0.41

0.49

TWA13

B1.66

e-14

±7.83

e-15

8.55

e-14

±5.30

e-14

1.22

e-13

±7.24

e-16

5.33

e-13

±4.54

e-15

2.13

e-13

±6.06

e-15

2.16

e-13

±4.83

e-15

−0.24

−0.61

−21

.91

0.01

0.22

0.32

TWA13

A4.33

e-14

±1.21

e-14

<1.73

e-14

1.83

e-13

±1.18

e-14

6.50

e-13

±5.87

e-15

2.93

e-13

±1.01

e-14

3.18

e-13

±7.42

e-15

−0.61

...−15

.88

-0.12

0.15

0.26

TWA2A

1.61

e-14

±5.31

e-15

<3.93

e-14

2.10

e-13

±7.81

e-16

1.23

e-12

±1.05

e-14

4.94

e-13

±1.38

e-14

3.84

e-13

±9.01

e-15

−0.16

...−21

.62

0.08

0.32

0.42

Sz12

22.93

e-15

±6.09

e-16

2.18

e-15

±2.18

e-15

1.88

e-14

±7.91

e-16

<4.84

e-16

<6.36

e-16

<2.94

e-16

−0.44

−0.06

−19

.58

......

...TW

A9B

1.96

e-15

±2.73

e-16

<3.74

e-15

1.72

e-14

±1.44

e-16

1.15

e-13

±1.33

e-15

5.84

e-14

±1.57

e-15

3.64

e-14

±9.35

e-16

−0.37

...−20

.82

0.02

0.18

0.32

TWA15

B2.63

e-15

±5.10

e-16

1.30

e-14

±4.50

e-15

1.51

e-14

±3.17

e-16

6.10

e-14

±8.13

e-16

2.69

e-14

±1.36

e-15

2.19

e-14

±5.98

e-16

−0.69

−0.58

−27

.41

-0.20

0.03

0.19

TWA7

1.66

e-14

±2.82

e-15

<1.57

e-14

1.42

e-13

±8.57

e-16

8.47

e-13

±9.56

e-15

3.90

e-13

±1.19

e-14

3.91

e-13

±6.34

e-15

−0.35

...−25

.00

-0.10

0.07

0.22

TWA15

A4.40

e-15

±6.80

e-16

1.32

e-14

±4.04

e-15

1.79

e-14

±8.08

e-16

7.13

e-14

±9.30

e-16

3.11

e-14

±1.59

e-15

2.84

e-14

±7.04

e-16

−1.05

−0.70

−19

.29

-0.31

-0.02

0.16

Sz12

1<1.45

e-16

3.50

e-15

±3.22

e-15

1.22

e-14

±5.84

e-16

<1.30

e-15

<2.23

e-15

<7.06

e-16

...−0.11

−23

.33

......

...Sz94

1.12

e-15

±1.41

e-16

<9.13

e-16

5.64

e-15

±1.53

e-16

<7.37

e-16

<9.93

e-16

<3.70

e-16

−0.59

...−18

.92

......

...SO

797

2.92

e-17

±7.82

e-18

<3.02

e+02

2.48

e-16

±2.09

e-17

<7.25

e-17

<1.11

e-16

<4.69

e-17

−0.35

...−20

.54

......

...SO

641

2.12

e-17

±3.81

e-18

<5.96

e-17

1.00

e-16

±2.47

e-18

<4.89

e-17

<6.87

e-17

<3.47

e-17

−0.65

...−16

.82

......

...Par−

Lup3

−2

3.00

e-16

±7.21

e-17

<9.93

e-16

1.39

e-15

±4.54

e-17

<9.00

e-16

<1.25

e-15

<5.15

e-16

−0.46

...−9.30

......

...SO

925

1.98

e-17

±3.89

e-18

<7.64

e-17

8.55

e-17

±7.46

e-18

<4.93

e-17

<7.12

e-17

<3.54

e-17

−0.79

...−18

.79

......

...SO

999

2.70

e-17

±8.90

e-18

<1.19

e-16

2.05

e-16

±3.06

e-17

<9.80

e-17

<1.65

e-16

<7.89

e-17

−0.42

...−15

.56

......

...Sz10

73.08

e-16

±1.66

e-16

<5.19

e-16

3.34

e-15

±1.76

e-16

<5.03

e-16

<1.06

e-15

<3.70

e-16

−0.57

...−25

.26

......

...Par−

Lup3

−1

6.02

e-17

±5.72

e-17

<3.71

e-16

<4.40

e-17

<1.97

e-16

<2.86

e-16

<1.58

e-16

−2.35

......

......

...TW

A26

1.89

e-17

±9.96

e-18

<4.28

e-16

6.20

e-17

±1.82

e-17

<1.10

e-16

<1.69

e-16

<8.50

e-17

−1.88

...−56

.20

......

...TW

A29

<2.27

e-17

<9.49

e-17

<2.02

e-17

<2.59

e-17

<4.37

e-17

<2.73

e-17

......

......

......

Notes.Fluxesarerepo

rtedin

[ergs−

1cm

−2]inth

efirstrow

s,equivalentwidths(Å)arereportedinth

esecond

row

s.Upp

erlimitsareshownwith

<.

65


Inthefollowing, wecharacterizethiseffectusingthe Lline derivedinSect. 3.5 andthemostrecent Lacc − Lline relationsforClassII YSOsderivedby Alcaláetal. (2014). Theserelationshavebeenderivedusingthesamemethodasdescribedin Rigliacoetal. (2012),where LaccisobtainedfromthecontinuumexcessemissionandthencomparedwiththeLline oftheseveralemissionlinediagnosticsineveryobject. In Alcaláetal. (2014)thesampleiscomposedof36YSOslocatedintheLupus I andLupus III clouds, togetherwitheightadditionalYSOslocatedinthe σ-Oriregionfrom Rigliacoetal. (2012). Thesere-lationsarenotquantitativelydifferentfromthoseintheliterature, buthavesignificantlysmalleruncertaintiesandhavebeenderivedforstarswithsimilarpropertiesthantheClassIII analyzedhere. ForeachClassIII object, wecompute Lacc fromanumberofdifferentlines, asdescribedinthefollowing. Thisprovidesameasurementofthe``noise''intro-ducedinthedeterminationof Lacc fromlineluminositiesinClassII.Itrepresentsatypicalthresholdfordetermining Lacc inClassII objectsbychromosphericactivity, assumingthatthis isapproximately thesamein the twodifferentclassesofobjects, withandwithoutongoingaccretion. Wedefineitinthefollowingas Lacc,noise.

3.6.1 Accretionluminositynoise

Alcaláetal. (2014)useasampleofClassII YSOsintheLupusstarformingregionobservedwithX-Shootertorefinethe Lacc − Lline relations. WeadopttheirrelationstoestimatetheLacc,noise forourClassIII YSOs, usinginparticulartheHα, Hβ, Hγ, Hδ, H8, H9, H10, H11,HeI (λλ587.6, and1083nm), CaII (λ393nm), CaII (λ849.8nm), CaII (λ854.2nm), andCaII (λ866.2nm)lines. Moreover, weusetherelationfrom Nattaetal. (2004)betweenMacc andthe10%Hα widthtoestimate Lacc,noise fromthisindicator.

WeshowinFig. 3.10-3.11 thevaluesof Lacc,noise foreveryobjectobtainedusingthedifferentindicators. Theuncertaintiesonthesevaluesaredominatedbytheerrorsintherelationsbetween Lline and Lacc. Upperlimitsforundetectedlinesindicatethe3σ upperlimits. EachBalmerlineleadstovaluesof Lacc,noise thatalwaysagreewiththeotherBalmerlinesbylessthan∼0.2dex, andsimilarlytheHeIλ587.6 linealmostinallcases. TheHeIλ1083andtheCaII lines, instead, invariouscasesdonotagreewiththeresultobtainedusingtheBalmerandHeIλ587.6 lines, withdifferencesevenlargerthan0.6dex. Itshouldbeconsid-eredthattheexactvalueoftheCaII IRT linesluminosityissubjecttomanyuncertainties,owingtothecomplicatedprocedureforestimatingtheexcessluminosity(seeSect. 3.5.3).

ThePaschenandBrackettHI emissionlinesarenotdetectedinourspectra, aspointedoutinSect. 3.5.1. WereportinFigs. 3.10-3.11 the3σ upperlimitsfor Lacc,noise obtainedusingthePaβ andBrγ lineluminositiesandtherelationsfrom Alcaláetal. (2014). Thesevaluesarealwaysbelowthemean Lacc,noise valueobtainedwiththeBalmerandHeIλ587.6lines. This implies that those linesare lesssensitive tochromosphericactivity than theBalmerlines.

The10%Hα widthistheaccretionindicatorthatleadstovaluesof Lacc,noise thataremorediscrepant fromthemean(Figs. 3.10-3.11). Thisclearlydoesnot followinmorethan50%ofthecasestheresultsobtainedusingtheotherindicators. Thisisnotsurprising

66


K5 K7 M1 M3 M5 M7 M9

200025003000350040004500Teff (K)

−8

−7

−6

−5

−4

−3

−2<l

og(L

acc,

nois

e/L

sun)>

TWALupIIIσOri

Figure 3.12: Meanvaluesof log(Lacc,noise/L⊙) obtainedwithdifferentaccretiondiagnosticsasafunctionofTeff. Errorbarsrepresentthestandarddeviationaroundthemean log(Lacc,noise/L⊙). Thesedatashouldbeintendedasthenoiseinthevaluesof Lacc duetochromosphericemissionflux.

sincetheHα widthismainlyakinematicsmeasurement, unlike Lline measurements. Itis tobeexpectedthat theapplicationofamethodcalibratedforaccretionprocessestochromosphericactivitywouldresultininconsistencies. WeknowthatthebroadeningoftheHα lineandtheotheraccretion-relatedlinesisduetothehigh-velocityinfallofmaterialintheaccretionflows, whiletheintensityoftheemissionlinesisduetoemissionfromthehigh-temperatureregion. Thelattercanbeeitheraccretionshocksonthestellarsurfaceorchromosphericemission. Thatinoursampleofnon-accretingobjectsrelationsconvertingLline to Lacc leadtosimilarresultswhenusinglinefluxes, whiletheresultisquitedifferentwhenusinglinebroadeningseemstoconfirmthattheHα 10%widthwedetectisonlyduetothermalbroadeninginthechromosphereofthesestarsandnottothegasflowkinematicsassociatedwiththeaccretionontothecentralobject.

For all objects, themean Lacc,noise value is alwaysbelow ∼ 10−3L⊙, and this valuedecreasesmonotonicallywiththeSpT.InFig. 3.12 thesemeanvaluesof log(Lacc,noise/L⊙)obtainedwith theBalmerandHeIλ587.6 linesareplottedasa functionof theTeff of theobjects. Theerrorbarsontheplotrepresentthestandarddeviationofthederivedvaluesof Lacc,noise

3. Thesevaluesshouldbeintendedasthenoiseinthe Lacc valuesarisingfrom

3For the objectTWA29, where only theHα line is detected, we report the error on the estimate of

67


K5 K7 M1 M3 M5 M7 M9

3.303.353.403.453.503.553.603.65log(T eff ) [K]

−4.5

−4.0

−3.5

−3.0

−2.5

−2.0

−1.5<

log

(Lac

c,no

ise

/L*)

>

TWALupIIIσOri

Figure 3.13: Meanvaluesof log(Lacc,noise/L⋆) obtainedwithdifferentaccretiondiagnosticsasafunctionoflogTeff. Thedashedlineisthebestfittothedata, whoseanalyticalformisreportedinEq. (3.2). Twoobjects(Sz122andTWA9A) areexcludedfromthefit(emptysymbols), asexplainedinthetext.

thechromosphericactivity. InFig. 3.13 weshowthemeanvaluesofthelogarithmicratioLacc,noise/L⋆ obtainedusingtheBalmerandHeIλ587.6 linesasafunctionoftheTeff. UnlikeFig. 3.12, thequantity Lacc,noise/L⋆ isunbiasedbyuncertaintiesondistancevaluesorbydifferentstellarages, leadingtosmallerspreads. WeseethatfromtheK7objectsdowntotheBDs, thevaluesoflog(Lacc,noise/L⋆)decreasewiththeTeff oftheYSOs. AfterfittingtheLacc,noise-Teff relationwithapowerlaw, usingonlytheobjectsintherangeK7-M9.5, andexcludingSz122(seeAppendix 3.A fordetails), weobtainthefollowinganalyticalrelation(Fig. 3.13):

log(Lacc,noise/L⋆) = (6.17± 0.53) · logTeff − (24.54± 1.88). (3.2)

The only clear deviation from the general trend, apart from Sz122, is the K5YSOTWA9A,whichshowsavalueof log(Lacc,noise/L⋆) lowerby ∼0.6dexwithrespecttowhatshouldbeexpectedbytheextrapolationofthepreviousrelation. Unfortunately, oursam-pleistoosmall, andwedonothaveotherobjectswithearlierSpT toverifywhetherthislowvalueisactuallyadifferenttrendduetodifferentchromosphericactivityforearlierSpT YSOsorifthesourceispeculiar. Therearealsosignaturesofdifferentchromosphericactivity intensityamongobjectswith thesameSpT andlocatedin thesameregion; for

Lacc,noise/L⊙ fromthislineinFigs. 3.12-3.13 withadashedline.

68


−1.0 −0.8 −0.6 −0.4 −0.2 0.0 0.2log(M/M sun )

−12.0

−11.5

−11.0

−10.5

−10.0

−9.5

−9.0lo

g M

acc,

nois

e [M

sun/y

r]

1 Myr3 Myr10 Myr

Figure 3.14: logMacc,noise asa functionof logM⋆, withvaluesof Macc,noise obtainedusing threedifferentisochronesfrom Baraffeetal. (1998)andthevaluesof Lacc,noise/L⋆ derivedfromthefitinEq. (3.2)atany Teff.Resultsusingthe1Myrisochronearereportedwithfilledcircles, thoseusingthe3Myrisochronewithfilledtriangles, andthoseusingthe10Myrisochronewithfilledsquares.

example, thetwoTW HyaM1YSOs, whicharetwocomponentsofabinarysystem, thuscoevalobjects, haveaspreadin log(Lacc,noise/L⋆) of ∼0.5dex.

Valuesof Lline and Lacc inClassII YSOsthatareclosetothoseestimatedinthisworkshouldbeconsideredverycarefully, becausethechromosphericactivitycouldbeanim-portantfactorintheexcessluminosityinthelineandcouldproducemisleadingresults.

3.6.2 Massaccretionratenoise

Inthissection, wedeterminewhatthetypical``chromosphericnoise"on Macc wouldbewhenderivedfromindirectmethodsifthechromosphericemissionisnotsubtractedbeforecomputing Lline. Werefertothisquantityas Macc,noise.

Theprocedureisthefollowing. Weselectthreeisochrones(1, 3, and10Myr)fromBaraffeetal. (1998)modelsandninedifferentYSOsmasses(0.11, 0.20, 0.35, 0.40, 0.60,0.75, 0.85, 1.00, and1.10 M⊙), whichalwayscorrespondto Teff intherange2500-4000K,whereourresultsareapplicable. Then, foreach Teff wederive Lacc,noise/L⋆ usingthefitreportedinEq. (3.2). Finally, weuseEq. (3.1), adoptingtheproper R⋆, M⋆, and L⋆ atanyagefromthe Baraffeetal. (1998)tracks, inordertodeterminethetypical Macc,noise

atdifferentagesasafunctionof M⋆. TheresultsareshowninFig. 3.14, whereastrongcorrelationbetweenthetwoparametersisevident, withincreasing Macc,noise with M⋆. Atthesametime, thevariationin Macc,noise withageatanygiven M⋆ islarge, upto0.5dex

69


ρ−Oph∼ 1 Myr

−12

−11

−10

−9

−8

−7

ChaII ∼ 4 Myr

−1.5 −1.0 −0.5 0.0

L1630N−L1641 ∼ 1 Myr

−1.5 −1.0 −0.5 0.0−12

−11

−10

−9

−8

−7

ONC∼ 2−3 Myr

Paβ,Br γ

Hα,H β,HeI Hα,H β,HeI,L1630

Hα,H β,HeI,L1641

U−band Hα

log (M * /M sun )

log

Mac

c [M

sun/y

r]

Figure 3.15: logMacc asafunctionof logM⋆ forClassII objectslocatedindifferentstarformingregions. Datafor ρ-Opharefrom Nattaetal. (2006), correctedfornewdistancemeasurementsby Rigliacoetal. (2011a);ONC pointsarefrom Manaraetal. (2012), dataforL1630N andL1641arefrom Fangetal. (2009), andthoseforChaII from Biazzoetal. (2012). Macc valuesobtainedwithindirectmethodsareshownwithcolored,filledpoints, whilemeasurementsbasedonthedirect U -bandexcessmethodareshownwithgrayemptypointsasareference. Downwardstrianglesrefertoupperlimits. Thethickredsolidlineisthelowerlimittothemeasurementsof Macc setbychromosphericactivityinthelineemission. Weusethevaluesforthecorrectisochroneaccordingtothemeanvalueoftheageforeachregion, asreportedontheplot.

fordifferencesof2MyrattheH-burninglimit, butdecreaseswithincreasing M⋆. Similarresultsareobtainedwhenusingotherevolutionarymodels.

Wederivealimitonthedetectable Macc of ∼ 6.6 · 10−10 M⊙/yrforsolar-mass, young(1Myr)objects, decreasingto 2.5 · 10−12 M⊙/yrforlow-mass, older(10Myr)objects. Wereportthese Macc,noise valuesforthethreeisochronesanalyzedinTable 3.8.

Comparisonwithliteraturedata

Asdiscussed, Macc,noise isalowerlimittothevaluesof Macc thatcanbederivedfromtheluminosityof linesemittedby stellar chromospheres. In this section, wecompare thischromosphericlimitwithestimatesof Macc basedon Lline fromtheliterature. WeshowinFig. 3.15 valuesof Macc asafunctionofM⋆ obtainedwithopticalorNIR emissionlinesasaccretiondiagnosticforobjectslocatedinfourstarformingregionswithdifferentages: ρ-Ophiucus, theOrionNebulaCluster, theL1630N andL1641regions, andtheChameleonII region. Weoverplotthelociiof Macc,noise obtainedusingtheproperisochroneforthemeanageofeachregion. Inthiswaywedonotaddresspossibledifferencesin Macc dueto

70


Table 3.8: Valuesof logMacc,noise atdifferent M⋆ andages

M⋆ Age[Myr][M⊙] 1 3 100.11 −10.59 −11.08 −11.620.20 −10.15 −10.80 −11.260.35 −9.96 −10.41 −10.940.40 −9.90 −10.32 −10.830.60 −9.66 −10.05 −10.510.75 −9.50 −9.90 −10.330.85 −9.40 −9.80 −10.201.00 −9.28 −9.67 −9.971.10 −9.18 −9.52 −9.80

Notes. ValuesoflogMacc,noise atdifferentM⋆ anddifferentagesobtainedusingisochronesfrom Baraffeetal.(1998)andtheprocedureexplainedinSect. 3.6.2.

agespreadsintheseregions, butthesespreads, ifpresent, are ≲1-2Myr(seee.g. Reggianietal., 2011). Theeffectofasimilaragespreadonthe Macc,noise thresholdwouldbe ∼0.5dexforlow-massobjectsand ∼0.3dexforsolar-massstars(seeFig. 3.14).

We consider in this analysis Macc values obtained using optical emission lines. InFig. 3.15 weshow thevalues for the ∼1Myrold regionsL1630N andL1641 (Fangetal., 2009), wheretheHα, Hβ, andHeI lineswereused, thoseforthe ∼ 2-3MyroldOrionNebulaCluster(Manaraetal., 2012)obtainedwithphotometricnarrow-bandHα lumi-nosityestimates4, andthoseobtainedwiththeHα, Hβ, andHeI linesforthe ∼4MyroldChameleon II region (Biazzoetal., 2012). Inall thesecases, thevastmajorityofdat-apointsare found, asexpected, wellabove the Macc,noise threshold, confirming that thechromosphericcontributiontothelineemissionisnegligibleforstronglyaccretingYSOs.Nevertheless, thereareafew(11)objectsinL1630N andL1641, wheremeasuredvaluesofMacc arelowerthantheexpected Macc,noise. Onepossibilitytoexplainthisresultisvariableaccretionintheseobjects, whichhasbeenfoundtovaryasmuchas0.4dexontimescalesofoneyear(Costiganetal., 2012). Still, thisdoesnotexplainwhyonlylowermassobjectshappentobebelowthethreshold. Inanycase, thesepointsshouldbeconsideredwithcaution, since themeasured Lline couldbecompletelyduetochromosphericemission,leadingtoerroneousestimatesof Macc.

Forthe ∼1Myrold ρ-Ophiucusregionweconsiderthe Lline derivedthroughPaβ andBrγ lines (Nattaetal., 2006), corrected foramore recentestimateof thedistance (seeRigliacoetal., 2011a). AswenotedinSects. 3.5.1 and 3.6.1, Paβ andBrγ linesarenotdetectedinourClassIII YSOsspectra. InFig. 3.15, allthedetectionsarelocatedwellabovethe Macc,noise locus, whiletheupperlimitsaredistributedalsoattheedgeofthe Macc,noise

threshold. ThisconfirmsthevalidityoftheseNIR linesasgoodtracersofaccretionandthefactthattheyaremostlikelylesssubjecttochromosphericnoisethantheBalmerandHeIλ587.6 lines.

4Wealsoreportthevaluesof Macc obtainedthrough U -bandexcessby Manaraetal. (2012), whichareshownasacomparison.

71


3.7 Conclusion

Inthispaper, wepresentedtheanalysisof24diskless, hencenonaccreting, ClassIII YSOs,observedwiththebroad-band, medium-resolution, high-sensitivityVLT/X-Shooterspectro-graph. Thetargetsarelocatedinthreenearbystarformingregions(LupusIII, σ-Ori, andTW Hya)andhaveSpT intherangefromK5toM9.5. WecheckedtheSpT classifications,usingbothspectralindicesandbroadmolecularbands. Moreover, usingthefluxcalibratedspectra, wederivedthestellarluminosity. Then, weanalyzedtheemissionlinesrelatedtoaccretionprocessesinaccretingobjectsthatarepresentinthesespectraand, fromtheirluminosities, westudiedtheimplicationsofchromosphericactivityfor Macc determinationinaccreting(ClassII) objects. Thiswasdonebyderivingtheparameter Lacc,noise, whichisthesystematic``noise"introducedbychromosphericemissioninthemeasurementsof Lacc

fromlineemissioninClassII objects. Forthisanalysisweassumedasimilarchromosphericactivityinthetwoclassesofobjects.

Ourmainconclusionsare:

1. AllhydrogenrecombinationemissionlinesoftheBalmerseriesaredetectedinoursampleofClassIII YSOsspectrawhentheS/N ishighenough. Incontrast, Paschenand Brackett series lines are not detected in emission, and they are significantlyweaker, whencomparedtoBalmerlines, thaninClassII objects. Thechromospheric``noise''intheselinesislowerthanintheopticallines(seeFigs. 3.10-3.11), andtheyareverygoodtracersofaccretioninlow-mass, lowaccretionrateobjects.

2. UsingBalmerandHeIλ587.6 linesandthecalibratedrelationsbetween Lline and Lacc

from the literature, wederived Lacc,noise values that always showgoodagreementamongallthelines.

3. CalciumemissionlinesintheNIR spectralrange(λλ 849.8, 854.2, 866.2nm)aredetectedin11objects(45%ofthesample)superposedonthephotosphericabsorp-tionlines. Thisresultsinamorecomplicatedlinefluxmeasurementthanforotherlines. Theirbehaviorwithrespecttothehydrogenlineluminositiesisdifferent; inparticular, thevaluesof Lacc,noise obtainedusingtheselinesoftendonotagreewellwiththoseobtainedusingBalmerandHeIλ587.6 lines.

4. Themeanvaluesof Lacc,noise fortheobjectsinoursamplearelowerthan ∼10−3L⊙andhaveacleardependencewithTeff forK7-M9.5objects. Therefore, Lacc ofthisorder or smallermeasured inClass II objects using line luminosity as aproxyofaccretionmaybesignificantlyoverestimated if thechromosphericcontribution tothelineluminosityisnottakenintoaccount.

5. Ourresultsshowthatthe``noise"duetochromosphericactivityontheestimateofMacc inClassII YSOsobtainedusingsecondaryindicatorsforaccretionhasastrongdependenceon M⋆ andage. Typicalvaluesoflog(Macc,noise)forM-typeYSOsareintherangefrom ∼ −9.2 forsolar-massyoung(1Myr)objectsto −11.6 M⊙/yrforlow-mass, older(10Myr)objects. Therefore, derivedaccretionratesbelowthisthreshold

72

3.A Comments on individual objects

shouldbetreatedwithcautionbecausethelineemissionmaybedominatedbychro-mosphericactivity.

Aknowledgements C.F.M.acknowledgesthePhD fellowshipoftheInternationalMax-Planck-ResearchSchool.WethankAleksScholz, GregHerczeg, andLucaRicciforinsightfuldiscussions.ThisresearchmadeuseoftheSIMBAD databaseandoftheVizieR catalogaccesstool, operatedattheCDS,Strasbourg, France.

3.A Commentsonindividualobjects

Sz94

Sz94wasdiscoveredasanHα emittingstarinthesurveybySchwarz(1977). Sincethen,ithasbeenconsideredasaPMS starinreviewsandinvestigations(Krautter, 1992; Hughesetal., 1994; Comerón, 2008; Mortieretal., 2011), butnothinghasbeenmentionedaboutthepresenceof the lithiumabsorption line at670.8nm. Basedon its spectral energydistribution, Sz94hasbeenclassifiedasaClass III IR YSO by Merínetal. (2008). Forallthesereasons, weincludedthestarinourprogramasaClassIII templateofM4SpT.NotwithstandingthehighqualityoftheX-ShooterdataintermsofS/N andresolution, theLi I λ 670.8nmlineisnotpresentinthespectrum, andwedetermineanupperlimitofEWLi < 0.1Å.Thequestionthenrisewhetherlithiummayhavealreadybeendepletedinthestar. However, lithiumissignificantlydepletedbylargefactorsonlyafterseveraltensofMyr, inconsistentlywiththeaverageageofafewMyroftheLupusmembers. AnalysisoftheradialvelocityofthisobjectshowsthatitsvalueisinthetypicalrangeforLupussources(Stelzeretal., 2013). WeconsiderherethesourceasaPMS,givenitspositionintheHRD,theHα emission, andtheradialvelocitymeasurement, butmoreanalysisshouldbedonetoconfirmitsPMS status.

Sz122

Sz122isaClassIII objectclassifiedwith Spitzer (Merínetal., 2008). Allthelinesofthisobjectappearverybroadened. Nevertheless, thespectrumcannotbefittedwithsyntheticspectrabroadenedat reasonable valuesof v sin i, meaning that this is not a single fastrotator. Wethink, therefore, thatthisobjectisabinarysystem. ThiscouldexplaintheveryfaintLiI line(EWLi < 0.25Å) andthelackofemissionintheCaII IRT absorptionfeatures,whichisusuallyfoundinotherearlyM-typeobjectsofoursample. EventhepresenceofabroadHα line(10%width > 600km/s)canbeexplainedbythepresenceoftwoobjects.Wedonotconsiderthisobjectintheanalysisoftheimplicationsofchromosphericactivityon Macc measurements, becausetheeffectofitsbinaritycannotbeaccountedfor.

73

3. Photospheric templates of young stellar objects and the impact of chromospheric emissionon accretion rate estimatesSz121

Similar toSz122, thelinesof thisobjectappearverybroadened. Alsointhiscase, theobjectisclassifiedwith Spitzer asaClassIII (Merínetal., 2008). Wecanfitthespectrumwitha synthetic spectrumof the sameTeff, logg=4.0, and v sin i =70km/s. Thisvalueof v sin i isratherhighforatypicalM4-typeYSO.Iftrue, itwouldbeanextremeultrafastrotator. Thelarge10%Hαwidth(∼380km/s)isthenduetoitsveryhighrotationalvelocity.Anotherpossibilityisthatthisobjectmayalsobeanunresolvedspectroscopicbinary.

TWA6

Thisobjecthasbeendiscoveredandclassifiedasayoungnonaccretingobjectby Webbetal. (1999). ItisamemberoftheTWA association, anditisaknownfastrotator(v sin i=72km/s, Skellyetal. (2008)). Wemeasurea10%Hαwidthof362km/s. Thisislargerthanthethresholdtodistinguishtheaccretingandnonaccretingobjectsproposedby White&Basri(2003). Nevertheless, thisobjectcanbeconsideredaClassIII YSO forthesmallEWHα, thesmallBalmerjump, andinparticular, theIR classification. Forthesereasonsweconsiderthisobjectinouranalysis.

3.B NIR spectralindices

SpectralindicesintheNIR partofthespectrumareparticularlyusefulforclassifyingVLMstarsandBDs, whichemitmostoftheirradiationinthisspectralregion. Therefore, variousanalysestocalibratereliableNIR indicestoclassifylateM-typeobjectsandBDshavebeencarriedoutinthepast(e.g. Kirkpatricketal., 1999). Withoursample, weareabletoverifythevalidityofvariousNIR indicesforM-typeYSOs, comparingtheSpT obtainedthroughopticalspectroscopy(Sect. 3.3). Weconsider in thisanalysisdifferentNIR indices thathavebeencalibratedusingeitherdwarfsoryoungstars(subgiants) forobjectswithSpTM orlater. Inparticular, Rojas-Ayalaetal. (2012)calibratedthe H2O − K2 indexusingasampleofM dwarfs, andderivedarelationvalid for thewholeM class. Allersetal.(2007)calibratedthegravity-independentspectralindex H2O onasampleofyoungBDsanddwarfs. ThisisvalidforobjectswithSpT intherangeM5-L0. Testietal. (2001)andTesti (2009)proposedvariousspectralindices(sH2OJ , sH2OK , sKJ,sHJ,sH2OH1, sH2OH2,IJ , IH , andIK )toclassifyM-, L-, andT-typeBDs, calibratingthoseonalargesampleofdwarfs; finally, Scholzetal. (2012)usedasampleofVLM YSOstocalibratetheHP index,which isvalid forobjectswithSpT in the rangeM7-M9.5. We report those indices inTable 3.9.

Figure 3.16 showstheresultsofthisanalysis. TheSpTsobtainedinSect. 3.3 arereportedasafunctionofeachspectralindexforourobjects. Forthespectralindicesderivedin Testietal. (2001)and Testi (2009), wereportalsothedataforthesampleofL-andM-typedwarfstheyadoptedintheanalysis. Westressthatthelattersampleshouldbeconsideredwith

74

3.B NIR spectral indices

H2O−K2

0.6 0.7 0.8 0.9 1.0K3

M0

M5

L0

L5

T0

iH 2O

0.9 1.0 1.1 1.2K3

M0

M5

L0

L5

T0

sKJ

0.2 0.4 0.6 0.8 1.0 1.2K3

M0

M5

L0

L5

T0

sHJ

0.0 0.2 0.4 0.6K3

M0

M5

L0

L5

T0

sH2OJ

−0.2 0.0 0.2 0.4 0.6K3

M0

M5

L0

L5

T0

sH2OH1

0.0 0.2 0.4 0.6 0.8K3

M0

M5

L0

L5

T0

sH2OH2

0.2 0.3 0.4 0.5 0.6 0.7K3

M0

M5

L0

L5

T0

sH2OK

−0.2 0.0 0.2 0.4 0.6K3

M0

M5

L0

L5

T0

I J

0.6 0.8 1.0 1.2K3

M0

M5

L0

L5

T0

I H

0.5 0.6 0.7 0.8 0.9 1.0K3

M0

M5

L0

L5

T0

I K

0.6 0.7 0.8 0.9 1.0K3

M0

M5

L0

L5

T0

HP

0.9 1.0 1.1 1.2 1.3 1.4K3

M0

M5

L0

L5

T0

Spectral index values

SpT

Figure 3.16: SpectraltypeoftheobjectsasafunctionofthespectralindexvaluesobtainedwithdifferentNIRindices(Table 3.9). Redsymbolsarevaluesfromthiswork, whilegreencrossesarefrom Testietal. (2001)and Testi (2009). RedstarsareusedforobjectswithSpT intherangeofvalidityof theindex, whileredpentagonsforthosenotintherangeofvalidityoftheindex. DashedlinesarethebestfitfromthereferencecitedinTable 3.9 foreachindex, whenavailable. NopublishedrelationsareavailablefortheindicesIJ andIK . Blackandredsolidlinesarethebestfits(black=linear, red=polynomial)fromthiswork, consideringbothoursampleandthevaluesfromtheliterature.

75

3. Photospheric templates of young stellar objects and the impact of chromospheric emissionon accretion rate estimatescaution, sincegravitydependentspectralindicescalibratedusingsamplesofdwarfsmaynotbereliableforYSOs, whicharesubgiants.

WeseethattheindicessH2OH2, sKJ,sHJ,andIK cannotbeusedtoclassifyYSOsofM-typeclass. Incontrast, wefindagoodcorrelationwith the indices sH2OK , sH2OJ ,sH2OH1, IJ , and IH . For these indiceswedecide tofitourobjectsand those from theliteraturetogetherinallcases, usingeitheralinearorasecond-degreepolynomialfit. WeshowthebestfitobtainedinFig. 3.16. Usingtheserelations, wederivedtheSpT foreachofourYSOs, andwereporttheseresultsinTable 3.4. WethusproposenewSpT-spectralindexrelationsforthefollowingfiveindicesfrom Testietal. (2001)and Testi (2009). ThefirstthreearevalidforobjectswithSpT laterandequaltoM1:

SpT− code = 0.55 + 2.48 · sH2OK (3.3)

SpT− code = 0.38 + 2.89 · sH2OH1 − 1.56 · (sH2O

H1)2 (3.4)

SpT− code = 1.11 + 3.92 · IH − 5.35 · (IH)2. (3.5)

ThefollowingtwoindicesarevalidforobjectswithSpT laterandequaltoM2:

SpT− code = 0.70 + 2.90 · sH2OJ − 1.78 · (sH2O

J)2 (3.6)

SpT− code = 1.83 + 1.08 · IJ − 2.59 · (IJ)2 (3.7)

where the SpT are coded in the followingway: M0 ≡ 0.0, M9 ≡ 0.9, L5 ≡ 1.5, andavariationof0.1corresponds toastepofonesubclass. For thesespectral indicesweconcludethatresultsforobjectswithSpT inthenominalrangeofvalidityofeachindexarereliablewithinatypicaluncertaintyofaboutonesubclass.

A goodcorrelationisalsofoundforthespectralindex H2O−K2. UsingtheanalyticalrelationbetweenSpT andspectralindexfromtheliterature, weobtainfor13outof22objectswithM SpT resultsthatarecompatiblewithinonesubclasswiththecorrectSpT(seeTable 3.4). Thedifferencescanbeduetotheimperfecttelluricremovalinthefirstintervalof interestof this index. Moreover, weconfirmthat theH2O index isvalid forYSOswithSpT intherangeM5-M9.5, findingagreementwithinonesubclassforallourobjects(seeTable 3.4). RegardingtheHP index, weconfirmthatitisnotvalidforYSOswithSpT earlierthanM7, andweobservethattheSpT obtainedwiththisindexconfirmthosefromtheliteratureforourtwolaterSpT objects(seeTable 3.4).

76

3.B NIR spectral indices

Table3.9:

NIR

spectralin

dicesanalyzedin

App

endix3.B

Index

Rangeofvalidity

Num

erator[nm

]Denom

inator[nm

]Reference

H2O-K2

M0-M9

(207

0-20

90)/(2

235-22

55)

(223

5-22

55)/(2

360-23

80)

Rojas-Ayalaetal.(201

2)I J

M0-T9

(109

0-11

30)+

(133

0-13

50)

2·(126

5-13

05)

Testi(20

09)

I HM0-T9

(144

0-14

80)+

(176

0-18

00)

2·(156

0-16

00)

Testi(20

09)

I KM0-T9

(196

0-19

90)+

(235

0-23

90)

2·(212

0-21

60)

Testi(20

09)

H2O

M5-L5

1550

-156

014

92-150

2Allersetal.(200

7)sH

JL0-L9

(126

5-13

05)-(1

600-17

00)

0.5·[(12

65-130

5)+(1

600-17

00)]

Testietal.(200

1)sKJ

L0-L9

(126

5-13

05)-(2

120-21

60)

0.5·[(12

65-130

5)+(2

120-21

60)]

Testietal.(200

1)sH

2O

JL0-L9

(126

5-13

05)-(1

090-11

30)

0.5·[(12

65-130

5)+(1

090-11

30)]

Testietal.(200

1)sH

2O

H1

L0-L9

(160

0-17

00)-(1

450-14

80)

0.5·[(16

00-170

0)+(1

450-14

80)]

Testietal.(200

1)sH

2O

H2

L0-L9

(160

0-17

00)-(1

770-18

10)

0.5·[(16

00-170

0)+(1

770-18

10)]

Testietal.(200

1)sH

2O

KL0-L9

(212

0-21

60)-(1

960-19

90)

0.5·[(21

20-216

0)+(1

960-19

90)]

Testietal.(200

1)HIP

M7-M9.5

(167

5-16

85)

(149

5-15

05)

Scho

lzetal.(201

2)

77


3.C On-linematerial

NIR spectra

Figure 3.17: SpectraofClassIII YSOswithspectraltypeearlierthanM3intheNIR arm. Allthespectraarenormalizedat1700nmandoffsetintheverticaldirectionby0.5forclarity. Thespectraarealsosmoothedtotheresolutionof2000at2000nmtomakeidentificationoffeatureseasier.

78

3.C On-line material

Figure 3.18: SameasFig. 3.17, butforspectraltypesbetweenM3andM5.

79


Figure 3.19: SameasFig. 3.17, butforspectraltypeslaterthanM5.

80


UVB spectra

Figure 3.20: SpectraofClassIII YSOswithspectraltypeearlierthanM2intheUVB arm. Allthespectraarenormalizedat450nmandoffsetintheverticaldirectionforclarity. Thespectraarealsosmoothedtotheresolutionof1500at400nmtomakeidentificationoffeatureseasier.

81


Figure 3.21: SameasFig. 3.20, butforspectraltypesbetweenM2andM4.

82


Figure 3.22: SameasFig. 3.20, butforspectraltypeslaterthanM4.

83


84

4Accuratedeterminationofaccretionandphotosphericparametersinyoungstellar

objects

PublishedasManara, Beccari, DaRio, DeMarchi, Natta, Ricci, TestiA&A,2013, 558,114; ``Accuratedeterminationofaccretionandphotosphericparameters

inyoungstellarobjects: ThecaseoftwocandidateolddisksintheOrionNebulaCluster''∗

Abstract

Context: Currentplanetformationmodelsarelargelybasedontheobservationalconstraintthatprotoplanetarydiskshavealifetimeof ∼3Myr. Recentstudies, however, reporttheexistenceofpre-main-sequencestarswithsignaturesofaccretion(strictlyconnectedwiththepresenceofcircumstellardisks)andphotometricallydeterminedagesof30Myrormore.Aims: Here, wepresentaspectroscopicstudyoftwomajorageoutliersintheOrionNe-bulaCluster. Weusebroadband, intermediateresolutionVLT/X-Shooterspectracombinedwithanaccuratemethodtodeterminethestellarparametersandtherelatedageofthetar-getstoconfirmtheirpeculiarageestimatesandthepresenceofongoingaccretion.Methods: Theanalysisisbasedonamulticomponentfittingtechnique, whichderivessi-multaneouslyspectraltype, extinction, andaccretionpropertiesoftheobjects. Withthismethod, weconfirmandquantifytheongoingaccretion. Fromthephotosphericparame-tersofthestars, wederivetheirpositionontheH-R diagramandtheagegivenbyevolu-tionarymodels. Withotherageindicatorslikethelithium-equivalentwidth, weestimatetheageoftheobjectswithhighaccuracy.Results: Ourstudyshowsthatthetwoobjectsanalyzedarenotolderthanthetypicalpop-ulationoftheOrionNebulaCluster. Whilephotometricdeterminationofthephotospheric

∗ThischapteristheresultofacollaborativeworkthatI startedandlead. InparticularI wrotetheDDTproposaltoaskfortheobservingtimetocollectthedata. I thenpreparedtheobservationsthathavebeencarriedoutinservicemode. I alsocarriedoutallthedatareductionofthespectra, I developedthemethod-ologytoanalyzethedata, andI havetheninterpretedthedata. Finally, I wasresponsibleofthecompositionofthemanuscript, wherethecoauthorshelpedwithseveralcommentsandsuggestions.

85

4. Accurate determination of accretion and photospheric parameters in young stellar objects

parametersareanaccuratemethodtoestimatetheparametersofthebulkofyoungstellarpopulations, ourresultsshowthatthoseofindividualobjectswithhighaccretionratesandextinctionmaybeaffectedbylargeuncertainties. Broadbandspectroscopicdetermina-tionsshouldthusbeusedtoconfirmthenatureofindividualobjects.Conclusions: Theanalysiscarriedoutshowsthatthismethodallowsustoobtainanaccu-ratedeterminationofthephotosphericparametersofaccretingyoungstellarobjectsinanynearbystar-formingregion. Wesuggestthatourdetailed, broadbandspectroscopymethodshouldbeusedtoderiveaccuratepropertiesofcandidateoldandaccretingyoungstellarobjectsinstar-formingregions. Wealsodiscusshowasimilarlyaccuratedeterminationofstellarpropertiescanbeobtainedthroughacombinationofphotometricandspectroscopicdata.

4.1 Introduction

Theformationofplanetarysystemsisstronglyconnectedtothepresence, structure, andevolutionofprotoplanetarydisksinwhichtheyareborn. Inparticular, thetimescaleofdisksurvivalsetsanupperlimitonthetimescaleofplanetformation, becomingastringentconstraint forplanet formationtheories (Haischetal., 2001; Wolfetal., 2012). As theobservedtimescalefortheevolutionoftheinnerdiskaroundpre-MainSequence(PMS)starsisontheorderofafewMyr(e.g., Williams&Cieza, 2011), allmodelsproposedtoexplainthegasgiantplanetformation(coreaccretion, gravitationalinstability)aregenerallyconstrainedtoagreewithadisklifetimeofmuchlessthan10Myr.

Observationsofnearbyyoungclusters(age ≲ 3Myr)showthepresenceoffewoutlierswithderivedisochronalagesofmorethan10Myr. Forexample, thisisfoundintheOrionNebulaCluster(ONC) (DaRioetal., 2010a, 2012)andinTaurus(White&Hillenbrand,2005). TheageoftheobjectsisderivedfromtheirpositionontheH-R diagram(HRD) andthussubject toseveraluncertainties, whichareeitherobservational (e.g., spectral type,extinction), intrinsictothetargets(e.g., strongvariability, edge-ondiskpresence, Huélamoetal. 2010), orrelatedtotheassumedevolutionarymodels (e.g., DaRioetal., 2010b;Baraffe&Chabrier, 2010; Barentsenetal., 2011). On theotherhand, observationsoffar(d >1kpc)andextragalacticmoremassiveclustersrevealedthepresenceofalargepopulationofaccretingobjectsolder(ages>30Myr)thanthetypicalagesassumedforthecluster(∼ 3-5Myr). ExamplesofthesefindingsarethestudiesofNGC 3603(Beccarietal., 2010), NGC 6823(Riazetal., 2012)andofvariousregionsoftheMagellanicClouds(DeMarchietal., 2010, 2011; Spezzietal., 2012). Observations in theseclustersaredominatedbysolar-andintermediate-massstars, forwhichtheagedeterminationissubjecttothesameuncertaintiesasfornearbyclusters, butalsotootheruncertainties: Forexample,theageof thesemoremassive targets ismoresensitive toassumptionson thebirthline(Hartmann, 2003).

Thesefindingschallengethepresentunderstandingofprotoplanetarydiskevolutionandcanimplyaentirelynewscenariofortheplanetformationmechanism. Ononeside, theexistenceofoneorfewolderdisksinyoungregionsdoesnotchangetheaforementioned

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diskevolutiontimescalesbutrepresentsagreatbenchmarkofapossiblenewclassofob-jects, whereplanetformationishappeningonlongertimescales. Incontrast, thepresenceofalargepopulationofolderobjects, representingasignificantfractionofthetotalclustermembers, couldimplyatotallydifferentdiskevolutionscenariointhoseenvironments.

Wethuswanttoverifytheexistenceandthenatureofolderandstillactiveprotoplan-etarydisksinnearbyregions. Forthisreasonwehavedevelopedaspectroscopicmethod,whichallowsustoconsistentlyandaccuratelydeterminethestellarandaccretionprop-ertiesofthetargetedobjects. Here, wepresentapilotstudycarriedoutwiththeESO/VLTX-ShooterspectrographtargetingtwomajorageoutlierswithstrongaccretionactivityintheONC.Thisisanidealregionforthisstudy, givenitsyoungmeanage(∼2.2Myr, Reg-gianietal. 2011), itsvicinity(d =414pc, Mentenetal. 2007), thelargenumberofobjects(morethan2000, DaRioetal. 2010a), andthelargenumberofpreviousstudies(e.g., Hil-lenbrand, 1997; DaRioetal., 2010a, 2012; Megeathetal., 2012; Robbertoetal., 2013).Ourobjectivesarethereforea)toverifythepreviouslyderivedisochronalagesofthesetwoobjectsbyusingdifferentandmoreaccurateageindicatorsandb)toassessthepresenceofanactivediskwithongoingaccretion.

Thestructureofthepaperisthefollowing. InSect. 4.2, wereportthetargetsselectioncriteria, theobservationstrategy, andthedatareductionprocedure. InSect. 4.3, wede-scribetheprocedureadoptedtoderivethestellarparametersoftheobjects; andinSect. 4.4,wereportourresults. InSect. 4.5, wediscusstheimplicationsofourfindings. Finally, wesummarizeourconclusionsinSect. 4.6.

4.2 Sample, observations, anddatareduction

4.2.1 Targetsselectionanddescription

Wehave selected the twoolderPMS candidates from the sampleof theHST TreasuryProgramontheONC (Robbertoetal., 2013). Ourselectioncriteriawereasfollows: clearindicationsofongoingaccretionandofthepresenceofaprotoplanetarydisk; anestimatedisochronalagemuchlargerthanthemeanageofthecluster, i.e. ≳ 30Myr; alocationwelloutsidethebrightcentralregionof thenebulatoavoidintensebackgroundcontamina-tion, whichcorrespondstoadistancefrom θ1 OrionisC largerthan5′(∼0.7pc); andlowforegroundextinction(AV ≲ 2mag). Tofindthebestcandidates, wehavecombinedHSTbroadbanddata(Robbertoetal., 2013)withnarrow-bandphotometricandspectroscopicdata(Hillenbrand, 1997; Stassunetal., 1999; DaRioetal., 2010a, 2012)withmid-infraredphotometricdata(Megeathetal., 2012). Accordingto DaRioetal. (2010a, 2012), thetotalnumberofPMS starcandidatesintheONC fieldwithaderivedisochronalage ≳ 10Myr,whichisderivedusingvariousevolutionarymodels(D'Antona&Mazzitelli, 1994; Siessetal., 2000; Palla&Stahler, 1999; Baraffeetal., 1998), is ∼165includingbothaccretingandnon-accretingtargets, whichcorrespondsto ∼10%ofthetotalpopulation. Amongthese,∼ 90objects(∼5%)haveages ≳30Myr. Thepresenceofongoingaccretionhasbeenesti-matedthroughtheHα lineequivalentwidth(EWHα)reportedin DaRioetal. (2010a). We

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Table 4.1: Parametersavailableintheliteraturefortheobjectsanalyzedinthiswork

Name Other RA DEC SpT Teff AV L⋆ M⋆ agenames (J2000) (J2000) [K] [mag] [L⊙] [M⊙] [Myr]

OM1186 MZ Ori 05:35:20.982 -05:31:21.55 K5 4350 2.40 0.13 0.66 64OM3125 AG Ori 05:35:21.687 -05:34:46.90 G8 5320 1.47 0.63 0.95 25

Notes. Datatakenfrom DaRioetal. (2010a, andreferencestherein).

useathresholdvalueofEWHα > 20 Å,whichimpliesnonnegligiblemassaccretionrates(Macc ≳ 10−9M⊙/yr). Thisisahighthresholdthatleadstoselectstrongeraccretors. Fol-lowing White&Basri (2003)and Manaraetal. (2013a), objectswithEWHα < 20 Åcanstillbeaccretorsbutwithaloweraccretionrate. Theavailable Spitzer photometry(Megeathet al., 2012)hasbeenused toconfirm thepresenceof anoptically thickcircumstellardisk, whichisalreadysuggestedbythestrongHα emission. Thisisperformedbylookingatthespectralenergydistribution(SED) ofthetargetstoseeclearexcesswithrespecttothephotosphericemissioninthemid-infraredwavelengthrange. Amongtheobjectswithisochronalage ≳10Myr, therearetenobjects(<1%)showingverystrongHα excess, orEWHα > 20 Å.Verystriking, twoPMS starcandidates, eachwithanisochronalage ≳30Myr, haveclearindicationsofHα excessandthepresenceofthediskfromIR photometry.Thesetwoextremecasesofolder-PMS starcandidatesareselectedforthiswork.

ThetwotargetsselectedareOM1186andOM3125, andwereporttheirprincipalpa-rametersavailablefrom DaRioetal. (2010a, andreference therein) inTable 4.1. Bothtargetsare locatedon theHRD almoston themainsequence, asweshow inFig. 4.1,wherethepositionofthesesourcesisreportedusinggreenstars. WealsoplotthepositionofotherPMS starcandidatesintheONC field, asshownwithbluecirclesiftheirEWHα < 20ÅandwithreddiamondsifEWHα ≥ 20 Å.TheirpositionontheHRD clearlyindicatesthattheyhaveanisochronalagemuchlargerthanthebulkoftheONC populationwithagesbetween ∼ 60Myrand ∼ 90MyrforOM1186, accordingtodifferentevolutionarytracks,andbetween ∼ 25Myrand ∼ 70MyrforOM3125. Thespectraltype(SpT) ofthetwotargetshasbeendeterminedin Hillenbrand (1997)tobeK5forOM1186andG8-K0forOM3125.

4.2.2 Observationsanddatareduction

ObservationswiththeESO/VLT X-ShooterspectrographhavebeencarriedoutinservicemodebetweenFebruaryandMarch2012(ESO/DDT program288.C-5038, PI Manara).Thisinstrumentcoversthewavelengthrangebetween ∼300nmand ∼ 2500nmsimulta-neously, dividingthespectruminthreearms-namely, theUVB armintheregion λλ ∼300-550nm, theVIS armbetween λλ ∼ 550-1050nm, andNIR from λ ∼ 1050nmtoλ ∼ 2500nm(Vernetetal., 2011). Thetargetshavebeenobservedinslit-noddingmodetohaveagoodskysubtractionintheNIR arm. Forbothtargetsweusedthesameslitwidths(0.5′′ intheUVB armand0.4′′ intheothertwoarms)andthesameexposuretimes(300sx4inallthreearms)toensurethehighestpossibleresolutionoftheobservations(R=9100,

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4.3 Method

Figure 4.1: Hertzsprung-Russelldiagramdiagramof theONC from DaRioetal. (2012)withgreenstarsshowingthepositionsofthetwotargetsofthisstudy. Theoverplottedevolutionarytracksarefrom D'Antona&Mazzitelli (1994). Weplot(fromtoptobottom)the0.3, 1, 3, 10, 30, and100Myrisochrones.

17400, and10500intheUVB,VIS,andNIR arms, respectively)andenoughS/N intheUVB arm. Thereadoutmodeusedinbothcaseswas``100,1x1,hg". Theseeingconditionsoftheobservatoryduringtheobservationswere1′′ forOM1186and0.95′′ forOM3125.

Datareductionhasbeencarriedoutusingtheversion1.3.7oftheX-Shooterpipeline(Modiglianietal., 2010), whichisrunthroughthe EsoRex tool. Thespectrawerereducedindependentlyforthethreespectrographarms. Thepipelinewiththestandardreductionsteps(i.e., biasordarksubtraction, flat-fielding, spectrumextraction, wavelengthcalibra-tion, andskysubtraction)alsoconsiderstheflexurecompensationandtheinstrumentalprofile. Particularcarehasbeenpaidtothefluxcalibrationandtelluricremovalof thespectra. Fluxcalibrationhasbeencarriedoutwithinthepipelineandthencomparedwiththeavailablephotometry (Robbertoetal., 2013) tocorrect forpossible slit losses. Wehavealsocheckedtheconjunctionsbetweenthethreearms. Theoverallfinalagreementisverygood. TelluricremovalhasbeencarriedoutusingthestandardtelluricspectrathathavebeenprovidedaspartoftheX-Shootercalibrationplanoneachnightofobservations.ThecorrectionhasbeenaccomplishedusingtheIRAF1 task telluric, usingtheprocedureexplainedin Alcaláetal. (2014).

4.3 Method

ThedeterminationofSpT andstellarpropertiesinaccretingyoungstellarobjects(YSOs)isnottrivialforavarietyofreasons. First, YSOsareusuallystillembeddedintheirparental


89


molecularcloud, whichoriginatesdifferentialreddeningeffectsintheregion. Thiswiththepresenceofacircumstellardisksurroundingthestarcanmodifytheactualvalueoftheextinction(AV )onthecentralYSO fromoneobjecttoanother. Second, YSOsmaystillbeaccretingmaterialfromtheprotoplanetarydiskonthecentralstar. ThisprocessaffectstheobservedspectrumofaYSO -producingexcesscontinuumemissioninthebluepartofthespectrum, veilingofthephotosphericabsorptionfeaturesatallopticalwavelengthsandaddingseveralemissionlines(e.g., Hartmannetal., 1998). Thetwoprocessesmodifytheobservedspectruminoppositeways: Extinctiontowardthecentralobjectsuppressesthebluepartofthespectrum, makingthecentralobjectappearredderandthuscolder,whileaccretionproducesanexcesscontinuumemission, whichisstrongerinthebluepartofthespectrum, makingtheobservedcentralobjectlookhotter.

Forthesereasons, SpT, AV , andaccretionpropertiesshouldbeconsideredtogetherintheanalysisoftheseYSOs. Here, wepresenttheminimum χ2

like methodthatweusetode-termineSpT, AV , andtheaccretionluminosity(Lacc)simultaneously. Withthisprocedure,weareabletoestimate L⋆, whichisusedtoderiveM⋆, theageofthetargetusingdifferentevolutionarymodels, andthemassaccretionrate(Macc). Finally, wedeterminethesurfacegravity(log g)fortheinputobjectbycomparisontosyntheticspectra.

4.3.1 Parametersofthemulticomponentfit

Tofittheopticalspectrumofourobjects, weconsiderthreecomponents: asetofphoto-spherictemplates, whichareusedtodeterminetheSpT andthereforetheeffectivetem-perature(Teff)oftheinputspectrum; differentvaluesoftheextinctionandareddeninglawtoobtain AV ; andasetofmodels, whichdescribetheaccretionspectrumthatweusetoderive Lacc.

Photospherictemplates. Thesetofphotospherictemplatesisobtainedfromtheonecollectedin Manaraetal. (2013a). Thisisasampleof24well-characterizedX-Shooterspectraofnon-accreting(Class III) YSOswhicharerepresentativeofobjectsofSpT classesfromlateK toM.WeextendthisgridwithtwonewX-Shooterobservationsofnon-accretingYSOsfromtheESO programs089.C-0840and090.C-0050(PI Manara): oneobjectwithSpT G4andtheotheronewithSpT K2. Intotal, ourphotospherictemplatesgridconsistsof26non-accretingYSOswithSpT betweenG4andM9.5. WeuseClass III YSOsasaphotospherictemplate, becausesyntheticspectraorfielddwarfsspectrawouldbeinac-curateforthisanalysisforthefollowingreasons. First, YSOsarehighlyactive, andtheirphotosphere is stronglymodifiedby thischromosphericactivity inboth thecontinuumandthelineemission(e.g., Inglebyetal., 2011; Manaraetal., 2013a). Second, fielddwarfstarshaveadifferentsurfacegravitywithrespecttoPMS stars, whicharesub-giants. Usingspectraofnon-accretingYSOsastemplatesmitigatestheseproblems.

Extinction. Intheanalysisweconsidervaluesof AV intherange[0-10]magwithstepsof0.1magintherange AV =[0-3]magand0.5magathighervaluesof AV . Thisincludesallthepossibletypicalvaluesof AV fornonembeddedobjectsinthisregion(e.g., DaRioetal., 2010a, 2012). Thereddeninglawadoptedinthisworkisfrom Cardellietal. (1989)

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4.3 Method

with RV =3.1, whichisappropriatefortheONC region(DaRioetal., 2010a).Accretionspectrum. Todeterminetheexcessemissionduetoaccretionandthus Lacc,

weuseagridofisothermalhydrogenslabmodels, whichhasalreadybeenusedandproventobeadequateforthisanalysis(e.g., Valentietal., 1993; Herczeg&Hillenbrand, 2008;Rigliacoet al., 2012; Alcalá et al., 2014). Wedescribe theemissiondue toaccretionwiththeslabmodeltohaveagooddescriptionoftheshapeofthisexcessandtocorrectfortheemissionarisinginthespectralregionatwavelengthsshorterthantheminimumwavelengthscoveredbytheX-Shooterspectraat λ ≲ 330nm. Inthesemodels, weassumelocal thermodynamicequilibrium(LTE) conditions, andweincludeboth theH andH−

emission. Eachmodelisdescribedbythreeparameters: theelectrontemperature(Tslab),theelectrondensity(ne), andtheopticaldepthat λ=300nm(τ ), whichisrelatedtotheslablength. The Lacc isgivenbythetotalluminosityemittedbytheslab. Thegridofslabmodelsadoptedcoversthetypicalvaluesforthethreeparameters: Tslab areselectedintherangefrom5000to11000K, ne variesfrom1011 to1016 cm−3, and τ hasvaluesbetween0.01and5.

Additionalparameters. Inadditiontothefiveparametersjustintroduced-namely, thephotospherictemplate, AV , andthethreeslabmodelparameters(Tslab, ne, τ ), thereistheneed toalso include twonormalizationconstantparameters: one for thephotospherictemplate(Kphot)andonefortheslabmodel(Kslab). Thefirstrescalestheemittedfluxofthephotospherictemplatetothecorrectdistanceandradiusoftheinputtarget, whilethelatterconvertstheslabemissionfluxasitwouldhavebeenemittedatthestellarsurfacebyaregionwiththeareagivenbytheslabparameters.

4.3.2 Determinationofthebestfit

Toconsiderthethreecomponents(SpT,AV , Lacc)together, wedevelopaPythonprocedure,whichdeterminesthemodelthatbestfitstheobservedspectrum. Wecalculatealikelihoodfunctionforeachpointofthemodelgrid, whichcanbecomparedtoa χ2 distribution, bycomparingtheobservedandmodelspectrainanumberofspectralfeatures. ThesearechoseninordertoconsiderboththeregionaroundtheBalmerjump, wheretheemittedfluxmostlyoriginatesintheaccretionshock, andregionsatlongerwavelengths, wherethephotosphericemissiondominatestheobservedspectrum. Theformofthisfunctionthatisaddressedas χ2

like functionisthefollowing:

χ2like =

∑features

[fobs − fmod

σobs

]2, (4.1)

where f isthevalueofthemeasuredfeature, obs referstomeasurementsoperatedontheobservedspectrum,mod referstothoseonthemodelspectrum, and σobs istheerroronthevalueofthefeatureintheobservedspectrum. ThefeaturesconsideredherearetheBalmerjumpratio, definedastheratiobetweenthefluxat∼360nmand∼ 400nm; theslopeoftheBalmercontinuumbetween ∼335nmand ∼ 360nm; theslopeofthePaschencontinuumbetween ∼ 400nmand ∼ 475nm; thevalueoftheBalmercontinuumat ∼360nm; that

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Table 4.2: Spectralfeaturesusedtocalculatethebest-fitinourmulticomponentfitprocedure

Name Wavelengthrange[nm]

BalmerJumpratio (355-360)/(398-402)Balmercontinuum(slope) 332.5-360Paschencontinuum(slope) 398-477Continuumat ∼ 360 352-358Continuumat ∼ 460 459.5-462.5Continuumat ∼ 703 702-704Continuumat ∼ 707 706-708Continuumat ∼ 710 709-711Continuumat ∼ 715 714-716

ofthePaschencontinuumat ∼ 460nm; andthevalueofthecontinuumindifferentbandsat ∼ 710nm. TheexactwavelengthrangesofthesefeaturesarereportedinTable 4.2. Thebestfitmodelisdeterminedbyminimizingthe χ2

like distribution. Theexactvalueofthebestfit χ2

like isnotreported, becausethisvalueitselfshouldnotbeconsideredasanaccurateestimateofthegoodnessofthefit. Whereasafitwithavalueof χ2

like whichismuchlargerthantheminimumleadstoaverypoorfitoftheobservedspectrum, thisfunctionisnotaproper χ2, giventhatitconsidersonlytheerrorsontheobservedspectrumandonlysomeregionsinthespectrum.

Theprocedureisasfollows. Foreachphotospherictemplatewedereddentheobservedspectrumwithincreasingvaluesof AV . Consideringeachslabmodel, wethendeterminethevalueofthetwonormalizationconstants Kphot and Kslab foreveryvalueof AV . Thisisdonematchingthenormalizedsumofthephotosphereandtheslabmodeltotheobserveddereddenedspectrumat λ ∼ 360nmandat λ ∼ 710nm. Afterthat, wecalculatethe χ2

likevalueusingEq. (4.1). Thisisdoneforeachpointofthegrid(SpT, AV , slabparameters).Aftertheiterationoneachpointofthegridterminates, wefindtheminimumvalueoftheχ2like and thecorrespondentvaluesof thebestfitparameters (SpT, AV , slabparameters,

Kphot, Kslab).Wealsoderivetheuncertaintiesonthesetwoparametersandthuson Lacc fromthe

∆χ2like distributionwithrespecttotheSpT ofthephotospherictemplatesandthedifferent

valuesof AV . Indeed, thesearethemainsourcesofuncertaintyinthedeterminationofLacc, whichisameasurementoftheexcessemissionwithrespecttophotosphericemissionduetoaccretion. Mostoftheaccretionexcess(≳70%)isemittedinregionscoveredbyourX-Shooterspectraandoriginatesmostlyinthewavelengthrangeof λ ∼ 330-1000nm,whilemostof theexcessemissionat longerwavelengths isduetodiskemissionandisnotconsidered inouranalysis. Toderive the totalexcessdue toaccretion, weneedabolometriccorrectionfortheemissionatwavelengthsshorterthanthoseintheX-Shooterrangeat λ ≲330nm. Thisiscalculatedwiththebestfitslabmodel. Ouranalysisoftheslabmodelsleadtotheconclusionthat thiscontributionaccountsfor lessthan30%oftheexcessemissionandthattheshapeofthisemissioniswellconstrainedbytheBalmercontinuumslope. Different slabmodelparameterswith reasonableBalmercontinuum

92

4.3 Method

slopesthatwouldimplysimilarlygoodfitsasthebestonewouldleadtovaluesof Lacc

alwayswithin10%ofeachother, asithasalsobeenpointedoutby Rigliacoetal. (2012).Therefore, onceSpT and AV areconstrained, theresultswithdifferentslabparametersaresimilar. Withourprocedure, wecanverywellconstrainall theparameters. Typically,thesolutionswith χ2

like valuesclosertothebest-fitarethosewithinonespectralsubclassofdifferenceinthephotospherictemplateat ±0.1-0.2maginextinctionvaluesandwithdifferencesof Lacc oflessthan ∼10%. Othersourcesofuncertaintyontheestimateof Lacc

arethenoiseintheobservedandtemplatespectra, theuncertaintiesinthedistancesofthetarget, andtheuncertaintygivenbytheexclusionofemissionlinesintheestimateoftheexcessemission(seee.g., Herczeg&Hillenbrand 2008; Rigliacoetal. 2012; Alcaláetal.2014). Typicalerrorsaltogetheronestimatesof Lacc withourmethodareof0.2-0.3dex.

Aspreviouslymentioned, theaccretionemissionveilsthephotosphericabsorptionfea-turesoftheobservedYSO spectrum. Theseveiledfeaturescanbeusedtovisuallyverifytheagreementofthebestfitobtainedwiththe χ2

like minimizationprocedure. Todothis,wealwaysplottheobservedspectrumandthebestfitintheBalmerjumpregion, intheCa I absorptionlineregionat λ ∼420nm, inthecontinuumbandsat λ ∼ 710nm, andinthespectralrangeofdifferentphotosphericabsorptionlines, whichdependsmainlyonthetemperatureofthestar(Teff)-namely, TiO linesat λλ 843.2, 844.2, and845.2nm,andtheCa I lineat λ ∼ 616.5nm. TheagreementbetweenthebestfitandtheobservedspectrumintheBalmerjumpregionat λ ∼ 420 nmandinthecontinuumbandsat λ ∼ 710nmisusuallyexcellent. Similarly, thebestfitspectrumalsoreproducesthephotosphericfeaturesoftheobservedoneatlongerwavelengthsverywell.

4.3.3 Comparisontosyntheticspectra

Asanadditionalcheckofourresultsandtoderivethesurfacegravity(log g)forthetarget,wecomparethetargetspectrumwithagridofsyntheticspectra. Inparticular, weadoptsyntheticBT-Settlspectra(Allardetal., 2011)withsolarmetallicity, suchthat log g isintherangefrom3.0 to5.0 (instepsof0.5)andTeff isequaltothatof thebestfitphoto-spherictemplateusingtheSpT-Teff relationfrom Luhmanetal. (2003), andwithaTeff thatcorrespondstothenextupperandlowerspectralsub-classes.

Theprocedureusedisthefollowing. First, wecorrecttheinputspectrumforreddeningusingthebestfitvalueof AV . Subsequently, weremovetheeffectofveilingbysubtractingthebestfitslabmodel, whichisrescaledwiththeconstant Kslab derivedinthefit, tothedereddenedinputspectrum. Then, wedegradethesyntheticspectratothesameresolutionoftheobservedone(R=17400intheVIS),andweresamplethosetothesamewavelengthscaleof the target. Wethenselectaregion, following Stelzeretal. (2013), withmanystrongabsorptionlinesdependentonlyonthestarTeff andwithsmallcontaminationfrommolecularbandsandbroadlines. Thisischosentoderivetheprojectedrotationalvelocity(vsini)ofthetargetbycomparisonofitsspectrumwithrotationallybroadenedsyntheticspectra. Theregionchosen is in theVIS armfrom960to980nmandischaracterizedbyseveralTi I absorptionlinesandbysomeshallowerCr I lines. Then, webroadenthe

93


synthetic spectra tomatch the vsini of theobservedone, andwe compare itwith thereddened-andveiling-correctedtargetspectrumintworegions, whicharechosenbecausetheyincludeabsorptionlinessensitivetoboth Teff and log g ofthestar, following Stelzeretal. (2013, andreferencestherein). Oneoccursinthewavelengthrangefrom765to773nm, wheretheK I doublet(λλ ∼ 766-770nm)exists, andanotheroccursintherangefrom817to822nm, wheretheNa I doublet(λλ ∼ 818.3-819.5nm)exists. Thebestfit log goccursatthesyntheticspectrumwithsmallerresidualsfromtheobserved-syntheticspectrainthelineregions.

4.3.4 Stellarparameters

From thedeterminedbestfitparametersusing theexplainedprocedure in this section,weobtainTeff, AV , and Lacc. ThefirstcorrespondstotheTeff ofthebestfitphotospherictemplate, whichisconvertedfromtheSpT usingthe Luhmanetal. (2003)relation, derivedwiththemulticomponentfitandverifiedwiththecomparisontothesyntheticspectra. ThevalueofAV isalsoderivedinthemulticomponentfit, while Lacc iscalculatedbyintegratingthetotalfluxofthebestfitslabmodelfrom50nmto2500nm. Thefluxisrescaledusingthenormalizationconstant Kslab derivedbefore. This total accretionflux (Facc) is thenconvertedin Lacc usingtherelation Lacc = 4πd2Facc, where d isthedistanceofthetarget.

Toderive L⋆ oftheinputspectrum, weusethevaluesof L⋆ ofthenon-accretingYSOsusedasaphotospherictemplateforouranalysis, whichhavebeenderivedin Manaraetal. (2013a). Fromthebestfitresult, weknowthat fobs,dereddened = Kphot · fphot +Kslab · fslab,where f isthefluxofthespectrumofthedereddenedobservedYSO (obs, dereddened), ofthephotospherictemplate(phot), andoftheslabmodel(slab). Thephotosphericemissionoftheinputtargetisthengivensolelybytheemissionofthephotospherictemplaterescaledwiththeconstant Kphot. Therefore, weusetherelation:

L⋆,obs = Kphot · (dobs/dphot)2L⋆,phot, (4.2)

where d isthedistance, L⋆,phot isthebolometricluminosityofthephotospherictemplate,and L⋆,obs isthatoftheinputobject. Withthisrelation, wederive L⋆ fortheinputYSO.

FromTeff and L⋆ wethenderive R⋆, whoseerrorisderivedbypropagationoftheuncer-taintiesonTeff and L⋆. ThetargetM⋆ andageareobtainedbyinterpolationofevolutionarymodels(Siessetal., 2000; Baraffeetal., 1998; Palla&Stahler, 1999; D'Antona&Mazzitelli,1994)inthepositionofthetargetontheHRD.TheirtypicaluncertaintiesaredeterminedbyallowingtheirpositionontheHRD tovaryaccordingtotheerrorsonTeff and L⋆. Fi-nally, wederive Maccusingtherelation Macc = 1.25 ·LaccR⋆/(GM⋆) (Gullbringetal., 1998),anditserrorisdeterminedpropagatingtheuncertaintiesonthevariousquantitiesintherelation. Wereportthevaluesderivedusingalltheevolutionarymodels.

Anotherimportantquantity, whichcanbeusedtoassestheyoungageofaPMS star,is thepresenceand thedepthof the lithiumabsorption lineat λ ∼ 670.8. Given thatveilingsubstantiallymodifiestheequivalentwidthofthisline(EWLi), weneedtousethereddening-andveiling-correctedspectrumof the target toderive thisquantity. Onthis

94

4.4 Results

Figure 4.2: BestfitfortheobjectOM1186. Balmerjump, normalizationregion, CaI absorptionfeature, andphotosphericfeaturesusedtocheckderivedveiling.

correctedspectrum, wecalculateEWLi, integratingtheGaussianfitoftheline, whichispreviouslynormalizedtothelocalpseduo-continuumattheedgesoftheline. Thestatis-ticalerrorderivedonthisquantityisgivenbythepropagationoftheuncertaintyonthecontinuumestimate.

4.4 Results

Inthefollowing, wereporttheaccretionandstellarpropertiesofthetwoolderPMS can-didatesobtainedbyusingtheprocedureexplainedinSect. 4.3.

4.4.1 OM1186

ThebestfitforOM1186isshowninFig.4.2: Theredlineistheobservedspectrum, thegreenisthephotospherictemplate, thecyanlineistheslabmodel, andtheblueisthebest

95


Figure 4.3: Comparisonoftheextinction-andveiling-correctedspectrumofOM1186withasyntheticspec-trumwithTeff =4350K and log g=4.5.

fit. TheregionaroundtheBalmerjumpisshownintheupperplot. Theotherpanelsshowthenormalization regionaround ∼710nm, thecontinuumnormalizedCa I absorptionlineat ∼ 422nm, andthephotosphericfeaturesat λ ∼ 616nmand λ ∼ 844nm. Weclearlyseethattheagreementbetweentheobservedandbestfitspectraisverygoodinalltheregionanalyzed. ThisisobtainedusingaphotospherictemplateofSpT K5, whichcorrespondstoTeff=4350K withanestimateduncertaintyof350K, AV =0.9±0.4mag,andslabparametersleadingtoavalueof Lacc=0.20±0.09 L⊙. WethenconfirmtheSpTreportedintheliteratureforthisobject, butwederiveadifferentvaluefortheextinction,whichwasfoundtobe AV =2.4mag.

TheresultofthecomparisonoftheinputspectrumtothegridofsyntheticspectraisshowninFig. 4.3. ThebestagreementisfoundusingasyntheticspectrumatthesamenominalTeff ofthebestfitphotosphere: Teff =4350K with log g=4.5. InFig. 4.3, bothregionsadoptedforthe log g analysisareshown, andtheredlinereferstothereddening-andveiling-correctedobservedspectrum, whiletheblackdottedlineisthesyntheticspec-trum, whichreproducestheobservedonebetter. Wealsoreporttheresidualsinthebottompanel.

Fromthebestfit, wederiveavalueof L⋆ =1.15±0.36 L⊙ forthisobject. WiththisvalueandusingTeff =4350K,wederivethemassandagefortheseobjectswiththeevolutionarymodelsof Siessetal. (2000), Baraffeetal. (1998), Palla&Stahler (1999), and D'Antona&Mazzitelli (1994). Thevaluesderivedare, respectively, M⋆=1.1±0.4, 1.4±0.3, 1.1±0.4,0.6±0.3 M⊙ andage=3.2+4.8

−2.0, 4.7+7.2−2.5, 2.8

+2.9−1.7, 0.8

+1.4−0.4Myr. Moreover, wederive Macc

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4.5 Discussion

withtheparametersderivedfromthefitandfromtheevolutionarymodels, andweobtainMacc=1.3 ±0.8, 1.1±0.6, 1.4±0.8, 2.4±1.8 ·10−8M⊙/yrforthisobject, accordingtothedifferent evolutionary tracks. From the reddening- and veiling-corrected spectrum, wederiveEWLi =658 ± 85mÅ.AlltheparametersderivedfromthebestfitarereportedinTable 4.3, whilethosederivedusingtheevolutionarymodelsareinTable 4.4.

4.4.2 OM3125

ForOM3125, thevaluesreportedintheliteratureareSpT G8-K0and AV =1.47mag. Usingthesameprocedure, weobtainaminimum χ2

like valuewithaphotospherictemplateofSpTK7, whichcorrespondstoTeff=4060K withanuncertaintyof250K andAV =1.2±0.3mag.Withthisbestfitmodel, wedonotreproducetheCa I absorptionfeatureat λ ∼616.5nmverywell, becausetheamountofveilinginthislineistoohigh. Lookingatthesolutionswithvaluesof χ2

like similartotheminimumvalue, wefindthatthebestagreementbetweentheobservedandfittingspectruminthisfeatureiswithavalueof AV =1.0mag. Pleasenotethatthechoiceofthissolutioninsteadoftheonewith AV =1.2magimpliesthesamederivedparameters L⋆ and Lacc wellwithintheerrors. WeshowthisadoptedbestfitinFig. 4.4, usingthesamecolor-codeasinFig. 4.2. Theslabmodelusedhereleadstoavalueof Lacc=1.25±0.60 L⊙. TheeffectofveilinginthisobjectisstrongerthaninOM1186,andthisisclearlyseeninboththeBalmerjumpexcessandtheCa I absorptionfeatureat λ ∼420nm, whichisalmostcompletelyveiled. TheCa I absorptionfeatureshasalsoemissiononreversaloftheveryfaintabsorptionfeature. Moreover, theotherphotosphericfeaturesarealsomuchmoreveiled, asitisshowninthebottompanelsofFig. 4.4. Inthebottomrightpanel, therearealsohydrogenandheliumemissionlinesduetoaccretionincorrespondencewiththeTiO absorptionfeaturesnormallypresentinthephotosphereofobjectswiththisSpT (seeFig. 4.2 forcomparison).

InFig. 4.5 weshowthesyntheticspectrumanalysisforthisobject, usingthesamecolorcodeasinFig. 4.3. Inthiscase, thebestagreementisfoundwithasyntheticspectrumwithTeff =4000K and log g=4.0.

The luminosityderived for thisobject from itsbestfit is L⋆ =0.81±0.44 L⊙. WiththisvalueandusingTeff =4060K,wederivethemassandagefortheseobjectswiththeevolutionarymodelsof Siessetal. (2000); Baraffeetal. (1998); Palla&Stahler (1999);D'Antona&Mazzitelli (1994). Thevaluesderivedare M⋆=0.8±0.3, 1.2±0.2, 0.8±0.2,0.5±0.2 M⊙ andage=2.2+6.6

−1.0, 4.32+8.7−2.0, 2.4

+4.3−1.3, 0.8

+2.7−0.3Myr, respectively. According

to thedifferentevolutionarymodels, thisobjecthas Macc=1.2±0.9, 0.8±0.5, 1.2±0.7,1.9±1.2 ·10−7M⊙/yr, andEWLi =735± 42mÅ.TheseresultsarealsoreportedinTable 4.3and 4.4.

4.5 Discussion

Withtheresultspresentedintheprevioussection, wedeterminenewpositionsforourtargetontheHRD.ThisisshowninFig. 4.6 withgreenstarsrepresentingthetwoYSOsanalyzed

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Figure 4.4: BestfitfortheobjectOM3125. SameasFig. 4.2.

Table 4.3: Newlyderivedparametersfromthemulticomponentfit

Name SpT Teff AV L⋆ R⋆ log g EWLi Lacc

[K] [mag] [L⊙] [R⊙] [cm/s2] [mÅ] [L⊙]OM1186 K5 4350±350 0.9±0.4 1.15±0.36 1.9±0.4 4.5±0.5 658±85 0.20±0.09OM3125 K7 4060±250 1.0±0.3 0.81±0.44 1.8±0.5 4.0±0.5 735±42 1.25±0.60

Table 4.4: Parametersderivedfromevolutionarymodelsusingthenewlyderivedphotosphericparameters

OM1186 OM3125Evolutionary M⋆ age Macc M⋆ age Macc

model [M⊙] [Myr] [10−8M⊙/yr] [M⊙] [Myr] [10−8M⊙/yr]Siessetal. (2000) 1.1±0.4 3.2+4.8

−2.0 1.3±0.8 0.8±0.3 2.2+6.6−1.0 12.0±8.6

Baraffeetal. (1998) 1.4±0.3 4.7+7.2−2.5 1.1±0.6 1.2±0.2 4.3+8.7

−2.0 7.9±4.7Palla&Stahler (1999) 1.1±0.4 2.8+2.9

−1.7 1.4±0.8 0.8±0.2 2.4+4.3−1.3 11.6±7.5

D'Antona&Mazzitelli (1994) 0.6±0.3 0.8+1.4−0.4 2.4±1.8 0.5±0.2 0.8+2.7

−0.3 18.6±12.3

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4.5 Discussion

Figure 4.5: Comparisonoftheextinction-andveiling-correctedspectrumofOM3125withasyntheticspec-trumwithTeff =4000K and log g=4.0. SameasFig. 4.3.

in this studyand theother symbols representing the restof theONC population, as inFig. 4.1. TheirrevisedpositionsarecompatiblewiththebulkofthepopulationoftheONC,andtheirrevisedmeanages,∼2.9MyrforOM1186and∼2.4MyrforOM3125, aretypicalagesforobjectsinthisregion, forwhichthemeanagehasbeenestimatedaround2.2Myr(Reggianietal., 2011). Wealsocheckthatthefinalresultsdonotdependonthevaluethatwehavechosenforthereddeninglaw, i.e. RV =3.1. Withavalueof RV =5.0, weobtainages, whicharesystematicallyyoungerthantheoneobtainedinouranalysisbyafactor∼30%forOM1186and∼50%forOM3125. Withthenewlydeterminedparameters, theseobjectsareclearlynotolderthantherestofthepopulation, andtheirstatusofcandidateolderPMS isduetoanincorrectestimateofthephotosphericparametersintheliterature.Figure 4.6 alsoshowsthatmostoftheobjectsthatthenappearolderthan10MyrhavesmallornegligibleHα excess(bluecircles). Amongtheobjectswithage ≳ 10Myr, onlyeightobjectsactuallyshowsignaturesofintenseaccretion(onereddiamondvisibleintheplot; theothersevenhaveTeff < 3550 K andare, therefore, outsidetheplotrange)andatthesametimeofageolderthan10Myr. Theseobjectsshouldbeobservedinthefuturewithtechniquessimilartotheoneweusedhere(seeSect. 4.5.3 fordetails)tounderstandtheirrealnature. Inthefollowing, weanalyzeotherderivedparametersfortheseobjectsthatconfirmtheageestimatedwiththeHRD.Then, weaddresspossiblereasonswhichleadtomisclassificationofthesetargetsintheliterature.

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4.5.1 Agerelatedparameters

Lithiumabundance: UsingthevaluesofEWLi andthestellarparametersobtainedintheprevioussection, wecalculate the lithiumabundance (log N (Li)) for the two targetsbyinterpolationofthecurvesofgrowthprovidedby Pavlenko&Magazzu (1996). Wederivelog N (Li)=3.324±0.187forOM1186andlogN (Li)=3.196±0.068forOM3125. Thesevaluesarecompatiblewiththeyoungagesofthetargets, accordingtovariousevolutionarymodels(Siessetal., 2000; D'Antona&Mazzitelli, 1994; Baraffeetal., 1998). Indeed, thesemodelspredictalmostnodepletionoflithiumforobjectswiththeseTeff atageslessthan3Myr, whichmeansthatthemeasuredlithiumabundanceforyoungerobjectsshouldbecompatiblewiththeinterstellarabundanceoflog N (Li)∼3.1-3.3(Pallaetal., 2007).

Surfacegravity: Thederivedvaluesofthesurfacegravityforthetwotargetsarecom-patiblewiththetheoreticalvaluesforobjectswithTeff ∼4000-4350K andanagebetween∼1-4.5Myr. Indeed, bothmodelsof Siessetal. (2000)and Baraffeetal. (1998)predictavalueof log g∼ 4.0forobjectswiththeseproperties. Nevertheless, thederivedvalueoflog g forOM1186isalsotypicalofmucholderobjects, giventhatmodelspredict log g∼4.5atages ∼20Myrwithaverysmallincreaseinfutureevolutionarystages. However,theuncertaintiesonthedeterminationofthisparameterarenotsmall, andtheincreaseofthisvalueduringthePMS phaseisusuallywithintheerrorsofthemeasurements.

4.5.2 Sourcesoferrorinthepreviousclassifications

BothtargetshavebeenmisplacedontheHRD,butthereasonsweredifferent. ForOM1186,weconfirmedtheSpT availablefromtheliterature, butwefounddifferentvaluesofAV andLacc withrespectto DaRioetal. (2010a). Intheirwork, theyuseacolor-colorBV I diagramtosimultaneouslyderive AV and Lacc withtheassumptionoftheSpT.Tomodeltheexcessemissionduetoaccretion, theyuseasuperpositionofanopticallythickemission, whichreproducestheheatedphotosphere, andofanopticallythinemission, whichmodelstheinfallingaccretionflow. Fromthismodelspectrumtheyderivethecontributionofaccretiontothephotometriccolorsofthetargetsbysyntheticphotometry. Theirmethodassumesthatthepositionsoftheobjectsonthe BV I diagramaredisplacedfromthetheoreticalisochroneduetoacombinationofextinctionandaccretion. WiththeassumptionoftheSpT,theycanfindthecombinationofparameters(AV , Lacc), whichbestreproducesthepositionsofeachtargetonthe BV I diagram. Withthesevalues, theycorrectedthe I-bandphotometryfortheexcessduetoaccretion, derivedusingtheaccretionspectrummodel,andforreddeningeffects. Finally, theyderived L⋆ fromthiscorrected I-bandphotometryusingabolometriccorrection. Usingthismethod, theyfoundasolutionforOM1186withalargevalueof AV and, subsequently, accretion. Giventhelargeamountofexcessduetoaccretiontheyderiveinthe I-band, theyunderestimated L⋆ andassignedanoldagetothistarget. Ontheotherhand, ourrevisedphotosphericparametersarecompatiblewiththoseof Manaraetal. (2012), whofound AV =1.16mag, L⋆=0.77 L⊙, andage∼6Myr. Intheiranalysis, theyusedthesamemethodas DaRioetal. (2010a), buttheyalsohad U -bandphotometricdataattheirdisposal, andthususedan UBI color-colordiagram. Given

100

4.5 Discussion

Figure 4.6: Hertzsprung-RusselldiagramdiagramoftheONC from DaRioetal. (2012)withcoloredstarsshowing thenewpositionsof the two targetsof this study. Theoverplottedevolutionary tracksare fromD'Antona&Mazzitelli (1994). Weplot(fromtoptobottom)the0.3, 1, 3, 10, 30, and100Myrisochrones.

thattheexcessemissionduetoaccretionwithrespecttothephotosphereinthe U -bandismuchstronger, theywereabletodeterminetheaccretionpropertiesofthetargetsmoreaccuratelyandtofindauniquecorrectsolution.

RegardingOM3125, wederivedadifferentSpT withrespecttotheoneusuallyassumedintheliterature(G8-K0; Hillenbrand, 1997)withouranalysis. Thisestimatewasobtainedusinganopticalspectrumcoveringthewavelengthregionfrom∼500nmto∼900nm, andtheiranalysisdidnotconsiderthecontributionofaccretiontotheobservedspectrum. Thiswasassumednottobeastrongcontaminantforthephotosphericfeaturesinthiswave-lengthrange. This isa reasonableassumption forobjectswith lowaccretionrates, butitwaslatershownnottobeaccurateforstrongeraccretors(Fischeretal., 2011), aswealreadypointedoutforthistarget. Inparticular, strongveilingmakesthespectralfeaturesshallower, whichleadtoanincorrectearlierclassificationofthetarget. Hillenbrand (1997)markedthisSpT classificationasuncertain, andtheyalsoreportedthepreviousclassifica-tionforthisobjectfrom Cohen&Kuhi (1979), whoclassifieditasbeingofSpT K6, avaluewhichagreeswithourfinding. Thispreviousclassification isobtainedusingspectraatshorterwavelengths(λ ∼ 420-680nm)withrespect to Hillenbrand (1997)andwithnomodelingoftheaccretioncontribution. ThedifferenceintheclassificationismostlikelybecausesomeTiO featuresat λλ ∼476, 479nm, whicharetypicalofobjectsofspectralclasslate-K,werecoveredinthespectraof Cohen&Kuhi (1979). Giventhelargeamountofveilingduetoaccretionthatispresentinthespectrumofthisobject, itrepresentsaclearcasewherethedetailedanalysiscarriedoutinourworkisneeded. Weconcludethatevenopticalspectracoveringlargeregionsofthespectra, likethoseusedin Hillenbrand (1997)andin Cohen&Kuhi (1979), canleadtodifferentandsometimesincorrectresults, iftheveilingcontributionisnotproperlymodeled.

101


4.5.3 Implicationsofourfindings

Accretionveiling, extinction, andspectral typearedifficult toestimateaccurately fromlimiteddatasets, especiallyiftheyaremostlybasedononlyfewphotometricbands. Whenclassifyingyoungstellarobjects, thedifficultyincreases, giventhataccretionandextinctionmaysubstantiallymodifytheobservedspectra. Forthisreason, theuseoffewphotometricbands, whereaccretion, extinction, andspectraltypeeffectscanbeverydegenerate(e.g.,B-I bandrange), orofspectrathatcoveronlysmallwavelengthregions(e.g., λ ∼500-900nm), canleadtodifferentsolutions, whichmaybeincorrect.

Withourwork, weshowthatananalysisofthewholeopticalspectrum, whichincludesadetailedmodelingofthevariouscomponentsoftheobservedspectrum, leadstoanac-curateestimateofthestellarparameters. Whengoodqualityandintermediateresolutionspectrawithlargewavelengthcoveragefrom λ ∼330nmto λ ∼ 1000nmarenotavail-able, wesuggestthatacombinationofphotometricandspectroscopicdatacouldalsoleadtoarobustestimateofthestellarparametersofthetargetsindependentlyoftheirSpT.Inparticular, thephotometricdatashouldcovertheregionaroundtheBalmerjump(usingboth U -and B-band)andaround λ ∼700nm(R-and I-band). Withthisdataset, itwouldbepossible toderiveaccretionandveilingproperties, extinction, andSpT bymeansofphotometricalmethodssimilartothoseof Manaraetal. (2012). Thiscouldbeexpandedtoincludedifferentphotospherictemplatesanddifferentaccretionspectra. Atthesametime,spectrainselectedwavelengthregions, wherephotosphericlinessensitivetoTeff and/orlog g arepresent, arenecessarytopin-downdegeneraciesontheSpT.Finally, thelithiumabsorptionlineshouldbeincludedasanothertestoftheage.

Ourworkalsoimpliesthatsingleobjects, whichdeviatefromthebulkofthepopulationinnearbyyoungclusters(age ≲ 3Myr), couldbeaffectedbyanincorrectestimateofthephotosphericparameters, especially in caseswhere thedetermination is basedon fewphotometricbandsandtheeffectsofveilingduetoaccretionandextinctionarestrong.On theother hand, wehaveno reason to believe that also a largenumber of objectspositionedalongtheisochronesthatrepresentsthebulkofthepopulationinnearbyregionsshouldbeaffectedbysimilarproblems. Wethinkthatthevastmajorityoftheestimatesavailablearecorrectforthefollowingreasons. Firstofall, objectswithlowornegligibleaccretionshouldbeeasiertoclassify, giventhattheirspectralfeaturesarenotaffectedbyveiling. Then, therearesmalleffectsinvariousregionsduetodifferentialextinction. Inparticular, weexpect theamountofmisclassifiedobjects tobeparticularlysmallwhendealingwithobjectslocatedinregionslessaffectedbyextinctioneffects, suchas σ-Ori.Largernumbersofobjectscouldbemisclassifiedinveryyoungregions(age≲1Myr, e.g.,ρ-Oph), wherebothaccretionandextinctioneffectsarestrong. Wecouldhaveherebothanunderestimationof L⋆, asinthecaseofOM1186, duetoanoverestimationof Lacc andanoverestimationof L⋆ duetoanunderestimationof Lacc. Theseeffectscouldcontributetothespreadof L⋆, whichisobservedinalmostallstarformingregions. Finally, accretionvariabilityeffectsshouldnotsubstantiallyaffectthepropertiesofmostofthetargets, giventhattheseeffectsareusuallysmall(e.g., Costiganetal., 2012).

Eventhoughanyconclusiononthenatureoftheolderpopulationsobservedinmassive

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4.6 Conclusion

clusterscannotbedrawnfromthiswork, wesuggestthatthemethodexplainedhereorthealternativeapproachsuggestedinthissectionshouldbeusedtostudytheseobjectsinmoredetail.

4.6 Conclusion

Wehaveobservedtwocandidateold(age>10Myr)accretingPMS intheONC withtheESO/VLT X-Shooter spectrograph to confirmprevious accretion rate andage estimates.Usingadetailedanalysisoftheobservedspectrabasedonamulticomponentfit, thatin-cludesphotosphericemission, theeffectofreddening, andthecontinuumexcessduetoaccretion, wederivedrevisedaccretionratesandstellarparametersforthetwotargets. Weconfirmthattheobjectsareaccretorsasfrompreviousstudies, buttherevisedphotosphericparametersplacetheseobjectsinthesamelocationasthebulkoftheONC youngstel-larpopulation(age∼2-3Myr). Therefore, theycannotbeconsideredolderPMS,buttheyareclassicalaccretingYSOs. Inparticular, weconfirmedthepreviouslyestimatedSpT forOM1186, butwederiveddifferentvaluesof AV and Lacc withrespecttopreviousworksintheliterature, findingthattherealpositionofthisobjectontheHRD leadstoameanageestimateof ∼2.9Myr. Ontheotherhand, wederivedadifferentSpT forOM3125withrespecttotheusuallyassumedvalueintheliterature. Withtheotherparameters, thismovedthistargettoapositionontheHRD leadingtoanageof ∼2.4Myr. Theanalysisofthelithiumabundanceandthatofthesurfacegravityconfirmthisfinding, eveniftheuncertaintiesforthelatterwerelarge.

WeshowedthatweareabletoaccuratelydeterminethestellarparametersofYSOswithouranalysis, whiletheuseoffewphotometricbandsalone(e.g., onlythe B-and I-band)orofonlyspectracoveringsmallwavelengthregionscanleadtolargeerrorsinthederivedpositionontheHRD.Wethussuggestthatsingleobjectsinnearbyyoungclusterswhosepositionon theHRD isnotcompatiblewith thebulkof thepopulation in their region,couldbemisplaced, especiallyifthereisthesuspicionofhighopticalveilingconnectedtolargevaluesoftheaccretionrate. Thenatureoftheseindividualobjectswillneedtobeverifiedusingadetailedanalysissimilartotheonereportedinthisstudytoverifytheexistenceandtostudythepropertiesoflong-liveddisksaroundyoungstellarobjects.

Aknowledgements We thank theanonymous referee forprovidinguseful commentswhichhelpedus toimprove thepaper. We thank theESO DirectorGeneral forawardingDDT time to thisprojectand theESO staffinParanalforcarryingouttheobservationsinServicemode. WethankAndreaBanzattiforusefulcommentsanddiscussionwhichhelpedtoimprovethefittingprocedure. C.F.M.acknowledgesthePhDfellowshipoftheInternationalMax-Planck-ResearchSchool. G.B.acknowledgestheEuropeanCommunity'sSeventhFrameworkProgrammeundergrantagreementno. 229517.

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104

5Onthegascontentoftransitionaldisks:aVLT/X-Shooterstudyofaccretionand

winds

FromManara, Testi, Natta, Rosotti, Benisty, Ercolano, Ricci, submittedtoA&A∗

Abstract

Context: Transitionaldisksarethoughttobealateevolutionarystageofprotoplanetarydiskswhoseinnerregionshavebeendepletedofdust. Themechanismresponsibleforthisdepletionisstillunderdebate. Toconstrainthevariousmodelsproposedtoexplainthisphaseitismandatorytohaveagoodunderstandingofthepropertiesofthegascontentintheinnerpartofthedisk.Aims: UsingX-Shooterbroadband-UV toNIR -mediumresolutionspectroscopywederive thestellar, accretion, andwindpropertiesofasampleof transitionaldisks. Theanalysisofthesepropertiesallowsustoputstrongconstraintsonthegascontentinare-gionveryclosetothestar(≲ 0.2AU) whichisnotaccessiblewithanyotherobservationaltechnique.Methods: Wefitthemedium-resolutionbroad-bandspectraof22transitionaldiskswithaself-consistentproceduretoderivesimultaneouslyspectraltype, extinction, andaccre-tionpropertiesofthetargets. Thesearedeterminedfromthefitofthewholebroad-bandspectrum, includingtheUV-excessandvariousabsorptionfeaturesatlongerwavelengths.Fromthecontinuumexcessatnear-infraredwavelengthwedistinguishwhetherourtargetshavedustfreeinnerholes. Analyzingforbiddenemissionlinespresentinthespectrawederivealsothewindpropertiesof thetargets. Wethencompareourfindings toresultsobtainedinclassicalTTauristars.Results: Theaccretionratesandwindpropertiesof80%ofthetransitionaldisksinoursam-plearecomparabletothoseofclassicalTTauristars. Weshowthat, inoursample, whichis

∗ThischapteristheresultofacollaborativeworkthatI startedandlead. InparticularI wrotethetwoproposalstoaskfortheobservingtimetocollectthedata. I thenpreparedtheobservationsthathavebeencarriedoutinservicemode. I alsocarriedoutallthedatareductionofthespectraandI analyzedthedatawithvariousdiscussionswiththecoauthors. Finally, I wasresponsibleofthecompositionofthemanuscript,wherethecoauthorshelpedwithseveralcommentsandsuggestions.

105

5. On the gas content of transitional disks

stronglybiasedtowardsstonglyaccretingobjects, thereare(atleast)sometransitionaldiskswithaccretionpropertiescompatiblewiththoseofclassicalTTauristars, irrespectiveofthesizeofthedustinnerhole. Onlyin2casesthemassaccretionratesaremuchlower, whilethewindpropertiesremainsimilar. Wedonotseeanystrongtrendofthemassaccretionrateswiththesizeofthedustdepletedcavity, norwiththepresenceofadustyopticallythickdiskveryclosetothestar. Theseresultssuggestthat, closetothecentralstar, thereisagasrichinnerdisk. ThedensityofgasintheinnerregionofthesedisksissimilartothatofclassicalTTauristarsdisks.Conclusions: Thesampleanalyzedhere, thatisbiasedtowardsknownaccretingtransi-tionaldisks, suggeststhat, atleastforsomeobjects, theprocessresponsibleoftheinnerdiskclearingshouldallowforatransferofgasfromtheouterdisktotheinnerregion. Thisshouldproceedataratethatdoesnotdependonthephysicalmechanismproducingthegapseeninthedustemissionandresultsinagasdensityintheinnerdisksimilartothatofunperturbeddisksaroundstarsofsimilarmass.

5.1 Introduction

Atthebeginningoftheirevolution, protoplanetarydiskssurroundingformingstarsappearasacontinuousdistributionofgasandsmalldustparticles. Forthefirst fewMyrstheyevolvebyviscousaccretion(Hartmannetal., 1998), andinthemeantimegraingrowthandplanetformationtakeplace. Theobservationsshowthat, inarelativelysmallfractionofobjects(∼10%, e.g., Espaillatetal. 2014), asignificantchangeinthediskmorphologyisdetected: adust-depletedregionintheinnerpartofthediskappearsinmm-interferometryobservations(e.g. Andrewsetal., 2011)and/orasadipinthemid-IR spectralenergydi-stribution(SED;e.g., Merínetal., 2010). Thiscanbeeithera hole -absenceofgasfromthedustsublimationradiusouttosomemuchlargerradius-ora gap -absenceofgasinarelativelynarrowregion, likearing. Thesedisksareknownastransitionaldisks(TDs;e.g., Calvetetal., 2005; Espaillatetal., 2014). Insomecasesanexcessemissionisde-tectedatnear-infraredwavelengths; thisemissioncomesfromasmallannulusofwarmdustclosetothestar(e.g., Benistyetal., 2010). Theseobjectsarecommonlyreferredtoaspre-transitionaldisks(PTDs; e.g., Espaillatetal., 2010, 2014).

Differentprocesseshavebeenproposedtoexplaintheformationofthesegaps/holes.Themostplausiblemechanismssofararephotoevaporation, graingrowth, orplanetforma-tion(e.g., Espaillatetal., 2014). Still, noneoftheseprocessesalonehasbeenshowntobesufficienttoexplainallobservations. GraingrowthmodelsexplaintheobservedIR SEDsofTDs, butareunabletoreproducemm-observations(Birnstieletal., 2012). Thedetectionoflargemassaccretionrates(Macc)inseveralTDswithlargeinnerholesizesisatoddswithphotoevaporativemodelpredictions, whichexpectaveryfastdepletionof thegasmassreservoirforaccretionbytheinnerdiskontothestar(e.g. Owenetal., 2011, 2012).Moreover, planetformationmodels, whichneedtoincludemultipleaccretingplanetstoperturbsufficientlytheinnerdisksurfacedensity, stillcannotexplainTDswithlargeinnerholesizesandhighmassaccretionrates(Zhuetal., 2011). Atthesametime, thepresence

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5.1 Introduction

ofaplanetcouldexplaintheradialandazimuthaldistributionofmm-sizedgrainparticlesinTDs(Pinillaetal., 2012). Newattemptsarebeingmadetoincludevariousprocessesinonemodel. Asanexample, Rosottietal. (2013)combinedphotoevaporationandplanetformation, buttheywerestillnotabletoexplaintheobservedaccretionpropertiesofmanyTDs. SimilarconstraintsarisefromobservationsofwindsinTDs. Analysisofforbiddenlineemissionshowthatwindsareemittedfromtheinnermostregionofthediskinvariousob-jects(Alexanderetal., 2014), andtheseobservationssuggestthatphotoevaporationcouldplayaroleintheclearingofdisks. Atthesametime, itisnotyetclearwhatistheoriginandwhatarethepropertiesofthesewinds, andmorestudiesareneededtohaveaclearerunderstandingofthisaspect.

Inthiscontext, thecombinationofinnerholesize, Macc, andwindpropertiesisapow-erfulobservationaldiagnosticofdiskevolutionmodels. Inparticular, Macc andthewindproperties allowus toplace strongconstrainton thegaseous contentof the innermostregionofthesediskswhichcanbecomparedwiththemodels. Asexplainedbymagneto-sphericaccretionmodels(e.g. Hartmannetal., 1998), theprocessofaccretionisrelatedto thegaseouscontentof the innermostregionof thediskat radii ≲ 0.2AU.Similarly,forbiddenlinesareemittedinregionsinthediskascloseas ∼ 0.2AU fromthecentralstar. Themeasurementsof Macc forTDsavailable in the literaturearemostlybasedonsecondary indicators (suchas the10%Hα width)andhavebeenobtainedusingdiffe-rentnon-homogeneoustechniques. Inmanycasesthesevaluesarehighlyuncertainand,therefore, notreliable. Atthesametime, veryfewdataonthewindpropertiesforTDsareavailable. Inordertoremedythesedeficiencieswehavecollectedasampleof22spectraofTDswiththeESO VLT/X-Shooterspectrograph. Weaimatderivingwithanhighlyreliablemethodthestellarandaccretionpropertiesoftheseobjectsandtostudysimultaneouslytheirwindpropertiesfromopticalforbiddenlines.

TheanalysisofthissampleofTDsthatwepresenthereisfocusedfirstlyonderivingthevaluesof Macc fortheseobjectsinordertoverifythereliabilityofthosereportedintheliterature. Weuseaverydetailedandself-consistentanalysistoderiveaccretionratesandsimultaneouslythespectraltypesandstellarpropertiesoftheobjectsfromthefitofthewholespectrumfromUV toNIR.Inparticular, wewanttocheckthehighvaluesofMacc forobjectswithlargeinnerholesizes, thatcannotbeexplainedbycurrentmodels.Atthesametime, wewanttodeterminewhetherthereisanydependenceoftheaccretionpropertiesofTDswiththeirdiskmorphology, inparticulartestingpossiblecorrelationwiththeinnerholesize. Moreover, weinvestigatethedifferencesandsimilaritiesinaccretionandwindpropertiesofTDswithrespecttoclassicalTTauristars. Finally, weputconstraintonthepropertiesofthegaseousinnermostregionsofthediskintheseobjectsfromthederivedvaluesof Macc andfromthewindproperties.

Thepaperisorganizedasfollows. InSect. 5.2 wepresenttheobservations, thedatareductionprocedure, andthepropertiesofthetargetsinoursample. InSect. 5.3 webrieflydescribethemethodusedtoderivethestellarandaccretionpropertiesoftheobjects, andwereportthederivedvalues. Then, inSect. 5.4 wederivethewindpropertiesofourtargets.In Sect. 5.5 wediscussour results anddescribe the additionaldata from the literaturecollectedtoderiveourconclusions, whichwesummarizeinSect. 5.6.

107


5.2 Observations

AlltheobservationsincludedinthisworkhavebeenobtainedwiththeESO/VLT X-Shooterspectrograph. Thismediumresolutionandhigh-sensitivityinstrumentcoverssimultane-ouslythewavelengthrangebetween ∼300nmand ∼ 2500nm, dividingthespectruminthreearms, namelytheUVB armintheregion λλ ∼ 300-560nm, theVIS armbetweenλλ ∼ 560-1020nm, and theNIR arm from λ ∼ 1020nmto λ ∼ 2500nm(Vernetetal., 2011). In the following, wepresent thepropertiesof thesample, thedetailsof theobservations, thatarereportedalsoinTable 5.1-5.2, andthedatareductionprocedure.

5.2.1 Sampledescription

Thefirstcriterionusedtoselecttheobjectsinoursamplewastoincludeallthetargetswithknowninnerholesizes(Rin)largerthan ∼ 20AU andlarge Macc (≳ 10−9M⊙/yr). Thesewereselectedmainlyfromthesampleof Andrewsetal. (2011), wherethevalueof Rin

hasbeenmeasuredusingresolvedmm-interferometryobservations. Fromthissampleweselectedthe8objectswithspectraltypelaterthanG2. Fortwooftheseobjects(LkCa15,ISO-Oph196)theX-ShooterspectrawereavailableintheESO archive, whileweobservedtheremainingsixtargets(LkHα330, DM Tau, GM Aur, RX J1615-3255, SR21, andDoAr44)duringourprograms(seeTable 5.1). WeaddedtotheseobjectsalsofourTDsforwhichRin wasderivedfromIR SED fittingby Merínetal. (2010)and Kimetal. (2009), namelySZ Cha, CS Cha, Sz84, andSer34. OnlyforthelatterthespectrumwasnotavailableintheESO archive.

Then, weincludedsomeobjectswithsmallerinnerholesizesanddifferentvaluesofMacc, bothashighastheobjectwithlarge Rin andsmallerthanthose. Inparticular, weincludedfivetargetswhosespectrawerenotavailableintheESO archiveandwith Rin≲15AU andassmallas1AU,namelyOph22, Oph24, andSer29fromthesampleof Merínetal. (2010), RX J1842.9andRX J1852.3from Hughesetal. (2010). Finally, wecollectedall the spectraofTDsclassifiedby Kimetal. (2009)available in theESO archive (fourobjects, CHXR22E,Sz18, Sz27, andSz45)andthespectrumofTWHya, whose Rin hasbeenmeasuredwithresolvedmm-observationsby Hughesetal. (2007). Intotalthesampleanalyzedherecomprises22objects.

For9oftheobjectsanalyzedherethevalueofRin hasbeendirectlydeterminedfromre-solvedmm-interferometryobservations(Hughesetal., 2007; Andrewsetal., 2011), whilefortheremaining13targetstheclassificationasTD andthesizeoftheinnerholehasbeendeterminedfromIR SED fitting(Kimetal., 2009; Merínetal., 2010; Hughesetal., 2010;Espaillatetal., 2013). Thelistoftargets, theirdistances, andthevaluesof Rin availableintheliteraturearereportedinthefirst threecolumnsofTable 5.3. Theobjectsarelo-catedindifferentstarformingregions(Perseus, Taurus, Chameleon, TW Hydrae, Lupus,ρ-Ophiucus, Serpens, CoronaAustralis)andhavevaluesof Rin between ∼1to ∼ 70AU,beingrepresentativeofthewholerangeofmeasuredvaluesofRin. WhenbothvaluesofRin

obtainedusingIR SED fittingandmm-interferometryresolvedobservationsareavailable

108

5.2 Observations

weadopt in theanalysis themm-interferometryresult. MoreinformationonindividualobjectsinthesamplearereportedinAppendix 5.A.1.

EvenifthesampleiscontainingobjectsofdifferentTD morphologies, suchasvariousinnerholesizes, thisisnotstatisticallycompleteanditisingeneralbiasedtowardaccretingTDs. Asexplainedbefore, ourselectioncriteriawereaimedatobservingTDswithalreadyknownandhighaccretion rates, thusourownobservations representabiasedsample.Ontheotherhand, thetargetscollectedintheliteraturewereinsomecasesselectedwithdifferentcriteriathatcouldmitigateourbiases. Unfortunately, itisnotpossibletoestimatecompletelythebiasinitsselection.

Class III properties

Herewepresentthepropertiesofthreenon-accreting(Class III) YSOs, thatweuseaspho-tospherictemplatesinouranalysis(seeSect. 5.3)toenlargetheavailablesampleofClass IIIYSOsobservedwithX-Shooterandpresentedin Manaraetal. (2013a). Wefollowthesameprocedureasin Manaraetal. (2013a)toderivetheirspectraltypesandstellarproperties,whicharereportedinTable 5.2.

TheYSO IC348-127hasspectraltype(SpT) G4(Luhmanetal., 1998, 2003)andhasbeenclassifiedasClass III from Ladaetal. (2006)usingSpitzerphotometry. Thisclassifi-cation, withvaluesofextinction AV ∼ 6mag, hasbeenconfirmedby Ciezaetal. (2007)and Dahm (2008). Weconfirmthespectralclassificationandtheextinction, andwederiveL⋆ =12.9±5.9 L⊙ forthisobject. Duetotheveryhigh AV , thespectrumofthistargetatλ ≲350nmisverynoisy.

ThesecondClass III YSO includedinthisworkisT21, whichhasbeenclassifiedasaClass III YSO withSpT G5by Manojetal. (2011, andreferencestherein). Thetypicalred-deninglawoftheChameleon I regioninwhichthisobjectislocatedisnotwellconstrained(Luhman, 2008), andcouldbedescribedusingvaluesof RV (Cardellietal., 1989)upto5.5. Bycomparisonofthedereddenedspectrumwithablackbodyat T=5770K,whichisthetypical Teff ofastarwithSpT G5, weobtainthattheextinctiontowardsthisobjectisbetterrepresentedusing RV =3.1and AV =3.2mag. Adoptingthesevalues, thederivedluminosityofthetargetis L⋆ =18.5±8.5 L⊙.

Finally, weincludeinthesampleCrA75, whichhasbeenclassifiedasaClass III YSOwithSpT K2from Forbrich&Preibisch (2007, andreferencestherein). Thishasbeenlaterconfirmedby Petersonetal. (2011)and Currie&Sicilia-Aguilar (2011), whosuggestedthatthecorrectvaluesofextinctionforthisobjectis AV =1.5, assuming RV =5.5, representativeofobjects in theCoronaAustralis region (Petersonetal., 2011; Chapmanetal., 2009).WiththeseparameterswederiveforCrA75 L⋆ =0.4±0.2 L⊙.

TheobjectswhosepropertieshavebeendescribedinthissectionexpandthecoverageinSpT ofour libraryofphotospheric template. This, however, remains incomplete. Inparticular, theobjectspresentedin Manaraetal. (2013a)haveanalmostuniformcoverageintheSpT rangefromK5toM6.5. Thethreeobjectspresentedhere, instead, donotcoverentirelytherangeofSpT fromG3toK5. Thisincompletenessofphotospherictemplatesinthisrangewillbeconsideredintheanalysis.

109


5.2.2 Observationalstrategy

Asexplainedbefore, wehaveincludedintheanalysisbothnewobservationsandarchivaldata. Inthefollowing, wedescribeseparatelyourobservationalstrategyandthesettingsusedinthearchivalobservations.

Newobservations

NewobservationswiththeESO/VLT X-Shooterspectrographhavebeencarriedoutinser-vicemodebetweenAprilandNovember2012(ESO Pr.Id. 089.C-0840and090.0050, PIManara). ThetargetshavebeenobservedinABBA slit-noddingmodetoachievethebestpossibleskysubtractionalsointheNIR arm. Theobjectshavebeenobservedusingdiffe-rentslitwidthsintheUVB arm. Forthebrightestobjectstheslit0.5x11′′, whichleadstothehighestspectralresolutioninthisarm(R=9100), hasbeenadopted, whileforthefainterobjects-namelyOph22, Oph24, Ser29, andSer34-wehaveusedthe1.0x11′′slit, whichleadstoalowerresolution(R=5100)butallowstoachieveanhigherS/N.IntheVIS andNIR armsthe0.4x11′′slithasbeenadoptedforallthetargets. Thisslitwidthleadstothehighestpossibleresolution(R=17400, 10500intheVIS andNIR arms, respectively)andtoenoughS/N inthespectra. Thereadoutmodeusedwasinallcases``100,1x1,hg". Toobtainabetterfluxcalibration, weobservedthetargetsofPr.Id. 090.0050withthelargeslit(5.0x11′′)immediatelyaftertheexposurewiththenarrowslit. Withthelargeslitweobtainspectrawithalowerresolutionbut, atthesametime, weavoidslitlossesandwegetareliablefluxcalibrationforthisspectrum, whichisthenusedtocalibratethenarrowslitone(seeSect. 5.2.3). Thenamesofthetargets, theircoordinates, observingdate, andexposuretimesoftheobservationsaresummarizedinTable 5.1.

WeobservedinourprogramsalsotwoClass III YSOs(seeSect. 5.2.1 fordetails). WeobservedCrA 75usingthenarrowerslitsineacharm-0.5x11′′ intheUVB arm, 0.4x11′′ intheVIS andNIR arms-toobtainthehighestpossiblespectralresolution. ForIC348-127,weadoptedtheslitwidths1.6x11′′, 1.5x11′′, and1.2x11′′ intheUVB,VIS,andNIR arms,respectively. ThiswasdoneinordertoachieveenoughS/N intheavailableobservingtime.WerecapinTable 5.2 theseinformation.

Archivaldata

Thedata included in our analysis collected from the ESO archive havebeenobtainedusingdifferentobservational strategies. Theobservationaldata arepresentedhere andsummarizedinTable 5.1 and 5.2.

ThetransitionaldiskSz84hasbeenobservedduringtheINAFGTO timeinPr.Id. 089.C-0143(PI Alcalà). Theadoptedslitswidthforthisobjectwere1.0x11′′intheUVB armand0.9x11′′intheVIS andNIR arms. Moredetailsontheobservingprocedureandonthedatareductionforthistargetsaregivenin Alcaláetal. (2014).

ThetargetISO-Oph196wasobservedfortheprogramPr.Id. 085.C-0876(PI Testi)usingthesamesettingsastheoneadoptedinourobservations. Weincludeinouranalysistwo

110

5.2 Observations

targets-namelyDoAr44andTW Hya-fromtheprogramPr.Id. 085.C-0764(PI Guenther).Bothtargetshavebeenobservedwiththenarrowslits. Foralltheseobservationsthereadoutmodeusedwas``100,1x1,hg", asinourprograms, andforeachobject4exposuresintheABBA slit-noddingmodeweretaken.

WeconsiderinourworkalsosevenobjectsfromtheprogramPr.Id. 084.C-1095(PIHerczeg). Sixofthose-namelyCS Cha, CHXR22E,Sz18, Sz27, Sz45, andSzCha-areTDs, whileT21isaClass III YSOs. Thesetargetshavebeenobservedbothwithanarrow-slitsetting(slitwidths1.0x11′′intheUVB and0.4x11′′ intheVIS andNIR arms)andwiththelargeslittohaveabetterfluxcalibrationofthespectra. Thenarrow-slitobservationshavebeencarriedoutwiththe``400,1x2,lg"modewithaAB slit-noddingmode. Thelarge-slitexposureshavebeenobtainedinstaremode.

ThedatafortheTD LkCa15havebeenobtainedintheprogramPr.Id. 288.C-5013(PIHuelamo). Weuseinouranalysisonly4exposuresobtainedinoneepoch(2011-12-01),whichcorrespondtoanentireABBA slit-noddingcycle. Observationshavebeenmadeusingthe0.8x11′′ slitintheUVB arm, the0.7x11′′ oneintheVIS arm, andthe0.9x11′′

slitintheNIR arm. Thereadoutmodeusedwas``100,1x1,hg". Usingonly4framesweobtainaspectrumwithenoughS/N forourpurpose.

5.2.3 Datareduction

Data reduction has been carried out using the version 1.3.7 of theX-Shooter pipeline(Modiglianietal., 2010), runthroughthe EsoRex tool. Thespectrawerereducedinde-pendentlyforthethreespectrographarms. Thepipelinetakesintoaccount, togetherwiththestandardreductionsteps(i.e. biasordarksubtraction, flatfielding, spectrumextraction,wavelengthcalibration, andskysubtraction), alsotheflexurecompensationandtheinstru-mentalprofile. Wecheckedwithparticularcarethefluxcalibrationandtelluricremovalofthespectra.

TelluricremovalhasbeenperformedusingthestandardtelluricspectrathathavebeenprovidedaspartofthestandardX-Shootercalibrationplanoneachnightofobservations.Spectraoftelluricstandardstarsobservedatsimilarairmassesrightbeforeorafterthetargethavebeenselected. ThecorrectionhasbeenaccomplishedusingtheIRAF1 task telluricadoptingthesameprocedurefortelluricnormalizationintheVIS andforresponse-functionpreparationintheNIR asexplainedby Alcaláetal. (2014).

Fluxcalibrationhasbeencarriedoutwithinthepipeline. Then, forthetargetswhereonlynarrow-slitobservationswereavailable, wecheckedtheflux-calibratedpipelineprod-uctscomparingthemwiththeavailablephotometrytoquantifyslitlosses. Thesespectrawerethenrescaledtothephotometricdata, andafinalcheckwasperformedtoverifyacorrectconjunctionsbetweenthethreearms. Theoverallfinalagreementisverygood.Ontheotherhand, inthecaseswhereobservationswiththelargeslitwereavailable, we


111


firstcheckedthattheflux-calibrationofthespectraobtainedwiththisslitwerecompatiblewiththeavailablephotometry. Then, werescaledthenarrow-slitspectratothelarge-slitflux-calibratedones, thusachievingthebestpossiblefluxcalibration. Alsointhiscasethefinalproductshaveverygoodconjunctionsbetweenthearms.

112

5.2 ObservationsTable5.1:

Transitio

nald

isksobservinglo

g

Nam

eOthernam

esRA(200

0)DEC

(200

0)OBS.DATE

T exp(sec)

Pr.Id

(PI)

hms

′′′

YY-MM-D

DUVB

VIS

NIR

LkHα33

0...

034548

.29

322411

.920

12-12-18

4x15

04x15

04x15

009

0.C-005

0(M

anara)

DM

Tau

...04

3348

.74

181009

.720

12-11-23

4x30

04x30

04x30

009

0.C-005

0(M

anara)

LkCa15

...04

3917

.79

222103

.420

12-03-06

4x20

04x22

04x20

028

8.C-501

3(Huelamo)

GM

Aur

...04

5510

.98

302159

.320

12-11-23

4x30

04x30

04x30

009

0.C-005

0(M

anara)

SZCha

AssChaT2-6

105816

.77

-7717

17.0

2010

-01-18

4x14

04x15

04x15

008

4.C-109

5(Herczeg)

TWHya

...11

0151

.91

-3442

17.0

2010

-05-03

4x15

04x60

4x10

008

5.C-076

4(Guenther)

CSCha

ISO

ChaI3

110224

.91

-7733

35.7

2010

-01-18

2x55

2x60

2x60

084.C-109

5(Herczeg)

CHXR22

E...

110713

.30

-7743

49.9

2010

-01-19

2x28

02x30

02x30

008

4.C-109

5(Herczeg)

Sz18

AssChaT2-25

110719

.15

-7603

04.8

2010

-01-17

2x60

2x53

2x60

084.C-109

5(Herczeg)

Sz27

AssChaT2-35

110839

.05

-7716

04.2

2010

-01-18

2x41

02x42

02x42

008

4.C-109

5(Herczeg)

Sz45

AssChaT2-56

111737

.00

-7704

38.1

2010

-01-17

2x45

2x40

2x45

084.C-109

5(Herczeg)

Sz84

...15

5802

.53

-3736

02.7

2012

-04-18

2x35

02x30

02x11

508

9.C-014

3(Alcalà)

RXJ161

5-32

552M

ASS

J161

5202

3-32

5505

116

1520

.23

-3255

05.1

2012

-05-21

4x30

04x30

04x30

008

9.C-084

0(M

anara)

Oph

22SSTc2d

J162

245.4-24

3124

162245

.40

-2431

23.7

2012

-04-25

4x60

04x60

04x60

008

9.C-084

0(M

anara)

Oph

24SSTc2d

J162

506.9-23

5050

162506

.91

-2350

50.3

2012

-06-07

4x60

04x60

04x60

008

9.C-084

0(M

anara)

SR21

EM*SR21

A,ISO

-Oph

110

162710

.28

-2419

12.7

2012

-04-14

4x30

04x30

04x30

008

9.C-084

0(M

anara)

ISO-O

ph196

WSB

6016

2816

.51

-2436

57.9

2010

-07-28

4x75

04x75

04x75

008

5.C-087

6(Testi)

DoA

r44

...16

3133

.46

-2427

37.2

2010

-05-03

4x30

04x14

04x15

008

5.C-076

4(Guenther)

Ser29

SSTc2d

J182

911.5+

0020

3918

2911

.50

002038

.620

12-06-03

4x60

04x60

04x60

008

9.C-084

0(M

anara)

Ser34

SSTc2d

J182

944.1+

0033

5618

2944

.11

003356

.020

12-06-03

4x60

04x60

04x60

008

9.C-084

0(M

anara)

RXJ184

2.9-35

32...

184257

.95

-3532

42.7

2012

-05-21

4x30

04x30

04x30

008

9.C-084

0(M

anara)

RXJ185

2.3-37

00...

185217

.29

-3700

11.9

2012

-06-03

4x30

04x30

04x30

008

9.C-084

0(M

anara)

Table5.2:

ClassIII

YSO

sprop

ertiesandob

servinglog

Nam

eOthernam

esRA(200

0)DEC

(200

0)OBS.DATE

T exp

(sec)

SpT

T eff

AV

dL⋆

Pr.Id

(PI)

hms

′′′

YY-MM-D

DUVB

VIS

NIR

[K]

[mag]

[pc]

[L⊙]

IC34

8-12

7Cl*IC

348CPS

127

034507

.932

0401

.820

12-11-11

4x15

04x15

04x15

0G4

5800

6.0

320

12.9±5.9

090.C-005

0(M

anara)

T21

AssChaT2-21

110615

.4-7721

56.9

2010

-01-19

2x70

2x60

2x60

G5

5770

3.2

160

18.5±8.5

084.C-109

5(Herczeg)

CrA75

RXJ190

222.0-36

5541

190222

.1-3655

40.9

2012

-05-17

2x30

02x30

02x30

0K2

4900

1.5

130

0.4±

0.2

089.C-084

0(M

anara)

113


5.3 Accretionandphotosphericparameters

Inthefollowing, webrieflydescribetheprocedureadoptedtoderiveself-consistentlyfromthecompleteX-ShooterspectrumSpT, AV , andtheaccretionluminosity(Lacc)forourtar-gets. Themethodisdescribedindetailin Manaraetal. (2013b)andisbasedonthefitofvariouspartsoftheobservedspectratoderivetheseparameters. Inparticular, theanalysisoftheUV-excesstogetherwiththatofabsorptionfeaturesatlongerwavelengthsallowsustodetermineproperlythestellarpropertiesand, atthesametime, leadstoanaccurateanddirectdeterminationoftheaccretionproperties. Wethenreporttheresultsobtainedandcomparethesetothevaluesderivedintheliterature.

5.3.1 Methoddescription

Ourmethodisbasedonafittingprocedurethatconsidersthefollowingthreecomponentstoreproducetheobservedspectrum. Weincludearangeofphotospherictemplatespectra-ClassIII YSOsfrom Manaraetal. (2013a), augmentedwithsomeearlierSpT templates, asexplainedinSect. 5.2.1 -. WeconsiderarangeofpossiblevaluesforAV , andwemodeltheexcessspectrumproducedbythediskaccretionprocesswithasetofisothermalhydrogenslabemissionspectra. Thephotospherictemplatespectrumandtheslabmodelarenor-malizedusingtwodifferentnormalizationconstantsdeterminedinthefittingprocedure.

Thebestfitmodelisderivedbyminimizinga χ2like functiondefinedasthesumofthe

squareddeviations(data-model)dividedbytheerror. The χ2like iscomputedindifferent

regionsoftheUVB andVIS armsofthespectra, includingtheBalmerandPaschencontinuaregionandsomespectralregionsaround λ ∼ 700nmcharacterizedbymolecularfeaturesparticularly strong in late-type stars. Wealsoperforma visual checkof thebest fit indifferentphotosphericfeaturessensitivetotheSpT ofthetargetandveiledbytheaccretionemission.

TheSpT of thebestfitphotospheric template isassumed for the input targetwithatypicaluncertaintyofonespectralsub-class. Thebestfitdetermined AV hasanuncer-tainty ≲0.4mag, whichtakesintoaccountboththeuncertaintyonthetemplate AV (0.3mag; Manaraetal., 2013a)andtheoneonthebest-fitestimate(∼ 0.2mag; Manaraetal.,2013b). Wethenderive Lacc byintegratingthenormalizedbestfitslabmodelspectrumfrom50nmto2478nmtoincludealltheemissionofthemodel. Thisvaluehasanesti-mateduncertaintyof ∼0.2dex(Manaraetal., 2013b). Thevalueof L⋆ isobtainedfromtheluminosityofthebestfitphotospherictemplateafterproperlytakingintoaccountthenormalizationfactor. Theuncertaintyon L⋆, obtainedconsideringtheoneonthe L⋆ ofthetemplate(∼ 0.2dex; Manaraetal., 2013a), onthebest-fit, andonthedistance, is ∼0.25dex.

FromtheSpT ofthephotospherictemplatewederivethe Teff oftheobjectusingtheSpT-Teff relationfrom Luhmanetal. (2003)forM-typestarsand Kenyon&Hartmann (1995)forearlierSpT objects. Thestellarmass (M⋆) is thenderivedby interpolatingevolutionarymodelsof Baraffeetal. (1998)inthepositionoftheobjectontheHRD,anditsuncertaintyis computedperturbing thepositionon theHRD with the aforementioneduncertainty.

114

5.3 Accretion and photospheric parameters

Finally, Macc isderivedusingtheclassicalrelation Macc=1.25 · LaccR⋆/(GM⋆) (Hartmannetal., 1998), andthisvaluehasatypicaluncertaintyof ∼ 0.4dex, obtainedpropagatingtheuncertaintieson R⋆, M⋆, and Lacc.

Weadoptthereddeningcurvefrom Cardellietal. (1989). Thevalueof RV , whichisusuallyuncertaininyoungstarformingregions, isassumedtobe RV =5.5forCrA (Petersonetal., 2011, andreferencetherein)and RV =3.1fortheotherregions. Inparticular, theanalysisofthebestfitresultsfortheobjectslocatedinthe ρ-Ophiucusregionshowthattheuseofthestandardextinctioncurveatopticalwavelengthismoreappropriatetoreproducetheobservationsofour targets. Also forobjects located in theChameleon I regionwefindthatthestandardvalue RV =3.1betterdescribestheobservedspectrumofT21, aswereportedinSect. 5.2.1.

InadditiontothefittingoftheUVB andVIS armsofourspectradescribedabove, wealsoperformacheckofthebestfitresultsusingtheNIR armspectra, i.e. at λ ≳ 1000nm.Fordefinition, TDshavelowornegligibleemissioninexcesstothephotosphereatnear-infraredwavelengths, andstrongexcessatmid-infraredandfar-infraredwavelengths(e.g.,Calvetetal., 2005). Forthisreason, weexpectourbestfitphotospherictemplatetomatchthetargetspectrumalsointheNIR arm. ThisdoesnotapplywhenfittingPTDs, wherethecontributionofinner-diskemissionatnear-infraredwavelengths, whichisnotincludedinourmodels, isnotnegligible. Inthelattercaseweexpectthephotospherictemplatespectrumtolaybelowthetargetoneinthenear-infrared. Inthischeckweincludealso,whenavailable, the3.6 µmSpitzermagnitudeoftheobject, aftercorrectingitforextinctionfollowing theprescriptionof McClure (2009), and themagnitudeof the template. TheanalysisoftheIR colorexcesswillbedescribedindetailinSect. 5.3.2. ThebestfitstellarandaccretionparametersforthetargetsarereportedinTable 5.3.

Finally, weusetherelationsbetweentheluminosityofsomeemissionlines(Lline)andLacc calibratedby Alcaláetal. (2014)toverifyourderivedparameters. Ifthebestfit Lacc

and AV arecorrect, weexpect toderivecompatiblevaluesof Lacc fromthe luminosityofemission lines located indifferentpartsof thespectrawithnoparticularwavelengthdependence. Weselect for thischeck the following5emission lines spreadalong thewholespectrum: Hα (λ 656.3nm), Hβ (λ 486.1nm), Hγ (λ 434.0nm), Paβ (λ 1281.8nm), andBrγ (λ 2166.1nm). WereportthefluxesoftheselinesinTable 5.4. Wecollectinthesametablealsotheequivalentwidth(EW) ofthelithiumlineat λ 670.8nm, thatisanindicatorofyoungagesandconfirmstheYSO statusofallourobjects.

In Manaraetal. (2013b), Alcaláetal. (2014), and Rigliacoetal. (2012)theproceduredescribedabovehasbeentestedonlow-massstarswithSpT laterthan ∼K5. WeshowhereitsvalidityalsoforYSOswithearly-K SpT.AsreportedinSect. 5.2.1, westressthatthesampleofClass III YSOsavailableishighlyincompletewhenconsideringobjectswithSpT intheintervalfromG5toK5, giventhatwehaveatdisposalonlyoneClass III YSOwith SpT K2. Finally, amoredetailed analysis is needed for objectswithG-type SpT,theso-calledintermediatemassstars, becausefortheseobjectstheexcessemissionduetoaccretioncanbehardlydetectedinthewavelengthregioncoveredbyX-Shooter(e.g.Calvetetal., 2004). Wethusdiscussinthefollowingsectionsthesethreedifferenttypesofobjectsseparately.

115


Resultsforlowmassstars

ThebestfitsobtainedforTDswithM SpT areshowninFig. 5.1, whilethoseforTDswithSpT laterorequaltoK5inFig. 5.2. Theobservedandreddeningcorrectedspectraareshownwitharedline, thegreenlinerepresentsthephotospherictemplatesused, thelightbluelinetheslabmodel, andthebluelinethebestfit, whichisthesumofthephotospherictemplateandtheslabmodel. Thebestfitisplottedonlyintheregionswhereitiscalculated,i.e. λλ ∼330-1000nm. Theagreementbetweenthebestfitandtheobservedspectruminthiswavelengthrangeisalwaysverygood. Atwavelengthslongerthan ∼ 1000nmweplotonlythephotospherictemplateandtheobservedspectra, includingalsotheir3.6 µmSpitzermagnitudes, whenavailable. Asmentionedearlier, weexpect thephotospherictemplatespectrumnottoexceedthetargetoneinthisregion. Thisisthecaseformostofthetargets, butnotforOph22, Oph24, DM Tau, andGM Aur. Foroneobject, Sz27, theexcessemissionatnear-infraredwavelengthsconfirmsthepreviousclassificationasPTD.Accordingtoourbestfits, alsoCHXR22E andISO-Oph196shouldbeclassifiedasPTD.

ThestellarandaccretionparametersobtainedforthesetargetsarereportedinTable 5.3.Fortheobjectsconsideredinthissection, i.e. withSpT laterorequaltoK5, thebestfitSpTisthesamewithinuptooneortwospectralsub-classastheonereportedintheliterature.Inmostcasesthedifferencewithrespecttotheliteraturevaluesissmallalsofortheotherstellarandaccretionparameters. Inparticular, valuesof Macc agreewithin0.3dexwiththosereported in the literature. Theobjectswith largerdifferencesareSz18, Sz45, RXJ1615, Oph24, Ser29, andSer34. Wesuggestthatthesedifferencesareduetothedifferentmethodologiesofpreviousstudieswithrespecttoour. Variabilityofaccretionwouldresultinasmallerdifference. Recentstudiesshowedthatinmostyoungaccretingstarsthesevariationsareingeneralsmallerthan0.3dex(e.g., Costiganetal., 2012). ForSer29weareonlyabletoprovideanupperlimiton Lacc fromthefittingduetothelowsignal-to-noiseofthespectruminthewholeUVB arm. ThisvalueiscompatiblewiththemeasurementoftheHα line, whichistheonlylineseeninemissioninthespectrum.

116


Figure 5.1: BestfitforM-typetransitionaldisks. Theredlineistheobserveddereddenedspectrum, greenlinethephotospherictemplate, lightbluelinetheslabmodel, andbluelinethebestfit.

117


Table5.3:

Stellar,disk,and

accretio

nparametersofth

etargets

Nam

edist

Rin

SpT

T eff

AV

L ⋆logLacc

M⋆

logM

acc

R⋆

Disk

Ref

[pc]

[AU]

[K]

[mag]

[L⊙]

[L⊙]

[M⊙]

[M⊙/yr]

[R⊙]

type

LkHα33

025

068

G4

5800

3.0

14.40

-0.6

2.35

±0.57

-7.8

3.74

±1.08

PTD

1D

DM

Tau

140

19M2

3560

1.1

0.36

-1.3

0.56

±0.08

-8.2

1.57

±0.46

TD1

BLkCa15

140

50K2

4900

1.2

1.21

-1.1

1.24

±0.33

-8.4

1.52

±0.44

PTD

1B

GM

Aur

140

28K5

4350

0.6

0.99

-1.0

1.36

±0.36

-8.3

1.75

±0.51

PTD

1B

Sz-Cha

160

29K2

4900

1.3

1.17

-0.5

1.22

±0.32

-7.8

1.50

±0.43

PTD

2B

TWHya

554

K7

4060

0.0

0.18

-1.6

0.79

±0.17

-8.9

0.85

±0.25

TD3

BCSCha

160

43K2

4900

0.8

1.45

-1.0

1.32

±0.37

-8.3

1.66

±0.48

TD4

BCHXR22

E16

07

M4

3270

2.6

0.07

-4.1

0.24

±0.06

-10.9

0.82

±0.24

PTD

2B

Sz18

160

8M2

3560

1.3

0.26

-1.9

0.54

±0.08

-8.9

1.34

±0.39

TD2

BSz27

160

15K7

4060

2.9

0.33

-1.6

0.96

±0.24

-8.9

1.16

±0.34

PTD

2B

Sz45

160

18M0.5

3780

0.7

0.42

-1.2

0.85

±0.11

-8.3

1.51

±0.44

TD2

BSz84

150

55M5

3125

0.5

0.24

-2.3

0.24

±0.06

-8.9

1.67

±0.49

TD5

BRXJ161

518

530

K7

4060

0.0

0.89

-1.3

1.16

±0.16

-8.5

1.90

±0.55

TD1

BOph

2212

51

M3

3415

3.0

0.56

-2.9

0.53

±0.14

-9.7

2.13

±0.62

TD5

BOph

2412

53

M0

3850

4.0

0.42

-2.0

0.92

±0.13

-9.2

1.45

±0.42

TD5

BSR

2112

536

G4

5800

6.0

8.11

-0.7

a1.95

±0.50

-7.9

a2.81

±0.81

PTD

1D

ISO-O

ph19

612

515

M5.5

3060

3.0

0.08

-2.3

0.14

±0.04

-8.9

1.00

±0.29

PTD

1B

DoA

r44

125

30K2

4900

1.7

0.64

-0.9

0.97

±0.19

-8.2

1.11

±0.32

PTD

1B

Ser29

230

8M2

3560

2.6

0.04

<-3.8

0.47

±0.08

<-11.2

0.52

±0.15

TD5

BSer34

230

25M1

3705

2.7

0.26

-2.7

0.71

±0.08

-9.8

1.23

±0.36

TD5

BRXJ184

2.9

130

5K2

4900

0.4

0.56

-1.5

0.93

±0.16

-8.8

1.03

±0.30

PTD

6B

RXJ185

2.3

130

16K2

4900

1.0

0.77

-1.4

1.04

±0.19

-8.7

1.21

±0.35

TD6

B

Notes.R

eferenceforR

in:(1)A

ndrewsetal.(201

1),(2)Kimetal.(200

9),(3)Hughesetal.(200

7),(4)Espaillatetal.(201

3),(5)Merínetal.(201

0),

(6)H

ughesetal.(201

0),Evolutionarym

odelsusedto

deriveM

⋆andM

acc:

(B)Baraffeetal.(199

8),(D)D

'Anton

a&M

azzitelli

(199

4).

aHighly

uncertainvalue.

118


Figure 5.2: Bestfitforlate-K typetransitionaldisks. ColorsasinFig. 5.1

ResultsforearlyK-typestars

ForsixofourobjectsweobtainabestfitusingtheClass III YSO templatewithSpT K2.TheseresultsarealsoreportedinTable 5.3, whiletheirbestfitareshowninFig. 5.3. InallthesecasesthebestfitisverygoodwiththeonlyexceptionofRX J1852.3. Withrespecttotheliterature, typicaldifferencesoftheSpT fromthebestfitK2areofuptotwospectralsub-classesapartfromCS Cha, whichwaspreviouslyclassifiedasK6. WeadoptforallthesetargetsSpT K2inouranalysis, withthecaveatthattheuncertaintyonthisparameterislargerfortheseobjectswithrespecttolaterSpT targetsduetothealreadymentionedincompletenessofphotospherictemplatesofSpT late-G andearly-K.OurbestfitsconfirmthatDoAr44, LkCa15, andSzChaarePTD (seealsoSect. 5.3.2). WefindahintofexcessintheK-bandspectrumofRX J1842.9, whichbecomesclearerattheSpitzer[3.6]datapoint.Thisconfirmstheobservationsofinfraredexcessinthisobjectreportedin Hughesetal.(2010)andimpliesthatalsothisobjectisaPTD.Thelargestdifferenceinthederivedvaluesof Macc isforSzCha, whichresultstobeastrongeraccretorthanpreviouslydetermined.

Resultsforintermediate-massstars

Twoobjectsinoursampleareofearly-G SpT,namelyLkHα330andSR21. FortheseTDswehavenotbeenabletodetectexcessemissionwithourfitter. Asalso Calvetetal. (2004)pointedout, theexcessemissionforintermediate-massstarslikethesetwoishardtobedetectedat λ >330nmdueto thesimilar temperaturesof theaccretionshockandthestellarphotosphere. Wehaveonlybeenabletofitthesespectratoderivetheir AV andL⋆, andweshowthesebestfitsinFig. 5.4. TheirpositionsontheHRD arenotcoveredbytheevolutionarytracksof Baraffeetal. (1998), sowederivethevaluesof M⋆ forthesetwotargetsusingthemodelsof D'Antona&Mazzitelli (1994). Inbothobjectswedetect

119


Figure 5.3: BestfitofearlyK-typetransitionaldisks. ColorsasinFig. 5.1

anexcessemissioninthenear-infraredwavelengthswhichcouldimplytheseobjectsarePTD.

InthespectrumofLkHα330variousemissionlinesarepresent, suchastheHα, Hβ, Paβ,andBrγ. Theonly Lacc-Lline relationavailableforthisclassofobjectsistheonereportedin Calvetetal. (2004)fortheBrγ line. Weusethisrelationtoderiveavalueof Lacc∼ 0.23L⊙ whichleadstoavalueof Macc consistentwiththosereportedintheliterature.

Thehydrogenrecombination linesofSR21appear inabsorption in thewholespec-trum. ThesameisfoundwhenlookingattheCaII IRT lines. Moreover, thephotosphericlinesofthisobjectappearmuchbroaderthanthecorrespondingClass III YSO spectrum.Nevertheless, thewingsofthehydrogenlines, inparticularthoseoftheHα line, appearinemissionatveryhighvelocitiesupto ∼ 250km/s, thussuggestingthattheyoriginateinanaccretion-relatedinfallregion. ThereforeweclassifythisobjectasanaccretingTD.GiventhatnoBrγ emissionisdetectedinthisspectrum, wederive Lacc fromtheluminosityoftheHα line. Thisisderivedfromthedereddenedspectrumcorrectedforthephotosphericlinecontribution, whichisestimatedusingasyntheticspectrumofthesame Teff andbroadentomatchphotosfericlinesclosetotheHα. ToconverttheluminosityoftheHα linein Lacc

120


Figure 5.4: BestfitofG-typetransitionaldisks. ColorsasinFig. 5.1.

weusetherelationprovidedby Alcaláetal. (2014). Givenalltheassumptionsadoptedtoestimatethisvalueweconsiderthederived Lacc veryuncertain.

121


Table5.4:

Derivedpropertiesofanalyzedlin

es

Nam

eF H

αF H

βF H

γF P

aβF B

rγEW

Liλ67

0.8

[ergs−1cm

−2]

[ergs−1cm

−2]

[ergs−1cm

−2]

[ergs−1cm

−2]

[ergs−1cm

−2]

[mÅ]

LkHα33

0(1.11±

0.02)×

10−11

(6.6

±2.9)×

10−13

<3.2

×10−13

(6.5

±0.3)×

10−13

(5.9

±3.4)×

10−14

110±

2DM

Tau

(4.48±

0.02)×

10−12

(3.5

±0.1)×

10−13

(2.2

±0.1)×

10−13

(1.06±

0.07)×

10−13

(1.4

±0.2)×

10−14

410±

21LkCa15

(3.1

±0.1)×

10−12

(3.7

±1.6)×

10−13

(1.5

±0.1)×

10−13

(1.2

±0.2)×

10−13

(1.9

±1.5)×

10−14

460±

23GM

Aur

(1.06±

0.02)×

10−11

(2. 1

±0. 07)×

10−12

(7. 9

±0. 4)×

10−13

(1. 01±

0. 02)×

10−12

(1. 6

±0. 1)×

10−13

440±

22SzCha

(2.6

±0.1)×

10−12

<4.7

×10−14

<4.0

×10−14

(1.9

±0.3)×

10−13

<1.3

×10−14

350±

10TW

Hya

(2.39±

0.04)×

10−11

(4.52±

0.07)×

10−12

(2.2

±0.05)×

10−12

(2.21±

0.04)×

10−12

(2.61±

0.08)×

10−13

430±

21CSCha

(5.23±

0.09)×

10−12

(4.7

±1.1)×

10−13

(2.2

±0.4)×

10−13

(1.1

±0.1)×

10−13

(2.6

±1.1)×

10−14

510±

28CHXR22

E(1.1

±0.1)×

10−14

(4.4

±0.6)×

10−15

(9.1

±4.9)×

10−16

<9.1

×10−15

<2.4

×10−15

230±

56Sz18

(3.8

±0.2)×

10−13

(2.5

±0.6)×

10−14

(7.9

±0.9)×

10−15

(4.3

±1.7)×

10−15

<2.7

×10−15

590±

77Sz27

(1.83±

0.08)×

10−12

(7.8

±0.6)×

10−14

(3.2

±0.2)×

10−14

(4.9

±0.5)×

10−14

(1.0

±0.2)×

10−14

510±

29Sz45

(2.83±

0.02)×

10−12

(4.6

±0.1)×

10−13

(2.9

±0.1)×

10−13

(1.17±

0.08)×

10−13

(2.0

±0.5)×

10−14

440±

32Sz84

(7.0

±0.1)×

10−13

(5.59±

0.09)×

10−14

(2.7

±0.04)×

10−14

(3.1

±0.3)×

10−14

(1.1

±0.5)×

10−14

460±

20RXJ161

5(2.24±

0.05)×

10−12

(2.5

±0.3)×

10−13

(6.9

±1.1)×

10−14

(7.5

±1.6)×

10−14

(1.5

±0.7)×

10−14

550±

32Oph

22(1.9

±0.09)×

10−13

(5.4

±0.1)×

10−14

(2.8

±0.1)×

10−14

<1.7

×10−14

<7.3

×10−15

570±

40Oph

24(3.2

±0.2)×

10−13

(1.08±

0.03)×

10−13

(7.7

±0.3)×

10−14

<6.4

×10−14

(5.0

±1.5)×

10−15

570±

37SR

21a

......

......

...14

0±2

ISO−Oph

196

(3.54±

0.01)×

10−13

(8.8

±0.6)×

10−14

(8.8

±0.8)×

10−14

<7.4

×10−14

(2.2

±0.3)×

10−14

370±

31DoA

r44

(9.4

±0.1)×

10−12

(2.66±

0.08)×

10−12

(1.04±

0.06)×

10−12

(1.12±

0.01)×

10−12

(2.6

±0.1)×

10−13

420±

19Ser29

(1.3

±0.1)×

10−14

<1.7

×10−15

<8.0

×10−15

<1.9

×10−15

<4.8

×10−17

380±

199

Ser34

(1.02±

0.05)×

10−13

(7.8

±0.7)×

10−15

(5.9

±1.1)×

10−15

<1.6

×10−15

(1.8

±0.4)×

10−15

630±

40RXJ184

2.9

(3.41±

0.08)×

10−12

(4.6

±0.6)×

10−13

(2.0

±0.2)×

10−13

(1.2

±0.1)×

10−13

(1.5

±0.7)×

10−14

440±

21RXJ185

2.3

(5.67±

0.09)×

10−12

(1.01±

0.07)×

10−12

(2.8

±0.4)×

10−13

(2.0

±0.2)×

10−13

(4.0

±2.5)×

10−14

510±

30

Notes.Fluxesarerepo

rtedin

theform

at(fl

ux±err)m

ultip

liedbyth

eorderofm

agnitude.aTheestim

ateofth

eflu

xofth

eem

ission

linesofSR21

isveryun

certain,

thuswedo

notreportthesevaluesforthisobject.

122


2000 2500 3000 3500 4000 4500 5000 5500 6000Teff [K]

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

J-K

LkHa330

LkCa 15

GM Aur

Sz Cha

CS Cha

CHXR22ESz27

SR21

ISO-Oph196DoAr 44

RX J1842.9

Figure 5.5: (J −K)colorcalculatedwithsyntheticphotometryonthebestfitdereddenedTD spectra(bluecrosses) vs Teff of the targets. The red circles represent theClass III YSOs colors derivedwith syntheticphotometryontheirspectra. ThedashedlinerepresentsthephotosphericcolorofYSOsaccordingto Luhmanetal. (2010).

5.3.2 Infraredcolorexcess

FromthebestfitderivedasexplainedaboveweanalyzethecolorexcessintheIR colorsinordertodetectemissionfromtheinnermostdustydisk. WeperformsyntheticphotometryonthedereddenedTD spectraandontheClass III YSO spectra. WethenplotinFig. 5.5-5.6 the J−K and J−[3.6]colorsasafunctionof Teff bothfortheClass III YSOs(redcircles)andtheTDs(bluecrosses). Thesecolorstracethepresenceofaninnerdisk, whichwouldresultinanexcessemissionwithrespecttothephotosphereinthe K bandandat3.6 µm.Asareference, weplotwithadashedlinethephotosphericcolorlocusofdisklessYSOsderivedby Luhmanetal. (2010). WeseethatthecolorsoftheClass III YSOsaredistributedontheseplotsaroundthisempiricallycalibratedlocuswithasmalldispersion. AlsomostoftheTDshavecolorscompatiblewiththeClass III YSOsatthesame Teff, meaningthattheirIR colorsarecompatiblewithphotosphericones, thustheyhavenodust-richinnerdisk. Insomecases, however, theexcessisdetectableandtheobjectsshouldbeclassifiedasPTD.Thisisthecasefortheobjectsalreadylistedintheprevioussections, namelyISOOph196, CHXR22E,Sz27, DoAr44, SzCha, LkCa15, RX J1842.9, LkHα330, andSR21,andforGM Aur, giventheexcessinthe J−[3.6]color(seeFig. 5.6). ThisobjectwasalsopreviouslyclassifiedasPTD by Calvetetal. (2005)and Espaillatetal. (2010). Foroneobject, CS Cha, theexcessisdetectedinthe J −K colorbutnotinthe J-[3.6]one, andintheformeritiscompatiblewiththeClass III YSOscolor. WethusclassifythisobjectasTD.WereportthisclassificationinTable 5.3. TheobjectsclassifiedasPTD have Rin valuesthatrangesmoothlyfrom5to68AU.Thepresenceofadustyinnermostregionofthediskisthusuncorrelatedwiththesizeofthedust-depletedgap.

123


2000 2500 3000 3500 4000 4500 5000 5500 6000Teff [K]

0.0

0.5

1.0

1.5

2.0

2.5

J-[3

.6]

LkHa330

LkCa 15

GM Aur

CS Cha

Sz27SR21

ISO-Oph196

DoAr 44

RX J1842.9

Figure 5.6: (J−[3.6])colorvs Teff ofthetargets. The J magnitudeiscalculatedonthebest-fitdereddenedTD spectra(bluecrosses), whilethe[3.6]magnitudeisderivedfromtheliterature. ColorsandsymbolsarethesameasinFig. 5.5.

5.4 Windsignatures

ThemostprominentforbiddenlinepresentinthespectraofourTDsisthe[OI] λ 630nmline. ThislinehasbeendetectedinmanyaccretingYSOs(e.g., Hartiganetal., 1995)andcanpresent twodistinctcomponents. Thehigh-velocitycomponent (HVC, ∆v ∼ 100-200kms−1)ofthislineisknowntotracecollimatedjets. Theoriginofthelow-velocitycomponent(LVC,∆v ∼ 2-3kms−1), instead, isstillunclear. Itisbelievedtooriginateinthediskorfromthebaseofaslowdiskwind(Hartiganetal., 1995), buttherearesuggestionsthatitcanbeoriginatedinaphotoevaporativewind(Rigliacoetal., 2013, andreferencestherein). WedetecttheLVC ofthislinein17(∼80%)ofthespectraofourTDs, withacleardetectionoftheHVC onlyinISO-Oph196.

WederivethefluxandthepeakvelocityoftheLVC ofthe[OI]λ 630nmlineinthefollowingway. WefirstlyrefinethewavelengthcalibrationbyfittingthephotosphericLi Ilineat λ 670.78nmandshiftingthespectratomatchthenominalcentralwavelengthofthisline. ThislineisdetectedinalltheobjectsanditsEW arereportedinColumn 7ofTable 5.4. Then, wefitwithagaussianprofileonthedereddendspectrumtheLVC ofthe[OI]λ630nmlineandweintegratethefluxofthebestfittoderivethelineflux. Theerroronthefluxisderivedfromthestandarddeviationofthecontinuumestimatedaroundtheline. Thederivedflux, error, peakvelocity(v0), andFWHM ofthegaussianfitisreportedinTable 5.5. ThelinesandtheirbestfitsareshowninFig. 5.7. Inall theobjectswithdetected[OI]λ630nmline, withtheexceptionofLkHα330, thelineisslightlyblueshifted,withvaluesof v0 rangingfrom ∼ -2km/sto ∼ -8km/sinmostcases, andonlytwoobjects(Oph24andISO-Oph196)with v0 < -10km/s. Eveniftheexactvalueof v0 ineachobjectisstilluncertainalsoaftertheproceduretocorrectthewavelengthcalibrationdescribed

124

5.5 Discussion

200 0 200

0.0

0.5

1.0 LkHα330

200 0 200

0.0

0.5

1.0 DM Tau

200 0 200

0.0

0.5

1.0 LkCa 15

200 0 200

0.0

0.5

1.0 GM Aur

200 0 200

0.0

0.5

1.0 Sz Cha

200 0 200

0.0

0.5

1.0 TW Hya

200 0 200

0.0

0.5

1.0 CS Cha

200 0 200

0.0

0.5

1.0 Sz27

200 0 200

0.0

0.5

1.0 Sz45

200 0 200

0.0

0.5

1.0 Sz84

200 0 200

0.0

0.5

1.0 RX J1615

200 0 200

0.0

0.5

1.0 Oph24

200 0 200

0.0

0.5

1.0 ISO−Oph196

200 0 200v [km/s]

0.0

0.5

1.0 DoAr 44

200 0 200v [km/s]

0.0

0.5

1.0 Ser34

200 0 200v [km/s]

0.0

0.5

1.0 RX J1842.9

200 0 200v [km/s]

0.0

0.5

1.0 RX J1852.3

Norm

aliz

ed f

lux

Figure 5.7: Normalized[OI] 630nmlineforthetransitionaldisksinoursamplewherethislinehasbeendetected. Thereddashedlineisthebestgaussianfitofthelow-velocitycomponentoftheline.

above, weseethatthe[OI]λ630nmlineissystematicallyblueshifted, meaningthatitisoriginatedinsomekindofwind. ThemeanvalueoftheFWHM ofthe[OI]λ630nmlinederivedfromthespectraofourtargetsis ∼ 40km/s. Wenote, however, thatthevaluesofFWHM ≲ 30km/sshouldbeconsideredwithcaution, asthesevaluesareclosetothenominalresolutionoftheinstrument.

5.5 Discussion

Inthissectionwediscusstheaccretionandwindpropertiesofourtargetsandweesti-matetheamountofgaseousmaterialintheirinnerdisks. Itshouldbekeptinmindthatoursampleiscomposedmostlybyobjectsalreadyknowntobestrongaccretorsanditisnot anunbiasedsample. Nevertheless, thepropertiesofthesestrong-accretingTDshaveimportantconsequencesonourunderstandingoftheTDsformationandevolution, aswe

125


Table 5.5: Derivedpropertiesof[OI] lineat λ 630nm

Name F[OI]λ630 v0,[OI]λ630 FWHM[OI]λ630[ergs−1 cm−2] [km/s] [km/s]

LkHα330 (6.1± 1.9)× 10−14 7.7 24DM Tau (9.1± 1.8)× 10−15 -4.3 38LkCa15 (4.6± 1.9)× 10−14 -8.7 43GM Aur (3.9± 1.0)× 10−14 -2.2 42SzCha (2.4± 0.9)× 10−14 -4.7 30TW Hya (9.0± 0.8)× 10−14 -4.8 24CS Cha (2.8± 1.5)× 10−14 -4.4 37CHXR22E < 8.0× 10−16 ... ...Sz18 < 2.6× 10−15 ... ...Sz27 (5.4± 0.3)× 10−14 -7.6 49Sz45 (1.8± 0.5)× 10−14 -4.8 50Sz84 (2.4± 0.2)× 10−15 -6.5 44RX J1615 (1.7± 0.8)× 10−14 -8.7 49Oph22 < 7.3× 10−15 ... ...Oph24 (3.0± 1.3)× 10−14 -13.0 67SR 21 < 6.4× 10−14 ... ...ISO−Oph196 (1.4± 0.4)× 10−15 -20.4 68DoAr44 (2.6± 1.2)× 10−14 -7.7 50Ser29 < 4.2× 10−16 ... ...Ser34 (4.1± 0.8)× 10−15 -7.3 47RX J1842.9 (5.0± 0.8)× 10−14 -5.5 43RX J1852.3 (2.6± 1.3)× 10−14 -3.8 36

Notes. Fluxesarereportedintheformat(flux ± err)multipliedbytheorderofmagnitude.

discussinthefollowing.

5.5.1 Accretionproperties

Hereweaimatunderstandingwhetherthereisadependenceoftheaccretionpropertiesofourobjectswiththemorphologyofthedisk, inparticularwith Rin, andwhethertherearedifferenceswithrespecttoaccretionpropertiesincTTs.

InFig. 5.8 weshowthelogarithmicvaluesof Macc determinedinSect. 5.3 asafunctionofthevaluesof Rin reportedintheliterature(seeTable 5.3). Werepresentthesevaluesusingdifferent symbols todifferentiatemeasurementof Rin derivedwith resolvedmm-interferometryobservations(redcircles) fromthoseobtainedbymodelingtheoptical tomid-infraredSEDs(bluesquares). Theuncertaintiesonthevaluesof Rin arevariousanddependstronglyontheassumptionsonthemodels. Inparticular, valuesof Rin determinedwithSED fittingarestronglymodel-dependentandcanbeanoverestimationoftherealgapsize(Rodgers-Leeetal., 2014). Wedonotfindanystrongtrendof Macc increasingwith Rin overthewholerangeof Rin wehaveexplored. IfwecompareourresultswithmorecompletesamplesofTDs, e.g., Kimetal. (2013), weseethatourresultsagreewiththeupperboundaryoftheirsample, andthatindeedthereisanincreaseof Macc with Rin

126

5.5 Discussion

uptovaluesof Rin∼ 20AU.At Rin largerthanabout20AU,however, Macc isessentiallyconstantinoursample. Infact, asimilartrendisalsopresentintheupperenvelopeoftheTDsconsideredin Kimetal. (2013), where, however, therearejusttwoaccretingTDswithMacc> 10−8M⊙/yrfor Rin≳ 20AU andnonefor Rin≳30-40AU.Allobjectsinoursamplehave Macc intherange 10−9−10−8M⊙/yr, independentlyofthevalueof Rin. Therefore, thedensityoftheirinnermostgaseousdisk, whichaccretesontothestar, doesnotdependonthemechanismthatproducesthegaporthehole, andmustbehighenoughtosustaintheobservedaccretionrates.

Wewantnow tocompare thederivedvaluesof Macc forour sampleofTDswithasampleofclassicalTTauristars(cTTs)todeterminewhethertheaccretionpropertiesaredifferentinthesetwoclassesofobjects. Itiswellestablishedthatthevaluesof Macc incTTsdependonM⋆ withapowerof ∼1.6-1.8(e.g., Muzerolleetal., 2003; Rigliacoetal.,2011a; Manaraetal., 2012; Alcaláetal., 2014; Ercolanoetal., 2014). A comparisonofthevaluesof Macc betweendifferentsamplesshouldbebasedonacomparisonofthisrelationandnotonthevaluesof Macc alone. AnotherwellknowndependenceistheonebetweenMacc andtheageofthetargets(e.g., Hartmannetal., 1998; Sicilia-Aguilaretal., 2010;Manaraetal., 2012), whichisaconsequenceoftheviscousevolutionofprotoplanetarydisks. Therefore, acomparisonshouldbecarriedoutbetweenobjectsof similarmeanage. Finally, differentmethodologyandevolutionarymodelscanleadtodifferentvaluesof Macc; itisthusneededtocomparesamplesanalyzedwithasimilarmethodology. Forthesereasonsweselectasacomparisonsampletheobjectsstudiedby Alcaláetal. (2014).ThesearelocatedintheLupusI andIII cloudsandhaveages∼ 3Myr, similartotheobjectsinoursample. Theanalysisofthatsamplewascarriedoutwiththesamemethodologyastheoneweusedhere. WeshowinFig. 5.9 thelogarithmicrelationbetween Macc and M⋆

forthesetwosamples. Ourdataarereportedasbluecircles, whiledatafrom Alcaláetal.(2014)asgreendiamonds. Thesolidlineontheplotisthebestfitrelationfrom Alcaláetal. (2014), andthedashedlinesrepresentthedispersionof0.4dexaroundthisbestfitrelation. Thetypicalerrorsonthequantitiesareshownasablackcross. Weseethat∼80%oftheTDshavevaluesof Macc consistentwiththevaluesfoundby Alcaláetal. (2014)forLupusobjectsofthesame M⋆. Therefore, fortheseobjectswedonotseeanydifferenceintheaccretionpropertieswithrespecttothoseincTTs. WealsoperformonthesetwosamplesaKolmogorov-Smirnofstatisticaltest(K-S test). Whenrestrictingtoobjectsinthesame M⋆ rangetheprobabilitythatthetwosamplesaredrawnfromthesamedistributionis80%. Wecanthenconcludethatforoursample, theamountofaccretiondependsonthemassofthecentralobject, andnotontheevolutionarystage(cTTsorTD) ofthesystem.

Thisresultdiffersfromwhatfoundintheliterature. Forexample, Najitaetal. (2007)notedthatTDshaveasistematicallysmallervalueof Macc atanygivenvalueofthemassofthedisk(Md). TheyinferredthattheaccretionratesforTDsareingeneralsmallerthanforcTTs. Similarly, variousfurtheranalysesoflargersamplesofTDsfoundvaluesof Macc

typicallylowerthanthoseofcTTsbyafactor ∼10(e.g., Kimetal., 2009, 2013; Muzerolleetal., 2010; Espaillatetal., 2012). A criticalreviewoftheseresultsby Espaillatetal. (2014)foundsomehowdifferentresultswithrespectto Najitaetal. (2007), astheyreportvaluesof Macc for3TDsin ρ-OphiucuswhicharecompatiblewiththelocusofcTTsinthesame

127


0 10 20 30 40 50 60 70Rin [AU]

12

11

10

9

8

7

logM

acc [

Msu

n/y

r]

SED fitmm image

Figure 5.8: Logarithmofthemassaccretionratevsinnerholesizeforoursample. Differentsymbolsareusedtodistinguishthemethodsusedintheliteraturetoderivethesizeoftheinnerhole. Bluesquares areadoptedwhenthishasbeenderivedusingIR-SED fitting, while redcircles whenthevaluesarederivedfromresolvedmm-interferometryobservations. Downwardarrowsareupperlimits. Thetwolowestpointsare,fromlefttoright, CHXR22E andSer29. Theobjectat Rin=25AU andlogMacc=−9.8isSer34.

regionandinTaurusonthe Macc-Md plane. Theysuggestthatdifferentresultsmayarisefromdifferentsampleselectionand/ordifferentmethodstoestimate Macc. Toavoidthispossiblesystematicbias, wehaveshownhereonlythecomparisonbetweenoursampleofTDsandthesampleofcTTsinLupus, whichisanalyzedinthesamewayasourobjects.WestresshereagainthatourTDshavebeenselectedtobemostlystrongaccretors, andthatwedonotderiveconclusionsforthewholeTD population. Nevertheless, ourresultsprovethatthereareTDsthataccreteatthesamerateofcTTs.

ThetwomainoutliersinFig. 5.8-5.9 aretheobjectwithanupperlimiton Lacc, Ser29,andCHXR22E.ThelowerintensityofaccretionforthesetargetsimpliesthatthegasdensityintheinnerdiskissubstantiallydepletedwithrespecttotheoneofcTTs. Theseobjectsdonothaveanypeculiarpropertyreportedintheliterature. FromFig. 5.9 wenotethatinthesamerangeof M⋆ oftheseobjectsthereareotherTDswithvaluesof Macc comparableorevenhigherthancTTs. Therefore, theseobjectsarenotpeculiarintheirstellarproperties.Wewilldiscussinmoredetailabouttheseobjectslaterafterconsideringtheirwindanddustyinnerdiskproperties. ItispossiblethattheseobjectsarepartofapopulationofTDswithlowervaluesof Macc notincludedinoursample.

5.5.2 Windproperties

AsdiscussedinSect. 5.4, wehavemeasuredthefluxoftheLVC ofthe[OI]λ630nmline,whichisatracerofwindsinYSOs. Todeterminewhetherthewindpropertiesofourob-jectsdependonthediskmorphologywecompareinFig. 5.10 thelogarithmicluminosity

128

5.5 Discussion

1.5 1.0 0.5 0.0 0.5logM ∗ [Msun]

12

11

10

9

8

7lo

gM

acc [M

sun/y

r]Alcala+14TDs

Figure 5.9: LogarithmofthemassaccretionratevslogarithmofthestellarmassforoursampleoftransitionaldisksandforasampleofclassicalTTauristarsfrom Alcaláetal. (2014). Ourtargetsareshownas bluecircles,whiledatafromtheliteraturearereportedwith greendiamonds. Thelinesarethebestfittothedatareportedin Alcaláetal. (2014)(solidline)andthe0.4dexspreadreportedinthatstudy. Downwardarrowsareupperlimits. Thetwolowestpointsare, fromlefttoright, CHXR22E andSer29. Typicalerrorsareshownwiththeblackcross.

ofthislinewiththevaluesof Rin availablefromtheliterature. Wedonotseeanyclearcorrelationbetweenthesequantities. TheluminosityoftheLVC ofthe[OI]λ630nmline(L[OI]630)appearsconstantregardlessthesizeofthedustdepletedcavityinthediskwithvaluesbetween ∼ 10−6 and ∼ 10−4 L⊙. Thisimpliesthatthepropertiesofthewindtracedbythe[OI]λ630nmline-thatcanbeadiskwind, anaccretion-drivenwind, orapho-toevaporativewind-aresimilarinmostoftheTDsinoursample. Thequestiontheniswhereinthediskthewindisoriginated. Withthedatainourhandswecannotputanyconstraintontheemittingregion. Analysisofhigher-resolutionspectraofthisline(e.g.,Rigliacoetal., 2013)showedthat, incTTs, theemissionregioncanbeasclosetothestaras ∼ 0.2AU,whichiswellwithinthedustdepletedcavityinallourobjects. ModelsofX-rayphotoevaporation(Ercolano&Owen, 2010)predictthattheluminosityofthislineisinsensitivetothesizeoftheinnerholeanddependmostlyontheEUV andX-rayluminosity(LX )ofthecentralstar. TheX-rayphotonsareresponsiblefordrivingthewindinthefirstplace, whiletheEUV photonsheatuptheinnerregionofthewindandexcitethe[OI] line.Inthiscontext, thecorrelationof Lacc with L[OI] (seediscussioninthenextparagraphandFig. 5.11)couldbeduetotheheatingofthewindbytheUV photons. Thereforealackof

129


0 10 20 30 40 50 60 70Rin [AU]

8

7

6

5

4

3

log(L

[OI]6300/L

sun)

Figure 5.10: Logarithmicluminosityofthelow-velocitycomponentofthe[OI] 630nmlinevsinnerholesizeforthetransitionaldisksinoursample. Downwardarrowsareupperlimits.

correlationof L[OI] with LX , thatisfoundwhencomparingourdatawiththedatareportedintheliteraturefor LX (seeTable 5.9), isnotatallsurprising, astheemissionmeasureofthislineisdeterminedbytheUV luminosity, whichisinsteadcorrelatedto Lacc. Forthisreason L[OI] cannotbeusedasaquantitativetracerofthephotoevaporatedwind. Higherresolutionspectraandmorecompletegridsofmodelsareneededtobetterconstrainttheoriginofthisline.

It is thenimportant tocomparethepropertiesof this lineinourTDsandincTTstounderstandwhethertheyaresimilarinthetwoclassesofobjects. Ascomparisonsamplesweselecttheobjectsstudiedby Rigliacoetal. (2013, andreferencestherein)andthoseobservedwithX-Shooterandanalyzedby Nattaetal. (inprep.). Thesetwosamplesarerepresentativeofdifferentstellar, accretion, andwindpropertiesofcTTs. Inparticular, thesampleof Rigliacoetal. (2013, andreferencestherein)comprisesmostlystrongaccretorswithlowtointermediatestellarmass, while Nattaetal. (inprep.) hasasampleoflow-andvery low-massYSOswith loweraccretionrates. HerewecompareonlytheluminosityoftheLVC ofthe[OI]λ630nmlinederivedinourworkandinthecomparisonsamples.Thebestwaytocomparethesevaluesistoanalyzethe L[OI]630-Lacc relation, whichiswellcharacterizedintheliterature(seee.g., Rigliacoetal., 2013, andreferencestherein). ThisisshowninFig. 5.11, whereweplot logL[OI]630 asafunctionof log Lacc foroursampleofTDs(redfilledcircles)andforthetwosamplesofcTTs(blueemptycircles fordatafromRigliacoetal. 2013 and greenemptycircles for those from Nattaetal. inprep.). Therelationbetweenthesetwoquantitiesspansover ∼ 7ordersofmagnitudeinbothaxeswithatypicalspreadof ∼ 1dexforthecTTs, andourobjectsfollowitverywellinallthecases. ThelocationofourTDsrightinthemiddlebetweenthetwocomparisonsamplesreflectsthefactthattheiraccretionratesaretypicalof ∼0.5-1 M⊙ YSOs, i.e. smallerthan

130

5.5 Discussion

6 5 4 3 2 1 0 1 2log(Lacc/Lsun)

9

8

7

6

5

4

3

2

log(L

[OI]63

00/

L sun)

Rigliaco+13Natta+14TDs

Figure 5.11: Logarithmicluminosityofthelow-velocitycomponentofthe[OI] 630nmlinevsthelogarithmoftheaccretionluminosityofourobjects(redfilledsymbols)andfortwosamplesofclassicalTTauristars(blueemptycircles from Rigliacoetal. 2013, greenemptysymbols from Nattaetal. inprep.). Downwardarrowsareupperlimits.

thoseinthesampleof Rigliacoetal. (2013)andlargerthanlow-massYSOs. Atthesametime, thisimpliesthattheirwindpropertiestracedbythe[OI]λ630nmlinescalewiththeaccretionpropertiesinthesamefashionasincTTs. Therefore, theprocessresponsiblefortheformationofthislineshouldbethesameinobjectssurroundedbydust-richdisksandinTDs.

[NeII] fromtheliterature

Tobetterunderstandthepropertiesofthewindsinourobjectsweincludealsodatafromtheliteratureonthe[NeII]λ12.8 µm, whichisawellknowtracerofdiskwind. Thislinehasbeendetectedinemissioninthemid-infraredspectraofprotoplanetarydisksusing Spitzer(e.g. Pascuccietal., 2007; Güdeletal., 2010; Espaillatetal., 2013, andreferencetherein)orground-basedobservations(e.g., Pascucci&Sterzik, 2009; Pascuccietal., 2011; Saccoetal., 2012). Thislineisofinteresttostudytheinnergaseousdiskpropertiesasittraceswarmgas(T ∼ 5000K) andtheeffectsofextremeultraviolet(EUV) andX-rayemissionfromthestaronthedisk(Glassgoldetal., 2007). High-resolutionspectroscopicstudiesconstrainedtheemittingregionofthislinewithin20-40AU fromthecentralstar(Saccoetal., 2012). High resolutionobservationsofTW Hya, inparticular, showedthatmost(≳80%)ofthe[NeII] emissionarisesfromtheregionwherethediskisstillopticallythick,butstillwithin ∼10AU fromthecentralstar(Pascuccietal., 2011).

Amongtheobjectsinoursample, 13havebeenobservedwithMIR spectroscopyand

131


Table 5.6: Propertiesof[NeII] λ12.8µmlinefromtheliterature

Name F[NeII]_hires FWHM v0 F[NeII]_Spitzer RefLkHα330 ... ... ... 0.38± 0.19 G10DM Tau ... ... ... 0.55 G10LkCa15 < 0.5 ... ... 0.28± 0.02 S12GM Aur ... ... ... 1.2± 0.06 G10SzCha ... ... ... 1.62± 0.20 E13TW Hya 3.8± 0.3 14.6 -6.2 5.9± 1.1 P09,G10CS Cha 2.3± 0.2 27 -3.3 3.63± 0.07 P09,E13CHXR22E ... ... ... ... ...Sz18 ... ... ... ... ...Sz27 ... ... ... 0.63± 0.07 E13Sz45 ... ... ... ... ...Sz84 ... ... ... ... ...RX J1615 1.4± 0.2 20.5 -7.5 2.76± 0.46 S12Oph22 ... ... ... ... ...Oph24 ... ... ... ... ...SR 21 0.5± 0.1 15.1 -8.3 < 3.0 S12,G10ISO-Oph196 ... ... ... ... ...DoAr44 < 0.3 ... ... 0.68± 0.33 S12,G10Ser29 ... ... ... ... ...Ser34 ... ... ... ... ...RX J1842.9 < 0.2 ... ... 0.43± 0.13 S12,G10RX J1852.3 ... ... ... 0.72± 0.04 G10

References. P11: Pascuccietal. (2011); P09: Pascucci&Sterzik (2009); S12: Saccoetal. (2012); E13:Espaillatetal. (2013); G10: Güdeletal. (2010)

Notes. Fluxesarereportedinunitsof 10−14 ergs−1 cm−2; v0 andFWHM inunitsofkm/s.

allofthemhavea[NeII] linedetection, aswereportinTable 5.6. Inalltheseobjectswedetectedalsothe[OI] λ 630nmline, withtheonlyexceptionbeingSR21.

5.5.3 Accretionandwindpropertiesinobjectswithinnerdiskemission

FollowingtheanalysisdescribedinSect. 5.3, wedividethesampleintwoclassesofob-jects: werefertoobjectswithnonear-infraredcolorexcessasTD,whilethosewithexcessarereferredtoasPTD,asreportedinTable 5.3. Themorphologicaldifferencebetweenthese twoclasses is thepresenceofwarmdust in the inner regionof thediskofPTD,whichcouldbeasmallringofdustatfewtenthsofAU fromthestar(e.g., Benistyetal.,2010; Espaillatetal., 2010). Thisdifferenceintheinnerdiskmorphologycouldbeduetodifferentevolutionarystagesofthesetwoclassesofobjectsortodifferentdustdepletingmechanisms. Herewecomparetheaccretionandwindpropertiesoftheobjectsinthesetwoclassespresentinoursampletoverifywhetherweseeanydifferenceamongtheseobjects.

ThecomparisonoftheaccretionpropertiesofTDsandPTDsiscarriedoutusingthelogartithmic Macc-M⋆ relation, whichisshowninFig. 5.12. Inthisfiguredifferentsymbols

132

5.5 Discussion

1.5 1.0 0.5 0.0 0.5logM ∗ [Msun]

12

11

10

9

8

7

logM

acc [

Msu

n/y

r]

PTDsTDs

Figure 5.12: Logarithmofthemassaccretionratevslogarithmofthestellarmassofoursample. Differentsymbolsareusedtodistinguishobjectswithinnerdiskemission(bluesquares)fromTDswithnoIR-excess(redcircles).

areusedtoplotTDs(redcircles)andPTDs(bluesquares). Weoverplotherethebestfitrelationfrom Alcaláetal. (2014)asinFig. 5.9 thatisusedasareference. Weclearlyseethatthereisnotasignificantdifferencebetweenthetwoclassesofobjects. SimilarlytowhatwediscussinSect. 5.5.1, alsothisresultisapparentlyatoddwithpreviousstudies,whichshowedthatPTDsaccreteatalowerratethanTDs(e.g., Espaillatetal., 2010; Kimetal., 2013). ThisimpliesthattheaccretionpropertiesinoursampleofTDsareindependentonthedustyinnerdiskmorphology.

WeproceedwiththisanalysisbycomparinginFig. 5.13 thelogarithmicrelationbe-tween Lacc and L[OI]630 foroursample, usingdifferentsymbolsforTDs(redcircles)andforPTDs(bluesquares)toseewhetherwindpropertiesdependontheinnerdiskproperties.Alsointhiscasethereisnocorrelationbetweenthepositionontheplotandtheinnerdiskmorphology. Thewindpropertiestracedbythe[OI] λ 630nmlinearethusindependentofthepresenceofdustintheinnermostregionofthedisk.

5.5.4 Constraintonthegascontentoftheinnerdisk

Herewepresentadditionalconstraintontheregionintheinnerdiskwheregasispresentandonitsproperties. FromobservationsobtainedintheliteraturewehavemeasurementontheemissionfromCO fromtheinnerpartof thedisk, whichwediscussinthenextsubsection. Then, usingtheinformationontheaccretionpropertiesofourtargetswederivetheextentofthegas-richinnerdisk, whichextendsdowntothemagnetosphericradius(Rm)atfewstellarradii. Wethenalsoderivethedensityofgasintheinnerdiskneededtosustaintheobserved Macc ifthediskisassumedin``steady-state''. Wealsodiscusspossibilitiestoexplaintheobserved Macc, andthusforagas-richinnerdisk, allowingforasignificantgas

133


4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0log(Lacc/Lsun)

8

7

6

5

4

3

log(L

[OI]6300/L

sun)

PTDsTDs

Figure 5.13: Logarithmicluminosityofthelow-velocitycomponentofthe[OI] 630nmlinevsthelogarithmoftheaccretionluminosityofourobjects. SymbolsandcolorsarethesameasinFig. 5.12.

depletioninthediskgaps. Finally, wegiveacompleteviewofthegascontentoftheinnerdiskaddingtotheseinformationsalsothewindpropertiesofourTDs.

CO emissionfromtheliterature

Thefundamental(∆v =1)rovibrationallineofCO at4.7 µmisanimportantdiagnostictoconstraintthepresenceofgasintheinnerregionofTDs. Thisissensitivetogastempera-turesof100-1000K,whichcorrespondtoradiiof0.1-10AU intypicalprotoplanetarydisksaroundsolar-massstarsYSOs, assumingthatitoriginatesintheso-called``warmmolec-ularlayer"ofthedisk. Studyofhigh-resolutionspectraofthislinehavedeterminedwithhighprecisionitsemittingregionwithinthedisk(Najitaetal., 2003; Salyketal., 2007;Pontoppidanetal., 2008; Salyketal., 2009; Pontoppidanetal., 2011). Thesestudiesob-served7objectsincludedinthiswork, anddetectedthelineinallofthembesidesDMTau. WereportinTable 5.7 theinnerradiusoftheCO emissioninthedisks(Rin,CO)derivedfromthestudiesintheliterature. AdditionalstudiesonthreeotherobjectsofoursamplehavedetectedthislineinRX J1842.9, whilenon-detectionareobtainedinthespectraofSzChaandRX J1852.3(A.Carmona, personalcommunication). SevenoutofthetenobjectswherethislinehasbeenstudiedareclassifiedasPTD,sothisemissioncouldarisefromthedustyinnerdisk. However, wealsoseethatCO canbeemittedinthedust-depletedinnerdiskofsomeTDs, suchasTW HyaorRX J1852.3. A possibleexplanationfortheemissionofCO inthecaseofTW Hyaislocalwarmingduetothepresenceofacompan-ionorbitinginthegap(Arnoldetal., 2012). Inanycase, thedetectionofCO emissionintheaforementionedobjectsconfirmsthattheirinnerdiskisgas-rich, confirmingtheresultsobtainedbydetectingongoingaccretioninthesametargets.

134

5.5 Discussion

Table 5.7: PropertiesofCO fundamentaltransitionfromtheliterature

Name Rin,CO Ref[AU]

LkHα330 4±1 P11DM Tau ...a S09LkCa15 0.093 N03GM Aur 0.5±0.2 S09SzCha ...c ...TW Hya 0.11±0.07 P08CS Cha ... ...CHXR22E ... ...Sz18 ... ...Sz27 ... ...Sz45 ... ...Sz84 ... ...RX J1615 ... ...Oph22 ... ...Oph24 ... ...SR 21 7.6±0.4 P08ISO-Oph196 ... ...DoAr44 0.4±0.1 S09Ser29 ... ...Ser34 ... ...RX J1842.9 ...b ...RX J1852.3 ...c ...

References. S09: Salyk et al. (2009); N03: Najita et al. (2003); P08: Pontoppidan et al. (2008); P11:Pontoppidanetal. (2011). a Nondetection. PersonalcommunicationsfromA.Carmona: b detectionofCOline, c nondetectionofCO line.

Magnetosphericradius

Inthecontextofmagnetosphericaccretionmodels(e.g., Hartmannetal., 1998)theposi-tioninthediskfromwherethegasisaccretedontothestarisdeterminedby Rm. Thisistheradiuswheretheexternaltorqueduetostar-diskmagneticinteractiondominatesovertheviscoustorque. Following Armitage (2010), thiscanbederivedbyequatingtheexpres-sionsrepresentingthetwotimescalesinvolvedinthisprocess, namelythemagnetosphericaccretiontimescale:

tm ∼ 2πΣ√GM⋆r

BszB

sϕr

, (5.1)

where r istheradialdistanceinthediskfromthecentralstar, B isthemagneticfield, thesuperscript s standsformagneticfieldevaluatedatthedisksurface, and Σ isthesurfacedensityofthegas, andtheviscoustimescales:

tν ∼ r2

ν, (5.2)

135


where ν isthediskviscosity. Weassumethesteady-statediskrelationfortheviscosity:

νΣ =Macc

3π, (5.3)

thatimpliesaconstant Macc inthedisk, andweconsiderthesimplecasewherethestellarmagneticfieldisbipolarandorientedinthesamedirectionastherotationaxisofthestar.Withtheseassumptions, wederivetheusualrelationfor Rm (e.g. Hartmann, 2009):

Rm ∼(

3B2⋆R

6⋆

2Macc√GM⋆

)2/7

. (5.4)

It is important tonote that thisquantitydependsweaklyon Macc, M⋆, and to B⋆. Thestrongerdependenceison R⋆. Usingthisrelationweareabletoderivethevaluesof Rm

foralltheaccretingTDsinoursampleusingthevaluesof Macc2, M⋆, and R⋆ derivedin

Sect. 5.3, andassumingatypicalvalueforthemagneticfieldofthestar B⋆ ∼ 1 kG (e.g.Johnstoneetal., 2014). TheeffectofthearbitrarychoiceofthevalueofB⋆ istheprominentsourceofuncertaintyinourestimateof Rm. Wehavetoadoptatypicalvaluefor B⋆ sinceonlyfor twotargets inoursamplethisquantityhasbeenmeasured. Thisis thecaseofTW HyaandGM Aur, where B⋆ is1.76kG and1kG,respectively(Johns-Krull, 2007). Byvaryingthevaluesof B⋆ from2kG to0.5kG weestimatearelativeuncertaintyon Rm oflessthan0.5. Thisisthentheassumeduncertaintyofourestimate.

ThevaluesofRm wehavederivedarereportedinTable 5.8. InalltheobjectsRm > 5R⋆,inaccordancewithmagnetosphericaccretionmodels. Thisradiusisalwayslocatedatadistancefromthecentralstarmuchsmallerthan Rin. Thedetectionofongoingaccretionimpliesthatgasispresentinthediskatthisdistancefromthestar. Thegasdensityintheregionofthediskatradii ∼ Rm canbeestimatedasweexplaininthenextsubsection.

Densityofgasintheinnerdisk

Assumingsteady-statediskconditionthesurfacedensityofthegasisrelatedtotheaccretiondiskviscosityand Macc by the relationreported inEq. (5.3). Wedescribe theviscosityusingthe α viscosityprescription(ν = αcsH, Shakura&Sunyaev, 1973)andweassumethat thedisk isvertically isothermal, so that H = cs/Ω(r), where cs = (kT/µmp)

1/2 isthesoundspeed, µ=2.3isthemeanmolecularweight, mp isthemassoftheproton, andΩ = (GM⋆/r

3)1/2 istheangularvelocityofthedisk. Wethenderivethefollowingrelationforthesurfacedensityofthegasinthedisk:

Σ(r) ∼ 2mp

3παkBT (r)Macc

√GM⋆

r3. (5.5)

2Thevaluesof Macc derivedpreviouslyhavebeenobtained assuming Rm =5 R⋆. This is theusualassumptionmadeintheliterature, thusthisvalueistheappropriateonetoderive Macc consistentlytopreviousanalyses. Thesamevaluesof Macc canbeadoptedherebecauseoftheweakdependenceof Rm on Macc

(Rm ∝ M−2/7acc ). Byre-deriving Macc usingthenewlydetermined Rm weobtainvaluesof Macc withatypical

differenceof∼0.05dexandalwayssmallerthan0.1dex. Thistranslatesinrelativeuncertaintiesonthevalueof Rm oflessthan0.06.

136

5.5 Discussion

Table 5.8: Derivedpropertiesofthegas

Name Rm Rm Σ1AU

[R⋆] [AU] [gcm−2]LkHα330 10.05 0.175 411.36DM Tau 7.94 0.058 106.87LkCa15 8.35 0.059 82.81GM Aur 7.46 0.061 174.28SzCha 5.54 0.039 337.56TW Hya 8.37 0.033 19.17CS Cha 8.07 0.062 116.45CHXR22E 35.18 0.134 0.12Sz18 10.88 0.068 23.91Sz27 9.73 0.052 24.61Sz45 7.50 0.053 118.54Sz84 14.55 0.113 15.01RX J1615 9.49 0.084 92.24Oph22 24.22 0.240 4.63Oph24 13.13 0.089 15.08SR 21 8.97 0.117 299.26ISO−Oph196 11.51 0.054 9.45DoAr44 6.28 0.032 102.45Ser29 ... ... ...Ser34 18.61 0.106 2.95RX J1842.9 8.88 0.043 25.22RX J1852.3 9.08 0.051 34.91

Weestimatethesurfacedensityofthegasatadistanceof1AU fromthecentralstar(Σ1AU ).ThisradiusischosenbecauseitismuchlargerthanRm butstillwithinRin forallourtargets.Assuming α = 10−2 and T (1AU) =200K (representativevaluederivedfrom Andrews&Williams, 2007), wederivethevaluesof Σ1AU fromthecentralstarreportedinTable 5.8.Thesevaluesvaryfromfewgcm−2 to ∼ 4× 102 gcm−2 forourobjects, andrepresenttheexpecteddensitiesofgasinthediskinnerregionneededtosustaintheobservedaccretionratesassumingsteadystateviscousinnerdisk. Anotherpossibility is that thedensityofthegas in thecavity is lower than theonederivedhere if theradial inflowofgas isathighvelocity, approachingfree-fall(Rosenfeldetal., 2014). Finally, episodiceventsthatreplenishthegascontentoftheinnerdiskfromtheouterdiskcouldalsopossiblyexplainourobserved Macc withasignificantlygasdepletedholeformostoftheTD lifetime.

5.5.5 Discussiononthegascontentoftheinnerdisk

Wenowwant to put together all the information collected fromour spectra and fromtheliteratureontheobjectsinoursampletounderstandwhatisthemorphologyoftheirgaseousinnerdisk. Thediscussionisdividedbetweenaccretingandnon-accretingobjects.All theobjects analyzed in thisworkbesides Ser 29have accretiondetectedwithourmethod. AlsoCHXR22E hasameasuredvalueof Macc lowerthanotherobjectswithsimilar

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stellarproperties, aswepointedoutwhendiscussingtheresultofFig. 5.9. WediscussthesetwoobjectsinSect. 5.5.5, whiletheother20accretingobjectsarediscussedinthenextsubsection.

Accretingtransitionaldisks

Asdiscussed in the introduction, thedetectionofmeasurable accretion rates in youngstellarobjects implies that theinnermostregionof thediskisgasrich. This is thecaseforouraccretingTDs, andwederivedinSect. 5.5.4 and 5.5.4 theinnerboundaryofthegaseousdiskintheseobjectsandthedensitiesofthegasintheinnerdiskneededtosustaintheobservedaccretionratesassumingasteady-stateviscousdisk. Weconstrainwithouranalysisthatgasispresentinthesedisksinregionsasclosetothestaras ∼ 0.03-0.3AU,thatarethevaluesof Rm. Theevidenceofgaspresenceinthisregionisconfirmedin17ofthe20accretingTDswiththedetectionofthe[OI] λ 630nmlineintheirspectra, whichisoriginatedascloseas ∼ 0.2AU fromthestar. Atsimilardiskradii(∼ 0.1-0.5AU) theCOemissionisdetectedin4objects(LkCa15, GM Aur, TW Hya, andDoAr44, seeTable 5.7),confirmingthepresenceofgasintheirinnerdisk. ForTW Hyathisregionisknowntobestronglydust-depleted. Ontheotherhand, itisplausiblethattheCO emissionarisesfromthedustyinnerdiskintheotherthreeobjects, knowntobePTDs. TotheseobjectsweshouldaddRX J1842.9where theCO line isalsodetected, butnoanalysishasyetbeencarriedouttodeterminethedistancetothestaroftheregionemittingthisline. ForLkHα330andSR21theemissionoftheCO linearisesfromlargerradii(RCO ≥ 4AU) duetothehighertemperatureofthediskrelatedtothelarger L⋆ oftheseobjectscomparedtotherestofthesample. Finally, in13ofthe20accretingTDswecouldfindadetectionofthe[NeII] intheliterature. Thislineisalsooriginatedinawindcomingfromagasrichregionofthediskinsideadistancefromthecentralstarof ∼20-40AU.Inonly9ofthe20accretingTDswefoundevidenceofinfraredexcess, asignatureofthepresenceofadustyinnerdisk.

ThepictureoftheseaccretingTDscomingfromouranalysisisthenthefollowing. Theseareobjectswithagasrichdiskwellwithintheobserved Rin, i.e. attheinnerdiskedge.Giventhatwedonotfindanycorrelationbetweenthedust-depletedholeandtheaccretionorwindproperties, andthatweseebothaccretingTDswithdustyinnerdisksandwithout,themodelneededtoexplaintheformationofthedust-depletedinnerregionshouldleavealmostunalteredthegaspropertiesoftheinnermostregion. FromthepointofviewofthegascontentoftheinnerdiskthereisnoobservabledifferencebetweenaccretingTDsandcTTs.

Non-accretingtransitionaldisks

Thetwoobjectswediscusshere(Ser29andCHXR22E) haveagas-depletedinnerregionofthedisk. Thenon-detectionofaccretionsignaturesintheseobjectsortheverylowdetectedMacc ofCHXR22E implythatthedensityofthegasinthisregionofthediskissmallerintheseobjectsbyatleastoneorderofmagnitudethaninanyotheraccretingobject. Thisis

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5.6 Conclusions

clearlyseeninthevalueof Σ1AU =0.1gcm−2 reportedforCHXR22E inTable 5.8, smallerbyafactor ∼ 30-40thantheonecomputedforOph22andSer34, thathavesimilarstellarproperties. Withtheadditionofthenon-detectionofthe[OI] λ 630nminthespectraofbothobjectsweconcludethattheregionaround∼ 0.1-0.3AU issignificantlygas-depletedinthesenon-accretingTDs. NofurtherinformationonthegascontentoftheinnerdiskofSer29andCHXR22E areavailable. Thesetwotargetsshouldbeobservedinthefuturewiththeaimofdetecting[NeII] and/orCO emissionintheinnerpartsoftheseobjects, inordertoconstrainttheinnerboundaryofthegas-richdisk.

5.6 Conclusions

Inthisworkweanalyzedasampleof22X-ShooterspectraofTDs. Thissamplecomprisesobjectswithdifferentouterdiskmorphologies, inparticularwithvaluesofRin rangingfrom∼1AU to ∼ 70AU,andincludesmainlyTDswithpreviousaccretionrateestimates. ThissamplecannotprovideaconclusivestatisticalresultonthegeneralpropertiesoftheTDclass, butitisagoodbenchmarktostudywithanhighlyreliablemethodtheseobjects.Weusedamulti-componentfittingmethodtoderivesimultaneouslytheSpT, AV , and Lacc

oftheobjectsfittingourbroad-bandspectra. Atthesametimewederivedfromthesamespectratheintensityofthe[OI]λ 630nmline. Fromtheanalysisoftheresultswederivedthefollowingconclusions:

-ThedependenceoftheaccretionpropertiesofoursampleofstronglyaccretingTDswiththesizeofthedust-depletedcavity(Rin)issmall, inparticularthereisnoevidenceforincreasing Macc with Rin atvaluesof Rin≳ 20− 30 AU;

-TherearestronglyaccretingTDs, likethemajorityoftheobjectsinoursample, whoseaccretionpropertiesareconsistentwiththosereportedintheliteratureforcTTs;

-ThewindpropertiesoftheTDsanalyzedherehavenodependencewiththesizeofthedust-depletedcavity(Rin)andareconsistentwiththewindpropertiesofcTTs;

-Therearenodifferencesintheaccretionandwindpropertiesbetweentheobjectsinoursamplewithinnerdiskemission(PTD) orwithout(TD);

-StronglyaccretingTDssuchasthoseanalyzedherearegas-richdowntodistancesfromthecentralstarassmallas ∼0.03-0.3AU ascanbeobtainedfromthederivationofthevaluesof Rm, fromthedetectionofthe[OI] λ 630nmline, andfromthedetectionoftheCO and[NeII] lines. Thisdistanceisalwayssmallerthanthevaluesof Rin reportedintheliteraturefortheseobjects, meaningthatthereisagaseousinnerdiskmuchclosertothestarthanthedustyone;

-Non-accretingTDshavegasdepletedinnerdisks. Thegaseousdiskissignificantlydepletedofgasatadistancefromthestarofatleast ∼0.03-0.3AU.Alsofortheseobjectstheinnerextentofgasanddustinthediskareuncoupled;

-TheprocessneededtoexplaintheformationofTDsshouldactdifferentlyonthegasandthedustcomponentsofthedisk.

FuturestudiesaimedatunderstandingtheprocessresponsiblefortheformationofthedustdepletedcavityinTDsshouldaimat:

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-conductingasimilaranalysisonalargerandmorecompletesampleofTDs, includingalargeramountofobjectsknowntoaccreteatlowerratesthanthoseincludedinthiswork;

-determiningtheprocessresponsiblefortheformationoftheforbiddenlines-pho-toevaporation, diskwind, orotherpossibilities-fromtheanalysisofhigh-resolutionandhigh-signaltonoiseforbiddenlinesandthecomparisonwiththeoreticalmodelscoveringmorecompletelytheparameterspaceofstellarproperties;

-determiningtheextentoftheregionemittingthe[OI] and[NeII] lineandthedensityofgasinthisregioninordertoputstrongerconstraintonthedistancefromthecentralstaratwhichgasispresent;

-studyingtheCO linewithhigh-resolutionspectroscopyinnon-accretingTDstoverifythedecouplingofthegas-richanddust-richdiskintheseobjects.

Aknowledgements Wethank theESO staff inParanal forcarryingout theobservations inServicemode.WethankJ.Alcalaandthe``JEtsandDiskatInaf''(JEDI) teamforprovidingthereducedspectrumofSz84.WethankA.CarmonaforsharingtheinformationontheCO linedetection. C.F.M.acknowledgesthePhDfellowshipoftheInternationalMax-Planck-ResearchSchool.

5.A Commentsonindividualobjects

5.A.1 Sampleproperties

LkCa15: thisobjecthasbeenresolvedwith880 µminterferometricobservationsby An-drewsetal. (2011), anditscavitywaspreviouslyresolvedat1.3mmby Piétuetal. (2006).Themodellingofthistargetcarriedoutby Andrewsetal. (2011)hasadiscrepancywiththeobserved880 µmfluxinsidethecavityprobablyduetodustemission.SzCha: weobservedonlytheprimarycomponentofthiswidebinarysystemwithsep-aration5.122′′. Thecompanionof thisobject isnotaconfirmedmemberof theCha Iassociation(Luhman, 2008).CS Cha: Guentheretal. (2007)classifiedthisobjectsasaspectroscopicbinarywithape-riodofmorethan2482days. Theminimummassofthecompanionis0.1 M⊙.Sz84: ItisunderdebatewhetherthisobjectshouldbeclassifiedasTD. Merínetal. (2010),whoclassifieditasaTD infirstplace, derivedRin =55±5AU,buttheypointedoutthattheclassificationwasratheruncertainduetopossibleextendedemissioncontamination. AlsoMatràetal. (2012)suggestthatthisclassificationisdubious. Theypointoutthatthisobjecthasno10 µmsilicatefeatureinthespectrum, whichisatypicalfeatureinthespectraofTDs. Then, theyreportthatithasaSED verysimilartotheoneofT54, whichtheyproposeisnotaTD duetoextendedemissioninthe Herschel images.RX J1615-3255: differentdistancesforthisobjectarereportedintheliterature. Merínetal. (2010)considerthisobjecttobelocatedinthe ρ-Ophiucicloud, thusat d=125pc. An-drewsetal. (2011), instead, adoptadistancetothisobjectof185pcbecausetheyassumeitislocatedintheLupuscloud. Thislocationisthenadoptedalsoby Saccoetal. (2012),buttheyuseadistanceof150pcfortheobject. Wedecidetoadoptthedistanceof185pc

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5.B Additional literature data

usedby Andrewsetal. (2011)forconsistencywiththevaluesof Rin derivedinthatwork.Wethencorrectthevalueof Rin derivedin Merínetal. (2010)tothedistanceadoptedhere. Thisisthevalueof Rin,SED reportedinTable 5.9.SR21: thisobject isknowntobeawidebinarywithaseparationof ∼ 6.4′′∼ 770AU.Weobservedonlytheprimarycomponentofthesystem, whichistheoneobservedbyAndrewsetal. (2011). Regardingthedustemissionofthisobject, Andrewsetal. (2011)reportapoormatchingof theobservationswhichmay indicate thata smallamountof∼mm-sizeddustparticlesispresentinthecavity.ISO-Oph196: theinnerdustdepletedcavityhasbeenbarelyresolvedwithSMA by An-drewsetal. (2011). LookingattheSED ofthisobjectthereisnosignatureofdustdepletion,i.e. thereisnodipintheMIR SED.Thissuggeststhatthedustisnotstronglydepletedintheinnerdiskofthisobject.

5.B Additionalliteraturedata

WereportinTable 5.9 additionaldatacollectedintheliteratureonourtargets. Thesedatahavenotbeenusedinthisworkbutareusefulforfurtheranalysis. ThespectraltypeandMacc reportedherehavenotbeenusedinouranalysis.

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Table 5.9: Stellaranddiskparametersavailableintheliterature

Name Spectral Macc Rgap,in,SED Rin,SED Rin,mm i logLX Disk Reftype [10−8 M⊙/yr] [AU] [AU] [AU] [] [erg/s] Type

LkHα330 G3 0.20 0.8 50 68 35 ... ... 1,12,22,26DM Tau M1 0.60 ... 3 19 35 30.30 TD 1,10,20,21,28LkCa15 K3 0.30 4 48 50 49 <29.6 PTD 1,15,16,21,24GM Aur K5 1.00 1 24 28 55 30.20 TD 6,9,10,20,21,28SZ Cha K0 0.24 ... 29 ... ... 29.90 PTD 16,17,21,25TW Hya K7 0.20 ... 4 4 4 30.32 ... 10,14,23,27CS Cha K6 0.53 ... 43 ... 45 30.56 TD 10,16,24,25,30CHXR22E M3.5 ... ... 7 ... ... 29.41 TD 16,19Sz18 M3 1.5e-10 ... 8 ... ... ... TD 16,19Sz27 M0 1.2e-9 ... 15 ... ... 29.76a PTD 16,19Sz45 M0.5 7.6e-10 ... 18 ... ... ... WTD 16,19Sz84 M5.5 1.00 ... 55 ... ... ... ... 8RX J1615 K5 0.04 ... 3b 30 41 30.40 ... 1,2,8Oph22 M2 ... ... 1 ... ... ... ... 8Oph24 M0.5 1.00 ... 3 ... ... ... ... 8SR 21 G3 <0.1 0.45 18 36 22 30.00 ... 1,3,4,5,18,26ISO-Oph196 M4 0.20 ... ... 15 28 ... ... 1,18DoAr44 K3 0.90 ... 27 30 35 29.9 PTD 1,13,29Ser29 M0 30.00 ... 8 ... ... ... ... 8Ser34 M0 0.25 ... 25 ... ... ... ... 8RX J1842.9 K2 0.10 ... 5 ... ... 30.34 ... 6,7,19RX J1852.3 K3 0.05 ... 16 ... ... 30.41 ... 6,7,19

References. (1) Andrewsetal. (2011), (2) Krautteretal. (1997), (3) Brownetal. (2009), (4) Andrewsetal. (2009), (5) Grossoetal. (2000), (6) Hughesetal. (2010), (7) Neuhäuseretal. (2000), (8) Merínetal. (2010), (9) Hughesetal. (2008), (10) Güdeletal.(2010), (11) Königetal. (2001), (12) Brownetal. (2008), (13) Montmerleetal. (1983), (14) Hughesetal. (2007), (15) Neuhaeuseretal. (1995), (16) Kimetal. (2009), (17) Whiteetal. (2000), (18) Nattaetal. (2006), (19) Pascuccietal. (2007), (20) Inglebyetal. (2009),(21) Espaillatetal. (2010), (22) Salyketal. (2009), (23) Herczeg&Hillenbrand (2008), (24) Inglebyetal. (2013), (25) Espaillatetal.(2013), (26) Brownetal. (2007), (27) Calvetetal. (2002), (28) Calvetetal. (2005), (29) Kimetal. (2013), (30) Pascucci&Sterzik (2009)----- Rgap,in,SED istheinnerradiusofthegapinPTDsobtainedfromMIR SED fitting, whereas Rin istheinnerradiusofthedustyouterdisk, i.e. theouterradiusofthegapinPTDsandoftheholeinTDs. a Highlyuncertainparameter. b ValuecorrectedforthedifferentdistanceasexplainedinAppendix 5.A.1.

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6Accretionasafunctionofstellarproperties

innearbystarformingregions

TobeincludedinManara, Testietal., tobesubmittedtoA&A∗

6.1 Introduction

Toaddress thequestionsraisedinSection 1.4.2 I collectherea largesampleof88ac-cretingYSOsinvariousstarformingregions(ρ-Ophiucus, ONC,Lupus, σ-Orionis, UpperScorpius, andothers). Thisdatasetcoversarangeofstellarmassesofmorethan2dex, andnominalageoftheregionsfrom ∼1to ∼10Myr. ThebasicdataandreferenceforeachdatasetincludedinthisChapterarereportedinTable 6.1. Allthetargetshavebeenob-servedwiththeVLT/X-Shooterspectrographandanalyzed, whenpossible, withthemethoddescribedinChapter 4 orwithsimilarproceduresdescribedinthefollowing. Therangeofstellarpropertiescovered, theuseofthesameinstrument, andofthesameanalysismethodmakethissampleauniquebenchmarktostudythepropertiesofaccretioninYSOs.

A significantfractionofthedataanalyzedherehavebeenobtainedduringguaranteedtimeobservations(GTO) oftheINAF collaboration(e.g., Alcaláetal., 2011). Otherdatahavebeencollectedinindependentprograms, suchasthe ρ-Ophiucusdata(ESO Pr.Id.085.C-0876, PI Testi), theUpperScorpiusdata(ESO Pr.Id. 085.C-0482, PI Montesinos),andthedatadiscussedinChapters 4 and 5.

InthisChapterI willmainlyreportontheanalysisofeachdatasetandonthederivationofthemassaccretionratesforthetargetedClass II YSOs. Thiswillallowmetodiscussthedependenceofaccretionwithmassofthecentralobject, withage, andwiththeluminosityofthecentralstarinSect. 6.6. I refertothepublishedpapers(e.g., Rigliacoetal., 2012;Alcaláetal., 2014; Manaraetal., inprep.) formoredetailsontheobservationsandthedatareductionofeachdatasetandonmoregeneraldiscussiononthepropertiesof the

∗Thischapterincludesworkdoneincollaborationwithotherscientistsandalsoworkinterelycarriedoutbyme. When including theadapted textofapublishedpaper I willclarifymycontribution to thatspecificwork. Withregardstothe ρ-OphiucusandtheUpperScorpiusdatasets, I havecarriedoutallthedatareductionandanalysisworkthatisreportedhere. Finally, thediscussionandanalysisofthepropertiesofthefulldatasetisreportedforthefirsttimeinthisThesis.

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6. Accretion as a function of stellar properties in nearby star forming regions

targets. Inanycase, foralltheobjectsdiscussedherethereductionhasbeencarriedoutusingthesameproceduresasinChapters 3, 4, and 5, whiletheobservingstrategieshavealwaysbeensimilar, withsmalldifferences, suchasdifferentslitwidthsorexposuretimes,mostlyduetothebrightnessofthetargets.

ThestructureofthisChapterisasfollows. InSect. 6.2 I reportontheanalysisoftheaccretionpropertiesof36Class II YSOsintheLupusclouds, andinSect. 6.3 thoseof8Class II YSOsinthe σ-Oricluster. Then, inSect. 6.4 I willshowforthefirsttimetheanalysisandresultsontheaccretionpropertiesof17accretingYSOsinthe ρ-Ophiucuscloud, andinSect. 6.5 for3Class II YSOsintheUpperScorpiusassociation. Finally, inSect. 6.6 IcollecttogetherthesedataandthoseanalyzedinChapters 4 and 5 toanalyzetheaccretionpropertiesofthewholesampleofthisThesis.

Table 6.1: Dataincludedinthisstudy

Region dist Age ClassII Reference[pc] [Myr] [#]

ρ-Ophiucus 125 1 17 Manaraetal. (inprep.)OrionNebulaCluster 420 2 2 Manaraetal. (2013b)Lupus 150-200 2-3 36 Alcaláetal. (2014)σ-Orionis 360 3-5 8 Rigliacoetal. (2012)UpperSco 145 ≳5 3 Montesinosetal. (inprep.)Transitionaldisks various ∼1-10 22 Manaraetal. (2014a)TOTAL - 1-10 88 Manaraetal. (inprep.)

6.2 AccretionintheLupusclouds

BasedonAlcalà, Natta, Manaraetal. 2014, A&A,561, A2†

TheLupuscloudcomplex (RA 15h to16handDEC −43d to −33d) isoneof the low-mass star-forming regions locatedclosest (d <200 pc) to the Sun (see Comerón, 2008,forareview). Similarlytootherregions(e.g., Taurus, Chamaeleon, and ρ-Ophiucus), alargevarietyofobjectsinvariousstagesofevolutionarepresentinthisregion. Thestellarpopulationof thiscomplexiscomposedof ∼250YSOs (e.g., Evansetal., 2009)andisdominatedbymidM-typePMS objects, whileseveralsubstellarobjectshavealsobeendiscoveredintheregion(Comerón, 2008; LópezMartíetal., 2005). Withanageof ∼2-4Myr, Lupusliesbetweentheepochwhenmoststarsarestillembeddedintheparentalcloudorsurroundedbyopticallythickdisks(Class I/II,1-2Myr)andtheepochwherestarsarenotsurroundedbyanopticallythickdisk(ClassIII, ∼10Myr). TheYSO population

†InthisworkI havepersonallycarriedoutalltheanalysisoftheUV-excessofthetargetsandthederivationoftheaccretionrates. I havealsocontributedsignificantlytothetextofthemanuscript, bothintheanalysisandinthediscussionsections. HereI reportonlythepartsofthisworkmorerelevantforthisThesismodifyingsubstantiallythetextoftheoriginalpaper.

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6.2 Accretion in the Lupus clouds

inthiscomplexismainlydistributedinfiveclouds(LupusI,III,IV,V andVI) whicharesubstantiallydifferent fromeachother inthestellarcontent. Inparticular, theLupus IIIcloudcontains the largestandmostconcentratedYSO population, followedbyLupus IwithafilamentarystructureandbythesmallerLupus IV,whereaveryhighdensitycorewithlittlestarformationactivityhasbeenfound. InthesethreecloudsthelargemajorityoftheobjectsarefoundtobeClass II (∼50%)orClass III (28%)YSOs, withonly12%ofthetargetsbeingClass I sources(Merínetal., 2008). Theremainingtwoclouds, Lupus V andVI,showasignificantlydifferentpicturetothepreviouslymentionedones, withthelargemajorityoftheYSOsbeingsurroundedbyopticallythindisks(Class III;79%inLupus Vand87%inLupus VI, Spezzietal. 2011). Thereasonforthisdifferencecouldbeanagegradientintheregion, butfurtherstudiesthatgobeyondtheaimofthisworkareneededtounderstandthis. InthissectionI reportontheworkcarriedoutonalargefraction(∼50%)oftheYSO populationoftheLupus I andIII cloudstoderivestellarparametersandaccretionratesfortheseobjects.

6.2.1 Sample

Thesampleanalyzedherecomprises36YSOsmainlylocatedintheLupus I andIII cloudsthatsatisfythefollowingcriteriaascloselyaspossible: i)YSOswithinfraredclass II char-acteristicsandlowextinctiontotakefulladvantageofthebroadX-ShooterspectralrangefromUV tonear-infrared (NIR); ii) targetsextensivelysurveyed inasmanyphotometricbandsaspossible: mainlySpitzer(IRAC &MIPS) surveysandcomplementaryWide-fieldInfraredSurveyExplorer(WISE; Wrightetal., 2010)data, aswellasopticalphotometryavailable; andiii)mostlyverylowmass(VLM, M⋆ < 0.3M⊙)objects, butalsoanumberofmoremassive(M⋆ ≲ 1M⊙)starstoexploreawiderrangeofaccretionluminosity.

Thetwomainbibliographicsourcesfromwhichthissamplewascompiledare Allenetal. (2007)and Merínetal. (2008). TheformerreportedseveralnewVLM YSOswithSpitzercolors, whilethelatterprovidedaclearlycharacterisedsampleintermsofspectralenergydistribution(SED) andSED spectralindex, basedontheSpitzerc2dcriteria(Evansetal.,2009). Additionalclass II YSOsinLupus III thatextendthesampletoabroadermassrangeandeventually tohigheraccretionluminositywereselectedfrom Hughesetal. (1994),followingcriteriai)andii)aboveascloselyaspossible, althoughseveralofthesetargetsdonotpossessSpitzerfluxesbecausetheywerenotcoveredbythec2dorotherSpitzersurveys. Thus, theavailableWISE datawereused.

Amongtheselectedobjects, therearetwovisualbinaries, namelySz 88andSz 123,wherebothcomponentswereobserved. InanothereightoftheLupusYSOstheSpitzerimagesrevealedobjectswithseparationsbetween2.′′0and10.′′0(seeTable 6.2). Amongthese, sixhaveseparationslargerthan4.′′0(seeTable 1inMerínetal. 2008andTable 9inComerón2008). ThespatialresolutionofX-Shooterissufficientlyhigh, allowingtheobservationofallthetargetswithoutlightpollutionfromanyofthosenearbyobjects. Noneofthesetargetshasbeenreportedasaspectroscopicbinaryinpreviousinvestigationsusinghigh-resolutionspectroscopy(e.g. Melo, 2003; Guentheretal., 2007). Thesamplealso

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G0 G5 K0 K5 M0 M5 L0SpT

0

2

4

6

8

10

12

14

16

18

Num

ber

All ClassIIThis work

Figure 6.1: HistogramcomparingthenumberofobjectsintheLupusI andIII cloudsanalyzedherewiththetotalClass II YSO populationoftheregion. ThehistogramofSpT oftheobjectsanalyzedhereisshowningreen, whilethetotalpopulationofClass II YSOsfrom Merínetal. (2008)and Mortieretal. (2011)isshownwithabluehistogram.

includesPar-Lup3-4, oneof the lowest-massYSOs in Lupus known tohost anoutflow(Comerónetal., 2003).

This samplecoversamass rangebetween ∼0.05M⊙ and ∼1M⊙ ratherwell (40%with M⋆ < 0.2M⊙, 35%with M⋆ intherange0.2-0.5M⊙, and25%with M⋆ >0.5M⊙).Nevertheless, thisisincompleteateachmassbin. However, itrepresentsabout50%ofthetotalpopulationofClass II YSOsintheLupus I andLupus III clouds(seeFig. 6.1), beingthusarepresentativesampletostudytheaccretionpropertiesofthisregion.

Table 6.2 providesthelistofthetargets. Theobservationsanddatareductionprocedureareexplainedindetailin Alcaláetal. (2014).

6.2.2 Stellarandsubstellarproperties

A firstestimateoftheSpT oftheLupustargetswasderivedusingvariousspectralindicesfrom Riddicketal. (2007) foropticalwavelengths (thedetailsof these indicesarealsoreportedinTable 3.5), andtheH2O-K2indexfrom Rojas-Ayalaetal. (2012)fortheNIRspectra (seeTable 3.9). The Riddicketal. (2007) spectral indices in theVIS arealmostindependentofextinction1. TheSpT assignedtoagivenobjectwasestimatedastheaverageSpT resultingfromthevariousindicesintheVIS.Fromthedispersionovertheaverageofalltheavailablespectralindicesweobtainanuncertaintyofhalfaspectralsubclass.

TheNIR indicesprovideSpTsthatareconsistentwiththeVIS resultstypicallywithinonespectralsubclass. Therefore, theSpTsderivedfromtheVIS wereadoptedfortheana-

1Notethatthespectralindicesmaybeaffectedbyhighextinction(AV >5mag). NoneoftheLupustargetshassuchhigh AV.

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Table 6.2: SelectedYSOsintheLupusregion

Object/othername RA(2000) DEC(2000) Lupush :m :s ' '' cloud

Sz66† 15:39:28.28 −34:46:18.0 IAKC2006-19 15:44:57.90 −34:23:39.5 ISz69/HW Lup† 15:45:17.42 −34:18:28.5 ISz71/GW Lup 15:46:44.73 −34:30:35.5 ISz72/HM Lup 15:47:50.63 −35:28:35.4 ISz73 15:47:56.94 −35:14:34.8 ISz74/HN Lup 15:48:05.23 −35:15:52.8 ISz83/RU Lup 15:56:42.31 −37:49:15.5 ISz84 15:58:02.53 −37:36:02.7 ISz130 16:00:31.05 −41:43:37.2 IVSz88A (SW) /HO Lup(SW) 16:07:00.54 −39:02:19.3 ISz88B (NE) /HO Lup(NE) 16:07:00.62 −39:02:18.1 IIISz91 16:07:11.61 −39:03:47.1 IIILup713† 16:07:37.72 −39:21:38.8 IIILup604s 16:08:00.20 −39:02:59.7 IIISz97 16:08:21.79 −39:04:21.5 IIISz99 16:08:24.04 −39:05:49.4 IIISz100† 16:08:25.76 −39:06:01.1 IIISz103 16:08:30.26 −39:06:11.1 IIISz104 16:08:30.81 −39:05:48.8 IIILup706 16:08:37.30 −39:23:10.8 IIISz106 16:08:39.76 −39:06:25.3 IIIPar-Lup3-3 16:08:49.40 −39:05:39.3 IIIPar-Lup3-4† 16:08:51.43 −39:05:30.4 IIISz110/V1193Sco 16:08:51.57 −39:03:17.7 IIISz111/Hen3-1145 16:08:54.69 −39:37:43.1 IIISz112 16:08:55.52 −39:02:33.9 IIISz113 16:08:57.80 −39:02:22.7 III2MASS J16085953-3856275† 16:08:59.53 −38:56:27.6 IIISSTc2d160901.4-392512 16:09:01.40 −39:25:11.9 IIISz114/V908Sco 16:09:01.84 −39:05:12.5 IIISz115 16:09:06.21 −39:08:51.8 IIILup818s† 16:09:56.29 −38:59:51.7 IIISz123A (S) 16:10:51.34 −38:53:14.6 IIISz123B (N) 16:10:51.31 −38:53:12.8 IIISST-Lup3-1† 16:11:59.81 −38:23:38.5 III

Notes. † : nearby(2.′′0 < d < 10.′′0)objectdetectedinSpitzerimages(see Merínetal., 2008; Comerón,2008).

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lysis, consistentlywiththeSpT assignmentfortheclass III templates(Manaraetal., 2013a,andChapter 3). TheSpTsare listed inTable 6.3. For the twoearliest-typestars in thissample(Sz73andSz83), anaccurateSpT ofK7isreportedintheliterature(see Herczeg&Hillenbrand, 2008; Comerón, 2008). AlltheseestimatesarecheckedusingthefittingproceduresofChapter 4 asdescribedinthefollowing.

SomedifferenceindeterminingtheSpT intheliteraturecanbeascribedtothedifferentspectralrangesusedinthedifferentinvestigations(Comerónetal., 2003; Hughesetal.,1994; Mortieretal., 2011). WiththewidespectralrangeofX-Shooterandtheaccuratefittingprocedureadoptedthisproblemisovercome. Generally, theSpT derivedhereareconsistentwithin ±0.5subclasswith those in the literature, witha fewexceptions thatarediscussed in the following. TheSpTsof thissample range fromK7 toM8, withanoverabundanceofM4-M5objects.

ToderivethefirstestimateofAV foragivenClass II YSO,itsVIS spectrumwascomparedwiththeClass III templates(Manaraetal., 2013a, andChapter 3)thatbestmatchtheClass IISpT.AlltheClass III templateshavealowextinction(AV < 0.5). Thetemplateswerethenartificiallyreddenedby AV =0...4.0maginstepsof0.25mag, untilthebestmatchtotheClass II YSO was found. Theproceduresimultaneouslyprovidedanadditional test forthecorrectassignmentofthetemplatetoderivetheaccretionluminosity(seeSect. 6.2.3).The AV valuesderivedin thiswayare listed inTable 6.3. Thisprocedureconfirmsthatthemajorityof the targetspossesszeroextinctionbecause theywereselectedwith thiscriterion. Thehighestvalues, 2.2magand3.5mag, arefoundforPar-Lup3-3andSz73,respectively.

The combined effect of uncertainties in SpT and AV leads generally to an error of<0.5mag. However, anotherimportantsourceofuncertaintymaybeintroducedbystrongveiling, whichmakestheYSOsspectraintrinsicallybluerthanthetemplates. Thishasnotstrongimpactsontheestimatesofstellarparametersforlow-accretinglate-typeYSOslikethoseanalyzedhere. Forearlier-typestars(<K7)withmuchhigherlevelsofveilingthanthoseinthissample, thefittingmethodexplainedinChapter 4 mustbeusedtoderiveSpT,extinction, andaccretionpropertiessimultaneously. Here thisfittingmethodisused toverifyandvalidatetheestimatesderivedasjustexplained.

Tochecktheself-consistencyof theextinctionderivedinanotherspectral range thesameprocedurewas repeatedon theNIR spectra. The result is that the AV valuesareconsistentwithinthe0.5maguncertainty, butareaffectedbyalargererror(∼0.75mag).ThelatterismainlyduetothehigheruncertaintyintheSpT providedbythespectralindicesintheNIR thanintheVIS.ThereforethevaluesderivedfromtheVIS areadoptedforthefollowinganalysis. AnotherobviousreasonforpreferringtheextinctionintheVIS isthattheextinctionintheNIR islowandoneneedstomultiplyit(anditsuncertainty)byalargefactortoderive AV.

TheSpT andextinctiondeterminationsreportedhereagreewellwiththeliteratureval-uesexceptforafewcases. ForSz 69, Hughesetal. (1994)reportedaSpT M1withavisualextinctionof3.20mag, whileitisshownherethattheM4templatewithzeroextinctionfitstheentireX-Shooterspectrummuchbetter. ForSz 110, Hughesetal. (1994)reportedM2, while Mortieretal. (2011)claimedM3, moreconsistentwiththeM4determination

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reportedhere. InthecaseofSz 113, theM4SpT reportedby Hughesetal. (1994)agreeswith theM4.5determinationof thisstudy, while Mortieretal. (2011)reportedM1andComerónetal. (2003)M6. Thevisualextinctionvalues in the literature forPar-Lup3-4rangefrom2.4to5.6mag(Comerónetal., 2003). Theconfirmedunder-luminosityandedge-ongeometryofthisobject(Comerónetal., 2003; Huélamoetal., 2010)suggestthatthezeroextinctionadoptedherecanbeinterpretedaswavelength-independent, thatis,grayextinction, ratherthanasanullextinction(seealso Whelanetal., 2014). Interest-ingly, anullextinctionisconsistentwiththevaluederivedoff-sourceusingthe[Fe II] linesat1.27, 1.32, and1.64µm(Bacciottietal., 2011; Gianninietal., 2013), whichtracejetemission(Nisinietal., 2005).

Ontheotherhand, it isworthmentioningthat thezeroextinctionderivedforSz 83agreeswiththevaluederivedby Herczegetal. (2005)fromtheprofileoftheLyα line. ThefittingprocedureasinChapter 4 alsoconfirmstheSpT ofthisYSO,despiteitsstrongveiling(seeSection 6.2.3). The AV determinationforSz 113, themostveiledamongtheM-typeYSOsinthissample, alsoagreeswiththatreportedby Hughesetal. (1994).

Theeffectivetemperature, Teff, wasderivedusingthetemperaturescalesgivenby Kenyon&Hartmann (1995)forthetwoK-typestars, andby Luhmanetal. (2003)fortheM-typeYSOs. Thelatterscaleisintermediatebetweenthedwarfandgianttemperaturescales, andmoreappropriateforyoungobjectsthantemperaturescalesderivedforfieldM-dwarfs(e.g.Testi, 2009). Thestellarluminosityandradiuswerecomputedusingthemethodsdescribedin Spezzietal. (2008), adoptingtheextinctionanddistancevaluesgiveninTable 6.3. Thestellarradiuswasalsodeterminedusingtheflux-calibratedX-Shooterspectraasexplainedin Alcaláetal. (2011). Thegoodagreementbetweentheradiuscalculatedwiththetwomethods(cf. Figure 5in Alcaláetal., 2011)alsoconfirmsthereliabilityofthefluxca-librationofthespectra. MassandagewerederivedbycomparisonwiththeoreticalPMSevolutionarytracks(Baraffeetal., 1998)ontheHRD.ThephysicalparametersofthetargetsarelistedinTable 6.3. Uncertaintiesinluminosity, radius, andmasstakeintoaccounttheerrorpropagationofabouthalfaspectralsubclassinspectraltyping, aswellaserrorsinthephotometryanduncertaintyonextinction.

Theluminosityof fourobjects, namelyPar-Lup3-4, Lup706, Sz 123B,andSz 106, issignificantly lowerthanfor theotherYSOsofsimilarSpT ormass, hencetheiragesareapparentlyolderthan15Myr. ThemuchlowerluminosityoftheseobjectsascomparedtotheothersisevidentinFigure 6.2, wheretheHRD forthesampleisshown. Itisnotentirelyclearwhether therelatively lowluminosityof theseobjects isdue toevolutionor toobscurationeffectsbecauseofaparticulardiscgeometry. Sz 106andPar-Lup3-4havebeenreported tobesubluminous (Comerónetal., 2003), and for the latter ithasbeenshownthatthediscisedge-on(Huélamoetal., 2010). Noevidenceofsignificantlylowluminosityfortheothertwoobjectsisfoundintheliterature. I referto Alcaláetal.(2014)forargumentssuggestingthatthelowluminosityoftheseobjectsismostlikelyduetogeometricaleffects.

Theaverageageof3±2MyrfortheLupussample, excludingthesubluminousobjects,isconsistentwithpreviousageestimatesforLupusYSOs(Comerón, 2008, andreferencestherein). Finally, theLi I λ670.78nmabsorptionlineisdetectedinallthespectraanalyzed

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3.403.453.503.553.60logTeff [K]

3.0

2.5

2.0

1.5

1.0

0.5

0.0

0.5

log(L

/L¯)

0.02 M¯

0.05 M¯

0.10 M¯

0.2 M¯

0.4 M¯

0.6 M¯

0.8 M¯

1.0 M¯

1.2 M¯

Lup706

Sz106

Par-Lup3-4

Sz123B

Figure 6.2: Hertzsprung-RusselldiagramfortheLupussample. Thefoursubluminousobjectsdescribedinthetextarerepresentedwithopensymbolsandlabelled. Thecontinuouslinesshowthe1Myr, 3Myr, 10Myr,30Myr, and100Myrisochrones, reportedby Baraffeetal. (1998), whilethedashedlinesshowthelow-masspre-mainsequenceevolutionarytracksbythesameauthorsaslabelled. Adaptedfrom Alcaláetal. (2014).

here.

6.2.3 Accretionratedetermination

TheprocedureexplainedinChapter 4 isusedtodetermine Lacc fromthecontinuumexcessluminosityandtoconfirmandvalidatetheSpT and AV derivedasdescribedinSect. 6.2.2.BeingtheLupusobjectsanalyzedherenotstronglyveiled, low-extincted, andmostlyM-type, theresultsobtainedasexplainedintheprevioussectionarereliableandconfirmedbythefittingprocedureofChapter 4. ExamplesofthebestfitoftheBalmerjumpregionfortheLupustargetsareshowninFigure 6.3, whilethecompletesetofplotsshowingthebestfitsforalltargetscanbefoundin Alcaláetal. (2014).

TheadoptedClass III templatesand thederived Lacc valuescorresponding toeachClass II YSO are reported inTable 6.4. Theuncertaintieson Lacc aredominatedbytheuncertainty in theextinctionandby thechoiceof theClass III template (Manaraetal.,2013b). Anadditional, non-negligibleuncertaintyforlowvaluesof Lacc comesfromtheuncertaintyonthePaschencontinuumexcessemission, especiallyinobjectswithapoorsignal-to-noiseratio(e.g. Rigliacoetal., 2012). Ingeneral, theuncertaintyon Lacc isesti-matedtobe ∼0.2 dex. TheBalmerandPaschencontinuaaswellastheBalmerjumparewellreproducedbythefits. Onlyinonecase, namelySz123B,theslopeoftheBalmercontinuumisnotexactlyreproducedbyanycombinationofthefreeparameters. Thisisnotcausedbydatareductionproblems(e.g. flat-fieldcorrectionorincorrectspectrumextrac-tion), butperhapstoslit-losseffects. Smalldifferencesbetweentheobservedandbest-fitspectrainthePaschencontinuumarepresentinsomeobjects(e.g. AKC2006-19, Sz115),

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Table 6.3: Spectraltypes, extinction, andphysicalparametersoftheLupusClass II YSOs

Object SpT Teff AV d L⋆ R⋆ M⋆ Age[K] [mag] [pc] [L⊙] [R⊙] [M⊙] [Myr]

Sz66 M3.0 3415 1.00 150 0.200±0.092 1.29±0.30 0.45+0.05−0.15 4

AKC2006-19 M5.0 3125 0.00 150 0.016±0.008 0.44±0.10 0.10+0.03−0.02 13

Sz69 M4.5 3197 0.00 150 0.088±0.041 0.97±0.22 0.20+0.00−0.03 3

Sz71 M1.5 3632 0.50 150 0.309±0.142 1.43±0.33 0.62+0.02−0.17 4

Sz72 M2.0 3560 0.75 150 0.252±0.116 1.29±0.30 0.45+0.12−0.00 3

Sz73 K7 4060 3.50 150 0.419±0.193 1.35±0.31 1.00+0.00−0.00 9

Sz74 M3.5 3342 1.50 150 1.043±0.480 3.13±0.72 0.50+0.10−0.10 1

Sz83 K7 4060 0.00 150 1.313±0.605 2.39±0.55 1.15+0.25−0.05 2

Sz84 M5.0 3125 0.00 150 0.122±0.056 1.21±0.28 0.17+0.08−0.02 1

Sz130 M2.0 3560 0.00 150 0.160±0.074 1.03±0.24 0.45+0.05−0.00 6

Sz88A (SW) M0 3850 0.25 200 0.488±0.225 1.61±0.37 0.85+0.10−0.10 4

Sz88B (NE) M4.5 3197 0.00 200 0.118±0.054 1.12±0.26 0.20+0.05−0.03 2

Sz91 M1 3705 1.20 200 0.311±0.143 1.36±0.31 0.62+0.13−0.08 4

Lup713 M5.5 3057 0.00 200 0.020±0.009 0.52±0.12 0.08+0.05−0.00 4

Lup604s M5.5 3057 0.00 200 0.057±0.026 0.83±0.19 0.11+0.04−0.02 2

Sz97 M4.0 3270 0.00 200 0.169±0.078 1.34±0.28 0.25+0.05−0.00 2

Sz99 M4.0 3270 0.00 200 0.074±0.034 0.89±0.20 0.17+0.08−0.00 3

Sz100 M5.5 3057 0.00 200 0.169±0.078 1.43±0.33 0.17+0.00−0.04 1

Sz103 M4.0 3270 0.70 200 0.188±0.087 1.41±0.30 0.25+0.05−0.00 1

Sz104 M5.0 3125 0.00 200 0.102±0.047 1.11±0.26 0.15+0.02−0.02 1

Lup706† M7.5 2795 0.00 200 0.003±0.001 0.22±0.05 0.06+0.03−0.02 32

Sz106† M0.5 3777 1.00 200 0.098±0.045 0.72±0.17 0.62+0.00−0.05 32

Par-Lup3-3 M4.0 3270 2.20 200 0.240±0.110 1.59±0.37 0.25+0.05−0.05 1

Par-Lup3-4† M4.5 3197 0.00 200 0.003±0.001 0.17±0.04 0.13+0.02−0.00 >50

Sz110 M4.0 3270 0.00 200 0.276±0.127 1.61±0.37 0.35+0.05−0.05 1

Sz111 M1 3705 0.00 200 0.330±0.152 1.40±0.32 0.75+0.05−0.13 6

Sz112 M5.0 3125 0.00 200 0.191±0.088 1.52±0.35 0.25+0.00−0.08 1

Sz113 M4.5 3197 1.00 200 0.064±0.030 0.83±0.19 0.17+0.03−0.04 3

2MASS J16085953-3856275 M8.5 2600 0.00 200 0.009±0.004 0.47±0.11 0.03+0.01−0.01 1

SSTc2d160901.4-392512 M4.0 3270 0.50 200 0.148±0.068 1.25±0.29 0.20+0.10−0.05 1

Sz114 M4.8 3175 0.30 200 0.312±0.144 1.82±0.42 0.30+0.05−0.10 1

Sz115 M4.5 3197 0.50 200 0.175±0.080 1.36±0.31 0.17+0.08−0.08 1

Lup818s M6.0 2990 0.00 200 0.025±0.011 0.58±0.13 0.08+0.02−0.02 3

Sz123A (S) M1 3705 1.25 200 0.203±0.093 1.10±0.25 0.60+0.20−0.03 7

Sz123B (N)† M2.0 3560 0.00 200 0.051±0.024 0.58±0.13 0.50+0.00−0.10 40

SST-Lup3-1 M5.0 3125 0.00 200 0.059±0.027 0.85±0.19 0.13+0.02−0.04 2

Notes.† objectsclassifiedassubluminousYSO by Alcaláetal. (2014).

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Figure 6.3: ExamplesofthebestfitofX-ShooterspectraofClass II YSOsinLupusintheregionoftheBalmerjump (red lines). The spectrumof theadoptedClass III templatesareoverplottedwithgreen lines. Thecontinuumemissionfromtheslabisshownbytheblackcontinuousline. Thebestfitwiththeemissionpredictedfromtheslabmodeladdedtothetemplateisgivenbythebluelines. From Alcaláetal. (2014).

butdifferencesare smallcomparedwith theexcessemission in theBalmercontinuumregion.

Theaccretionluminositiesareconvertedinto Macc usingtherelationofEq. (1.2)andadoptingfor R⋆ and M⋆ thevaluesreportedinTable 6.3. Theresultson Macc arelistedincolumn 7 ofTable 6.4. Thecalculated Macc values range from2×10−12 M⊙ yr−1 to4×10−8M⊙ yr−1. Thesourcesoferrorin Macc aretheuncertaintieson Lacc, stellarmassandradius(seeTable 6.3). Propagatingthese, theestimatedaverageerrorisof∼0.35 dexinMacc. However, additionalerrorsonthesequantitiescomefromtheuncertaintyindistance,aswellasfromdifferencesinevolutionarytracks, whichcouldaffectsignificantlythe M∗estimates, andthus Macc. TheuncertaintyontheLupusYSOsdistanceisestimatedtobe∼20%(see Comerón, 2008, andreferencestherein), yieldingarelativeuncertaintyofabout

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0.26 dexin Macc2. Ontheotherhand, usingthe D'Antona&Mazzitelli (1994)tracks, the

differencein M∗ rangesfrom10%to70%(withanaverageof30%)withrespecttotheBaraffeetal. (1998)tracks, leadingtouncertaintiesof0.04 dexto0.3 dexin Macc. Thecumulativerelativeuncertaintyin Macc isthenestimatedtobeofabout0.5 dex.

With Macc=3.4×10−8 M⊙ yr−1 , thestrongestaccretorintheLupussampleisSz 83. Avarietyof Macc estimatesforthisstarexistintheliteraturethatrangefrom10−7 toafew10−8 M⊙ yr−1 andmaybeashighas10−6 (Comerón, 2008). The Macc estimatederivedhereagreesverywellwiththatcalculatedbyHerczeg&Hillenbrand (2008)(1.8×10−8 M⊙ yr−1).Therearelargediscrepanciesbetween Macc determinedhereandthosederivedby Com-erónetal. (2003)forSz 100, Sz 106, Sz 113, andPar-Lup3-4. The Comerónetal. (2003)estimates, whicharebasedonthefluxofthecalcium λ854.2 nmline, arehigherbyupto1 dex. Althoughpartofthediscrepanciesmayinprinciplebeascribedtovariableaccre-tion, thisvariabilitymustbeenormousovertimescalesofyearstoexplainthedifferences.Costiganetal. (2012)and Costiganetal. (2014)haveobservedvariableaccretionoveryears, buttheirresultsshowthatitisveryraretohaveYSOsthatvary Macc bylargefactors.Mostofthevariabilitytheyfoundoccursonrotationaltimescales, suggestingasymmetricandnotstronglyvariableaccretionflows.

Emissionlinesastracerofaccretion

In the spectraof theLupusYSOs therearea largenumberofpermittedand forbiddenemissionlinesthatdisplayavarietyofprofiles. ThedetectedemissionlinesincludeseveralfromtheBalmerandPaschenseries, theBrγ line, andseveralheliumandcalciumlines.

BalmerlinesaredetecteduptoH25insixobjects(Lup 713, Sz 113, Sz 69, Sz 72, Sz 83,Sz 88A).Oneofthese(Lup 713)isatthehydrogen-burninglimit, withitsspectrumresem-blingthatoftheyoungbrowndwarfJ 053825.4−024241reportedin Rigliacoetal. (2011b).InSz 88A BalmerlineemissionisdetecteduptoH27atthe2σ level. ThePa 8, Pa 9, andPa 10linesarelocatedinspectralregionsofdensetelluricabsorptionbands. Althoughthetelluriccorrectionwasperformedasaccuratelyaspossible, someresidualsfromthecor-rectionstillremain. Thus, thedetectionandanalysisofthesethreePaschenlinesismoreuncertainandleadtolargererrors, inparticularforPa 8.

Other linesdetected in thesespectraare thenineHe I lineswith thehighest transi-tionstrength. Ofthese, theHe I λ1082.9nmhasbeenfoundtobealsorelatedtowindsandoutflows(Edwardsetal., 2006). Thus, thelinemayincludebothaccretionandwindcontributions. InmostcasestheHe I λ492.2 nmisblendedwiththeFe I λ492.1 nmline.

TheCa II H &K linesaredetectedinallYSOs. TheCa II H-lineispartiallyblendedwithHϵ. TheCa II IRT λλλ 849.8, 854.2, 866.2 nm, andtheD-linesofthe Na i λλ 589.0,589.6 nmdoubletareverywellresolvedinallthespectra. InseveralobjectsboththeCa II IRT andthe Na i linesaredetectedasanemissionreversalsuperposedonthebroadphotosphericabsorptionlines. Thereforethestrengthoftheselineswascorrectedforthephotosphericcontributionforthecompletesample.

2Notethat Macc ∝ d3, as Lacc ∝ d2 and R⋆ ∝ d.

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Table 6.4: AccretionpropertiesofLupusYSOs

Object Template logLacc logMacc WHα(10%)[L⊙] [M⊙ yr−1] [kms−1]

Sz66 SO797 −1.8 −8.73 460AKC2006-19 SO641 −4.1 −10.85 228Sz69 SO797 −2.8 −9.50 403Sz71 TWA15A −2.2 −9.23 350Sz72 TWA9B −1.8 −8.73 455Sz73 SO879 −1.0 −8.26 504Sz74 TWA15A −1.5 −8.09 401Sz83 SO879 −0.3 −7.37 604Sz84 SO641 −2.7 −9.24 456Sz130 TWA2A −2.2 −9.23 266Sz88A (SW) TWA25 −1.2 −8.31 597Sz88B (NE) SO797 −3.1 −9.74 405Sz91 TWA13A −1.8 −8.85 374Lup713 Par-Lup3-2 −3.5 −10.08 378Lup604s SO925 −3.7 −10.21 264Sz97 Sz94 −2.9 −9.56 452Sz99 TWA9B −2.6 −9.27 373Sz100 SO641 −3.0 −9.47 251Sz103 Sz94 −2.4 −9.04 426Sz104 SO641 −3.2 −9.72 201Lup706† TWA26 −4.8 −11.63 328Sz106† TWA25 −2.5 −9.83 459Par-Lup3-3 TWA15A −2.9 −9.49 240Par-Lup3-4† SO641 −4.1 −11.37 393Sz110 Sz94 −2.0 −8.73 498Sz111 TWA13A −2.2 −9.32 455Sz112 SO641 −3.2 −9.81 160Sz113 SO797 −2.1 −8.80 3922MASS J16085953-3856275 TWA26 −4.6 −10.80 147SSTc2d160901.4-392512 Sz94 −3.0 −9.59 447Sz114 Sz94 −2.5 −9.11 222Sz115 SO797 −2.7 −9.19 338Lup818s SO925 −4.1 −10.63 200Sz123A (S) TWA2A −1.8 −8.93 487Sz123B (N)† TWA15B −2.7 −10.03 519SST-Lup3-1 SO641 −3.6 −10.17 254

Notes.† objectsclassifiedassubluminousYSO by Alcaláetal. (2014).

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Finally, thetwoO I linesat777.3 nmand844.6 nmareclearlydetectedin14and18YSOs, respectively. TheselinesareseenintheobjectswiththestrongestBalmer, He I,andCa II lines.

Thefluxinpermittedlineswascomputedbydirectlyintegratingtheflux-calibratedandextinction-correctedspectrausingthe splot packageunderIRAF3.Threemeasurementsperlineweremade, consideringthelowest, highest, andthemiddlepositionofthelocalcon-tinuum, dependingonthelocalnoiselevelofthespectra. Thefluxanditserrorwerethencomputedastheaverageandstandarddeviationofthethreemeasurements, respectively.Theextinction-correctedfluxes, equivalentwidths, andtheirerrorsarereportedin Alcaláetal. (2014). Inthecaseswherethelineswerenotdetected, 3σ upperlimitswereesti-matedusingtherelationship 3× Fnoise×∆λ, whereFnoise isthermsflux-noiseintheregionofthelineand ∆λ istheexpectedaveragelinewidth, assumedtobe0.2 nm.

Thecontributionofthephotosphericabsorptionlinesofthe Na iD linesandtheCa II IRT,strongest inthelate-K andearly-to-midM-typeobjects, wereremovedinallspectrabyusingthesyntheticBT-Settlspectraby Allardetal. (2011)ofthesame Teff and log g astheYSOs, binnedatthesameresolutionofX-Shooter, andwererotationallybroadenedatthesame v sin i astheYSOs. Forthispurpose, weappliedtheROTFIT code(Frascaetal.,2006), specificallymodifiedforX-Shooterdata(see Stelzeretal., 2013, fordetails).

Theluminosityofthedifferentemissionlineswascomputedas Lline = 4πd2 ·fline, whered istheYSO distanceinTable 6.3 and fline istheextinction-correctedabsolutefluxofthelines.

Inunitsof L⊙, thedynamicalrangeof Lacc fortheLupussamplecoversmorethanfourordersofmagnitude, whiletheluminosityofthelinediagnosticsdiscussedintheprevioussectionspansmorethanfiveordersofmagnitude. Therefore, itispossibletoinvestigatetherelationshipsbetweencontinuumexcessemissionandtheemissioninindividualpermittedlines. A detaileddiscussiononthis Lacc-Lline relationshipformostoftheaforementionedpermittedemissionlinesispresentedin Alcaláetal. (2014). HereI reportonlytheresultsandtherelationshipsderivedforthebrighteremissionlinesusuallypresentinthespectraofTTauristars. Thesewerecalculatedaslinearfitsofthe logLacc vs. logLline relationshipsusingthepackageASURV (Feigelson&Nelson, 1985)undertheIRAF environment. ASURVincludescensoringofupperor lower limits in thefits. In thevarious relationships theupperlimitsintheindependentvariable Lline arewellconsistentwiththetrendsseeninthe Lacc vs. Lline plots. Theresultsofthefitsincludingandexcludingupperlimitsareconsistentwithin theerrors. Theerrors in thecomputed relationshipsalsoaccount forupperlimitswhenincluded. Therelationsderivedfortheprincipalemissionlinesarethe

3IRAF isdistributedbytheNationalOpticalAstronomyObservatory, whichisoperatedbytheAssociationoftheUniversitiesforResearchinAstronomy, inc. (AURA) undercooperativeagreementwiththeNationalScienceFoundation

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following:

log(Lacc/L⊙) = (1.12± 0.07) · log(LHα/L⊙) + (1.50± 0.26)

log(Lacc/L⊙) = (1.11± 0.05) · log(LHβ/L⊙) + (2.31± 0.23)

log(Lacc/L⊙) = (1.09± 0.05) · log(LHγ/L⊙) + (2.50± 0.25)

log(Lacc/L⊙) = (1.06± 0.06) · log(LHδ/L⊙) + (2.50± 0.28)

log(Lacc/L⊙) = (1.04± 0.08) · log(LPaβ/L⊙) + (2.45± 0.39)

log(Lacc/L⊙) = (1.18± 0.06) · log(LPaγ/L⊙) + (3.17± 0.31)

log(Lacc/L⊙) = (1.18± 0.10) · log(LPaδ/L⊙) + (3.33± 0.47)

log(Lacc/L⊙) = (1.16± 0.07) · log(LBrγ/L⊙) + (3.60± 0.38)

log(Lacc/L⊙) = (1.13± 0.06) · log(LHeIλ587.6nm/L⊙) + (3.51± 0.30)

log(Lacc/L⊙) = (1.16± 0.08) · log(LHeIλ667.8nm/L⊙) + (4.12± 0.45)

log(Lacc/L⊙) = (0.96± 0.05) · log(LCaIIλ393.4nm/L⊙) + (2.06± 0.27)

(6.1)

Thereported Lacc vs. Lline relationshipsgenerallyagreewiththosefoundinpreviousinvestigations (Muzerolleetal., 1998a; Calvetetal., 2004; Nattaetal., 2004; Herczeg&Hillenbrand, 2008; Inglebyetal., 2013). The slopesandzeropointsof the Lacc vs.Lline relationsderivedhereareconsistentwithintheerrorswiththosereportedin Herczeg&Hillenbrand (2008)(seetheirTable 164). AlsotherelationshipsofEq. (6.1)fortheHα,Hβ, andCa II K linesarepracticallyidenticaltothoseof Inglebyetal. (2013), whichwerederivedbyfittingUV andopticalspectrawithmultipleaccretioncomponents. AlsotheLacc-LPaβ and Lacc-LBrγ relationsaresimilartothoseinpreviousworksby Muzerolleetal. (1998a), Nattaetal. (2004), Calvetetal. (2000), and Calvetetal. (2004), butextendtomuchlowervaluesof Lacc, towardtheverylow-massregime. However, becauseofthedifferentmethodologiesadoptedinderiving Lacc (Hα lineprofilemodelling, veilingintheFUV,UV,andVIS,etc.), systematicdifferencesmayexistatdifferentmassregimes(e.g.,Herczeg&Hillenbrand, 2008). Therefore, except for theYSOsin σ-Ori(Rigliacoetal.,2012, andSect. 6.3), whose Lacc and Lline valueswerecomputedinthesamewayashere,nootherliteraturedatawerecombinedtoderive Lacc-Lline relationships. ItshouldbenotedalsothattheLupussamplecomprises Lacc ≤ 1L⊙, whileliteraturedataextendtohigheraccretionluminosities.

Whiletheaccretionluminosityiswellcorrelatedwiththeluminosityofalltheemissionlines, thescatterinthecorrelationsdiffersforthevariouslines. AmongtherelationshipsofEq. (6.1)theonewiththelargerscatteristhatoftheHα line. Thisisexpectedbecauseitiswellknownthatseveralotherprocesses(e.g. outflows, hotspots, chromosphericactivity,complexmagneticfieldtopologyandgeometry, stellarrotation)inadditiontoaccretionmaycontributetothestrengthoftheline. Alltheseprocesseshaveanimportantimpactonthelineprofile, inparticularonitswidth. Theleastscattered Lacc-Lline relationsarethoseoftheBalmerlineswith n > 3, theBrγ line, andtheHeI lines. ThePaβ andtheBrγ

4Notethattheslopeandzeropointsinthe Herczeg&Hillenbrand (2008)relationshipsareswappedintheirTable 16.

156


2.0 1.5 1.0 0.5 0.0 0.5log(M /M¯)

12

11

10

9

8

7

log(M

acc/

[M¯/y

r])

Lupus

Figure 6.4: MassaccretionrateasafunctionofmassfortheLupussample. Theopensymbolsareusedforthe low-luminosityYSOs. ThecontinuouslinerepresentsthelinearfitofEquation 6.3, thatis, itdoesnotincludethesubluminousobjects. Thedashedlinesrepresentthe1σ deviationfromthefit. Averageerrorbarsareshownintheupperleftcorner. Adaptedfrom Alcaláetal. (2014).

relationsarerecommendedbecausetheselinesaretheleastaffectedfromchromosphericemission, asshownalsobytheanalysisofClass III YSO spectra(Manaraetal., 2013a, andChapter 3). Finally, asalsonotedby Rigliacoetal. (2012), accretionluminosityderivedusingsimultaneouslyvariousemissionlinesleadtoresultscompatiblewithintheerrorswith thoseobtained fromtheUV-excessfittingandwithamuchbetteragreement thanusingasingleemissionline.

6.2.4 AccretionpropertiesofstellarandsubstellarobjectsinLupus

AsI discussedinSect. 1.4.1 ofthisThesis, previousinvestigations(Muzerolleetal., 2003;Mohantyetal., 2005; Nattaetal., 2006; Herczeg&Hillenbrand, 2008; Rigliacoetal.,2011a; Antoniuccietal., 2011; Biazzoetal., 2012; Manaraetal., 2012, andreferencestherein)havefoundthat Macc goesroughlyasthesquareofM⋆ althoughwithasignificantscatter(upto3 dex)in Macc foragivenYSO mass.

Figure 6.4 showsthe Macc versus M⋆ diagramfortheLupusYSOsstudiedhere. ThescatterissignificantlyincreasedbythefoursubluminousYSOs. A linearfittothecompleteLupussampleyieldstothefollowingrelation:

log Macc = 1.89(±0.26) · logM⋆ − 8.35(±0.18), (6.2)

witha standarddeviationof0.6. The scatterdecreases if the subluminousobjectsare

157


Table 6.5: Coordinates, spectraltypes, physicalandaccretionparametersofthe σ-OriClass II YSOs

Object RA DEC SpT Teff L⋆ R⋆ M⋆ logLacc logMacc

[K] [L⊙] [R⊙] [M⊙] [L⊙] [M⊙/yr]SO397 05:38:13.18 −02:26:08.63 M4.5 3200 0.19 1.45 0.20 –2.71 –9.42SO490 05:38:23.58 −02:20:47.47 M5.5 3060 0.08 1.02 0.14 –3.10 –9.97SO500 05:38:25.41 −02:42:41.15 M6 2990 0.02 0.47 0.08 –3.95 –10.27SO587 05:38:34.04 −02:36:37.33 M4.5 3200 0.28 1.73 0.20 <−4.00 <–10.41SO646 05:38:39.01 −02:45:31.97 M3.5 3350 0.10 0.97 0.30 –3.00 –9.68SO848 05:39:1.938 −02:35:02.83 M4 3270 0.02 0.46 0.19 –3.50 –10.39SO1260 05:39:53.63 −02:33:42.88 M4 3270 0.13 1.12 0.26 –2.00 –8.97SO1266 05:39:54.22 −02:27:32.87 M4.5 3200 0.06 0.84 0.20 <–4.85 <–11.38

excludedfromthefit, andthebestfitrelationbecomesinthiscase:

log Macc = 1.81(±0.20) · logM⋆ − 8.25(±0.14), (6.3)

witha standarddeviationof0.4. The slopeandzeropointof the Macc- M⋆ fitdonotchangesignificantlyineithercasesbecausethesubluminousobjectsrepresentonly11%ofthesample. ItisthusreasonabletoconcludethatfortheLupussample Macc ∝M⋆

1.8(±0.2),whichagreeswithpreviousstudies(Nattaetal., 2006; Muzerolleetal., 2005; Herczeg&Hillenbrand, 2008; Rigliacoetal., 2011a; Antoniuccietal., 2011; Biazzoetal., 2012;Manaraetal., 2012), butisinconsistentwiththeresultsby Fangetal. (2009)(Macc ∝ M⋆

3)fortheirsubsolarmasssampleintheLynds 1630N and1641cloudsinOrion. I willdiscussinSect. 6.6.2 thereasonsofthisdifference.

In this sampleof LupusClass II YSOs thepower-law indexof the Macc-M⋆ relationis ∼2similarlytopreviousinvestigations, butthescatterisconsiderablysmallerthaninpreviousdatasets(cf. thestandarddeviationfortheLupusfitisafactor2lowerthanfortheTaurussamplein Herczeg&Hillenbrand, 2008). AspointedoutinSection 6.2.1, thissamplerepresentsabout50%ofthecompletepopulationofClass II YSOsintheLupus IandLupus III clouds. Itisthereforeunlikelythatthewiderangeof Macc (> 2 dex) atagivenmassobservedinotherdatasetswillbealsodetectedinLupus, inamorecompletesample.However, itwouldbeworthwhiletostudyYSOsmoremassivethanthoseinthissample,usingspectraofthesamequalityasthese. Inaddition, althoughthenumberstatisticsofthissampleislow, thereisnoevidenceforadoublepower-lawassuggestedby Vorobyov&Basu (2009), supportingtheconclusionthattheapparentbi-modalrelationssuggestedintheliteraturebetween Macc andotherYSOspropertiesareprobablytheresultofmixingMacc derivedwithdifferentmethods.

158

6.3 Accretion in the σ-Orionis region

6.3 Accretioninthe σ-Orionisregion

BasedonRigliaco, Natta, Testietal. 2012, A&A,548, A56†

The σ-Orioniscluster(RA 5h40mDEC -2d36′)islocatedatadistanceof ∼360pc(Hip-parcosdistance352+166

−85 pc, Brownetal. 1994; Perrymanetal. 1997)andhasanageof ∼3Myr(ZapateroOsorioetal., 2002; Fedeleetal., 2010). Itcontainsmorethan300YSOs,ranginginmassfromthebright, massivemultiplesystem σ-Oriitself(thespectraltypeofthebrighteststarisO9.5V, Caballero 2008)tobrowndwarfs. Theregionhasnegligibleextinction(Béjaretal., 1999; Oliveiraetal., 2004), andhasbeenextensivelystudiedintheoptical, X-rays, andinfrared(e.g., Rigliacoetal., 2011a; Kenyonetal., 2005; ZapateroOsorioetal., 2002; Jeffriesetal., 2006; Franciosinietal., 2006; Hernándezetal., 2007).

Thesampleofobjectsin σ-OriobservedwithX-Shootercomprises8Class II YSOsand4Class III YSOswhichhavebeendiscussedinChapter 3 and Manaraetal. (2013a). HereI reportontheaccretionratesderivedforthe8Class II YSOs, thathavebeenselectedfromthesampleof Rigliacoetal. (2011a)andcoverarangeofmassesfrom M⋆ ∼ 0.08M⊙ to∼ 0.3M⊙.

Spectraltypesforthe σ-Orisamplewereestimatedbyaveragingtheresultsobtainedusingthevariousindicesby Riddicketal. (2007)onthefluxcalibratedspectra, similarlytotheotherdata-setsdiscussedinthisThesis (seee.g., Chapter 3 andSect. 6.2.2). TheSpT wasthenconvertedin Teff usingthetemperaturescaleof Luhmanetal. (2003). Stellarluminositieswerethencomputedfromthe I-bandmagnitudes, andthebolometriccorrec-tionforzeroagemainsequence(ZAMS) starsasreportedby Luhmanetal. (2003). Thetypicaluncertaintyin L⋆ forthesetargetsisestimatedtobeof ∼0.2dex. ThestellarmassesandageswerederivedcomparingthelocationofthetargetsontheHR diagramwiththeevolutionarymodelsby Baraffeetal. (1998). TheadoptedstellarparametersforthetargetsarereportedinTable 6.5.

TheaccretionluminositywasthenderivedfortheseobjectsbyfittingtheUV-excesswiththesamesetofmodelsasthefittingproceduredescribedinChapter 4, i.e. withaClass III YSO template(seeChapter 3)ofthesameSpT oftheClass II targetandtheslabmodel todescribe theexcessfluxdue toaccretion (seeChapter 2). I stress that itwasassumed AV ∼ 0 forallthetargetsinthisregion. In6outofthe8objectsanalyzedheretheexcessluminosityisclearlydetected; intwocases(SO587andSO1266)onlyupperlimitscouldbedetermined. ThederivedvaluesarereportedinTable 6.5, whilethebestfitsoftheBalmerjumpareshowninFig. 6.5.

Figure 6.6 showsthevaluesof Macc vs M⋆ forthe8targetsdiscussedinthissection.Theoverplotted relation is thebest fit of the Macc-M⋆ relationderivedby Alcalá et al.(2014)fortheLupussample, shownasreference. ThevaluesforallthesixClass II YSOs

†InthisworkI hadamarginalcontribution. However, theaccretionratesforthesetargetshavebeenderivedwiththesameslabmodelastheoneI havebeenusingandhavebeencheckedandvalidatedwithmyfittingprocedure. GiventhattheseresultsareobtainedwithamethodologycompatibletothatoftheotherworksdiscussedinthisThesis, thesecanbeincludedinthefinalsample. ForthesereasonsI includehereashortsectiondescribingthisworktoreporttheresultsthatI willuseinthenextsections.

159


Figure 6.5: BestfitoftheUV-excessforthe σ-OriClass II YSOs. Thespectrumofthetargetisshowninred,theClass III templateingreen, theslabmodelincyan, andthebestfitinblue. From Rigliacoetal. (2012).

withdetected Macc arecompatiblewiththisbestfitrelation. ForSO1266theupperlimiton Macc issignificantlysmallerthanthemeasurementsforClass II YSOsofsimilar M⋆. Asdiscussedindetailin Rigliacoetal. (2012), thisobjecthasprobablyasignificantlydepletedgaseousinnerdisk, andisanexampleoflow-accretingTD.Interestingly, theluminosityoftheemissionlinesforthisobjectwouldimplyanhighervalueof Lacc. However, Rigliacoetal. (2012)suggestthatmost(∼80%)oftheemissioninthelinesisduetochromosphericemission, asdiscussedalsoin Manaraetal. (2013a).

6.4 Newdatainthe ρ-Ophiucusembeddedcomplex

TheOphiucusmolecularcloud(RA 16h25mDEC -24d20′)isawellknownlow-massstarformingregion. ItislocatedattheedgeoftheUpperScorpioussubgroupintheSco-CenOB association. Thiscomplexiscomposedofseriesoffilamentarydarkcloudsthatextendfromcoresofdensemoleculargas(deGeus, 1992). It iscomposedofthreemaindarkclouds, L1688, L1689, andL1709locatedatadistance d ∼ 125pc(Lombardietal., 2008;Loinardetal., 2008). Thisregionisstillhighlyembeddedintheparentalcloud, withvaluesof AV upto50maginthedensestcore(Wilking&Lada, 1983), andwitharatherhighvalueoftotal-to-selectiveextinctionratio RV ∼ 5.6(e.g., Kenyonetal., 1998; Chapmanetal., 2009; McClureetal., 2010; Comerónetal., 2010). Thisregionhasbeenextensivelystudiedinthepastatvariouswavelengthsfromoptical, near-andfar-infrared, millimeter,radio, toX-ray(e.g., Greene&Meyer, 1995; Luhman&Rieke, 1999; Wilkingetal., 2005;AlvesdeOliveiraetal., 2010, 2012; Bontempsetal., 2001; McClureetal., 2010; Gagné

160

6.4 New data in the ρ-Ophiucus embedded complex

2.0 1.5 1.0 0.5 0.0 0.5log(M /M¯)

12

11

10

9

8

7

log(M

acc/

[M¯/y

r])

σ-Ori

Figure 6.6: Massaccretionrate Macc asafunctionofmassforthe σ-Orisampleanalyzedby Rigliacoetal.(2012). ThecontinuouslinerepresentsthelinearfitofthisrelationfortheLupussample(seeEquation 6.3)shownhereasareference. Thedashedlinesrepresentthe1σ deviationfromthefit. Averageerrorbarsareshownintheupperleftcorner.

etal., 2004)andthetotalpopulationoftheregionisestimatedtobeofaround300YSOs(Evansetal., 2009), with ∼200YSOslocatedinthecentralcluster ρ-Oph(orL1688). Onepossiblescenariofortheformationofthedense ρ-OphcoreistriggeredformationfromtheOB starsintheUpperScorpiussubgroup(deGeus, 1992; Preibisch&Mamajek, 2008).Theageofthisregionisestimatedtobe≲1Myrassuggestedbythefactthatitisstillhighlyembedded. VariousworksaimedatunderstandingtheaccretionpropertiesofYSOsatearlyageshavetargeted ρ-Oph. Inparticular, Nattaetal. (2004, 2006)havestudiedasignificantfractionoftheClass II YSOsinthiscluster, selectedfromtheISO sampleof Bontempsetal. (2001), usingnear-infraredemissionlines(Paβ andBrγ). Theirresults, correctedforthenewlydetermineddistanceasexplainedby Rigliacoetal. (2011a), showedanincreaseofMacc with M⋆ withanexponentofthepowerlawof1.3±0.2usingtheevolutionarytracksof Baraffeetal. (1998), butalsoalargescatterof Macc (roughlytwoorderofmagnitudes,independentof M⋆). WhencomparedwithresultsinTaurusfrom Muzerolleetal. (2003,2005), thevaluesof Macc derivedin ρ-Ophshowedasimilartrendbutsignificantlylargervaluesof Macc atlower M⋆ (≲ 0.1M⊙). ThelattercouldbeduetoincompletenessatlowermassesinthesampleofClass II YSOsof Bontempsetal. (2001).

InthissectionI willreportonnewdatacollectedwithX-Shooter(Pr.Id.085.C-0876,PI Testi)onasmallsampleof ∼20verylowmassstars(VLM) andBDsanalyzedhereforthefirsttime. I willfocusonlyontheClass II objects, whiletheClass III targetswillbediscussedinafollowingpaper(Manaraetal., inprep.) andtheTD objectISO-Oph196hasbeendiscussedin Manaraetal. (2014a).

161


Table 6.6: SelectedYSOsinthe ρ-Ophiucusregionandobservinglog

Object/othername RA(2000) DEC(2000) Obs. date Exp. Timeh :m :s ' '' YY-MM-DD [s]

ISO−Oph023/SKS1 16:26:18.821 −24:26:10.52 2010-08-20 4×750sISO−Oph030/GY5 16:26:21.528 −24:26:00.96 2010-08-20 4×750sISO−Oph032/GY3 16:26:21.899 −24:44:39.76 2010-06-01 4×750sISO−Oph033/GY11 16:26:22.269 −24:24:07.06 2010-08-26 4×1800sISO−Oph037/LFAM3/GY21 16:26:23.580 −24:24:39.50 2010-08-21 4×750sISO−Oph072/WL18 16:26:48.980 −24:38:25.24 2010-04-06 4×480sISO−Oph087 16:26:58.639 −24:18:34.66 2010-08-28 4×750sISO−Oph094 16:27:03.591 −24:20:05.45 2010-08-28 4×750sISO−Oph102/GY204 16:27:06.596 −24:41:48.84 2010-07-23 4×750sISO−Oph115/WL11/GY229 16:27:12.131 −24:34:49.14 2010-07-30 4×750sISO−Oph117/WLY2-32b/GY235 16:27:13.823 −24:43:31.66 2010-06-08 4×480sISO−Oph123 16:27:17.590 −24:05:13.70 2010-08-27 4×750sISO−Oph160/B162737-241756 16:27:37.422 −24:17:54.87 2010-08-27 4×750sISO−Oph164/GY310 16:27:38.631 −24:38:39.19 2010-06-08 4×750sISO−Oph165/GY312 16:27:38.945 −24:40:20.67 2010-06-08 4×750sISO−Oph176/GY350 16:27:46.291 −24:31:41.19 2010-08-21 4×750sISO−Oph193/B162812-241138 16:28:12.720 −24:11:35.60 2010-08-21 4×750s

Notes. ExposuretimesarethesameinthethreeX-Shooterarms(UVB,VIS,andNIR).

6.4.1 Sample, observations, anddatareduction

Thesampleof ρ-Oph targetscomprises17Class II YSOsobservedwithX-Shooter. AllthesetargetswereincludedintheISO samplecompiledby Bontempsetal. (2001)andhavebeenpreviouslystudiedby Nattaetal. (2004, 2006). TheirISO number, othernames,coordinates, observingdates, andexposuretimesarereportedinTable 6.6. Thesamplewasselectedfromthesampleof Nattaetal. (2006)tocoverasmanyoftheobjectswithestimatedmass <0.1 M⊙ andmeasured Macc aspossible. ISO-Oph072wasreportedbyMcClureetal. (2010)tobememberofabinarysystemwithaseparationof3.62′′. IntheX-Shooterspectrumanalyzedherethereisonlytheprimarycomponentofthesystem, whilethesecondaryisnotintheslit.

Alltheobjectshavebeenobservedinservicemodeusingthe1.0′′ slitintheUVB armandthe0.9′′ slitsintheVIS andNIR arms, whichleadtoresolutionR∼ 4350, 7450, and5300inthethreearms, respectively. Differentexposuretimeshavebeenadoptedforeachtargetdependingontheirestimatedfluxes. ThelowmassofmostofthetargetsandthehighextinctionoftheregionresultinalowSNR formostspectraatleastintheUVB arm.TheonlyspectrawithSNR greaterthan ∼5at400nmarethoseofISO-Oph032andISO-Oph123. SomeobjectshavealsoverylowSNR intheVIS spectraat λ ≲ 600-700nm, orevenmore. TheNIR spectraofalltargetshaveagoodSNR.

Data reductionhas been carriedout using theX-Shooter pipeline Modigliani et al.(2010)version 1.3.7andthesameprocedureasintherestofthisThesis(see, e.g, Chap-ters 3, 4, 5, and Manaraetal., 2013a,b, 2014a; Alcaláetal., 2014). Thefluxcalibrationof

162


thespectrareducedwiththepipelinehasbeencomparedwithavailablephotometryandagoodagreementwasfoundinmostcases. Indeed, thesespectrahaveallbeenobtainedwithquitelargeslits, soslitlossesaresmall. Onlyinfourcasesasmallcorrectionfactorwasapplied(∼ 1.2-1.5)toadjusttheflux-calibratedspectratothephotometricdata. TheonlyspectrumwheretheconjunctionbetweenthearmsarenotverygoodisISO-Oph102.Inthisobjectthefluxcalibrationofthelast ∼100nmoftheVIS armisnotgood, andthisresultsinabadmatchingoftheVIS andNIR arms.

6.4.2 Stellarandsubstellarproperties

Asexplainedintheprevioussection, mostofthespectrahaveSNR∼0intheUVB arm,thereforethefittingprocedureexplainedinChapter 4 andusedintheanalysesdiscussedin thisThesiscannotbeused. For this reason I deriveSpT and AV fromthespectraofthetargetsfollowingaproceduresimilartothatdiscussedinSect. 6.2.2 foralltheobjectsin ρ-OphbutISO-Oph032andISO-Oph123, asI explainlaterinthissection. Moreover,giventhattheSNR islowinmostspectraalsointheVIS arm(atleastat λ ≳ 600-700nm), IsmoothallofthemintheVIS toenhancetheSNR.Thisisdoneusingthe boxcarsmoothingprocedure includedin the IRAF splot package. Thisprocedureconvolves thespectrumwitharectangularbox, whosewidthissetto7or11binsdependingontheinitialSNR ofthespectra. Thissmoothingprocedureresultsinabroadeningofthenarrowerabsorptionandemissionlinesandfeatures, butpreservestheirfluxesandequivalentwidths.

3.353.403.453.503.553.60logTeff [K]

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

log(L

/L¯)

0.02 M¯

0.05 M¯

0.10 M¯

0.2 M¯

0.4 M¯

0.6 M¯

0.8 M¯

1.0 M¯

1.2 M¯

033

037072

094

115

123

165

Figure 6.7: Hertzsprung-Russelldiagramforthe ρ-OphClass II YSOsanalyzedhere. Smallersymbolsareused for theobjectswithuncertainstellarparameters, whileemptysymbols for thestronglyveiledones.Thecontinuouslinesshowthe1Myr, 3Myr, 10Myr, 30Myr, and100Myrisochrones, reportedby Baraffeetal. (1998), whilethedashedlinesshowthelow-masspre-mainsequenceevolutionarytracksbythesameauthorsaslabelled. ISO numbersarereportedfortheobjectswithestimatedagesgreaterthan10MyrandforISO-Oph033.

163


ToderiveSpT andAV I firstusetheindicesof Riddicketal. (2007)discussedinChapter 3andin Manaraetal. (2013a). Theseareindependentofextinctionif AV islow, whichisnotthecaseformostoftheobjectsincludedinthissample. TheyarealsovalidonlyforobjectswithSpT laterthanM2-M3, asdiscussedby, e.g., Manaraetal. (2013a). Therefore,theseareusedonly tohaveafirstestimateof theSpT.Then, I compare theVIS armoftheobservedspectrawiththeClass III YSO spectraofChapter 3 (Manaraetal., 2013a)ofsimilarSpT totheonepredictedfromtheindicesortothatreportedintheliteraturefortheobject. TheClass III YSO spectrumisreddenedusingthereddeninglawof Cardellietal. (1989)and RV =5.65 withincreasingamountof AV untilabestmatchisfound. Thisvisualcomparison, whichaimsatfindingagoodmatchingoftargetandClass III spectraintheabsorptionfeaturesdiscussedinChapter 3, leadstoadeterminationoftheSpT ofthetargets. Startingfromthisestimate, theNIR armoftheobservedspectraiscomparedwiththereddenedClass III YSO spectratovalidatetheSpT determinationandtoderiveanaccuratevalueof AV . Indeed, veilingduetoaccretionmakestheobservedspectrumbluerandleadstounderestimatethevalueof AV whenusingtheVIS armalone. Veilingis,instead, smallerintheNIR,inparticularinthe J-band, wheretheeffectsofthepresenceofacircumstellardiskarestillsmall. Inmostcases AV derived from theVIS spectraisconfirmedoravalueof AV largerbylessthanafactor ∼1.2-1.3isfound.

FromthevaluesofSpT and AV derivedwiththisprocedureI thendetermine L⋆ com-paringtheextinctioncorrectedspectrumoftheClass II YSO targetwiththatofthebestmatchingClass III YSO.FirstI derivethefactorK torescaletheobservedClass III spectrumtotheexinctioncorrectedClass II YSO bymatchingthetwospectraat λ =1030nm. Thisfactor K correspondstothesquaredratioof thestellarradiiof thetwotargets. Then, IcomputetheClass II YSO bolometricluminosityfromtheoneoftheClass III YSO takingintoaccountthedistanceofthelatterandthedistanceof ρ-Oph, assumedtobe125pc,usingtheformula:

L⋆,ClassII = K · (dρ−Oph/dClassIII)2 · L⋆,ClassIII, (6.4)

asdonein, e.g., Chapter 4 (Manaraetal., 2013b). ConvertingtheSpT inTeff usingtheSpT-Teff relationof Luhmanetal. (2003)forM-typestarsandof Kenyon&Hartmann (1995)forearliertypeobjectsI amabletoassigntoeachobjectapositionontheHRD andderivethestellarparameters(M⋆, age)usingtheevolutionarytracksof Baraffeetal. (1998). ThisisshowninFig. 6.7. InthefollowingparagraphsI presenttheresultsobtainedwiththisanalysis, whicharereportedinTable 6.7. Finally, thecomparisonbetweentheobservedspectraandthecorrespondingClass III YSO templateisshowninFigs. 6.8 to 6.12.

SpectrawithgoodVIS arms

For 10 out of the 17 targets (ISO-Oph023,030,032,033,102,117,160,164,176,193) theanalysisexplainedinthepreviousparagraphsleadstoafirmestimateoftheSpT and AV .

5Thisanalysiswascarriedoutalsoadopting RV = 3.1, butabetteragreementbetweenthereddenedspectraoftheClass III andthetargetsisfoundusing RV = 5.6, especiallyintheregionoftheVIS spectraatλ ≳ 900nmandintheNIR J-and H-bands. Thefinal Macc valuesderivedusingthetwodifferentvaluesofRV differbyafactor ∼2-3, atmost.

164


Table 6.7: Spectraltypes, extinction, andphysicalparametersofthe ρ-OphClass II YSOs

Object SpT Teff AV L⋆ R⋆ M⋆ Age[K] [mag] [L⊙] [R⊙] [M⊙] [Myr]

ISO−Oph023 M6.5 2935 8.5 0.027 0.64 0.07 2ISO−Oph030 M5.5 3060 5.8 0.098 1.12 0.14 1ISO−Oph032 M6.5 2935 1.9 0.040 0.77 0.08 1ISO−Oph033 M9 2400 5.5 0.002 0.28 0.02 ∼1ISO−Oph037† M0 3850 15.0 0.140 0.84 0.74 28ISO−Oph072∗ K7 4060 10.5 0.191 0.88 0.81 33ISO−Oph087† M4 3270 13.0 0.055 0.73 0.23 6ISO−Oph094† M2 3560 11.0 0.008 0.23 0.40 >100ISO−Oph102 M5.5 3060 3.7 0.070 0.94 0.14 1ISO−Oph115† M0 3850 15.5 0.102 0.72 0.68 43ISO−Oph117 M3 3415 10.0 0.205 1.30 0.38 2ISO−Oph123∗ M4 3270 5.0 0.020 0.44 0.19 20ISO−Oph160 M5.5 3060 7.0 0.029 0.61 0.11 4ISO−Oph164 M6.5 2935 3.0 0.016 0.49 0.07 4ISO−Oph165† M2 3560 13.0 0.033 0.48 0.45 69ISO−Oph176 M5.5 3060 8.3 0.065 0.91 0.14 2ISO−Oph193 M4.5 3200 8.4 0.060 0.80 0.20 4

Notes.M∗ andagearederivedusingtheevolutionarytracksof Baraffeetal. (1998). † ObjectswithuncertainstellarparametersduetothelowSNR ofthespectraandthestrongextinction. ∗ Objectwithuncertainstellarparametersduetothestrongveiling.

IntheseobjectstheVIS spectrahavealwaysgoodSNR atleastfor λ ≳650-700nm, thusinthewavelengthregionwheremostoftheabsorptionfeaturesare. InallthesetargetsbuttwothebestfitSpT derivedconfirmsthefirstestimatedoneusingtheindicesfrom Riddicketal. (2007), withdifferencesoflessthan0.5spectralsubclasses. ThelargestdifferenceswiththevaluesderivedusingtheindicesareforISO-Oph033andISO-Oph117. Inthefor-mertheindicessuggestaSpT M6.1whileI deriveSpT M9, andthedifferenceisprobablyduetothelowSNR ofthespectrumat λ ≲ 800nm. Inthelatter, theSpT assignedtothisobjectisM3, whiletheindicessuggestaSpT betweenM3.5andM4, butclosertoM4.Amongthetargetsanalyzedinthisparagraphthisoneisthemoreextinctedone(AV =10mag), andthiscouldbethereasonforthediscrepancy. Indeed, theindicesof Riddicketal.(2007)areextinctionindependentfor AV ≲ 5, butmaybeaffectedbylargerextinctions.

TheSpT derivedhereagreeinmostcaseswiththosereportedintheliteraturewithin0.5spectralsubclasses. Insomecases, suchasISO-Oph033, thedifferenceof0.5subclassesisduetotheincompletenessofthesetofphotospherictemplatesused. ThisobjecthasbeenclassifiedasM8.5by Nattaetal. (2002), Testietal. (2002), and Comerónetal. (2010),whileI classifyitasM9becausethegridI amusinghasnospectrabetweenM6.5andM9(seeFig. 3.4). ThelargestdifferencewithSpT availableintheliteratureisfoundforISO-Oph032andISO-Oph193. TheformerhasbeenclassifiedasM7.5by Nattaetal. (2002)andasM8by Wilkingetal. (2005), while I deriveaspectral typeM6.5 for this target.Alsointhiscasethiscouldbeduetoincompletenessofthegridoftemplatespectra, but

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I relyonmyresultwhichisalsovalidatedbythespectralindicesresultandbythefittingprocedurediscussedlater. ForthelattertheSpT reportedintheliteratureisM6(Nattaetal.,2002), whileI deriveM4.5bothwiththespectralindicesandwiththevisualcomparison.Inthiscase, thisisnotduetoincompletenessofthetemplategrid, asthisiscompleteuntilM6.5. UsingtheX-ShooterspectraI amabletodeterminetheSpT forISO-Oph117andISO-Oph164, whoseclassificationwasdebatedintheliterature. ForISO-Oph117 Gattietal. (2006)derivedSpT K8, while Wilkingetal. (2005)foundSpT M5. MyanalysisleadstoaSpT ofM3, whileitisclearthatthisobjectcanbeneitherlate-K orM5, asthemolecularfeaturesat λ ∼ 700-800nmaretoostrongforalate-K star, andtooshallowforanM5YSO.Regarding ISO-Oph164, the spectral typesdetermined in the literaturevary fromM8.5(Wilkingetal., 1999), toM6(Nattaetal., 2002), andeventoM4(Wilkingetal., 2005).ThespectralindicessuggestaSpT M5.9forthistarget, whilethebestmatchdeterminedwiththeX-ShooterspectraisfoundtobeM6.5. However, theobservedspectrumcannotbewellreproducedwithanyofthetemplatesavailable, andfurtheranalysisareneededonthistarget.

Theagedeterminedwiththestellarparametersdeterminedhereandusingtheevolu-tionary tracksof Baraffeetal. (1998) isalwaysbetween ∼1Myrand ∼4Myr for theseobjects, asonewouldexpectgiventheyoungageoftheregionand, atthesametime, theuncertaintyintheagedeterminationwithisochroneinterpolation. OnlyISO-Oph033hasstellarparametersthatcorrespondtoapositionontheHRD wheretherearenoevolution-arytracks. I extrapolatethestellarmassandreportanage ∼1Myr.

Spectrawithstongveilingand/orgoodUVB spectra

Twospectra, ISO-Oph072andISO-Oph123, aretoostronglyveiledtogetSpT fromtheprocedureexplainedinthepreviousparagraphs. ThefirstonehasaspectrumwithSNR∼0intheUVB armduetothehighextinction(AV ∼ 10.5mag), soI cannotusetheprocedureexplainedinChapter 4 todetermineitsproperties. UsingtheproceduredescribedaboveIdetermineabestestimateofSpT,M0.5, and AV =10.5mag. However, thisvalueisratheruncertainasthestrongveilingcouldmodifysubstantiallythestellarparameters(see, e.g.,Manaraetal., 2013b; Herczeg&Hillenbrand, 2014). TheSpT determinedhereiscompat-iblewiththatavailableintheliterature(K6.5 Wilkingetal., 2005), buttheagedeterminedwiththesestellarparametersisratherhigh(∼33Myr), suggestingthatthestellarluminosityisunderestimatedortheSpT isincorrect, similarlytotheobjectsdiscussedinChapter 4(Manaraetal., 2013b).

ForISO-Oph123theUVB spectrumhasenoughSNR tousethefittingproceduredis-cussedinChapter 4. ParticularcareisusedwhenfittingthisobjectinselectingthecorrectwavelengthrangestodeterminethestellarcontinuumintheBalmerandPaschencontinuaregions. Indeed, thisspectrumshowsamultitudeofemissionlinesatallwavelengthswhichmakethecontinuumestimatemoredifficult. Evenifparticularcareistaken, thebestfitdoesnotreproducewelltheslopeoftheBalmercontinuumandprobablyoverestimatestheexcessemissionat λ ≲ 320nm, thustheaccretionluminosity. ThiswillbediscussedinSect. 6.4.3 whenI willcomparethe Lacc obtainedfromtheluminosityoftheemission

166


lineswiththatdeterminedbythisfittingprocedure. TheSpT determinedhereagreeswellwiththatsuggestedbythespectralindicesandfoundintheliterature. Atthesametime,thestellarparametersderived(SpT M4, AV =5mag, L⋆=0.02 L⊙)resultinanestimatedageof ∼ 20Myr, mucholderthantheactualageoftheregion. I thenusethe M⋆ derivedfromtheevolutionarytracksfortherestoftheanalysis, butI cautionthattheageisprobablyover-estimatedduetoanunderestimationofthestellarluminosityforthestrongveilingandthepresenceofmanyemissionlinesintheobservedspectrum.

Finally, thefittingprocedure isusedalso for ISO-Oph032, whoseUVB spectrum isgood. InthisobjectthebestfitisobtainedwiththesameSpT asdeterminedbefore(M6.5)butwith a slightly higher AV due to the fact thatwith thismethodology the effects ofveilingduetoaccretionaretakenintoaccount. I adoptthevalueof AV determinedwiththismethodinthefollowinganalysis.

SpectrawithlowSNR intheVIS arm

FiveobjectsincludedinthissamplehaveaverylowSNR intheVIS armduetotheirstrongextinction, alwaysAV > 10mag. Thespectralclassificationforthesetargetsisdifficult, andshouldbecarriedoutintheNIR,andtheresultsareratheruncertain. TheprocedureI followissimilartothatcarriedoutearlieronwiththeVIS armofspectrawithgoodSNR.StartingfromtheSpT(s)reportedintheliteratureandcheckingalsootherSpTs, I vary AV untilabestvisualmatchisfound. Thisleadstoreasonableagreements, butnotforallthetargets,asI discussinthefollowing. However, inallthecasesthestellarparametersdeterminedresultinanestimatedagemucholderthantherealageof ρ-Oph, thussuggestingthatthestellarluminosityisunderestimatedforthesetargets. OnlyforISO-Oph087I deriveage∼6Myr, buttheresultisstillratheruncertain. InthefollowingI discussthefiveobjectsseparately.

TheNIR spectrumofISO-Oph037lookslikethatofanobjectataveryearlystageofevolution. Itisraisingduetothestrongdisk/envelopecontributionandhasnoclearspectralfeatures. McClureetal. (2010)classifythisobjectas``disk''butreportthatthiscouldbeaflatspectrum, aswell. TheSpT determinedhere(M0)isthesameasthatdeterminedbyMcClureetal. (2010), while Gattietal. (2006)reportedaSpT K5forthistarget. However,theSpT I determineisonlyabestguessforthistarget.

ForISO-Oph087nopreviousestimatesofSpT arereportedintheliterature. Thisisanobjectknowntobevariable, withtwostrongflaresreportedby Parksetal. 2014. Nattaetal. (2006)derived Teff =3090K forthisobjectsassumingasingleisochroneandderivingthestellarluminosityfromtheNIR photometriccolors. ThiscorrespondstoSpT M5-M5.5accordingtotheSpT-Teff relationby Luhmanetal. (2003). ThebestmatchingoftheNIRspectrumisfoundwiththetemplateofSpT M4, andthisvalueleadstoareasonableageestimateof ∼6Myr. However, alsointhiscasethestellarparametersshouldbeconsidereduncertain.

AlvesdeOliveiraetal. (2010)alreadyanalyzedaNIR spectrumofISO-Oph094, andsuggestedthatthisobjectcouldhaveaSpT M3, butwithhighuncertainty. IntheX-ShooterspectraonlytheNIR armhasareasonableSNR,whiletheVIS armhasalmostnoflux. I

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deriveSpT M2forthisobject, butwithhighuncertainty. ThestellarparametersdeterminedhereresultinapositionofthisobjectontheHRD wellbelowthemainsequence. Thissuggeststhattheparametersareincorrect, orthatthisobjectisunder-luminous.

BothforISO-Oph115andISO-Oph165I determinethesameSpT asreportedintheliterature. FortheformerI getM0asreportedby Gattietal. (2006), whileforthelatterI getM2asobtainedby McClureetal. (2010). Inbothcases, however, thestellarparametersleadtoapositionontheHRD correspondingtoobjectswithage≳40Myr, suggestingthattheluminosityisunderestimatedorthatthesetargetsareunder-luminous.

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Figure

6.8:

Spectraof

ρ-O

phtargetsfrom

600

to245

0nm

.Theob

servedspectraareshowninblack.InredIrepo

rtth

eClassIII

YSO

spectrum

withthesameSpToftheobservedtarget.Thisisexinctedandnorm

alizedatλ=1250nmtomatchtheobservedspectrum.Veilingduetoaccretion

ordiskem

ission

isnotin

clud

ed.

169


Figure

6.9:

Spectraof

ρ-O

phtargetsfrom

600

to245

0nm

.SameasFig.6

.8.

170


Figure

6.10

:Spectraof

ρ-O

phtargetsfrom

600

to245

0nm

.SameasFig.6

.8.

171


Figure

6.11

:Spectraof

ρ-O

phtargetsfrom

600

to245

0nm

.SameasFig.6

.8.

172


Figure

6.12

:Spectraof

ρ-O

phtargetsfrom

600

to245

0nm

.SameasFig.6

.8.

173


Table 6.8: Accretionluminosityandmassaccretionratesofthe ρ-OphClass II YSOs

Object logLacc log Macc Detectedlines[L⊙] [M⊙/yr] [#]

ISO−Oph023 -3.76 -10.22 4ISO−Oph030 -2.85 -9.34 6ISO−Oph032 -3.82 -10.21 9ISO−Oph033 -5.10 -11.38 1ISO−Oph037 -1.78 -9.12 4ISO−Oph072 -0.97 -8.33 8ISO−Oph087 <-3.10 <-9.99 0ISO−Oph094 -3.70 -11.33 1ISO−Oph102 -2.71 -9.28 7ISO−Oph115 -2.13 -9.50 2ISO−Oph117 -2.56 -9.43 3ISO−Oph123 -1.93 -8.97 11ISO−Oph160 -3.24 -9.88 4ISO−Oph164 -3.75 -10.30 8ISO−Oph165 -3.00 -10.38 2ISO−Oph176 -4.45 -11.04 1ISO−Oph193 -2.75 -9.54 3

Notes.M∗ andagearederivedusingtheevolutionarytracksof Baraffeetal. (1998). Thelastcolumnreportsthenumberofdetectedemissionlinesinthespectrausedtoderive Lacc.

6.4.3 Accretionpropertiesof ρ-Ophiucusyoungstellarobjects

Theaccretionluminosityforthe ρ-Ophtargetsisderivedusingtheluminosityofvariousemissionlinespresentintheirspectra. Thisisconvertedinto Lacc usingthe Lline-Lacc re-lationsof Alcaláetal. (2014)andreportedhereinEq. (6.1). Aspreviouslydiscussed, thisindirectmethodisnotasaccurateasdirectfittingoftheUV-excess, but Lacc determinedinthiswayareconsistentwithdirectmeasurementsof Lacc whenmultiplelinesareused(e.g., Rigliacoetal., 2012; Alcaláetal., 2014). GiventhehighextinctionandthelowSNRoftheUVB spectraofthetargets, thisistheonlyavailablemethodtodetermine Lacc forthissample.

SimilarlytoSect. 6.2.3 and Alcaláetal. (2014), thefluxoftheemissionlinesisdeter-minedfromtheflux-calibratedandextinction-correctedspectrausingthe splot packageunderIRAF bydirectintegrationoftheline. Thelinefluxistheaverageofthreemeasure-mentsof theintegratedfluxconsideringthelowest, highest, andthemiddlepositionofthelocalcontinuum, dependingonthelocalnoiselevelofthespectra. Theerroristhestandarddeviationofthesethreemeasurements. Fornondetectedlines, I calculatethe3σupperlimitswiththerelationship 3× Fnoise ×∆λ, whereFnoise isthermsflux-noiseintheregionofthelineand ∆λ istheexpectedaveragelinewidth, assumedtobe0.2 nm. Theluminosityofthelinesiscalculatedas Lline = 4πd2 · fline, where d isthedistanceof ρ-Oph(125pc)and fline thefluxofthelinedeterminedasexplainedinthisparagraph. Thelineluminosityisconvertedin Lacc withtherelationsofEq. (6.1), andthefinalvalueof Lacc isdeterminedastheaverageofthevaluesofthedetectedlines, whiletheerroristhestandard

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5

4

3

2

1

ISO-Oph023 ISO-Oph030 ISO-Oph032

5

4

3

2

1

log(L

acc/

L¯) ISO-Oph033

ISO-Oph037 ISO-Oph072

CaK

Hδ

Hγ

Hβ

HeI587

Hα

HeI667

Paδ

Paγ

Paβ

Brγ

5

4

3

2

1

ISO-Oph087

CaK

Hδ

Hγ

Hβ

HeI587

Hα

HeI667

Paδ

Paγ

Paβ

Brγ

ISO-Oph094

CaK

Hδ

Hγ

Hβ

HeI587

Hα

HeI667

Paδ

Paγ

Paβ

Brγ

ISO-Oph102

Figure 6.13: Accretionluminosityderivedfromvariousemissionlinesluminosityforthe ρ-Ophtargets. Eachsubplotshowsthevalueof log(Lacc/L⊙) derivedusingthevariousindicatorsreportedonthex-axis(CaK,Hδ, Hγ, Hβ, Heλ587nm, Hα, Heλ667nm, Paδ, Paγ, Paβ, Brγ). Theredsolidlineistheaveragevaluesobtainedfromthedetectedlines, whilethedashedlinesarethe1σ standarddeviationofthisvalue.

deviationofthesevalues. Figures 6.13-6.14 showthevalueof Lacc determinedwitheachlineforallthetargets. TheaccretionluminosityforthetargetsarereportedinthesecondcolumnofTable 6.8, whilethenumberofdetectedemissionlinesusedtocalculate Lacc isavailableinthelastcolumn.

I usethevalueof Lacc determinedusingtheemissionlinesasthefinalvaluealsoforISO-Oph032andISO-Oph123, forwhichmyfittingprocedurecoulddeterminedirectlyLacc from thefitof theUVB spectrum. This choice isdone tokeep the results for thetargetsconsistent. Moreover, thevalueof Lacc determinedforISO-Oph032fromthefit-tingprocedure(log(Lacc/L⊙)=-3.87)iscompatiblewiththatdeterminedwiththeemissionlines(log(Lacc/L⊙)=-3.82). ForISO-Oph123, thevaluedeterminedfromthefitoftheUV-excess(log(Lacc/L⊙)=-1.22)ismuchhigherthantheoneobtainedfromtheluminosityoftheemissionlines(log(Lacc/L⊙)=-1.93). However, asI discussedintheprevioussection, thisdifferencecouldbeduetothebadfitoftheBalmercontinuumandtoanunderestimationofthestellarluminositywiththefittingprocedure.

InthespectrumofISO-Oph087therearenoemissionlines. Thereforeitisnotpossible

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5

4

3

2

1

ISO-Oph115 ISO-Oph117 ISO-Oph123

5

4

3

2

1

log(L

acc/

L¯)

ISO-Oph160 ISO-Oph164

CaK

Hδ

Hγ

Hβ

HeI587

Hα

HeI667

Paδ

Paγ

Paβ

Brγ

ISO-Oph165

CaK

Hδ

Hγ

Hβ

HeI587

Hα

HeI667

Paδ

Paγ

Paβ

Brγ

5

4

3

2

1

ISO-Oph176

CaK

Hδ

Hγ

Hβ

HeI587

Hα

HeI667

Paδ

Paγ

Paβ

Brγ

ISO-Oph193

Figure 6.14: Accretionluminosityderivedfromvariousemissionlinesluminosityforthe ρ-Ophtargets. SameasFig. 6.13.

todeterminetheaccretionrateofthisobject. I estimateanupperlimitontheaccretionratefromtheupperlimitontheHα lineandusethisvalueintheanalysis.

Fromthevaluesof Lacc andthestellarparametersM⋆ and R⋆ derivedasintheprevioussectionandreportedinTable 6.7, I derive Macc usingEq. (1.2). ThisisreportedforeveryobjectinthethirdcolumnofTable 6.8. InFig. 6.15 I showthevaluesof Macc vsM⋆ derivedhere(blackpoints)andthosefrom Nattaetal. (2004, 2006)andcorrectedfordistancebyRigliacoetal. (2011a)(graypoints). I alsoreportthebestfitofthe Macc-M⋆ relationfortheLupustargetsderivedby Alcaláetal. (2014)asareference. Thevaluesderivedhereforthe ρ-Ophsamplearereportedwith smallfilledcircles fortargetswithuncertainstellarparametersandwith opencircles forobjectswithstrongveiling. InFig. 6.16, instead, Ishowonlythedataforthetargetsincommon, with redcrosses reportingthevaluesfromtheliterature. Withrespecttotheresultsof Nattaetal. (2004, 2006), thevaluesof M⋆ aredifferentinmanycases, andthisisduetothemoreprecisederivationofstellarparametersdonehere. Ingeneral, thevaluesof Macc derivedherearelower, andthiscouldbedueinparttothedifferent Lacc-Lline relationsadoptedhere, andinparttothebetterderivationoftheextinction. ThevaluesofM⋆ derivedhereare, instead, largerthanthoseintheliteratureformanytargets. However, thenewlyestimatedvaluesof M⋆ arecompatiblewiththose

176


2.0 1.5 1.0 0.5 0.0 0.5log(M /M¯)

12

11

10

9

8

7

log(M

acc/

[M¯/y

r])

Natta+06this work

Figure 6.15: Massaccretionrateasafunctionofmassforthe ρ-Ophsample. Valuesderivedfortheobjectsanalyzedherearereportedwith black markers, where bigfilledcircles areused forobjectswithreliablestellarpropertiesestimates, smallfilledcircles forthosewithuncertainstellarparameters, and opencirclesfortargetswithstrongveiling. Valuesfromtheliteraturefor ρ-Oph(Nattaetal., 2004, 2006, correctedfordistanceasin Rigliacoetal. 2011a)areshownwithgraysymbols. Downwardarrowsareusedforupperlimits. ThecontinuouslinerepresentsthelinearfitofthisrelationfortheLupussample(seeEquation 6.3).Thedashedlinesrepresent the1σ deviationfromthefit. Averageerrorbarsareshownintheupper leftcorner.

of Nattaetal. (2004, 2006)forthevastmajorityofobjectswithreliablestellarparametersestimatederivedinmyanalysis. ThelargestdifferencesarefoundforobjectswithhighlyuncertainSpT,asexpected. Inonecase, ISO-Oph176, thespectrumanalyzedherepresentadetectedemissionline(Hα)andI amabletomeasureavalueof Macc lowerthantheupperlimitreportedby Nattaetal. (2004, 2006). Thisobject, however, hasavalueof Macc

significantlylowerthantheoneexpectedbythe Macc-M⋆ relationofLupusforobjectswiththesamemass. Thismeasurementisprobablystronglyaffectedbychromosphericemissioncontributioninthelineluminosityandthusuncertain, asthevalueof Lacc iscompatibletothevalueof Lacc,noise determinedinChapter 3 fromtheluminosityoftheemissionlinesofClass III YSOs. However, this object could also possibly be a low-accreting objectwithasignificantlygas-depleteddisk. Ontheotherhand, I donotdetectanylineinthespectrumofISO-Oph087, forwhich Nattaetal. (2004, 2006)reportavalueof Lacc fromthemeasurementofthePaβ line. Onepossibilityisthatthisobjectisvariable, andthelargedifferenceinthevaluecouldbeinpartduetovariabilityandinparttothelowluminosityderivedhereforthistarget.

Interestingly, thenewlyderivedvaluesof Macc arecompatibleonthe Macc-M⋆ planewiththebestfitrelationoftheLupussampleatleastfornineofthetenspectrawithgood

177


2.0 1.5 1.0 0.5 0.0 0.5log(M /M¯)

12

11

10

9

8

7

log(M

acc/[

M¯/y

r])

Natta+06this work

Figure 6.16: Comparisonofmassaccretionrateasafunctionofmassforthe ρ-Ophsamplewiththeliteraturedata. Valuesfromtheliteraturefor ρ-Oph(Nattaetal., 2004, 2006, correctedfordistanceasin Rigliacoetal. 2011a)forthetargetsanalyzedhereareshownwith redcrosses. Theredlinesconnectthesevalueswiththosederivedhereforthesametargets. TheothersymbolsareasinFig. 6.15.

SNR andnotstrongveilingandforthetwostronglyveiledones. Inparticular, thespreadofvaluesonthisplotatanymassdecreaseswithrespecttotheoneofthesameobjectswiththevaluesderivedby Nattaetal. (2004, 2006). Thefiveobjectswithuncertainstellarparameters(foursmallblackcirclesandoneblackupperlimit)arealllocatedbelowthe1σ deviationfromthebest-fitline. However, giventheuncertaintiesontheirstellarparam-eters, thevaluesof Macc derivedforthesetargetsareprobablynotaccurate, andpossiblyunderestimated, giventhattheir L⋆ areprobablyunderestimated, aswell.

Thesmallerspreadof Macc atany M⋆ foundwiththenewX-Shooterspectraandtheanalysisdescribedinthissectionsuggestthatmostofthespreadobservedinthepastwasdue to themethodologyused. Inparticular, in thecaseof ρ-Oph theuncertain stellarparameterandthepossibilitytouseonlyoneortwoemissionlinestoderive Lacc wasaseverelimitationofpreviousinvestigations. However, thesmallsampleanalyzedinthissectiondoesnotallowtomakefinalstatementson this. I refer toSect. 6.6 foramoredetaileddiscussionincludingallthesamplesanalyzedinthisChapterandinthisThesis.

6.5 NewdataintheUpperScorpiusassociation

TheScorpius-CentaurusorScoOB2associationistheOB associationlocatedclosertotheSun. Itisdividedinthreedistinctgroupswithdifferentages: UpperScorpius(US),UpperCentaurus-Lupus(UCL),andLowerCentaurus-Crux(LCC).Theiragesareestimatedtobeofabout5, 17, and16Myr(Preibisch&Mamajek, 2008). InthewholeScoOB2associationthereisnosignofongoingstarformation, thusthisisanidealregiontostudypropertiesofprotoplanetarydisksafterthecompletionofthestarformationprocess. Strictlyrelatedtothis, thereisalmostnogasanddustcloudsintheregion, sothemembersshowonlyvery

178

6.5 New data in the Upper Scorpius association

Table 6.9: YSOsintheUpperScorpiusassociationanalyzedhere: coordinatesandobservinglog

Object RA(2000) DEC(2000) Obs. date Exp. Timeh :m :s ' '' YY-MM-DD [s]

J160525.5-203539 16:05:25.5 −20:35:39 2010-09-17 750x2J160702.1-201938 16:07:02.1 −20:19:38 2010-09-18 750x4J161420.2-190648 16:14:20.2 −19:06:48 2010-09-19 400x4

Notes. Namesarefrom Preibischetal. (2002). ExposuretimesarethesameinthethreeX-Shooterarms(UVB,VIS,andNIR).

moderateextinction, typically AV ≲2mag. Thisgasanddustfreeenvironmentispossiblytheconsequenceofseveralsupernovaexplosionsandmassivestarswinds, thatclearedtheregionfromdiffusematter. Theevolutionofthesemassivestarsalsocreatedahugesystemofloop-likeH I structuresaroundtheassociation, withatotalmassofabout 3 × 105M⊙.TheseloopsarethoughttobetheremnantsoftheoriginalgiantmolecularcloudinwhichtheOB subgroupsformed(e.g., deGeus, 1992; Preibisch&Mamajek, 2008).

Theyoungersubgroup, andobjectofstudyhere, isUpperScorpius(RA 16hDEC -21d),whichislocatedatadistanceof∼145pc(Blaauw, 1991; deGeusetal., 1989; Preibischetal., 1999; deZeeuwetal., 1999). Theageofthisregionhasbeendeterminedtobe ∼5-6Myrby deGeusetal. (1989)usingthemainsequenceturnoffintheHRD fortheB-typestarsofthissubgroup. ThetotalpopulationofPMS membersofthisassociationhasbeendiscussedin Preibischetal. (2002)and Preibisch&Mamajek (2008)andcomprises364members. Preibischetal. (2002)haveshownthat, using D'Antona&Mazzitelli (1994)models, thewholepopulationof this region, fromlow-massstars tohigh-massstars, islocatedontheHRD aroundthe5Myrisochronewithanobservationaldispersionof1-2Myr. IntheUpperScorpiusassociationonly ∼19%ofthewholesampleofPMS starsarestillsurroundedbyprotoplanetarydisks(Carpenteretal., 2006), asonewouldexpectgiventheageoftheregionandtheknowndiskdispersaltimescale(seeSect. 1.1). Finally,20disksinthisregionhavebeenrecentlyobservedwithALMA todeterminetheirmasses(Carpenteretal., 2014). TheaveragediskmassinthisregionhasbeenfoundtobelowerthaninyoungerregionssuchasTaurus, eventhoughthesignificanceofthisresultisbelow3σ.

HereI presentforthefirsttimetheanalysisoftheX-ShooterspectraofthreeobjectslocatedintheUpperScorpiusassociationandstillsurroundedbyaprotoplanetarydisk.ThesehavebeenobservedduringPr.Id.085.C-0482(PI Montesinos)usingthe1.0′′, 1.2′′,and0.9′′slitsintheUVB,VIS,andNIR arms, respectively. Thecoordinates, dateofobser-vation, andexposuretimesarereportedinTable 6.9.

Data reductionhasbeencarriedoutusing thesameprocedureas in the restof theThesis. Thefluxcalibrationofthepipelinehasbeenfoundtounderestimatesystematicallythefluxofthetargetsobtainedfromtheavailablephotometrybyafactor ∼2. Thiscouldbeduetothebadseeingofthethreenightsoftheobservations(∼2-2.5′′). Therefore, theflux-calibratedspectrareducedusingthepipelinehavebeenmultipliedbyafactor2inall

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Figure 6.17: BestfitoftheUV-excessforthetargetslocatedinUpperScorpius.

Table 6.10: StellarandaccretionparametersfortheUpperScorpiusYSOs

Object SpT Teff AV L⋆ M⋆ age Lacc Macc

[K] [mag] [L⊙] [M⊙] [Myr] [L⊙] [M⊙/yr]J160525.5-203539 M5 3125 0.9 0.05 0.16 3.63 2.16e-04 4.20e-11J160702.1-201938 M5.5 3060 1.2 0.03 0.11 3.59 4.13e-04 9.60e-11J161420.2-190648 K7 4060 3.0 0.12 0.68 62.37 4.13e-01 1.69e-08

Notes. M⋆ andagearedeterminedusingthe Baraffeetal. (1998)evolutionarytracks.

cases.The three spectra are analyzed using themethod described inChapter 4 assuming

RV = 3.1. InthecaseofJ160525.5-203539andJ160702.1-201938agoodfitisfound(seeFig. 6.17)andthebestfitparameters, whicharereportedinTable 6.10, arecompat-iblewiththosederivedintheliterature(Preibischetal., 2002). Ontheopposite, thebestfitforJ161420.2-190648, showninFig. 6.17 aswell, doesnotreproducetheBalmercon-tinuumwellandresultsinasignificantlyunderestimated L⋆, thusinaveryoldage, forthistarget. SimilarlytotheClass II YSO ISO-Oph123discussedintheprevioussection, alsothistargethasaspectrumwithmanyemissionlinesthatmakeitmoredifficulttoestimateaccuratelythevalueofthecontinuuminthespectrum. Forthetimebeing, I adoptthestellarparametersofthebestfitintheanalysisbutI stressthattheseareuncertain. I refertoMontesinosetal. (inprep.) foramoredetailedanalysisofthistarget.

I refertothenextsectionforadiscussiononthevaluesoftheaccretionratesderivedhere, asthesampleavailableinthisregionistoosmalltobediscussedonitsown.

6.6 Discussion

AsreportedinTable 6.1, inthisThesisI havecollectedstellarandaccretionpropertiesforatotalof88accretingYSOs, includingobjectsindifferentstarformingregionsandintwodifferentevolutionarystates, i.e. Class II YSOsandtransitionalobjects. AlltheseobjectshavebeenobservedwithX-ShooterandanalyzedusingtheprocedureexplainedinChap-

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6.6 Discussion

ter 4, orwithsimilarprocedures(e.g., Sect. 6.4). Thissampleisthereforehomogeneousintheanalysismethod, andthusthereshouldbenosystematicdifferencesinthesubsamplesduetodifferentmethods. Inparticular, thetargetedobjectshavedifferentstellarproperties,coveringawiderangeof L⋆, M⋆, andage, anddifferentaccretionproperties(Lacc, Macc).InthissectionI discussthedependenceofaccretiononthestellarpropertiestoderivegen-eralpropertiesoftheaccretionprocessandoftheinnerdiskproperties. Asnotedabove, IincludeinthesampleanalyzedherebothClass II YSOs, thusPMS objectsstillsurroundedbyfulldisks, andtheTDsanalyzedinChapter 5. Thelatterareknowntobeadifferentevolutionarystageofdiskevolution, but, atleastforthesampleanalyzedinthisThesis, Ihaveshownthatthegascontentoftheinnermostregionofthedisk, whichisrelatedtotheaccretionrate, isthesameintheseobjectsasinClass II.Moreover, forsomeobjectsconsideredhereasClass II theclassificationasTD orClass II isstillunderdebate, thusitcouldbepossiblethatsomeobjectsinthesamplehavedifferentouterdiskpropertiesthanitisthought. Therefore, I consideralltheobjectsaltogetherstressingthatI amanalyzingthepropertiesoftheinnerdisk, andnotoftheouterdisk, fortheentiresample6.

Thesampleanalyzedhereis, I stress, homogeneouslyanalyzedbutisnotcomplete.Eachofthesubsamplesdoesnotrepresenttheentirepopulationofitsregion, andthevastmajorityoftargetsincludedherewereknownaccretors. Thispossiblymakesthesample,ingeneral, biasedtowardshigheraccretionrateobjects. Nevertheless, thissample isagoodensemble tostudy thegeneralpropertiesofaccretingYSOs, asit isrepresentativeoftheseobjectsbyvariousmeans. Indeed, itcontainsPMS starscoveringalargestellarparameterspace, i.e. withSpT rangingfromM9toG3, L⋆ from ∼ 1L⊙ to ∼ 10−3L⊙, M⋆

from ∼ 2M⊙ to ∼ 0.01M⊙, andlocatedinregionswithagefrom ∼ 1Myrto ∼ 10Myr.However, incompletenesscouldaffectsomeoftheresultsI willderive, andI willdiscussthisindetailineachofthefollowingsubsections.

6.6.1 Accretionluminosityandstellarluminosityrelation

Thestellarandaccretionparametersderivedmoreindependentlyonevolutionarymodelsare L⋆ and Lacc. Theformerisdeterminedfromthederivedparameters(SpT, AV ), fromtheabsolutefluxcalibrationofthespectrum(orfromphotometry), andfromtheknowledgeofthedistanceofthetarget. ThelatterisobtainedmeasuringthefluxduetoaccretioninexcesstothephotosphereplusabolometriccorrectiontoaccountforthenotobservedfluxintheUV partofthespectrum(seeChapters 2 and 4), orconvertingthefluxofvariousemissionlinesinto Lacc usingcalibratedrelations(seeSect. 6.2.3). NeitherisbasedonmodelsofPMS stellarevolution, thusthesequantitiescanbeconsideredas``purelyob-servational''. Forthesereasons, itisinterestingtoanalyzetheobservedrelationbetweenthose. ThisisshowninFig. 6.18, wheretheaccretingobjectsfromthevarioussamplesareshownwithdifferentsymbols(cf. legendintheplotandcaption). Thedashedlinesontheplotrepresentdifferent Lacc/L⋆ ratios, goingdownwardsfrom1, to0.1, to0.01. The gray

6TheonlyobjectpresentindifferentsamplesisSz84, aTD locatedinLupus. ForthistargetI useinthefollowinganalysisonlytheresultsobtainedinChapter 5, whichdonotdiffersubstantiallyfromthosederivedasinSect. 6.2.

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3 2 1 0 1

log(L /L¯)

6

5

4

3

2

1

0

1

log(L

acc/

L¯)

L acc=L

L acc=0.1×

L

L acc=0.01× L

TDs

Lupus

ONC

ρ-Oph

Upper Sco

σ-Ori

Lacc,noise

Figure 6.18: Accretionluminosityvsstellarluminosityforthewholesample. Datafromthedifferentsamplesarereportedwithvarioussymbolsasinthelegend. Upperlimitsareshownasdownwardarrows. GreenemptydiamondsrepresenttheLupussub-luminousobjects. Theredemptystar, blackemptycircles, andtheblueemptysquarearethetargetswithstrongveiling. Thesmallblackcirclesrepresentthetargetsin ρ-Ophwithhighlyuncertainstellarproperties. Thegrayshadedregionshowsthevaluesof Lacc,noise derivedusingClass III YSOslineluminositiesinChapter 3, andisshownhereasareference. Thethreedottedlinesarethevaluesof Lacc,noise at10Myr(blue), 3Myr(green), and1Myr(black), respectively. Ontheupperleftsidethetypicalerrorsonthetwoquantitiesareshown. Finally, thedashedlinesarefordifferent Lacc/L⋆ ratios,goingdownwardsfrom1, to0.1, to0.01, aslabeled.

shadedregion showsthevaluesof Lacc,noise derivedusingClass III YSOslineluminositiesinChapter 3, andisshownhereasareference. Thethreedashedlinesonthatregionarethenthevaluesof Lacc,noise at10Myr(blue), 3Myr(green), and1Myr(black), respectively.

Mostofthetargets(∼50%)lieonthe logLacc-logL⋆ planebetweenthe Lacc/L⋆=0.1andLacc/L⋆=0.01lines, whileonlythe4stronglyveiledtargetslieclosetothe Lacc/L⋆=1line.AlltheClass III YSOsarelocatedbelowthe Lacc/L⋆=0.01, wherealsoseveralaccretingobjects(∼30%)arefound. Previousstudies(e.g., Nattaetal., 2006; Clarke&Pringle, 2006;Rigliacoetal., 2011a; Manaraetal., 2012)on(almost)completesamplesofClass II YSOsindifferentstarformingregionshaveshownacorrelation, althoughwithalargescatter, ofLacc with L⋆. Theresultsreportedherearesimilartothosefoundintheliteratureregardingtheobservedupperboundaryat Lacc∼L⋆, andaboutthefactthatmostofthetargetshappentohave Lacc/L⋆ ratiosbetween0.1and0.01. Withrespecttopreviousworks, suchas Natta

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6.6 Discussion

etal. (2006)or Manaraetal. (2012), I findasmallerspreadofvaluesof Lacc atany L⋆,whichcouldinpartbeduetoincompletenessofmysample, butismostlyaconsequenceofthemorerobustmethodologyusedinthisThesis. ItisalsoevidentfromFig. 6.18 thatthedistributionof targetsonthe logLacc-logL⋆ planehasaslopelargerthan1, i.e. notparalleltothedashedlines. I computethelinearbestfitrelationusingalltheaccretingobjects (i.e. excluding theClass III YSOs)and includingupper limitsusing theASURVpackage (Feigelson&Nelson, 1985)under the IRAF environment7 and I deriveaslopeof1.5±0.1withastandarddeviationof0.6excludingthefoursub-luminousobjectsintheLupussample, thefivehighlyuncertainones in the ρ-Ophsample, forwhich L⋆ isunreliable, andthefourstronglyveiledClass II YSOs. Thisrelationis, however, dominatedbythetwotargetswith L⋆ ≳ L⊙ (LkHα330andSR21)andbyISO-Oph033, theonlypointat L⋆ < 0.009L⊙. IfI excludealsothesethreepointsI obtain Lacc∝L⋆

1.7±0.1 andthesamestandarddeviationasbefore. ThesameresultisfoundwhenexcludingonlythetwotargetswithL⋆ ≳ L⊙, aswell. ThisvalueisinagreementwithwhatfoundforthesoleLupussample(Alcaláetal., 2014)andforthecompletesampleintheONC of Manaraetal. (2012). Alsoanon-parametricfitofthe logLacc-logL⋆ dataleadstoabestfitwhichiscompatiblewiththelinearfitbothintheslopeandintheshape.

Itisinterestingtonotethat Lacc,noise derivedfortheClass III YSOsinChapter 3 islocatedonthe Lacc-L⋆ planerightat thelowerboundaryofmytargets. Onlyfiveobjectshavemeasurementsinorbelowthischromosphericthreshold. Forfourofthefive, however,Lacc ismeasuredfromtheUV-excess, whileforISO-Oph176thisisderivedfromtheHαline luminosityalone. Thissuggests that Lacc for thisobject ishighlyuncertain, as thismeasurementcouldbesignificantlycontaminatedbychromosphericcontributionintheline. Theupperboundaryofthe Lacc-L⋆ relationseenhere, instead, seemstobearealfeatureandnotanobservationalbias. Indeed, I expectmysampletobeincompleteoflowaccretingobjects, ratherthanofstrongaccretors. Moreover, contraryto Nattaetal.(2006), most of the ρ-Oph targets in commonbetween their andmy sample andwithL⋆ ≲ 0.1L⊙ arenotfoundheretohavevaluesof Lacc>0.1L⊙. SimilartoFig. 6.15, thetargetsincommonarenowfoundtobeconsistentwiththoseintheotherregions, i.e. tofollowthe Lacc∝L⋆

1.7 slope. Thissuggestthattheobjectsfoundinthepastwithhigh Lacc

atlow L⋆ weremisclassified, i.e. Lacc wasprobablyoverestimated, or L⋆ underestimated,andthiscausedalargerspreadofvalueso Lacc atlow L⋆. Thus, I believethattheupperboundaryofthe Lacc-L⋆ relationpresentinFig. 6.18 isreal.

Startingfromtheseobservationalconclusionsmoretheoreticalworkshouldbecarriedouttounderstandthe Lacc-L⋆ relation. Sofar, theonlystudyofthisrelationhasbeencar-riedoutby Tillingetal. (2008)usingsimplifiedPMS stellarevolutionmodels. Theyderivedaslopeof1.7forthisrelation, assuming Macc todecreasewithtimewithanexponent1.5,andusingtheevolutionarymodelsof D'Antona&Mazzitelli (1994)intheanalysis. Tillingetal. (2008)alsosuggest thatunpopulatedregionson the Lacc-L⋆ planecouldbeusedtoconstraindiskmodelsdirectly fromthispurelyobservational relation. This iswhat I

7AllthelinearfitsdonewithASURV inthischapterareobtainedusingthe emmethod task. Resultsfromthistaskandfromthetaskbuckleyjames arecompatible. Othertasks, suchas schmittbin, arenotusedastheyhavemanydrawbacks(cf. IRAF usermanual)andthesampleanalyzedherecomprisescensoreddataonlyononevariable.

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findintheupperenvelopeofmydata. However, theirsimplifiedevolutionarymodelcan-notdescribetheregionintheplaneat L⋆<0.1L⊙, somoredetailedPMS stellarevolutioncalculationsareneededtoexplainmyfindings.

6.6.2 Accretionasafunctionofstellarmass

AsdiscussedinSect. 1.4.1 and 6.2.4 ofthisThesis, variouspreviousworks(Muzerolleetal.,2003; Mohantyetal., 2005; Nattaetal., 2006; Herczeg&Hillenbrand, 2008; Rigliacoetal., 2011a; Antoniuccietal., 2011; Biazzoetal., 2012; Manaraetal., 2012, andreferencestherein)studiedthe Macc-M⋆ relationindifferentsamplesofaccretingYSOsandfoundthatMacc∝M⋆

2, butwithasignificantscatter(upto3 dex)in Macc atanygivenM⋆. InSect. 6.2.4,inparticular, I showedthatthedependenceof Macc with M⋆ isconfirmedfromthedataI analyzehereusingtheLupussamplealone. TheslopeobtainedusingtheseobjectsisMacc∝M⋆

1.8, andthisrelationisfoundtohaveasignificantlysmallerscatterofthedataaroundthebestfit, withastandarddeviationof0.4dex. InFig. 6.19 I showthevaluesoflogMacc vs logMacc forthewholesampleofaccretingobjectsanalyzedhere(cf. captionandlegendforexplanationofthesymbols). WithrespecttotheLupussample(Fig. 6.4)thescatterofvaluesof Macc atany M⋆ isnotsubstantiallyincreased. Inparticular, therearenoobjectssignificantlyabovethe Macc-M⋆ locusoftheLupusClass II YSOs, andthissuggeststhattheupperboundaryofthisrelationisconstrainedbythedatashownhere.Thiscouldbethecaseasthissampleiscomposedmostlybyknownaccretors, thusitismostprobablymissingobjectsinthelowerpartofthe Macc-M⋆ relation, ratherthanintheupperenvelope. Ontheotherside, variousobjectsseemtohavevaluesof Macc attheirM⋆ lowerthantheLupustargets, andevenaslowasthesub-luminousobjectsinLupus.However, alsointhiscasethisdoesnotchangesubstantiallythespreadofvalues.

ConsideringthewholesampleandlinearlyfittingthepointswithASURV,thestandarddeviationaroundthelinearfitwithslope1.7±0.2isof0.7. Thisbecomessmaller(0.6)whileleavingunalteredtheslopewhenexcludingsub-luminousLupustargetsandhighlyuncertain ρ-OphYSOs. ThelatteristhebestfitrelationreportedinFig. 6.19, whoseanalyticformis:

log Macc = 1.71(±0.16) · logM⋆ − 8.45(±0.11). (6.5)

Differentslopes, butalwayscompatiblewithintheuncertainty, arefoundwhenexcludingtheveiledobjectsorthetwohighermassstars(slope∼1.6). Theexactvalueoftheslope,however, isuncertainanddependsonmanyparameters. Inparticular, thechoiceoftheevolutionarymodelusedtoderive M⋆ iscrucial, asthiscanleadtodifferentslopes, andeventodifferentspreadofvalues. HereI havebeenusingoneevolutionarymodel(Baraffeetal., 1998)forallthesubsamples, inordertohaveresultscompatibleamongeachother,buttheexactvaluesoftheslopereportedheredependstronglyonthechoiceofthispartic-ularmodel. Alsoforthe Macc-M⋆ fitanon-parametricanalysisleadstoabestfitcompatiblewithalinearone. However, theslopederivedhereiscompatiblewiththosederivedinthepast(e.g., Nattaetal., 2006; Rigliacoetal., 2011a; Manaraetal., 2012; Biazzoetal.,2012; Alcaláetal., 2014), apartfromthatderivedby Fangetal. (2009)(Macc∝M⋆

3). The

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2.0 1.5 1.0 0.5 0.0 0.5log(M /M¯)

12

11

10

9

8

7

log(M

acc/[

M¯/y

r])

TDs

Lupus

ONC

ρ-Oph

Upper Sco

σ-Ori

Figure 6.19: Massaccretionrateasafunctionofmassforthewholesample. SymbolsareasinFig. 6.18.Thesolidlineisthelinearfitobtainedexcludingonlythesub-luminousandhighlyuncertaintargets, andthedashedlinesrepresentthe1σ deviationfromthefit. Theblackdot-dashedlineisthebestfitfortheLupussampleby Alcaláetal. (2014). Typicalerrorbarsareshownintheupperpartoftheplot.

differencewiththelatteristobeascribedtothedifferentevolutionarymodelandanalysismethodusedinthatwork.

Theslightly larger spreadofvaluesof Macc around thebestfit in thewholesamplewithrespecttotheLupusonecouldbeexplainedasaneffectofage, ashereI includealsoyoungerandolderobjects, or toadifference in the intrinsicpropertiesof thevar-iousregions. Thiscannotbedeterminedwiththissample, aseffectsof incompletenesscouldmodifytheresults. However, thefactthatthespreadinthewholesampleremainssignificantlysmallerthaninpreviousinvestigationsuggeststhatthemethodologyusedinpreviousanalyseswasoneofthemainreasonsfortheobservedscatterofvalues. Indeed,even studies carriedout usingbroad-band spectroscopy including theobservation andmodelingoftheUV-excessinthepast(e.g., Herczeg&Hillenbrand, 2008)wereaffectedbyuncertaintiesduetothenon-simultaneousobservationsatdifferentwavelengthsandtothelowresolutionofthespectra. This, asalsodiscussedby Herczeg&Hillenbrand (2014),couldleadtoanerroneouslyestimateoftheSpT, AV , andveilingofthetargets. Allthesewillresultinalargerscatterintheobserved Macc atanygiven M⋆.

InFig. 6.20 I reportthevaluesof Macc and M⋆ forthewholesampleasinFig. 6.19,

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2.0 1.5 1.0 0.5 0.0 0.5log(M /M¯)

12

11

10

9

8

7

log(M

acc/[

M¯/y

r])

TDs

Lupus

ONC

ρ-Oph

Upper Sco

σ-Ori

Macc,noise

Figure 6.20: Massaccretionrateasafunctionofmassforthewholesample. SymbolsareasinFig. 6.18 andFig. 6.19. Thegrayshadedregionshowsthevaluesof Macc,noise derivedusingClass III YSOslineluminositiesinChapter 3, andisshownhereasareference. Thethreedottedlinesarethevaluesof Lacc,noise at1Myr(black), 3Myr(green), and10Myr(blue), respectively.

butI alsoshowasareferencethevaluesof Macc,noise derivedinChapter 3 asagrayshadedarea. ThisislocatedrightatthebottomofthedistributionofmostofthetargetsontheMacc-M⋆ plane. Mostofthetargetswhichareinthisregionhappentobesub-luminousorhighlyuncertainones. Inanycase, amongthetargetsanalyzedhereonlythoselocatedinthe ρ-Ophclusterhave Lacc measuredfromlineemission. Apartfromthetwohighlyun-certainvalues, onlyISO-Oph176islocatedinaregionoftheplanewhichsuggeststhatitsmeasuredaccretionisstronglycontaminatedfromchromosphericemission, asdiscussedpreviously.

Theanalysiscarriedouthereshowsthatthescatterof Macc atany M⋆ issignificantlyreducedusingthemethoddescribedinthisThesis. Ifthisrelationisduetotheinitialcondi-tionsatdiskformation(e.g., Dullemondetal., 2006), thenthiswouldimplythatthespreadofinitialcorerotationratesissmall, aswell. Then, I havederivedaslopeofthisrelationwhichisclearlynot1andissmallerthan2. Whencomparingwithmodelspredictingthatthe Macc-M⋆ relationisdrivenbyphotoevaporation, mydataareinagreementwiththepro-posedslopeof Ercolanoetal. (2014)obtainedusingX-rayphotoevaporation, andnotwiththatof Clarke&Pringle (2006)derivedfromEUV photoevaporation. AsdiscussedearlyonwhenconsideringtheLupussamplealone, thereisinthedataanalyzedherenoevidenceofthedoublepower-lawbehaviorsuggestedby Vorobyov&Basu (2009), andtheappar-entbi-modalityofpastdatacouldbeascribedtomixing Macc determinedwithdifferentmethodsandevolutionarymodels. Finally, theupperenvelopeofmymeasurementshasaslopesimilartothebulkofthepopulationandlargerthan1, contrarytothepredictionofHartmannetal. (2006), whichdescribesthe Macc-M⋆ relationasaconsequenceoflayereddiskaccretion. A morecompleteviewofthisrelationwillbeavailablewhenapplyingmy

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6.6 Discussion

methodtolargerandcompletesamplesofdata.

6.6.3 Accretionasafunctionofage

InSect. 1.3.1 I havediscussedhowtheevolutionof Macc withtheageofthecentralobjectisimportanttoputconstraintsonthepropertiesoftheprotoplanetarydisk. However, fromanobservationalpointofviewthemeasurementofthe Macc-agerelationisproblematic.

AsI discussedinthisThesis, estimatesof Macc inPMS starsarenottrivialandareaffectedbyvariousuncertainties. However, I havealsoshownhowthemethodologydevelopedhereleadtoreliablederivationsof Macc, andhowthesamplediscussedhereisagoodsampletostudypropertiesofaccretion.

Ontheotherhand, determiningtheexactagesofthetargetsisnotpossibleatpresentdayswiththeprecisionneededtostudythe Macc-agerelation(e.g., Soderblometal., 2014).Indeed, agesaredeterminedfromcomparisonofthederivedpositionofPMS starsontheHRD with theoreticalPMS evolutionarymodels (e.g., Baraffeetal., 1998; D'Antona&Mazzitelli, 1994; Palla&Stahler, 1999; Siessetal., 2000). Thesemodelsaresimplified,forexample theydonot includedifferentaccretionhistoriesof the targets, and lead tosystematicallydifferentvaluesofagesonewithrespecttotheothers. Thiseffectissmallerintheestimated M⋆, whicharethenmorereliable, andthisleadtoprivilegethestudyofthe Macc-M⋆ relationinaccretingYSOsoverthe Macc-ageone. Moreover, asI alsodiscussinthischapter(e.g., Sect. 6.4.2 andSect. 6.5)andinChapter 4, uncertaintiesinthestellarparameterscanleadtoestimatedpositionsontheHRD correspondingtoextremelyoldisochronalageswhichare, infact, unrealistic. EvenplacingtheobjectscorrectlyontheHRD,theintrinsicuncertaintieson L⋆ and Teff (thuson M⋆ andage)generatecorrelateduncertaintiesonthe Macc-ageplanethatleadtosignificantlyunderestimate(i.e. byafactor∼3)theslopeofthedecayof Macc withage(DaRioetal., 2014).

AsestimatesoftheageofeachYSO independentontheevolutionarymodelsarenotpossible, here I make thechoice toassumeall the targets ineachregion tobecoeval,thereforeI assumethemeanageoftheclusterastheageofeachobject. Discussionhasbeenongoingsincemanyyearswhetherstarformingregionsarereallycoeval, andnofinalanswerhasbeenfoundyet, mainlyduetothelargeuncertaintiesintheageestimatesdis-cussedalsohere. Itisplausiblethatarealagespreadof∼1-2Myrispresentineachregion(e.g., Reggianietal., 2011; DaRioetal., 2014; Soderblometal., 2014), butthechoiceofusingthemeanassumedageappeartobethemorereasonablegiventheuncertaintiesontheindividualisochronalages. However, I willnotattempttofitthedistributionofobjectsonthe Macc-ageplanebutonlytoshowsomegeneralproperties.

Thevaluesof Macc vsageareshowninFig. 6.21 foralltheobjectsinmysample(cf.legendandcaptionforexplanationofthesymbols). Asexplainedabove, themeanageofeachregionisusedtoplotthetargetshere. Noclearrelationbetweenthetwoquantitiesisevidentinthisplot, andI donotattempttofitthepointshere. A similarscatteredplotisfoundwhenplottingtheisochronalagesinsteadofthemeanageofthecluster. However,apossibleexplanationforthelargescatteristhatI amplottingobjectswithverydifferent

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2 4 6 8 10 12age [Myr]

12

11

10

9

8

7

log(M

acc/

[M¯/y

r])

TDs

Lupus

ONC

ρ-Oph

Upper Sco

σ-Ori

Figure 6.21: Massaccretionrateasafunctionofageforthewholesample. Themeanageoftheregioninwhichthetargetsarelocatedisused. SymbolsareasinFig. 6.18. Typicaluncertaintiesonthetwoquantitiesareshowninthelowerrightcorner.

M⋆. AsI discussedintheprevioussection, Macc variessteeplywith M⋆, andinmysamplethevaluesof Macc arespreadover ∼4dexfromonesidetotheotherofthe Macc-M⋆ plane(cf. Fig. 6.19). In the following, I firstconsider twosubsamplescomprisingobjects insmallerstellarmassrangestoshowthetypicalsetofparametersthatshouldbeexploredtoreproducetheobservationsthroughviscousevolutionmodels, thenI analyzethewholesamplenormalizingthevaluesof Macc fortheknowndependenceon M⋆ (seeSect. 6.6.2)asdoneby, e.g., CarattioGarattietal. (2012).

ThetwomassbinsI considerherearethatintherangeof M⋆ from0.1 M⊙ to0.3 M⊙,andtheonewith M⋆ rangingfrom0.5 M⊙ to1.0 M⊙. Thesearechosentohaveanar-rowmassrangebutwithenoughtargetsindifferentstarformingregionstosampleenoughstellarages. The Macc-agerelationforthesetwosubsamplesisshowninFig. 6.22. Asex-pected, thespreadofvaluesof Macc atanyageissignificantlyreducedwhenconsideringthetwosubsampleswithrespecttothetotalsample. Alsointhiscase, however, I donotattempttofitthedata, astheuncertaintiesontheagewouldmaketheresultsunreliable.Instead, I trytofindunderwhichdiskphysicalconditionsthepositionofthedataontheseMacc-ageplotscouldberepresentedusingthesimilaritysolutionobtainedassumingavis-cousdisk. AsI discussedinSect. 1.3.1, Hartmann (2009)derivedsimpleexpressionstodescribetheevolutionofaccretingYSOsundertheassumptionofsimilaritysolutionandofadependenceofthethediskviscosity(ν)tothediskradius(R)as ν ∝ R, whichimpliesthat Macc∝ t−1.5. Inparticular, theevolutionof Macc withageundertheassumptionsdis-cussedintheaforementionedsectionisexpressedusingEq. (1.23)andcalculating M(R)

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2 4 6 8 10 12age [Myr]

12

11

10

9

8

7

log(M

acc/[

M¯/y

r])

TDs

Lupus

ρ-Oph

Upper Sco

σ-Ori

(a) 0.1 M⊙≤ M⋆≤ 0.3 M⊙

2 4 6 8 10 12age [Myr]

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8

7

log(M

acc/[

M¯/y

r])

TDs

Lupus

Upper Sco

(b) 0.5 M⊙≤ M⋆≤ 1.0 M⊙

Figure 6.22: Massaccretionrateasafunctionofagefortwosubsampleswithasmallerstellarmassrange.Themeanageoftheregioninwhichthetargetsarelocatedisused. SymbolsareasinFig. 6.18. Therangeofmassesarereportedinthecaptionofeachplot. Inbothpanelsthe blacksolidline isthesimilaritysolutionmodelobtainedusingthefiducialparametersof Hartmann (2009). The redsolidline inpanel(a)isobtainedrescalingthefiducialparameterstothelowerstellarmassandradiusasdiscussedinthetext. Dottedlinesareobtaineddiminishingbyafactorof ∼3theinitialdiskmassoftherescaledfiducialmodelinpanel(a)andofthefiducialmodelinpanel(b)whileleavingtheotherparametersunaltered. Similarly, the dashed-dottedlines refertoa ∼3timessmallerradiusandthe dashedlines toa10timeslargervalueoftheviscosityparameter α. Finally, the greensolidlines arethesimilaritysolutionobtainedwhenthethreeaforementionedparametersaremodifiedaltogether.

at R = 5R⋆. Accordingtothisdescription, theevolutionof Macc withageisdeterminedbythefollowingparameters: theinitialdiskradiusinsidewhich ∼60%oftheinitialdiskmassiscontained(R0), theinitialdiskmass(Md(0)), theviscosityparameter(α), thestellarmass(M⋆), andthedisktemperatureat100AU (T100). Inthisexpressionthedependenceof Macc on M⋆ ismuchshallowerthanthatontheotherparameters.

ThesolutionusingthefiducialparametersofEq. (1.23)- Md(0)=0.1M⊙, R0=10AU,α=10−2, M⋆=0.5M⊙, T100=10K,and R⋆=2R⊙ -suggestedby Hartmann (2009)isshownonbothpanelofFig. 6.22 asa blacksolid line. TheseparametersarerepresentativeofatypicalcTTswith M⋆∼ 0.5 M⊙, and, indeed, seemtorepresentthevaluesof Macc forsometargetsinpanel(b)ofFig. 6.22, i.e. fortargetswith0.5M⊙≤M⋆≤1.0M⊙. However,therearevariousYSOsinthesameplotwhose Macc aresmallerthanthosepredictedbythefiducialmodel. Inorder to reproduce theseobserved Macc, I calculate Macc usingEq. (1.23)withoneparameteratatimedifferentfromthefiducialmodel. I firstlyassumea ∼3timessmallerinitialdiskmass(Md(0)=0.03M⊙, blackdottedline), thena ∼2timessmallerinitialradius(R0=5AU, blackdashed-dottedline), andfinallya10timeslargervalueof α (α = 10−1, blackdashedline). Eachofthesethreeassumptionsleadstosmallerestimatedvaluesof Macc attheagesofmytargetsandallowstoreproducetheobservedvaluesof Macc forsomeotherobjects. Whenmodifyingthesethreeparametersaltogether

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(greensolidline)I obtainevensmallervaluesof Macc intheobservedagerangeandthismodelisabletoreproducetheobserved Macc ofmostoftheremainingtargets. Thus, withacombinationofthesevariationsoftheparametersorwithothervaluesoftheparametersinthesamerangeI canreproducemostofthedata.

I proceedinasimilarwaytoreproducetheobserved Macc ofpanel(a)ofFig. 6.22, i.e. ofobjectswith0.1M⊙≤M⋆≤0.3M⊙. Thefiducialmodelisclearlyoverestimating Macc inthiscase, asitiscalculatedformoremassiveobjects. I rescalethestellarmassto0.2M⊙ and,accordingly, Md(0) to0.04AU,thestellarradiusto1 R⊙ and, accordingly, theinitialdiskradiusto5AU,whileusingthesamevaluefor α andfor T100. Thisrescaledfiducialmodelisshownontheplotwitha redsolidline andrepresentswithgoodagreementthebehaviorofmostofthestrongeraccretingobjectsinthissubsample. IfI wouldmodifylessoneoftheaforementionedparametersI wouldobtainabetteragreementwithsomeofthestrongeraccretorsinthissubsample. Asanexample, I shownwitha reddashed-dottedline therescaledfiducialmodelwithinitialdiskradius R0=10AU,whichwouldmakethevaluesof Macc atanyagelarger. I thencalculatethesolutionvaryingoneparameteratatimeusingthesameratiosasbeforetotherescaledfiducialmodel-a ∼3timessmallerinitialdiskmass(Md(0)=0.01M⊙, reddottedline)anda10timeslargervalueof α (α = 10−1, reddashedline)-andalsovaryingthetwoparametersaltogether(greensolidline). AlsointhiscasewiththeseassumptionsI amabletoreproducetheobserved Macc vsagedependenceformostofthetargets.

Thetypicalsetofparametersthatcanbeusedtoreproducetheobservationsisthenusedtocomparethewholesamplewithviscousevolutionmodels. I showninFig. 6.23thevaluesof Macc normalizedfortheknowndependence Macc∝M⋆

1.7 (seeSect. 6.6.2)asafunctionofage. IfcomparedwithFig. 6.21, thespreadofvaluesonthisplotissignificantlysmaller(≲ 2dex). Moreover, withthisrepresentationI cancomparetheobserveddatawiththeviscousevolutionmodelswithouttheneedtorescalethediskparameterstothoseoftypicalobjectsindifferentmassbins. Indeed, thefiducialmodelandtherescaledfiducialmodel for lowermassstars (cf. Fig. 6.22b, blackandredsolid lines)wouldoverlap inFig. 6.23, i.e. whendividingthepredicted Macc bytheassumed M1.7

⋆ . I canthencomparetheobservationswithasetofsimilaritysolutionviscousevolutionmodelswithvaluesofα rangingfrom10−3 to10−1, Md(0) from0.005 M⊙ to0.1 M⊙, and R0 from5AU to15AU.Theregioncoveredby thesemodels is showninFig. 6.23 asacyanregion, whilethefiducialmodelisshownasablackline. Withthissetofparametersviscousevolutionmodelscanreproducethewholerangeofobserved Macc normalizedtotheirdependencewith M⋆.

Even ifmy sample is incomplete, I have shownhere thatviscousevolutioncan re-producethegeneralobserveddependenceof Macc withagewithsomevariationsofthefiducialparameters. Thisconfirmsthatthegeneralevolutionofdiskscanbedescribedinfirstapproximationwiththesemodels.

However, viscousevolutionmodelsalonestillfailtoexplainsomeobservables. Asanexample, thepresenceofobjectssurroundedbyopticallythindisks(Class III) atallages(cf. Chapter 3)orthepropertiesoftransitionaldisks(cf. Chapter 5)isnotdescribedbythesemodels. Theclearingofdisksatyoungagescouldbeexplainedwithdisklifetimes

190

6.7 Conclusions

2 4 6 8 10 12age [Myr]

12

11

10

9

8

7

log(M

acc/

M1.

7)

TDs

Lupus

ONC

ρ-Oph

Upper Sco

σ-Ori

Figure 6.23: Massaccretionratenormalizedtothestellarmassasafunctionofageforthewholesample. Thevaluesofthemassaccretionratesaredividedforeachobjectbythestellarmasstothepowerof1.7, whichisthedependencethatI derivedinSect. 6.6.2. Themeanageoftheregioninwhichthetargetsarelocatedisused. SymbolsareasinFig. 6.18. Thecyanregionrepresentstheregionofthisplotthatcanbereproducedusingviscousevolutionsimilaritysolutionmodelsfrom Hartmann (2009)andwithreasonableassumptionsonthediskphysicalparameters. TheblacksolidlineisthemodelwithfiducialparameterssuggestedbyHartmann (2009).

verydifferentfromoneobjecttoanother, which, accordingtoEq. (1.22), couldbeduetosignificantlysmaller initialdisksorsignificantlymoreviscousdisks. However, othermechanismsareknowntoplayanimportantroleintheevolutionandcanleadtoafasterdispersalofsomedisksatyoungages. Possiblemechanismsresponsibleofthisfastdispersalcouldbe internal or external photoevaporation, or dynamical disruptionof thedisk inbinarysystemsorincrowdedenvironments, orevenplanetformationitself. ThisThesishasshownthat, instead, Class II objectswithdetectableaccretionrates, i.e. withLacc≳Lacc,noise,havepropertiescompatiblewiththosepredictedbyviscousaccretionmodels.

6.7 Conclusions

Inthischapter, I havediscussedthederivedaccretionpropertiesforatotalofalmost90accretingYSO locatedinvariousstarformingregions. Thissamplecoversalargerangeofstellarproperties, suchasluminosityandmass, andregionswithdifferentages. WithmyanalysismethodI havederivedconsistentlyforthewholesamplethestellarandaccretionpropertiesforthewholesampleandI derivedasignificantlysmallerspreadofaccretionratesatanyvalueofthecentralstarstellarproperties(e.g., L⋆, M⋆)thaninthepast. I have

191


shownwhythisisprobablynotaneffectofincompletenessinmysample, butratheraresultofthemoresophisticatedandreliableanalysismethoddevelopedhere. I havedetermi-nedthat Macc∝M⋆

−1.7, whichiscompatiblewiththepredictionsofX-rayphotoevaporationmodels. Thesmallobservedspreadofvaluesof Macc atany M⋆ couldalsobeexplainedbyasmallspreadofinitialconditionsoftheparentalcloudcoresatthebeginningofthestarformationprocess. Thankstothewelldetermined Macc-M⋆ relationI havebeenabletoremovethedependenceoftheaccretionrateswiththestellarmassandtosearchforapurelyevolutionarytrendofaccretion. I havethenshownthat, ingeneral, thederivedac-cretionarecompatiblewiththosepredictedbyviscousevolutionmodelsusingreasonableassumptionsonthediskproperties. Inparticular, smallchangesontheassumedinitialdiskmassorradius, andontheviscosityparameter α canexplaintheobservedvaluesandtheirspread. However, I havealsodiscussedhowviscousevolutionmodelsalonefallshortofexplainingsomeobservables, suchasthepresenceofdisk-lessYSOsatanyageinanystarformingregion.

192

7Conclusions

HereI summarizethemainachievementsandfindingsofthisThesis, andI discusssomeofpossiblefollow-upinvestigationsanddevelopmentsofthiswork.

DuringmyThesis, I haveworkedonvarioussamplesofaccretingYSOs, developingmyownanalysismethodtostudytheirproperties. I havedeterminedstellarandaccretionpropertiesofYSOstoultimatelyconstrainthecurrentmodelsofdiskevolution. MymethodisbasedonfittingvariouscontinuumandabsorptionfeaturesofobservedClass II YSO X-ShooterspectraatvariouswavelengthstodeterminesimultaneouslyandautomaticallySpT,AV , and Lacc. I codedtheslabmodelusedtoreproducetheemissionduetoaccretion,andcollectedalargesampleofspectraofnonaccreting(Class III) YSOsthatI havebeenusingasphotospherictemplatesinmyanalysis. ThismethodrepresentsabigleapforwardinthefieldasitallowsforthefirsttimetodeterminestellarandaccretionpropertiesofPMS starswithnopriorassumptions. ThisimportantdevelopmentwasmadepossiblealsothankstothespectraobtainedwithX-Shooter, asecondgenerationESO/VLT instrumentwithmedium-resolution, broad-bandcoverage, andgreatsensitivityalsointheUV partofthespectrum. Inthefutureitwouldbeprofitabletoincludeadiskmodelinthegridtostudythestellar, accretion, andinnerdustydiskpropertiessimultaneously. ThiswouldbepossiblealreadywiththedataanalyzedinthisThesis, astheX-Shooterspectraextendstothe K-bandintheNIR,wherethecontributionofthedustyinnerdisktotheobservedfluxisalreadyimportant. Furthermore, amorecompletegridofClass III YSO spectrawouldhelptostudythelowermassobjectsandthehighermassones.

ThestudyoftwoobjectswithanapparentlyoldageintheOrionNebulaCluster(ONC)hasshownthatthefittingproceduredevelopedinthisThesisleadstocorrectstellarandaccretionparametersforobjectswherephotometricstudiesorspectroscopicstudieswithsmallwavelengthcoveragefounddegeneratesolutions. Thisalsosuggeststhatothertargetswithanomalousageinthisandotherregionscouldbemisclassifiedintheliterature, buttherealextentofthiseffectisunknown. Toquantifytheseeffects, I amleadinganinternationalteamthathasproposedanextensiveX-ShootersurveyoftheONC.

I havealsostudied, inthisThesis, theaccretionpropertiesofamoreevolvedclassofobjects, thetransitionaldisks(TDs), whoseinnerdiskissignificantlydepletedofdust. IhaveshownthatatleastsomeoftheseobjectshaveinnergaseousdiskpropertiessimilartothoseofcTTsdisks. Thisisduetothefactthattheiraccretion, andalsowindproperties,areverysimilartothoseofClass II YSOs. Thisallowedmetoderivethepropertiesofthe

193

7. Conclusions

gascontentoftheirinnerdisk, suchasthedensityandtheradialextent. Toexplainthisobservationsundersteady-stateassumptions, anefficientmechanismtotransportgasfromtheouterdisktotheinnerregionsofthesystemthroughthedustdepletedgapisneeded.ThequestionnowiswhetherthisistrueonlyforthisrelativesmallsampleofobjectsorfortheentireclassofTDs. FuturecompletesurveysofTDsdonewithX-Shooterwillfinallyanswerthisquestion. Moreover, abetterunderstandingofthemechanismdrivingthewindtracedbytheforbiddenlinesdetectedinthesetargetswillhelptoconstrainthecurrentmodelsexplainingtheevolutionfromClass II toTD andtobetterconstraintheextentanddensityof thegaseous innerdisk inTDs. Forthisreason, I havealreadycollectedwithothercollaboratorshigher-resolutionspectrawith theVLT/UVES instrumenttostudythelineprofileofvariousforbiddenlinesinthespectraoftheTDsanalyzedinthisThesis.

The largeamountofdatacollected in thisThesis, meaningthestellarandaccretionpropertiesforalmost90accretingYSO locatedinvariousregions, allowedmetostudythedependenceofaccretionwithvariousstellarparameters. Thefirstresultofthisanalysisisthatthespreadofaccretionratesatanyvalueofthecentralstarstellarproperties(e.g., L⋆,M⋆)observedinthepastissignificantlysmallerwhenderivedusingmymoreaccurateana-lysis. Thisisprobablynotaneffectofincompletenessinmysample, butratheraresultofthemoresophisticatedandreliableanalysismethoddevelopedhere. Thederivedslopeofthe Macc-M⋆ relationiscompatiblewiththepredictionsofX-rayphotoevaporationmodels,whilethesmallobservedspreadofvaluescouldalsobeexplainedassimilarinitialcon-ditionsoftheparentalcloudcore. WithmydataI wasalsoabletoshowthatthegeneralpropertiesofaccretionarecompatiblewiththosepredictedbyviscousevolutionmodelsusingreasonableassumptionsonthediskproperties. Accordingtomyanalysis, someofthespreadof Macc atanygiven M⋆ andinagivenregioncouldbeascribedtodifferentinitialdiskmassandradiusortodifferentvaluesoftheviscosityparameter α. However, Ihavealsodiscussedhowviscousevolutionmodelsalonefallshortofexplainingsomeob-servables, suchasthepresenceofdisk-lessYSOsatanyageinanystarformingregion. Thedatapresentedherearealsoagreatbenchmarkforfuturetheoreticalstudies, eventhoughthisisnotacompletesampleofaccretingYSO.Inthefuture, withmycollaborators, I plantocontinuetocollectdataintheseandotherstarformingregionsinordertogetcompletesamplesofYSOstobestudiedwithmymethod, andtobetterconstraintheaccretiondiskphysics.

Toconclude, I haveshownthatthecurrentpictureofdiskevolutionisabletoreproducesomeof theobservedpropertiesofdisks. However, myanalysishasalso showed thatcurrentobservationsallowtobetterconstrainthemodelsastheyshowsignificantlysmallerspreadofvalues. WithcompletesamplesinvariousstarformingregionsI couldbeabletooffer an evenbetter observational benchmark to theorists. At the same time I havealsoshownthat thegeneralpictureofdiskevolutionstillcannotexplaineverything. Abetterobservationalandtheoreticalunderstandingoftheimportanceofothermechanismsdrivingtheevolutionofdisks, suchasphotoevaporationordynamicalinteraction, isneededtofirmlyunderstandtheevolutionofprotoplanetarydisksanditsconnectionwithplanetformation. Thefutureandpresentlargesurveyswillgiveusthepossibilitytoconstrainthevariousmodelswithmoreaccuracy. FutureX-Shootersurveysofaccretionincomplete

194

sampleswillsurelybecrucial. Atthesametime, on-goingsurveyssuchastheGAIA-ESOsurveywillhelpastheywillprovidepropertiesforcompleteandunbiasedsamplesofYSOstothecommunity. GAIA itselfwillhelpthefieldsubstantially, asitwilldrasticallylowertheuncertaintiesaffectingthepresentdaystudiesduetothepoorknowledgeofthedistanceoftheobjects. Finally, surveysofstarformingregionswithALMA willgiveusforthefirsttimethepossibilitytocomparestellaranddiskpropertiesinlargesamplesofdata. Forexample,I aminvolvedinasurveyoftheLupusI andIII cloudsaimedatstudyingthedustandgascontentofthediskspresentinthatregion. ThiswillprovideagreatsampletocomparewiththeobjectslocatedinLupusobservedwithX-ShooteranddiscussedinthisThesis.

Thewealthofdatawithunprecedenteddetails, thenewanalysismethoddeveloped,andtheknowledgeacquiredwillmakeitpossibletohaveanevenmorecompleteandpreciseunderstandingofthewholediskevolutionprocess, andthusoftheformationofplanetslikeours.

195

7. Conclusions

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209

Acknowledgments

FirstofallI sincerelywanttothankLeonardoforhissupervisionduringtheseyears. Youkeptmemotivated, onpath, andtaughtmealotaboutthepracticalaspectsandthespiritofthisjob. Youshowedmethatyouhavetofollowyourideas, thatyouhavetoputallyourselfinwhatyoudo. Andmanythingsmore. AlsothankstoyoursupervisionI willneverregrettohavedonemyPhD inMunichandnotinHonolulu!

A specialthanksalsotoAntonella, forherpatience, herguidance, andhersuggestions.I amalsoverygratefultoJuan, forhisavailabilityandhisdetailedexplanations. I thankBarbaraforherhelpandenthusiasm, andNinaforarrivingrightintimeformydefense!

I want to acknowledge the support, suggestions, andcomments froma greatmanypeople, includingThomasPreibisch, GiacomoBeccari, LucaRicci, MassimoRobberto,GiovanniRosotti, NicolaDaRio, LeeHartmann, GregHerczeg, BeateStelzer, andAndreaBanzatti. I amalsogratefulforthediscussionsI hadattheAccretion/StarFormationcoffeeatESO.

A specialthanksformytravelmateGrainneCostigan, inparticularforheravailability,herproofread, andthegreatdailydiscussionswehad. Andthatwewillhave.

I thankESO,andinparticularEricEmsellem, forthesupport, andmyfellowmentorsGiacomoandIzaskunfortheirsuggestions. I amgratefulfortheofficematesI had, Car-olina, Carina, Leticia, Julian, Karina, andalsoDominika, evenifforashorttime. I wouldalsoliketothankallthestudentsandfellowsthathavecometoESO inthelastyears. Ithasbeenagreatcompany! Inparticular, aspecialthankyoutoAnnaFeltre, thebestnext-officemateI haveeverhad, andalsotoGrainne, Leticia, Dominika, Margherita, Sebastian,Roberto, Peter, Alvaro, Leo, Anja, AnnaMcLeod, Marco(s), andmanymore.

I alsowanttothanktheinstitutesthatI visitedinthelastyears, inparticularArcetri,DIAS,andSTScI.A specialthankstoLuca, Lorenzo, Rachel, Andrea, andGiuliafortheirsupportinFlorence, Dublin, andBaltimore.

Finally, I wanttothankmyfriendsintheseyearsinMunich, inparticularLorenzo, Jack,Massi, Tommi, Dome, Vito, Marco, Anna, Thomas, Berno, Davide, Veronica, Michi, dieVero, dieFranci, Paolo, Diego, Andrea, BeppeeTitta, Fedeandmanyothers, includingthosearoundtheworld. Andaparticularthankstomytwogirls: Anna, forsustainingmeeveryday, andCaterina, formakingussmileatthebeginningofeverynewday.

211

Referredpublications

[11] `Ònthegascontentoftransitionaldisks: aVLT/X-Shooterstudyofaccretionandwinds''Manara, C.F., Testi, L., Natta, A., Rosotti, G., Benisty, M., B.Ercolano, Ricci, L.,A&A,submitted

[10] ``StrongBiasesinEstimatingtheTimeDependenceofMassAccretionRatesinYoungStars''DaRio,N., Jeffries,R.D., Manara,C.F., Robberto,M., 2014, MNRAS,439, 3308

[9] ``The M -M⋆ relationofpre-mainsequencestars: aconsequenceofX-raydrivendiscevolution"Ercolano, B., Mayr, D., Owen, J., Rosotti, G., Manara, C.F., 2014, MNRAS,439,256

[8] ``Timemonitoringofradiojetsandmagnetospheresinthenearbyyoungstellarclus-terR CoronaeAustralis''H.B.Liu, R.Galvan-Madrid, J.Forbrich, etal. (incl. Manara, C.F.), ApJ,780, 155

[7] `À simultaneousUV-to-MIR monitoringofDR TautoexplorehowwatervaporinthediskisaffectedbyaccretionvariabilityduringtheT Tauriphase''Banzatti, A., Meyer, M., Manara, C.F., Pontoppidan, K., Testi, L., 2014, ApJ,780,26

[6] ``X-Shooter spectroscopyofyoungstellarobjects: IV.Accretion in low-massandsub-stellarobjectsinLupus''Alcala, J., Natta, A., Manara, C.F., Spezzi, L., Stelzer, B., Frasca, A., Biazzo, K.,Covino, E., Randich, S., Rigliaco, E., Testi, L., etal., 2014, A&A,561, A2

[5] ``X-Shooter spectroscopyofyoungstellarobjects: III.Photosphericandchromo-sphericpropertiesofClassIII objects''B.Stelzer, A.Frasca, J.M.Alcala,Manara, C.F., K.Biazzo, E.Covino, E.Rigliaco, L.Testi, S.Covino, V.D'Elia, 2013, A&A,558, 141

[4] `ÀccuratedeterminationofaccretionandphotosphericparametersinYoungStellarObjects: thecaseoftwocandidateolddisksintheOrionNebulaCluster''Manara, C.F., Beccari, G., DaRio, N., DeMarchi, G., Natta, A., Ricci, L., Robberto,M., Testi, L., 2013, A&A,558, 114

[3] ``TheHubbleSpaceTelescopeTreasuryProgramontheOrionNebulaCluster''M.Robberto, D.Soderblom, E.Bergeron, V.Kozhurina-Platais, R.Makidon, etal.(incl. Manara, C.F.), 2013, ApJS,207, 10

[2] ``X-Shooterspectroscopyofyoungstellarobjects: II.Impactofchromosphericemis-siononaccretionrateestimates''Manara, C.F., Testi, L., Rigliaco, E., Alcala, J., Natta, A., Stelzer, B., Biazzo, K.,Covino, E., Covino, S., Cupani, G., DElia, V., Randich, S., 2013, A&A,551, 107

[1] ``HST measurementsofMassAccretionRatesintheOrionNebulaCluster''Manara, C.F., Robberto, M., DaRio, N., Lodato, G., Hillenbrand, L., A., Stassun,K., Soderblom, D., 2012, ApJ,755, 154

215

Conferencecontributions[13] `Àccretioninyoungstellarobjects: acompleteviewwiththeVLT/X-Shooterspec-

trograph.'', Manara, C.F.; talk attheconference: "CoolStars18", LowellObser-vatory, Flagstaff, USA,June2014.

[12] `Ònthegascontentoftransitionaldisks: aVLT/X-Shooterstudyofaccretionandwinds.'',Manara, C.F.; poster attheconference: "CoolStars18", LowellObserva-tory, Flagstaff, USA,June2014.

[11] ``Theevolutionofprotoplanetarydisksinyoungstellarclusters.'',Manara, C.F.; talkat theconference: "The formationof the solar system", MaxPlanck InstituteofRadioastronomy, Bonn, Germany, May2014.

[10] ``TheOrionNebulaClusterasabenchmarkforthestudyoftheaccretionprocess.'',Manara, C.F.; talk attheconference: "TheOrionNebulaClusterasaparadigmforstarformation", STScI,Baltimore, USA,October2013.

[9] ``TheimprintofaccretionontheUV spectrumofYSOs: anX-Shooterview.'', Ma-nara, C.F.; talk attheconference: "UV-challengesinAstronomy", ESO,Garching,October2013.

[8] `À VLT/X-ShooterstudyofaccretionandphotoevaporationinTransitionalDisks'',Manara, C.F., Testi, L., Natta, A., Ricci, L., etal.; poster attheconference: "Proto-stars&PlanetsVI", Heidelberg, Germany, July2013.

[7] `À VLT/X-ShooterstudyofaccretionandphotoevaporationinTransitionalDisks'',Manara, C.F., Testi, L., Natta, A., Ricci, L., etal.; poster attheconference: "IAUsymposium299: Exploringtheformationandevolutionofplanetarysystems", Vic-toria, BritishColumbia, Canada, June2013.

[6] ``Thetimescalesofprotoplanetarydiskevolution. Newscenarioopening?'', Man-ara, C.F., Beccari, G., Testi, L., Ricci, L., etal.; poster attheconference: "PlanetFormationandEvolution", Munich, Germany, September2012.

[5] `ÙnderstandingstarformationwiththeVLT/X-Shootereye", Manara, C.F., talk attheconference: "30yearsofItalianparticipationtoESO", Rome, Italy, July2012.

[4] `ÀnX-ShooteranalysisofchromosphericactivityofClassIII lowmasssources'',Manara, C.F., Testi, L., Natta, A., etal.; poster attheconference: "TheLabyrinthofStarFormation", Crete, Greece, June2012.

[3] ``TheHST TreasuryProgramontheONC:MassAccretionRatesEstimates'',Manara,C.F., talk attheconference: "FormationandevolutionofVeryLowMassStarsandBrownDwarfs", Garching, Germany, October2011.

[2] ``HST measurementsofmassaccretionratesintheOrionNebulaCluster'',Manara,C.F., Robberto, M., DaRio, N.etal.; poster attheconference: "TransportProcessesandAccretioninYSOs", SchlossRingberg, Germany, February2011.

[1] ``TheHST TreasuryProgramontheONC -MassAccretionRatesDetermination'',Robberto, M., Manara, C.F., DaRio, N.; poster attheconference: ``SciencewiththeHubbleSpaceTelescopeIII.Twodecadesandcounting'', Venice, Italy, October2010.

216

Acceptedproposals

[16] ``Disk Demographics in Lupus.'', Proposal 2013.1.00220.S, 6 hrs, ALMA, PIWilliams.

[15] ``ALMAmeasurementsofdiskturbulence.'', Proposal2013.1.00601.S,3hrs, ALMA,PI Isella.

[14] ``A VLT/UVES detailed study tounderstandwindemission in transitionaldisks.'',Pr.Id.093.C-0658, 9hrs, VLT/UVES, PI Manara.

[13] ``TheJet-AccretionConnectioninYoungStellarObjects: CoordinatedObservationswithKMOS andtheJVLA.'', Pr.Id.093.C-0657, 12hrs, VLT/KMOS,CoI,PI Galvan-Madrid.

[12] ``The Jet-AccretionConnection inYoungStellarObjects'', Pr.Id.VLA/14A-120, 18hrs, VLA,CoI,PI Liu.

[11] ``Spectroscopic followup of long living circumstellar discs in the star clus-ters NGC3572 and NGC3590 discovered withWFI'', Pr.Id.092.C-0820, 8 hrs,VLT/FLAMES,CoI,PI Beccari.

[10] ``TheJet-AccretionConnectioninYSOs: KMOS MappingofNear-InfraredLines.'',Pr.Id.060.A-9463, 2 hours, VLT/KMOS (Science Verification), CoI, PI Galvan-Madrid.

[9] ``The embedded stellar population of ρ-Oph.'', Pr.Id.060.A-9459, 4 hours,VLT/KMOS (ScienceVerification), CoI,PI Fedele.

[8] ``ConnectingAccretionVariationstoStellarRotation.'', Pr.Id.060.A-9453, 5hours,VLT/KMOS (ScienceVerification), CoI,PI Costigan.

[7] ``MonitoringMagneticFields: SearchingfortheConnectionBetweenVariableMag-netic Fields andAccretionVariability.'', Pr.Id.091.C-0768, 4nights, VLT/CRIRES,CoI,PI Costigan.

[6] ``CircumstellardiscsandstarformationhistoryinthemassiveclusterTrumpler14.'',Pr.Id.091.C-0876, 4hours, VLT/FLAMES,CoI,PI Beccari.

[5] ``ConstrainingtheevolutionanddispersalofTransitionalDisksthroughX-Shootersignaturesofaccretionandphotoevaporation'', Pr.Id.090.C-0050, 3hours, VLT/X-Shooter, PI Manara.

[4] ``WFI surveyof circumstellar discs innearby star clusters.'', Pr.Id.090.C-0647, 7nights, ESO/MPG 2.2m/WFI,CoI,PI Beccari.

[3] ``Characterizingthemassaccretionratesinyounglow-massstarsatlowmetallicity'',4orbits, HST/ACS-WFC3, CoI,PI DaRio.

[2] ``ConstrainingtheevolutionanddispersalofTransitionalDisksthroughX-Shootersignaturesofaccretionandphotoevaporation'', Pr.Id.089.C-0840, 8hours, VLT/X-Shooter, PI Manara.

[1] ``Canplanetformationhappenonverylongtimescales? PreparationofanALMAmassive and long-liveddisk confirmation.'', Pr.Id.288.C-5038, 1.5hours, VLT/X-Shooter, DDT, PI Manara.

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the physics of the accretion process in the formation and evolution

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