White Dwarf Spectra and White Dwarf Spectra and AtmospheresAtmospheres

Tala MonroeTala Monroe

A540 Stellar AtmospheresA540 Stellar Atmospheres

Apr. 6, 2005Apr. 6, 2005

OutlineOutline

• HistoryHistory

• Current Classification SchemeCurrent Classification Scheme

• SpectraSpectra

• AtmospheresAtmospheres

• Spectral EvolutionSpectral Evolution

• Future WorkFuture Work

HistoryHistory• Bessell (1844)-variability in proper motions of Sirius and Bessell (1844)-variability in proper motions of Sirius and

ProcyonProcyondark companionsdark companions• Clark (1861) visually sighted Sirius BClark (1861) visually sighted Sirius B• Schaeberle (1896) Lick Obs. announced Procyon’s companionSchaeberle (1896) Lick Obs. announced Procyon’s companion• 40 Eri (faint white and red stars)40 Eri (faint white and red stars)

– Class A0, Russell dismissed when 1Class A0, Russell dismissed when 1stst Russell diagram published Russell diagram published– Adams confirmed A-typeAdams confirmed A-type

• Adams (1915)-Sirius B spectrumAdams (1915)-Sirius B spectrum Type A0 Type A0 • Eddington (1924) Mass-Luminosity RelationshipEddington (1924) Mass-Luminosity Relationship

– Coined “white dwarfs” for 1Coined “white dwarfs” for 1stst time time– Deduced mass and radius of Sirius BDeduced mass and radius of Sirius B density=53,000x water density=53,000x water

• Fowler (1926) WDs supported by electron degeneracy Fowler (1926) WDs supported by electron degeneracy pressure, not thermal gas pressurepressure, not thermal gas pressure

• Chandrasekhar (early 1930s) worked out details of white Chandrasekhar (early 1930s) worked out details of white dwarf structure, predicted upper mass limit of 1.44 Mdwarf structure, predicted upper mass limit of 1.44 Msunsun, & , & found mass-radius relationfound mass-radius relation

Early ClassificationsEarly Classifications

• Kuiper (mid-1930s, Lick Obs.) WDs found Kuiper (mid-1930s, Lick Obs.) WDs found in increasing numbersin increasing numbers– 1941 introduced 11941 introduced 1stst WD classification scheme WD classification scheme

• w in front of spectral type and Con starsw in front of spectral type and Con stars

• Luyten (1921) proper motion studies from Luyten (1921) proper motion studies from faint blue star surveysfaint blue star surveys– 1952 presented new scheme for 44 WDs1952 presented new scheme for 44 WDs

• D for true degeneracy, followed by A, B, C, or FD for true degeneracy, followed by A, B, C, or F

• Greenstein (1958) introduced new schemeGreenstein (1958) introduced new scheme– 9 types9 types

Current ClassificationsCurrent ClassificationsSion (et al. 1983)Sion (et al. 1983)

• ~2200 WDs w/in ~500 pc of Sun~2200 WDs w/in ~500 pc of Sun• D=degenerateD=degenerate• Second Letter-primary spectroscopic signature in Second Letter-primary spectroscopic signature in

opticaloptical– DA-Hydrogen lines (5000K<TDA-Hydrogen lines (5000K<Teffeff<80000K)<80000K)– DB-He I lines (TDB-He I lines (Teffeff<30000K)<30000K)– DC-Continuous spectrum (TDC-Continuous spectrum (Teffeff<11,000K)<11,000K)– DZ-Metal lines (Mg, Ca, Fe)DZ-Metal lines (Mg, Ca, Fe)– DQ-Atomic/Molecular carbon featuresDQ-Atomic/Molecular carbon features– DO-He II lines (TDO-He II lines (Teffeff>45,000K)>45,000K)

• Additional letters indicate increasingly weaker or Additional letters indicate increasingly weaker or secondary features, e.g. DAZ, DQABsecondary features, e.g. DAZ, DQAB– P-polarized magnetic, H-non-polarized magnetic, V-variable P-polarized magnetic, H-non-polarized magnetic, V-variable

• TTeffeff indicated by digit at end; 50,400/T indicated by digit at end; 50,400/Teffeff, e.g. DA4.5, e.g. DA4.5• New class TNew class Teffeff<4000K, IR absorption for CIA by H<4000K, IR absorption for CIA by H22

DA SpectraDA Spectra

Rapid settling of elements Rapid settling of elements heavier than H in high log gheavier than H in high log g

DB SpectraDB Spectra

DQ Stars & SpectraDQ Stars & Spectra

• Helium-rich Helium-rich stars, stars, generally generally characterizecharacterized by Cd by C22--Swan bandsSwan bands

• Hotter DQs Hotter DQs have C Ihave C I

PG 1159 PG 1159 SpectraSpectra

• Features due to Features due to CNO ions, CNO ions, TTeffeff>100,000K>100,000K

• Absence of H or Absence of H or He I features; He He I features; He II, C IV, O VIII, C IV, O VI

ZZ Ceti

Magnetic WDsMagnetic WDs

• About 5% of field About 5% of field white dwarfs display white dwarfs display strong magnetismstrong magnetism

• 3 classes of H-3 classes of H-atmosphere MWDs atmosphere MWDs based on field based on field strengthstrength

• He-atmosphere He-atmosphere MWDs have unique MWDs have unique featuresfeatures

Basic PictureBasic Picture• 75% DA, 25% non-DA75% DA, 25% non-DA• Spectral classification provides info about Spectral classification provides info about

principal constituent, with some T infoprincipal constituent, with some T info• Progenitors: Post-AGB stars, central stars of Progenitors: Post-AGB stars, central stars of

planetary nebulae (CSPN), hot subdwarfsplanetary nebulae (CSPN), hot subdwarfs• Expected structure-stratified object with Expected structure-stratified object with

<M>~0.6M<M>~0.6Msunsun– C-O core, He-rich envelope, H-rich shellC-O core, He-rich envelope, H-rich shell

• O-Ne cores-most massiveO-Ne cores-most massive– Atmosphere contains <10Atmosphere contains <10-14-14 M M

• Many WDs have pure H or He atmospheresMany WDs have pure H or He atmospheres• Thicknesses of H and He Thicknesses of H and He

Mechanisms in AtmosphereMechanisms in Atmosphere

• Gravitational diffusionGravitational diffusion

• ConvectionConvection

• Radiative levitationRadiative levitation

• MagnetismMagnetism

• AccretionAccretion

• Wind-lossWind-loss

• T-sensitive T-sensitive T determines chemical T determines chemical abundancesabundances

Effects of MechanismsEffects of Mechanisms• Diffusion & SettlingDiffusion & Settling

– Gravitational separation leads to pure envelope of Gravitational separation leads to pure envelope of lightest element t<10lightest element t<108 8 yryr• But, observations show traces of heavier elementsBut, observations show traces of heavier elements

– radiative levitationradiative levitation– Cooler WDs result of recent accretion eventCooler WDs result of recent accretion event

• Radiative Levitation T>40kKRadiative Levitation T>40kK– Radiative acceleration on heavy elementsRadiative acceleration on heavy elements

• Convection for T<12kKConvection for T<12kK– Convection zone forms and increases inward as star Convection zone forms and increases inward as star

coolscools– For He envelopes, convection begins at high TFor He envelopes, convection begins at high T– Mixing changes surface compositionMixing changes surface composition– Need to couple models of atmospheres and interiorsNeed to couple models of atmospheres and interiors

StatisticsStatistics

• T>45kK DA far outnumber DOT>45kK DA far outnumber DO– Ratio increases to about 30kK (diffusion)Ratio increases to about 30kK (diffusion)

• DB gap in 45k-30kK rangeDB gap in 45k-30kK range– Float up of HFloat up of H

• Always enough H to form atmosphere?Always enough H to form atmosphere?– Dredge up of HeDredge up of He

• T<30kK He convection zone massive T<30kK He convection zone massive engulfs outer H layer if thinengulfs outer H layer if thin– 30kK-12kK 25% stars revert to DB spectral type 30kK-12kK 25% stars revert to DB spectral type

(edge of ZZ Ceti Strip)(edge of ZZ Ceti Strip)– Convection zone increases as T decreases. At Convection zone increases as T decreases. At

T~11kK, numbers of DAs and non-DAs are T~11kK, numbers of DAs and non-DAs are ~equal (ZZ Ceti Strip)~equal (ZZ Ceti Strip)

• ‘‘Non-DA gap’ for 5000-6000K dearth of He Non-DA gap’ for 5000-6000K dearth of He atmospheresatmospheres

Spectral EvolutionSpectral Evolution• GapsGapsindividual WDs undergo spectral evolutionindividual WDs undergo spectral evolution

– Compositions change, DACompositions change, DADBDBDA, as T changesDA, as T changes• Evolution of convection zone? Accretion?Evolution of convection zone? Accretion?

• Explanation of ‘non-DA gap’-opacity? Bergeron et Explanation of ‘non-DA gap’-opacity? Bergeron et al.al.– Low opacity of He I means small amounts of H dominates Low opacity of He I means small amounts of H dominates

opacityopacity– HH-- atomic energy levels destroyed when H added to dense atomic energy levels destroyed when H added to dense

atmosphere-reduces H opacity contributionatmosphere-reduces H opacity contribution– Must accrete a lot of H to make difference in photospheric Must accrete a lot of H to make difference in photospheric

conditionsconditionsDA (fixes 6000K edge)DA (fixes 6000K edge)– Re-appearance of DBs at 5000K b/c convection zone Re-appearance of DBs at 5000K b/c convection zone

grows, H is diluted with additional Hegrows, H is diluted with additional He– This fails! Destruction of HThis fails! Destruction of H-- bound level produces free e bound level produces free e--, ,

which provide opacitywhich provide opacity

ZZ Ceti

Cooling EvolutionCooling Evolution

CSPNCSPN

Hot DAZs (T>40kK)Hot DAZs (T>40kK)Radiative leviation makes ZRadiative leviation makes Z

No Z cooler than 35kKNo Z cooler than 35kK

ZZ Ceti w/ variable H layersZZ Ceti w/ variable H layers1010-8-8…………………10…………………10-4-4 M Msunsun

He-Rich DAHe-Rich DA(0.01<He/H<20)(0.01<He/H<20)

Pure DAPure DA(He/H<0.01)(He/H<0.01)

Some DC, DZSome DC, DZ Cool DAsCool DAsSome w/ T<5kKSome w/ T<5kK

Model AtmospheresModel Atmospheres

• Plane-parallel geometryPlane-parallel geometry

• Hydrostatic equilibrium (mass loss rates)Hydrostatic equilibrium (mass loss rates)

• NLTENLTE

• Stratisfied AtmospheresStratisfied Atmospheres– Parameters: degree of ionization, intensity of Parameters: degree of ionization, intensity of

radiation fieldradiation field• Make radiative cross sections of each element depth Make radiative cross sections of each element depth

dependentdependent

• ConvectionConvection– Parameters of Mixing Length theoryParameters of Mixing Length theory

Future/Active WorkFuture/Active Work

• Exact masses of H and He layersExact masses of H and He layers– Thin or Thick EnvelopesThin or Thick Envelopes

• Explanations for DB-gapExplanations for DB-gap• Explanations for ‘non-DA gap’Explanations for ‘non-DA gap’• DAs outnumber He-rich WDs, yet DAs outnumber He-rich WDs, yet

progenitor PNN have ~equal progenitor PNN have ~equal numbers of H- and He-rich stars. numbers of H- and He-rich stars. What rids degenerates of He?What rids degenerates of He?

• Couple core & atmosphere modelsCouple core & atmosphere models

ReferencesReferences• Dreizler, S. 1999, RvMA, 12, 255DDreizler, S. 1999, RvMA, 12, 255D• Fontaine et al. 2001, PASP, 113, 409Fontaine et al. 2001, PASP, 113, 409• Hansen, B. 2004, Physics Reports, 399, 1Hansen, B. 2004, Physics Reports, 399, 1• Hansen, B & Liebert, J. 2003 ARA&A, 41, 465Hansen, B & Liebert, J. 2003 ARA&A, 41, 465• Hearnshaw, J.B. 1986, Hearnshaw, J.B. 1986, The Analysis of The Analysis of

Starlight.Starlight.• Koester, D. & Chanmugam, G. 1990, Koester, D. & Chanmugam, G. 1990, RPPh, RPPh,

53, 837K53, 837K • Shipman, H. 1997, Shipman, H. 1997, White DwarfsWhite Dwarfs, p. 165. , p. 165.

KluwerKluwer• Wesemael et al. 1993, PASP, 105, 761Wesemael et al. 1993, PASP, 105, 761

WR Central Stars

PG 1159 stars

DO starsVia float-up, at 45 kK

DA stars w/ then H layersVia dredge-up, at 30 kK

DB Stars

Accretion DZ

Accretion DBA Dredge-up DQ

(from DAs)

DA StarsDA Stars

• 5,000-80,000K5,000-80,000K

• Heavily broadened Balmer linesHeavily broadened Balmer lines– Strongest near 12,000K at log g~8 (DA4)Strongest near 12,000K at log g~8 (DA4)

• No other features in optical spectrumNo other features in optical spectrum– Rapid settling of elements heavier than H in Rapid settling of elements heavier than H in

high log ghigh log g– Underabundances of elements by at least Underabundances of elements by at least

1/1001/100

• Higher dispersion revealed traces of Higher dispersion revealed traces of helium in a few-DAO (HeII) and DAB (HeI)helium in a few-DAO (HeII) and DAB (HeI)

DO StarsDO Stars

• Spectra dominated by He IISpectra dominated by He II

• TTeffeff >45,000 K >45,000 K

• 2 subclasses2 subclasses– Cool (TCool (Teffeff~45-70,000K), very strong ~45-70,000K), very strong

4686, also He I features4686, also He I features

– Hot (THot (Teffeff>80,000K), only >80,000K), only 46864686

• At TAt Teffeff <30,000K, He II can no longer <30,000K, He II can no longer be detected, only see He Ibe detected, only see He I

DO SpectraDO Spectra

DB StarsDB Stars

• Classical DB stars have rich spectra Classical DB stars have rich spectra of He I in optical, with nothing elseof He I in optical, with nothing else

• Coolest DB stars merge with He-rich Coolest DB stars merge with He-rich DQ starsDQ stars

• Many DBs have H, metals (Ca II), and Many DBs have H, metals (Ca II), and carbon (C I and Ccarbon (C I and C22))

DZ Stars & SpectraDZ Stars & Spectra

• He-rich He-rich stars to cool stars to cool to show He to show He I, below I, below (DB5-(DB5-9000K) still 9000K) still show metal show metal featuresfeatures

• Ca I, Ca II H Ca I, Ca II H and K, Mg I, and K, Mg I, Fe I, Na IFe I, Na I

PG 1159 StarsPG 1159 Stars

• Features due to CNO ions, TFeatures due to CNO ions, Teffeff>100,000K>100,000K• Absence of H or He I features; He II, C IV, Absence of H or He I features; He II, C IV,

O VIO VI• 3 groups3 groups

– A: Cooler TA: Cooler Teffeff~100,000K, He II, C IV, O VI~100,000K, He II, C IV, O VI– E: TE: Teffeff~140,000K, emission cores, He II, C IV, ~140,000K, emission cores, He II, C IV,

O VI, some have N V (DOQZ1)O VI, some have N V (DOQZ1)– lgE: Low g central stars of planetary nucleilgE: Low g central stars of planetary nuclei

•Characteristic emission cores, narrower Characteristic emission cores, narrower absorption featuresabsorption features

DC Stars & SpectraDC Stars & Spectra• Featureless, no line Featureless, no line

deeper than 5% of deeper than 5% of continuumcontinuum

• Higher resolution Higher resolution reveals weak reveals weak featuresfeatures

• Many reclassified as Many reclassified as DB or DADB or DA

• True DCs remain, True DCs remain, among coolest WDs, among coolest WDs, TTeffeff < 11,000 K < 11,000 K