Star formationStar formation

Suzanne RamsaySuzanne Ramsay

UK Astronomy Technology Centre,UK Astronomy Technology Centre,

Royal Observatory EdinburghRoyal Observatory Edinburgh

UKIRT+WFCAM infrared image of Orion

The ChallengeThe Challenge

A theory of star formation requires to explain A theory of star formation requires to explain the origins of stars over four orders of the origins of stars over four orders of magnitude in massmagnitude in mass From 0.01 MFrom 0.01 M brown dwarfs powered only by brown dwarfs powered only by

gravitational energygravitational energy To >100 MTo >100 M stars with lifetimes around 1million stars with lifetimes around 1million

yearsyears

The typical star has mass ~1 MThe typical star has mass ~1 M So, what do we know and how do we know it?So, what do we know and how do we know it?

Stars form in molecular Stars form in molecular cloudsclouds

From Dame, Hartmann and Thaddeus 2001.

although stars are generally not in clusters although stars are generally not in clusters youngyoung stars are and so these are identified stars are and so these are identified as the sites of star formationas the sites of star formation

GMC chemistryGMC chemistry >100 molecules discovered in MCs>100 molecules discovered in MCs HH22 most abundant most abundant CO commonly studied at 10CO commonly studied at 10-4-4 of H of H22 abundance abundance

since it emits from cold GMC which Hsince it emits from cold GMC which H22 does not does not complex molecules detected include complex molecules detected include

formaldehyde, amino acids.formaldehyde, amino acids. Important constituent (1% of ISM) is dust (C, Si) – Important constituent (1% of ISM) is dust (C, Si) –

much cloud chemistry takes place on dust grains much cloud chemistry takes place on dust grains Most dust mass is in grains size ~1000A, 10Most dust mass is in grains size ~1000A, 1099 atoms atoms

Properties of GMCsProperties of GMCs 2-4% of interstellar volume 2-4% of interstellar volume

The rest is the atomic interstellar mediumThe rest is the atomic interstellar medium Lifetime, debatable but <~ 10Lifetime, debatable but <~ 1077 years years

Free-fall timescale ~ 10Free-fall timescale ~ 1066 years years Typically dispersed by radiation from massive stars, Typically dispersed by radiation from massive stars,

timescale ~10timescale ~1077 years years Supported by magnetic fields and turbulence due to Supported by magnetic fields and turbulence due to

motion of clumps motion of clumps Observed galactic star formation rate 3 MObserved galactic star formation rate 3 M yryr-1-1

Star formation in clouds is relatively inefficient: 1-Star formation in clouds is relatively inefficient: 1-3% of the cloud ends up as stars3% of the cloud ends up as stars

Within the Orion Within the Orion molecular cloud molecular cloud higher density higher density clumps are readily clumps are readily identifiableidentifiable

Monoceros R2

Orion A

Orion B

Monoceros R2

Orion A

Orion B

Stars form from yet Stars form from yet smaller structures - smaller structures - corescores

OMC has stars of OMC has stars of various agesvarious ages

At 460pc, the Orion At 460pc, the Orion Nebula is our closest Nebula is our closest laboratory for studying laboratory for studying massive star formationmassive star formation

Star formation in clustersStar formation in clusters Embedded clustersEmbedded clusters T associations e.g. TaurusT associations e.g. Taurus R associations (AB stars) e.g. R associations (AB stars) e.g.

Mon R1Mon R1 OB associations (massive stars OB associations (massive stars

e.g. BN-KL in Orion) e.g. BN-KL in Orion) Open clusters (e.g. Hyades, Open clusters (e.g. Hyades,

Pleiades) can be very old Pleiades) can be very old

dense coresdense cores

Bok globule b335Bok globule b335 typical formation typical formation

site for an site for an individual starindividual star

PhasePhase GMCsGMCs ClumpsClumps CoresCores

Mass (MMass (M)) 6x106x1044 – – 2x102x1066

101022 1-101-10

Size (pc)Size (pc) 20-10020-100 0.2-40.2-4 0.1-0.40.1-0.4

Density (cmDensity (cm——

33))100-300100-300 101033-10-1044 101044-10-1055

Temp (K) Temp (K) 15-4015-40 7-157-15 1010

B (B (G)G) 1-101-10 3-303-30 10-5010-50

Line width Line width (kms(kms-1-1))

6-156-15 0.5-40.5-4 0.2-0.40.2-0.4

Dynamical Dynamical life (years)life (years)

3 x 103 x 1066 101066 6x106x1055

ExtinctionExtinction Some Some

valuesvalues AAVV~20mag~20mag AAKK~2mag~2mag

Much higher Much higher for dense for dense corescores

Star formation requires long Star formation requires long wavelength astronomywavelength astronomy

High obscuration means that many High obscuration means that many starformation phenomena require long starformation phenomena require long wavelength observationswavelength observations mm, submm and infraredmm, submm and infrared

Youngest sources are the most deeply Youngest sources are the most deeply embedded and therefore the hardest to embedded and therefore the hardest to studystudy

Evolution of a (low mass) Evolution of a (low mass) protostarprotostar

Evolutionary sequence From Andre, Ward-Thompson & Barsony 1993 Extended from original by Lada 1987

Starless coresStarless cores

Starless core or pre-stellar core Starless core or pre-stellar core Cold (<~15K)Cold (<~15K) Sufficient mass for protostar + envelope (0.05-Sufficient mass for protostar + envelope (0.05-

30 M30 M)) Gravitationally bound, but no protostarGravitationally bound, but no protostar

Core collapse Core collapse

Considering the core as an isothermal Considering the core as an isothermal spheresphere Density Density 1/r 1/r22

Maximum mass for such a sphere is the Bonor Maximum mass for such a sphere is the Bonor Ebert mass Ebert mass

M > MBE, collapse starts with central coreM > MBE, collapse starts with central core

21

23

2

)(18.1

s

BE

PGM

Balances surface pressure from the cloud, velocity dispersion from temperature and gravity.

21

2112 )

102()

10(96.0

cmdynx

P

K

TM s

BE MM

Core collapse Core collapse

If unmediated, free fall collapse with If unmediated, free fall collapse with Density Density 1/r 1/r3/23/2 and v and vffff

22 1/r 1/r1/21/2

Requires additional support otherwiseRequires additional support otherwise Timescales too fastTimescales too fast Velocities become supersonic and core Velocities become supersonic and core

fragmentsfragments

Magnetic SupportMagnetic Support Clouds are known to contain magnetic fieldsClouds are known to contain magnetic fields These support the cloud against collapseThese support the cloud against collapse

Mechanism to allow slow collapse requiredMechanism to allow slow collapse required Ambipolar diffusionAmbipolar diffusion

Neutral particles immune to magnetic field drift to the centre Neutral particles immune to magnetic field drift to the centre of the coreof the core

Ionised particles remain fixed by the field linesIonised particles remain fixed by the field lines Once the core mass reaches critical level, collapse proceedsOnce the core mass reaches critical level, collapse proceeds

AD timescales are too long for standard initial AD timescales are too long for standard initial conditionsconditions

Effect of AD increased by turbulenceEffect of AD increased by turbulence

Starless coresStarless cores

Observed Observed magnetic fields magnetic fields inadequate for inadequate for ambipolar diffusion ambipolar diffusion modelmodel

Turbulent support Turbulent support of the core of the core requiredrequired

Ward-Thompson, Motte, Andre 1999

Class 0 sourcesClass 0 sources Sources with a central protostar that are Sources with a central protostar that are

very faint/undetectable in the optical/NIRvery faint/undetectable in the optical/NIR LLsubmmsubmm/L/Lbol bol > 0.5%> 0.5% MMenvelopeenvelope>m>m**

TTbol bol < 70K< 70K

Class 0 sourcesClass 0 sources First Class 0 source, VLA1623, discovered First Class 0 source, VLA1623, discovered

in Rho Ophiucus (1993)in Rho Ophiucus (1993)

Andre, Ward-Thompson, Barsony 1993Andre, Ward-Thompson, Barsony 1993

Class 0 sourcesClass 0 sources Sources with a central protostar that are very Sources with a central protostar that are very

faint/undetectable in the optical/NIRfaint/undetectable in the optical/NIR LLsubmmsubmm/L/Lbol bol > 0.5%> 0.5% MMenvelopeenvelope>m>m**

TTbol bol < 70K< 70K

The deeply embedded protostar acquires most The deeply embedded protostar acquires most of its mass during this phaseof its mass during this phase

Bipolar molecular outflows are associated with Bipolar molecular outflows are associated with Class 0 sourcesClass 0 sources Mechanism for removing angular momentumMechanism for removing angular momentum

B335 revistedB335 revisted• Contains embedded source of 3

LL • Contains a disk, radius 100AU• Density profile – inner region of

r-1.5 and outer envelope r-2 (to 5000AU)

• Inner density profile consistent with gravitational free fall

H2CO map from Choi. A bipolar outflow is detected from the embedded young source

Harvey et al 2003 sub-mm imaging reveals. Disk of radius ~100AU.

Protostellar evolutionProtostellar evolution Most of the core mass must be ejected to Most of the core mass must be ejected to

evolve from Class 0 to Class Ievolve from Class 0 to Class I During their evolution, Class 0 sourcesDuring their evolution, Class 0 sources

Increase mass from ~ 0.3 MIncrease mass from ~ 0.3 M to 3 M to 3 M Mass accretion regulated by deuterium burningMass accretion regulated by deuterium burning

Luminosity reaches 10-100 LLuminosity reaches 10-100 L

solsolsolsol

bol LR

R

M

M

yrM

MdotL 1**

15)

5)()(

10(63

Class I sourcesClass I sourcesIR visible IR visible protostarsprotostars

Sources withSources with irir > 0 > 0 over the wavelength over the wavelength range fromrange from 2.2 2.2 toto 10-25 10-25mm ir ir is the slope on the spectral energy distributionis the slope on the spectral energy distribution

These sources have both disks and envelopesThese sources have both disks and envelopes 70K < T70K < Tbolbol < 650K < 650K Identifiable by their large infrared excessIdentifiable by their large infrared excess Infrared emission lines detectableInfrared emission lines detectable Outflows, less energetic than those from Outflows, less energetic than those from

Class 0Class 0

Class 0/I sourcesClass 0/I sourcestimescalestimescales

Time spent in Class I phase – 1-5 10Time spent in Class I phase – 1-5 1055 years years from statistical arguments on source from statistical arguments on source numbersnumbers This works under assumption that the various This works under assumption that the various

classes are an evolutionary trendclasses are an evolutionary trend 10 times fewer than Class II10 times fewer than Class II

Timescale for Class 0 - 10Timescale for Class 0 - 1044 years in Rho years in Rho OphOph 10 times fewer than Class I10 times fewer than Class I Implies mass accretion rate of 10Implies mass accretion rate of 10-5-5 M MYrYr-1-1 to form to form

half solar mass starhalf solar mass star

Class II sourcesClass II sourcesClassical T TaurisClassical T Tauris

Sources withSources with -1.5 < -1.5 < irir<0 – <0 – pre-main sequence sources pre-main sequence sources with large circumstellar diskswith large circumstellar disks

Optically visibleOptically visible H-alpha and forbidden lines H-alpha and forbidden lines

from outflowfrom outflow Stellar photospheric features, Stellar photospheric features,

but often veiled by disk/dust but often veiled by disk/dust continuumcontinuum

Ages 1-4 x 10Ages 1-4 x 1066yryr T Tauri.2MASS Atlas Image mosaics by E. Kopan, R. Cutri, and S. Van Dyk (IPAC).

Strong infrared excess initially hypothesised as an Strong infrared excess initially hypothesised as an obsuring disk, with later observational confirmationobsuring disk, with later observational confirmation

Class III sourcesClass III sourcesWeak line T TaurisWeak line T Tauris

Sources withSources with irir<-1.5 – <-1.5 – pre-main sequence pre-main sequence stars that are no longer strongly accretingstars that are no longer strongly accreting Disks disspipated, so optically visibleDisks disspipated, so optically visible ‘‘weak-lined’ - H-alpha equivalent width < 10 weak-lined’ - H-alpha equivalent width < 10 ÅÅ

Ages 1-20 x 10Ages 1-20 x 1066yryr Final state for our low mass protostarFinal state for our low mass protostar Somewhat ambiguous definition as e.g. not Somewhat ambiguous definition as e.g. not

all stars with disks have strong H-alpha and all stars with disks have strong H-alpha and vice versavice versa

Accretion and outflowAccretion and outflow Outflows and jets are a ubiquitous phenomenon Outflows and jets are a ubiquitous phenomenon

associated with star formationassociated with star formation They appear during all phases, but with trends They appear during all phases, but with trends

in their evolution with protostellar classin their evolution with protostellar class Class 0 – highly collimated, luminousClass 0 – highly collimated, luminous Class 1, lower collimation, less energeticClass 1, lower collimation, less energetic

Momentum flux of outflow predicted by Momentum flux of outflow predicted by modelling to be proportional to mass accretion modelling to be proportional to mass accretion so Class 0 sources have higher accretion than so Class 0 sources have higher accretion than Class 1Class 1

Accretion and outflowAccretion and outflow

HH212 (above) and HH211 (below) are class 0 sources: high collimation, highly luminous molecular outflow

HH-30HH-30

HH-47HH-47

Outflows and angular Outflows and angular momentum transportmomentum transport

Preferred launching mechanism for outflows is Preferred launching mechanism for outflows is magneticmagnetic Capable of explaining high degree of collimation and Capable of explaining high degree of collimation and

outflow strengthoutflow strength Material ejected along magnetic field lines from Material ejected along magnetic field lines from

the diskthe disk Field geometry is crucial, but a succesful model Field geometry is crucial, but a succesful model

can remove a large fraction of angular momentum can remove a large fraction of angular momentum with a small amount of materialwith a small amount of material

Launch sites: disk; disk-star interface; star’s Launch sites: disk; disk-star interface; star’s surfacesurface

High mass star formationHigh mass star formation

Stars above 8 MStars above 8 M can’t form by the same can’t form by the same process as low massprocess as low mass Hydrogen burning ignites during accretion phaseHydrogen burning ignites during accretion phase

Yet they conspicuously exist, though in small Yet they conspicuously exist, though in small numbers compared with low mass starsnumbers compared with low mass stars

Extreme examplesExtreme examples Eta Carinae: 100 MEta Carinae: 100 M; the Pistol ~150 M; the Pistol ~150 M; ;

LBV 1806–20 ~130-190 MLBV 1806–20 ~130-190 M

High mass star formationHigh mass star formation

Fundamental difficulties in observing high Fundamental difficulties in observing high mass star formation is due to the rarity of mass star formation is due to the rarity of the sources, the distance of the nearest the sources, the distance of the nearest examplesexamples

Recent intense effort is providing larger Recent intense effort is providing larger samples of candidate HMYSOs based on samples of candidate HMYSOs based on infrared colours, radio datainfrared colours, radio data

High mass star formationHigh mass star formation

Basic problem – Kelvin Helmholtz Basic problem – Kelvin Helmholtz timescale exceeds the free fall timescaletimescale exceeds the free fall timescale ttKHKH~10~1044 years for an O star (~10 years for an O star (~1077 for the Sun) for the Sun)

Contraction proceeds faster than accretion Contraction proceeds faster than accretion of material from the cloud and hydrogen of material from the cloud and hydrogen burning begins while still embedded in the burning begins while still embedded in the cloudcloud

Alternative formation mechanism? E.g. Alternative formation mechanism? E.g. coagulation from lower mass stars coagulation from lower mass stars

HII regions as signpostsHII regions as signposts HII regions form once Hydrogen burning HII regions form once Hydrogen burning

ignites producing Lyman continuum photons ignites producing Lyman continuum photons Electron free-free emission detected in radioElectron free-free emission detected in radio

Embedded HII regions are constrained as Embedded HII regions are constrained as compact or ultra-compact HII regionscompact or ultra-compact HII regions

High mass young stellar High mass young stellar objectsobjects

‘‘hot cores’ (T~100K) observed associated with hot cores’ (T~100K) observed associated with or as precursors to UCHII regionsor as precursors to UCHII regions

High mass young stellar High mass young stellar objectsobjects

Sub-mm imaging reveals dense cluster of Sub-mm imaging reveals dense cluster of sources analogous to the Trapezium sources analogous to the Trapezium cluster in Orioncluster in Orion

Beuther et al. 2007

Outflow activity in the region – SiO jetOutflow activity in the region – SiO jet

Outflows from HMYSOsOutflows from HMYSOs

Well know examples of high mass outflows Well know examples of high mass outflows have suggested low collimation compared have suggested low collimation compared with low mass sourceswith low mass sources

Different mechanism for generation or low Different mechanism for generation or low spatial resolution?spatial resolution?

IRAS20126+4104IRAS20126+4104Varricatt et al. 2008Varricatt et al. 2008

IRAS18151-1208IRAS18151-1208Davis et al. 2004Davis et al. 2004

Outflows from Outflows from high mass high mass sourcessources

Brown dwarfsBrown dwarfs

Stars with insufficient mass to star Stars with insufficient mass to star hydrogen burninghydrogen burning Mass limit ~0.011-0.013 MMass limit ~0.011-0.013 M (12-14M (12-14MJupJup))

Brown dwarfs represent bridge the gap Brown dwarfs represent bridge the gap between stars and planetsbetween stars and planets Stars form from collapsing cloud cores Stars form from collapsing cloud cores Planets from coagulation of material in Planets from coagulation of material in

circumstellar disks (during the Class II stage)circumstellar disks (during the Class II stage)

Formation of the lowest Formation of the lowest mass starsmass stars

Brown dwarf discoveriesBrown dwarf discoveries ‘‘L’ and ‘T’ dwarfs now numerous, identified L’ and ‘T’ dwarfs now numerous, identified

from their very red colours through 2MASS from their very red colours through 2MASS and Sloan surveysand Sloan surveys T dwarfs: M – 80MT dwarfs: M – 80MJupJup-10M-10MJupJup, Temp~800K, Temp~800K

Surveys with e.g. WFCAM on UKIRT, VISTA Surveys with e.g. WFCAM on UKIRT, VISTA promise the discovery of yet cooler, lower promise the discovery of yet cooler, lower mass objects – the (as yet) mythical Y dwarfmass objects – the (as yet) mythical Y dwarf NB 2-3 objects for 100s sq degrees of skyNB 2-3 objects for 100s sq degrees of sky

Formation of the lowest Formation of the lowest mass starsmass stars

Statistics suggest that brown dwarfs have Statistics suggest that brown dwarfs have much in common with starsmuch in common with stars

Possible formation mechanisms include Possible formation mechanisms include photo-evaporation of cores by HII regionsphoto-evaporation of cores by HII regions ejection from star forming coresejection from star forming cores fragmentation of low mass prestellar coresfragmentation of low mass prestellar cores

All supported by modelling – which All supported by modelling – which dominates?dominates?

Outflow from 2MASSW Outflow from 2MASSW J1207334-393254J1207334-393254

Subarcsecond outflow detected from a24 Jupiter Mass brown dwarf (Whelan et al. 2007, ApJ, 659, L45.

The initial mass functionThe initial mass function

From Salpeter (1955)From Salpeter (1955) The relative number of stars produced The relative number of stars produced

per unit mass intervalper unit mass interval Derived from the observed luminosity Derived from the observed luminosity

functionfunction Power law function of MPower law function of M**

, slope , slope

Initial mass functionInitial mass function

Turn off at brown dwarfMasses where sources are faint and hard to find

Salpeter mass function

Example observed IMF

The initial mass functionThe initial mass function

Salpeter power law slope Salpeter power law slope Now updated – Now updated –

C(M/ MC(M/ M ) )-1.2-1.2 0.1 < M*/ M0.1 < M*/ M < 1.0 < 1.0 C(M/ MC(M/ M ) )-2.7-2.7 1 < M*/ M1 < M*/ M < 10 < 10 0.4C(M/ M0.4C(M/ M ) )-2.3-2.3 10 < M*/ M10 < M*/ M

Determining the Initial mass Determining the Initial mass function using clustersfunction using clusters

Low end of the IMF needs deepIR observations and observations of openclusters

Establishing slope for high Mass stars requires observations Of OB associations

IMF in clustersIMF in clusters

The initial mass functionThe initial mass function

The IMF for field stars and those in The IMF for field stars and those in clusters shows it to be the same clusters shows it to be the same confirmation that the stars did form in confirmation that the stars did form in

clusters.clusters. More recently, the core mass function More recently, the core mass function

found to be consistent with the stellar IMFfound to be consistent with the stellar IMF The IMF is robust to a variety of clusters The IMF is robust to a variety of clusters

and environments, but so far lacking and environments, but so far lacking theoretical basistheoretical basis

The endThe end

These stars provide most of the mass in the galaxy

These stars dominate energy feedback and chemical enrichment

These stars provide most of the luminosity in the galaxy.