A Brief Summary of Star A Brief Summary of Star Formation in the Milky WayFormation in the Milky Way

Yancy L. Shirley

Star Formation Disucssion Group

April 1 2003 (no joke!)

OutlineOutlineBrief overview of Milky Way Star Formation (SF)Brief overview of Milky Way Star Formation (SF)

Where? How much? How long? Where? How much? How long? Molecular cloud lifetime & supportMolecular cloud lifetime & support

Dense Cores = sites of SFDense Cores = sites of SFCompare & Contrast low-mass vs. high-massCompare & Contrast low-mass vs. high-massDichotomy in understanding SF across mass spectrumDichotomy in understanding SF across mass spectrumIMF cores to starsIMF cores to stars

Observational ProbesObservational ProbesMolecules & dustMolecules & dust

Future Disucssion TopicsFuture Disucssion Topics

SF in the Milky WaySF in the Milky Way10101111 stars in the Milky Way stars in the Milky Way

Evidence for SF throughout history of the galaxy (Gilmore 2001)Evidence for SF throughout history of the galaxy (Gilmore 2001)

SF occurs in molecular gasSF occurs in molecular gasMolecular cloud complexes: M < 10Molecular cloud complexes: M < 1077 Msun (Elmegreen 1986) Msun (Elmegreen 1986)Isolated Bok globulesIsolated Bok globules M > 1 Msun (Bok & Reilly 1947) M > 1 Msun (Bok & Reilly 1947)

SF traces spiral structure SF traces spiral structure (Schweizer 1976)(Schweizer 1976)

NASA

M51 Central Region

SF Occurs throughout the GalaxySF Occurs throughout the GalaxyTotal molecular gas = 1 – 3 x 10Total molecular gas = 1 – 3 x 1099 Msun Msun (CO surveys)(CO surveys)

SF occurring within central 1 kpc SF occurring within central 1 kpc SF occurring in outer galaxy > 15 kpc SF occurring in outer galaxy > 15 kpc (Combes 1991)(Combes 1991)SF occurring nearbySF occurring nearby

Rho Oph 120 pc, Lupus 130 pc, Taurus 140 pc, Orion 400 pcRho Oph 120 pc, Lupus 130 pc, Taurus 140 pc, Orion 400 pcPleiades 70 pcPleiades 70 pc

SF occurs in isolated & clustered modesSF occurs in isolated & clustered modes

Blum, Conti, & Damineli 2000

W42

VLT

BHR-71

Molecular Cloud LifetimeMolecular Cloud LifetimeSurvey of CO towards clusters Survey of CO towards clusters

Leisawitz, Bash, & Thaddeus 1989Leisawitz, Bash, & Thaddeus 1989All cluster with t < 5 x 10All cluster with t < 5 x 1066 yrs have molecular clouds M > 10 yrs have molecular clouds M > 1044 Msun MsunClusters older than t > 10Clusters older than t > 1077 yrs have molecular clouds M < 10 yrs have molecular clouds M < 1033 Msun MsunLower limit to molecular cloud lifetimeLower limit to molecular cloud lifetime

Some young clusters show evidence for SF over periods of Some young clusters show evidence for SF over periods of t > 10t > 1088 yrs (Stauffer 1980) yrs (Stauffer 1980)

Lifetimes of 10Lifetimes of 1077 to 10 to 108 8 yrsyrs

Molecular Cloud StructureMolecular Cloud StructureMolecular clouds structure complicated:Molecular clouds structure complicated:

Clumpy and filamentaryClumpy and filamentarySelf-similar over a wide range of size scales (fractal?)Self-similar over a wide range of size scales (fractal?)May contain dense cores: with n > 10May contain dense cores: with n > 1066 cm cm-3-3

Transient coherent structures?Transient coherent structures?

L. Cambresy 1999

Lupus Serpens

Optical Av Optical Av

GravityGravity

Jeans MassJeans MassMinimum mass to overcome thermal pressure Minimum mass to overcome thermal pressure (Jeans 1927)(Jeans 1927)

Free-fall time for collapseFree-fall time for collapse

n = 10n = 1022 cm cm-3-3 => free-fall time = 3 x 10 => free-fall time = 3 x 1066 yrs yrsn = 10n = 1066 cm cm-3-3 => free-fall time = 3 x 10 => free-fall time = 3 x 1044 yrs yrs

2/12/32/12/3

18

nTM

GmTkM sunH

Jeans

yrsnG

t ff2/17

2/1

104.3323

Jeans MassJeans Mass

0.5 1 2510

2050100

200500

1000

Star Formation RateStar Formation RateCurrent SFR is 3 +/- 1 Msun yr Current SFR is 3 +/- 1 Msun yr -1-1 (Scalo 1986)(Scalo 1986)

Assuming 100% SF efficiency & free-fall collapseAssuming 100% SF efficiency & free-fall collapsePredicted SFR > 130 – 400 Msun yr Predicted SFR > 130 – 400 Msun yr -1-1 (Zuckerman & Palmer (Zuckerman & Palmer 1974)1974)TOO LARGE by 2 orders of magnitude!TOO LARGE by 2 orders of magnitude!

SF is NOT 100% efficientSF is NOT 100% efficientEfficiency is 1 – 2% for large molecular cloudsEfficiency is 1 – 2% for large molecular clouds

All clouds do not collapse at free-fallAll clouds do not collapse at free-fallAdditional support against gravity: rotation, magnetic fields, Additional support against gravity: rotation, magnetic fields, turbulenceturbulence

SFR per unit MassSFR per unit Mass Assume LFIR ~ SFR, then SFR per unit mass does not

vary over 4 orders of magnitude in mass (Evans 1991)

Plot for dense cores traced by CS J=5-4 shows same lack of correlation (Shirley et al. 2003)

Implies feedback & self-regulation of SFR ?

Rotational SupportRotational SupportNot important on large scale (i.e., molecular cloud Not important on large scale (i.e., molecular cloud support)support)

Arquilla & Goldsmith (1986)Arquilla & Goldsmith (1986) systematic study of dark clouds systematic study of dark clouds implies rotational support rareimplies rotational support rare

Rotational support becomes important on small scalesRotational support becomes important on small scalesConservation of angular momentum during collapseConservation of angular momentum during collapse

Results in angular momentum problem & solution via Results in angular momentum problem & solution via molecular outflowsmolecular outflows

Spherical symmetry breaking for dense coresSpherical symmetry breaking for dense coresFormation of disksFormation of disks

Centrifugal radius (Rotational support = Gravitational support) Centrifugal radius (Rotational support = Gravitational support) (Shu, Admas, & Lizano 1987)(Shu, Admas, & Lizano 1987) : :

8

233

16aMGRc

Magnetic SupportMagnetic SupportMagnetic field has a pressure (BMagnetic field has a pressure (B22/8/8) that can provide ) that can provide supportsupport

Define magnetic equivalent to Jeans Mass Define magnetic equivalent to Jeans Mass (Shu, Adams, & Lizano 1987):(Shu, Adams, & Lizano 1987):

Equivalently: Av < 4 mag (B/30 mG) cloud may be supportedEquivalently: Av < 4 mag (B/30 mG) cloud may be supportedM > MM > Mcrcr “Magnetically supercritical” “Magnetically supercritical”

Equation of hydrostatic equilibrium => support perpendicular to B-fieldEquation of hydrostatic equilibrium => support perpendicular to B-field

Dissipation through ambipolar-diffusion increases timescale for Dissipation through ambipolar-diffusion increases timescale for collapse collapse (Mckee et al. 1993):(Mckee et al. 1993):

Typical xTypical xee ~ 10 ~ 10-7-7 => t => tADAD ~ 7 x 10 ~ 7 x 1066 yrs yrs

232/1 2/30/1013.0 pcRGBMdABGM suncr

BBBP

dtrd

)(211 2

02

2

yrsxG

t eni

AD13103.7

43

Observed Magnetic FieldsObserved Magnetic Fields

Crutcher 1999

Turbulent SupportTurbulent SupportBoth rotation & magnetic fields can only support a cloud in Both rotation & magnetic fields can only support a cloud in one directionone direction

Turbulence characterized as a pressure: Turbulence characterized as a pressure: PPturbturb ~ ~ vvturbturb

22

General picture is turbulence injected on large scales with a power General picture is turbulence injected on large scales with a power spectrum of P(k) ~ kspectrum of P(k) ~ k-a-a

Potentially fast decay t ~ L / vPotentially fast decay t ~ L / vturbturb => need to replenish => need to replenish

Doppler linewidth is very narrow:Doppler linewidth is very narrow:

CO at 10K CO at 10K v = 0.13 km/sv = 0.13 km/sLow-mass regions typically have narrow linewidth => turbulence decays Low-mass regions typically have narrow linewidth => turbulence decays before SF proceeds? before SF proceeds? High-mass regions have very large linewidthsHigh-mass regions have very large linewidths

CS J=5-4 <CS J=5-4 <v> = 5.6 km/sv> = 5.6 km/s

amumTskm

mkTv /22.02ln22

Rho Oph Dense CoresRho Oph Dense Cores

Motte, Andre, & Neri 1998

Low-mass Dense CoresLow-mass Dense CoresB335

IRAS03282

10,000 AU

N2H+ J = 1 - 0

Caselli et al. 2002 Shirley et al. 2000

Star Formation within CoresStar Formation within Cores

Orion Dense CoresOrion Dense Cores

Lis, et al. 1998VST, IOA U Tokyo

CO J=2-1

Dust Continuum: Dense CoresDust Continuum: Dense Cores

350 m

Mueller et al. 2002

350 m

High-mass Dense CoresHigh-mass Dense CoresM8E S158

W44 S76E

RCW 38

CS J = 5-4, Shirley et al. 2003 J. ALves & C. Lada 2003

Optical

Near-IR

High-mass: Extreme ComplexityHigh-mass: Extreme Complexity

S106

Near- IR

SubaruH2

Orion-KL Winds & OutlfowsOrion-KL Winds & Outlfows

SF in Dense CoresSF in Dense CoresStar formation occurs within dense molecular coresStar formation occurs within dense molecular cores

High density gas in dense cores (n > 10High density gas in dense cores (n > 1066 cm cm-3-3))Clumpy/filamentary structures within molecular cloudClumpy/filamentary structures within molecular cloud

SF NOT evenly distributedSF NOT evenly distributedLow-mass star formation may occur in isolation or in clustered Low-mass star formation may occur in isolation or in clustered environmentsenvironments

Low-mass defined as M_core < few MsunLow-mass defined as M_core < few MsunHigh-mass star formation always appears to occur in a clustered High-mass star formation always appears to occur in a clustered environmentenvironment

Average Properties:Average Properties:Low-mass: R < 0.1 pc, narrow linewidths (~ few 0.1 km/s)Low-mass: R < 0.1 pc, narrow linewidths (~ few 0.1 km/s)High-mass: R ~ few 0.1 pc, wide linewidths (~ few km/s)High-mass: R ~ few 0.1 pc, wide linewidths (~ few km/s)

There is a dichotomy in our understanding of low-mass There is a dichotomy in our understanding of low-mass and high-mass protostar formation and evolutionand high-mass protostar formation and evolution

Low-mass Evolutionary SchemeLow-mass Evolutionary Scheme

P.Andre 2002

Low-mass: Pre-protostellar CoresLow-mass: Pre-protostellar Cores

10,000 AU

SCUBA 850 m

3.5’ x 3.5’

Ward-Thompson et al. 2002

ISO 200 m

12’ x 12’

L1544

Dense cores with no known internal luminosity source

SEDs peak longer than 100 m

Study the initial conditions of low-mass SF

B68

High-Mass Star FormationHigh-Mass Star FormationBasic formation mechanism debated:Basic formation mechanism debated:

Accretion Accretion (McKee & Tan 2002)(McKee & Tan 2002)How do you form a star with M > 10 Msun before radiation pressure How do you form a star with M > 10 Msun before radiation pressure stops accretion? stops accretion?

Coalescence Coalescence (Bonnell et al. 1998)(Bonnell et al. 1998)Requires high stellar density: n > 10Requires high stellar density: n > 1044 stars pc stars pc-3-3

Predicts high binary fraction among high-mass starsPredicts high binary fraction among high-mass stars

Observational complications:Observational complications:Farther away than low-mass regions = low resolutionFarther away than low-mass regions = low resolutionDense cores may be forming cluster of stars = SED dominated by Dense cores may be forming cluster of stars = SED dominated by most massive star = SED classification confused!most massive star = SED classification confused!Very broad linewidths consistent with turbulent gasVery broad linewidths consistent with turbulent gas

Potential evolutionary indicators from presence of :Potential evolutionary indicators from presence of :HH22O, CHO, CH33OH masersOH masersHot core or Hyper-compact HII or UCHII regionsHot core or Hyper-compact HII or UCHII regions

High-mass Evolutionary Sequence ?High-mass Evolutionary Sequence ?

A. Boonman thesis 2003

UCHII Regions & Hot CoresUCHII Regions & Hot Cores

VLA 7mm Cont. BIMA

UCHII Regions and Hot Cores observed in some high-mass regions such as W49A

DePree et al. 1997 Wilner et al. 1999

Chemical Tracers of Evolution?Chemical Tracers of Evolution?

High Mass Pre-protocluster Core?High Mass Pre-protocluster Core?

Have yet to identify initial Have yet to identify initial configuration of high-mass star configuration of high-mass star forming core!forming core!

No unbiased surveys for such No unbiased surveys for such an object made yetan object made yet

Based on dense gas surveys, Based on dense gas surveys, what would a 4500 Msun, cold what would a 4500 Msun, cold core (T ~ 10K) look like?core (T ~ 10K) look like?

Does this phase exist?Does this phase exist?

Evans et al. 2002

IMF: From Cores to StarsIMF: From Cores to Stars dN/dM ~ M-1.6 – 1.7 for molecular clouds & large CO

clumps

dN/dM ~ M-2.35 for Salpeter IMF of stars

How do we make the stellar IMF ?

Rho Oph (60 clumps): dN/dM ~ M-2.5, M>0.8 Msun (Motte et al. 1998)

Serpens: dN/dM ~ M-2.1 (Testi & Seargent 1998)

CO: Molecular Cloud TracerCO: Molecular Cloud TracerHubble

TelescopeCO J=3-2 Emission

NASA, Hubble Heritage TeamCSO

Dense Gas Tracers: CS & HCNDense Gas Tracers: CS & HCN

Shirley et al. 2003

CO 1-0 CS 2-1 HCN 1-0

Helfer & Blitz 1997

CS 5-4

Comparison of Molecular TracersComparison of Molecular TracersObservations of the low-mass PPC, L1517 Observations of the low-mass PPC, L1517 (Bergin et al.)(Bergin et al.)

AstrochemistryAstrochemistry

E. F. van Dishoeck 2003

Dust Extinction MappingDust Extinction Mapping

Good pencil beam probe for AGood pencil beam probe for Avv up to 30 mag up to 30 mag (Alves et al 1999)(Alves et al 1999)

Dust Continuum EmissionDust Continuum Emission

Optically thin at long Optically thin at long wavelengths => good wavelengths => good probe of density and probe of density and temperature structuretemperature structure

~ 1 at 1.2 mm for ~ 1 at 1.2 mm for AAvv = 4 x 10 = 4 x 1044 mag mag

Dust opacities Dust opacities uncertain to order of uncertain to order of magnitude!magnitude!

SCUBA map of Orion

Johnstone & Bally 1999

Some PuzzlesSome Puzzles

How do molecular clouds form?How do molecular clouds form?Does the same process induce star formation?Does the same process induce star formation?

What is the relative importance of spontaneous and What is the relative importance of spontaneous and stimulated processes in the formation of stars of various stimulated processes in the formation of stars of various mass?mass?What governs the SFR in a molecular cloud?What governs the SFR in a molecular cloud?What determined the IMF evolution from molecular cloud What determined the IMF evolution from molecular cloud clumps to stars?clumps to stars?Do stars form in a process of fragmentation of an overall Do stars form in a process of fragmentation of an overall collapse?collapse?Or rather, do individual stars form from condensed Or rather, do individual stars form from condensed regions within globally stable clouds?regions within globally stable clouds?

Based on question in Evans 1991

More PuzzlesMore PuzzlesHow do you form a 100 Msun star?How do you form a 100 Msun star?Is high-mass SF accretion dominated or coalescence Is high-mass SF accretion dominated or coalescence dominated?dominated?

Does the mechanism depend on mass?Does the mechanism depend on mass?

What are the initial conditions for high-mass cluster What are the initial conditions for high-mass cluster formation?formation?How does SF feedback disrupt/regulate star formation?How does SF feedback disrupt/regulate star formation?

Outflows, winds, SupernovaeOutflows, winds, Supernovae

What is a reasonable evolutionary sequence for high-What is a reasonable evolutionary sequence for high-mass star forming regions?mass star forming regions?IS SF in isolated globules spontaneous or stimulated?IS SF in isolated globules spontaneous or stimulated?Are we actually observing collapse in dense core Are we actually observing collapse in dense core envelopes?envelopes?