A Brief Summary of Star Formation in the Milky Way Yancy L. Shirley Star Formation Disucssion Group April (no joke!)

Outline Brief overview of Milky Way Star Formation (SF) Where? How much? How long? Molecular cloud lifetime & support Dense Cores = sites of SF Compare & Contrast low-mass vs. high-mass Dichotomy in understanding SF across mass spectrum IMF cores to stars Observational Probes Molecules & dust Future Disucssion Topics

SF in the Milky Way stars in the Milky Way Evidence for SF throughout history of the galaxy (Gilmore 2001) SF occurs in molecular gas Molecular cloud complexes: M < 10 7 Msun (Elmegreen 1986) Isolated Bok globules M > 1 Msun (Bok & Reilly 1947) SF traces spiral structure (Schweizer 1976) NASA M51 Central Region

SF Occurs throughout the Galaxy Total molecular gas = 1 – 3 x 10 9 Msun (CO surveys) SF occurring within central 1 kpc SF occurring in outer galaxy > 15 kpc (Combes 1991) SF occurring nearby Rho Oph 120 pc, Lupus 130 pc, Taurus 140 pc, Orion 400 pc Pleiades 70 pc SF occurs in isolated & clustered modes Blum, Conti, & Damineli 2000 W42 VLT BHR-71

Molecular Cloud Lifetime Survey of CO towards clusters Leisawitz, Bash, & Thaddeus 1989 All cluster with t 10 4 Msun Clusters older than t > 10 7 yrs have molecular clouds M 10 7 yrs have molecular clouds M < 10 3 Msun Lower limit to molecular cloud lifetime Some young clusters show evidence for SF over periods of t > 10 8 yrs (Stauffer 1980) Lifetimes of 10 7 to 10 8 yrs

Molecular Cloud Structure Molecular clouds structure complicated: Clumpy and filamentary Self-similar over a wide range of size scales (fractal?) May contain dense cores: with n > 10 6 cm -3 Transient coherent structures? L. Cambresy 1999 Lupus Serpens Optical A v

Gravity Jeans Mass Minimum mass to overcome thermal pressure (Jeans 1927) Free-fall time for collapse n = 10 2 cm -3 => free-fall time = 3 x 10 6 yrs n = 10 6 cm -3 => free-fall time = 3 x 10 4 yrs

Jeans Mass

Star Formation Rate Current SFR is 3 +/- 1 Msun yr -1 (Scalo 1986) Assuming 100% SF efficiency & free-fall collapse Predicted SFR > 130 – 400 Msun yr -1 (Zuckerman & Palmer 1974) TOO LARGE by 2 orders of magnitude! SF is NOT 100% efficient Efficiency is 1 – 2% for large molecular clouds All clouds do not collapse at free-fall Additional support against gravity: rotation, magnetic fields, turbulence

SFR per unit Mass Assume L FIR ~ 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 Support Not important on large scale (i.e., molecular cloud support) Arquilla & Goldsmith (1986) systematic study of dark clouds implies rotational support rare Rotational support becomes important on small scales Conservation of angular momentum during collapse Results in angular momentum problem & solution via molecular outflows Spherical symmetry breaking for dense cores Formation of disks Centrifugal radius (Rotational support = Gravitational support) (Shu, Admas, & Lizano 1987) :

Magnetic Support Magnetic field has a pressure (B 2 /8  ) that can provide support Define magnetic equivalent to Jeans Mass (Shu, Adams, & Lizano 1987): Equivalently: Av < 4 mag (B/30 mG) cloud may be supported M > M cr “Magnetically supercritical” Equation of hydrostatic equilibrium => support perpendicular to B-field Dissipation through ambipolar-diffusion increases timescale for collapse (Mckee et al. 1993): Typical x e ~ => t AD ~ 7 x 10 6 yrs

Observed Magnetic Fields Crutcher 1999

Turbulent Support Both rotation & magnetic fields can only support a cloud in one direction Turbulence characterized as a pressure: P turb ~  v turb 2 General picture is turbulence injected on large scales with a power spectrum of P(k) ~ k -a Potentially fast decay t ~ L / v turb => need to replenish Doppler linewidth is very narrow: CO at 10K  v = 0.13 km/s Low-mass regions typically have narrow linewidth => turbulence decays before SF proceeds? High-mass regions have very large linewidths CS J=5-4 = 5.6 km/s

Rho Oph Dense Cores Motte, Andre, & Neri 1998

Low-mass Dense Cores B335 IRAS ,000 AU N 2 H + J = Caselli et al. 2002Shirley et al. 2000

Star Formation within Cores

Orion Dense Cores Lis, et al VST, IOA U Tokyo CO J=2-1

Dust Continuum: Dense Cores 350  m Mueller et al  m

High-mass Dense Cores M8ES158 W44S76E RCW 38 CS J = 5-4, Shirley et al. 2003J. ALves & C. Lada 2003 Optical Near-IR

High-mass: Extreme Complexity S106 Near- IR Subaru H2H2

Orion-KL Winds & Outlfows

SF in Dense Cores Star formation occurs within dense molecular cores High density gas in dense cores (n > 10 6 cm -3 ) Clumpy/filamentary structures within molecular cloud SF NOT evenly distributed Low-mass star formation may occur in isolation or in clustered environments Low-mass defined as M_core < few Msun High-mass star formation always appears to occur in a clustered environment Average Properties: 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) There is a dichotomy in our understanding of low-mass and high-mass protostar formation and evolution

Low-mass Evolutionary Scheme P.Andre 2002

Low-mass: Pre-protostellar Cores 10,000 AU SCUBA 850  m 3.5’ x 3.5’ Ward-Thompson et al 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 Formation Basic formation mechanism debated: Accretion (McKee & Tan 2002) How do you form a star with M > 10 Msun before radiation pressure stops accretion? Coalescence (Bonnell et al. 1998) Requires high stellar density: n > 10 4 stars pc -3 Predicts high binary fraction among high-mass stars Observational complications: Farther away than low-mass regions = low resolution Dense cores may be forming cluster of stars = SED dominated by most massive star = SED classification confused! Very broad linewidths consistent with turbulent gas Potential evolutionary indicators from presence of : H 2 O, CH 3 OH masers Hot core or Hyper-compact HII or UCHII regions

High-mass Evolutionary Sequence ? A. Boonman thesis 2003

UCHII Regions & Hot Cores VLA 7mm Cont. BIMA UCHII Regions and Hot Cores observed in some high- mass regions such as W49A DePree et al. 1997Wilner et al. 1999

Chemical Tracers of Evolution?

High Mass Pre-protocluster Core? Have yet to identify initial configuration of high-mass star forming core! No unbiased surveys for such an object made yet Based on dense gas surveys, what would a 4500 Msun, cold core (T ~ 10K) look like? Does this phase exist? Evans et al. 2002

IMF: From Cores to Stars dN/dM ~ M -1.6 – 1.7 for molecular clouds & large CO clumps dN/dM ~ M for Salpeter IMF of stars How do we make the stellar IMF ? Rho Oph (60 clumps): dN/dM ~ M -2.5, M>0.8 M sun (Motte et al. 1998) Serpens: dN/dM ~ M -2.1 (Testi & Seargent 1998)

CO: Molecular Cloud Tracer Hubble Telescope CO J=3-2 Emission NASA, Hubble Heritage Team CSO

Dense Gas Tracers: CS & HCN Shirley et al CO 1-0CS 2-1HCN 1-0 Helfer & Blitz 1997 CS 5-4

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

Astrochemistry E. F. van Dishoeck 2003

Dust Extinction Mapping Good pencil beam probe for A v up to 30 mag (Alves et al 1999)

Dust Continuum Emission Optically thin at long wavelengths => good probe of density and temperature structure  ~ 1 at 1.2 mm for A v = 4 x 10 4 mag Dust opacities uncertain to order of magnitude! SCUBA map of Orion Johnstone & Bally 1999

Some Puzzles How do molecular clouds form? Does the same process induce star formation? What is the relative importance of spontaneous and stimulated processes in the formation of stars of various mass? What governs the SFR in a molecular cloud? What determined the IMF evolution from molecular cloud clumps to stars? Do stars form in a process of fragmentation of an overall collapse? Or rather, do individual stars form from condensed regions within globally stable clouds? Based on question in Evans 1991

More Puzzles How do you form a 100 Msun star? Is high-mass SF accretion dominated or coalescence dominated? Does the mechanism depend on mass? What are the initial conditions for high-mass cluster formation? How does SF feedback disrupt/regulate star formation? Outflows, winds, Supernovae What is a reasonable evolutionary sequence for high- mass star forming regions? IS SF in isolated globules spontaneous or stimulated? Are we actually observing collapse in dense core envelopes?