A Brief Summary of Star Formation in the Milky Way Yancy L. Shirley Star Formation Disucssion Group April 1 2003 (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 1011 stars in the Milky Way Evidence for SF throughout history of the galaxy (Gilmore 2001) SF occurs in molecular gas Molecular cloud complexes: M < 107 Msun (Elmegreen 1986) Isolated Bok globules M > 1 Msun (Bok & Reilly 1947) SF traces spiral structure (Schweizer 1976) M51 Central Region NASA SF Occurs throughout the Galaxy Total molecular gas = 1 – 3 x 109 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 BHR-71 W42 Blum, Conti, & VLT Damineli 2000 Molecular Cloud Lifetime Survey of CO towards clusters Leisawitz, Bash, & Thaddeus 1989 All cluster with t < 5 x 106 yrs have molecular clouds M > 104 Msun Clusters older than t > 107 yrs have molecular clouds M < 103 Msun Lower limit to molecular cloud lifetime Some young clusters show evidence for SF over periods of t > 108 yrs (Stauffer 1980) Lifetimes of 107 to 108 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 > 106 cm-3 Transient coherent structures? Lupus Serpens Optical Av Optical Av L. Cambresy 1999 Gravity Jeans Mass Minimum mass to overcome thermal pressure (Jeans 1927) 3/ 2 kT M Jeans m G 1/ 2 18M sunT 3 / 2 n 1/ 2 H Free-fall time for collapse 1/ 2 3 t ff 32G 3.4 107 n 1/ 2 yrs n = 102 cm-3 => free-fall time = 3 x 106 yrs n = 106 cm-3 => free-fall time = 3 x 104 yrs Jeans Mass 0.5 1 2 5 10 20 50 100 200 500 1000 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 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 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) : G 3 M 3 2 Rc 16a 8 Magnetic Support Magnetic field has a pressure (B2/8) that can provide support Define magnetic equivalent to Jeans Mass (Shu, Adams, & Lizano 1987): M cr 0.13G 1/ 2 B dA 103 M sun B / 30G R / 2 pc 2 Equivalently: Av < 4 mag (B/30 mG) cloud may be supported M > Mcr “Magnetically supercritical” Equation of hydrostatic equilibrium => support perpendicular to B-field 1 2 2 d r 1 2 P 2 B ( B ) B dt 0 Dissipation through ambipolar-diffusion increases timescale for collapse (Mckee et al. 1993): 3 t AD 7.3 10 13 xe yrs 4G ni Typical xe ~ 10-7 => tAD ~ 7 x 106 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: Pturb ~ vturb2 General picture is turbulence injected on large scales with a power spectrum of P(k) ~ k-a Potentially fast decay t ~ L / vturb => need to replenish Doppler linewidth is very narrow: 2 ln 2kT T Dv 2 0.22km / s m mamu CO at 10K Dv = 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 <Dv> = 5.6 km/s Rho Oph Dense Cores Motte, Andre, & Neri 1998 Low-mass Dense Cores B335 N 2H + J = 1 - 0 10,000 AU IRAS03282 Caselli et al. 2002 Shirley et al. 2000 Star Formation within Cores Orion Dense Cores CO J=2-1 VST, IOA U Tokyo Lis, et al. 1998 Dust Continuum: Dense Cores 350 m 350 m Mueller et al. 2002 High-mass Dense Cores RCW 38 M8E S158 Optical W44 S76E Near-IR CS J = 5-4, Shirley et al. 2003 J. ALves & C. Lada 2003 High-mass: Extreme Complexity S106 Near- IR Subaru H2 Orion-KL Winds & Outlfows SF in Dense Cores Star formation occurs within dense molecular cores High density gas in dense cores (n > 106 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 Dense cores with no known internal luminosity source SEDs peak longer than 100 m Study the initial conditions of low-mass SF B68 L1544 SCUBA 850 m ISO 200 m 10,000 AU Ward-Thompson et al. 2002 3.5’ x 3.5’ 12’ x 12’ 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 > 104 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 : H2O, CH3OH masers Hot core or Hyper-compact HII or UCHII regions High-mass Evolutionary Sequence ? A. Boonman thesis 2003 UCHII Regions & Hot Cores UCHII Regions and Hot Cores observed in some high- mass regions such as W49A VLA 7mm Cont. BIMA DePree et al. 1997 Wilner 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-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 Tracer Hubble CO J=3-2 Telescope Emission CSO NASA, Hubble Heritage Team Dense Gas Tracers: CS & HCN CS 5-4 CO 1-0 CS 2-1 HCN 1-0 Helfer & Blitz 1997 Shirley et al. 2003 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 Av 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 Av = 4 x 104 mag Dust opacities uncertain to order of magnitude! SCUBA map of Orion Johnstone & Bally 1999 Some Puzzles Based on question in Evans 1991 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? 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?
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