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A Brief Summary of Star Formation in the Milky Way

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

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A Brief Summary of Star Formation in the Milky Way

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  1. A Brief Summary of Star Formation in the Milky Way Yancy L. Shirley Star Formation Disucssion Group April 1 2003 (no joke!)

  2. 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

  3. 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

  4. 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 W42 BHR-71 Blum, Conti, & Damineli 2000 VLT

  5. 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

  6. 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

  7. Gravity • Jeans Mass • Minimum mass to overcome thermal pressure (Jeans 1927) • Free-fall time for collapse • n = 102 cm-3 => free-fall time = 3 x 106 yrs • n = 106 cm-3 => free-fall time = 3 x 104 yrs

  8. Jeans Mass 0.5 1 2 5 10 20 50 100 200 500 1000

  9. 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

  10. 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 ?

  11. 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) :

  12. Magnetic Support • Magnetic field has a pressure (B2/8p) 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 > Mcr “Magnetically supercritical” • Equation of hydrostatic equilibrium => support perpendicular to B-field • Dissipation through ambipolar-diffusion increases timescale for collapse (Mckee et al. 1993): • Typical xe ~ 10-7 => tAD ~ 7 x 106 yrs

  13. Observed Magnetic Fields Crutcher 1999

  14. Turbulent Support • Both rotation & magnetic fields can only support a cloud in one direction • Turbulence characterized as a pressure: • Pturb ~ rvturb2 • 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: • 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

  15. Rho Oph Dense Cores Motte, Andre, & Neri 1998

  16. Low-mass Dense Cores B335 N2H+ J = 1 - 0 10,000 AU IRAS03282 Caselli et al. 2002 Shirley et al. 2000

  17. Star Formation within Cores

  18. Orion Dense Cores CO J=2-1 VST, IOA U Tokyo Lis, et al. 1998

  19. Dust Continuum: Dense Cores 350 mm 350 mm Mueller et al. 2002

  20. 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

  21. High-mass: Extreme Complexity S106 Near- IR Subaru H2

  22. Orion-KL Winds & Outlfows

  23. 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

  24. Low-mass Evolutionary Scheme P.Andre 2002

  25. Low-mass: Pre-protostellar Cores • Dense cores with no known internal luminosity source • SEDs peak longer than 100 mm • Study the initial conditions of low-mass SF B68 L1544 SCUBA 850 mm ISO 200 mm 10,000 AU Ward-Thompson et al. 2002 3.5’ x 3.5’ 12’ x 12’

  26. 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 Star Formation

  27. High-mass Evolutionary Sequence ? A. Boonman thesis 2003

  28. 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

  29. Chemical Tracers of Evolution?

  30. 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

  31. 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)

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

  33. Dense Gas Tracers: CS & HCN CS 5-4 CO 1-0 CS 2-1 HCN 1-0 Helfer & Blitz 1997 Shirley et al. 2003

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

  35. Astrochemistry E. F. van Dishoeck 2003

  36. Dust Extinction Mapping • Good pencil beam probe for Av up to 30 mag (Alves et al 1999)

  37. Dust Continuum Emission • Optically thin at long wavelengths => good probe of density and temperature structure • t ~ 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

  38. 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?

  39. 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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