A brief summary of star formation in the milky way
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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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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

W42

BHR-71

Blum, Conti, & Damineli 2000

VLT


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)

  • 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


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) :


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


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 ~ 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


Rho Oph Dense Cores

Motte, Andre, & Neri 1998


Low-mass Dense Cores

B335

N2H+ 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 mm

350 mm

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 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’


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


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

CO J=3-2 Emission

Hubble Telescope

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

    • 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


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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