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

A Brief Summary of Star Formation in the Milky Way

Yancy L. Shirley

Star Formation Disucssion Group

April 1 2003 (no joke!)


Outline

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

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

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

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 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

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

Jeans Mass

0.5

1

2

5

10

20

50

100

200

500

1000


Star formation rate

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

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

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 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

Observed Magnetic Fields

Crutcher 1999


Turbulent support

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

Rho Oph Dense Cores

Motte, Andre, & Neri 1998


Low mass dense cores

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

Star Formation within Cores


Orion dense cores

Orion Dense Cores

CO J=2-1

VST, IOA U Tokyo

Lis, et al. 1998


Dust continuum dense cores

Dust Continuum: Dense Cores

350 mm

350 mm

Mueller et al. 2002


High mass dense cores

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

High-mass: Extreme Complexity

S106

Near- IR

Subaru

H2


Orion kl winds outlfows

Orion-KL Winds & Outlfows


Sf in dense cores

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

Low-mass Evolutionary Scheme

P.Andre 2002


Low mass pre protostellar cores

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’


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 Star Formation


High mass evolutionary sequence

High-mass Evolutionary Sequence ?

A. Boonman thesis 2003


Uchii regions hot cores

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

Chemical Tracers of Evolution?


High mass pre protocluster core

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

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: Molecular Cloud Tracer

CO J=3-2 Emission

Hubble Telescope

CSO

NASA, Hubble Heritage Team


Dense gas tracers cs hcn

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

Comparison of Molecular Tracers

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


Astrochemistry

Astrochemistry

E. F. van Dishoeck 2003


Dust extinction mapping

Dust Extinction Mapping

  • Good pencil beam probe for Av up to 30 mag (Alves et al 1999)


Dust continuum emission

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

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

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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