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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
BHR-71
VLT
W42
Blum, Conti, &
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)
M Jeans
  kT 

 
  mH G 
3/ 2
 1/ 2  18M sunT 3 / 2 n 1/ 2
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 / 30G 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



d r
1
 2  P   
dt
0
2
  
1  2
 2 B   ( B  ) B 
Dissipation through ambipolar-diffusion increases timescale for
collapse (Mckee et al. 1993):
3
t AD 
 7.3 1013 xe yrs
4G 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:
Dv  2
2 ln 2kT
T
 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
N 2H + J = 1 - 0
B335
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
M8E
S158
Optical
W44
S76E
Near-IR
CS J = 5-4, Shirley et al. 2003
RCW 38
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 highmass regions such as W49A
VLA 7mm Cont.
DePree et al. 1997
BIMA
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
Telescope
NASA, Hubble Heritage Team
CO J=3-2
Emission
CSO
Dense Gas Tracers: CS & HCN
CS 5-4
CO 1-0
CS 2-1
Helfer & Blitz 1997
HCN 1-0
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 highmass star forming regions?
IS SF in isolated globules spontaneous or stimulated?
Are we actually observing collapse in dense core
envelopes?