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!)
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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

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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 lt 107 Msun
  (Elmegreen 1986)
• Isolated Bok globules M gt 1 Msun (Bok
  Reilly 1947)
• SF traces spiral structure (Schweizer 1976)

M51 Central Region
NASA
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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 gt 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
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Molecular Cloud Lifetime

• Survey of CO towards clusters
• Leisawitz, Bash, Thaddeus 1989
• All cluster with t lt 5 x 106 yrs have molecular
  clouds M gt 104 Msun
• Clusters older than t gt 107 yrs have molecular
  clouds M lt 103 Msun
• Lower limit to molecular cloud lifetime
• Some young clusters show evidence for SF over
  periods of t gt 108 yrs (Stauffer 1980)
• Lifetimes of 107 to 108 yrs

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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 gt 106 cm-3
• Transient coherent structures?

Lupus
Serpens
Optical Av
Optical Av
L. Cambresy 1999
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Gravity

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

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Jeans Mass
0.5
1
2
5
10
20
50
100
200
500
1000
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Star Formation Rate

• Current SFR is 3 /- 1 Msun yr -1 (Scalo 1986)
• Assuming 100 SF efficiency free-fall collapse
• Predicted SFR gt 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

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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 J5-4 shows
  same lack of correlation (Shirley et al. 2003)
• Implies feedback self-regulation of SFR ?

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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)

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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 lt 4 mag (B/30 mG) cloud may be
  supported
• M gt Mcr Magnetically supercritical
• Equation of hydrostatic equilibrium gt support
  perpendicular to B-field
• Dissipation through ambipolar-diffusion increases
  timescale for collapse (Mckee et al. 1993)
• Typical xe 10-7 gt tAD 7 x 106 yrs

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Observed Magnetic Fields
Crutcher 1999
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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 gt need to
  replenish
• Doppler linewidth is very narrow
• CO at 10K Dv 0.13 km/s
• Low-mass regions typically have narrow linewidth
  gt turbulence decays before SF proceeds?
• High-mass regions have very large linewidths
• CS J5-4 ltDvgt 5.6 km/s

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Rho Oph Dense Cores
Motte, Andre, Neri 1998
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Low-mass Dense Cores
B335
N2H J 1 - 0
10,000 AU
IRAS03282
Caselli et al. 2002
Shirley et al. 2000
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Star Formation within Cores
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Orion Dense Cores
CO J2-1
VST, IOA U Tokyo
Lis, et al. 1998
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Dust Continuum Dense Cores
350 mm
350 mm
Mueller et al. 2002
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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
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High-mass Extreme Complexity
S106
Near- IR Subaru
H2
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Orion-KL Winds Outlfows
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SF in Dense Cores

• Star formation occurs within dense molecular
  cores
• High density gas in dense cores (n gt 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 lt few Msun
• High-mass star formation always appears to occur
  in a clustered environment
• Average Properties
• Low-mass R lt 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

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Low-mass Evolutionary Scheme
P.Andre 2002
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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

L1544
B68
SCUBA 850 mm
ISO 200 mm
10,000 AU
Ward-Thompson et al. 2002
3.5 x 3.5
12 x 12
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High-Mass Star Formation

• Basic formation mechanism debated
• Accretion (McKee Tan 2002)
• How do you form a star with M gt 10 Msun before
  radiation pressure stops accretion?
• Coalescence (Bonnell et al. 1998)
• Requires high stellar density n gt 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

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High-mass Evolutionary Sequence ?
A. Boonman thesis 2003
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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
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Chemical Tracers of Evolution?
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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
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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, Mgt0.8 Msun
  (Motte et al. 1998)
• Serpens dN/dM M-2.1
  (Testi Seargent 1998)

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(No Transcript)
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CO Molecular Cloud Tracer
CO J3-2 Emission
Hubble Telescope
CSO
NASA, Hubble Heritage Team
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Dense Gas Tracers CS HCN
CS 5-4
CO 1-0
CS 2-1
HCN 1-0
Shirley et al. 2003
Helfer Blitz 1997
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Comparison of Molecular Tracers

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

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Astrochemistry
E. F. van Dishoeck 2003
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Dust Extinction Mapping

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

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Dust Continuum Emission

• Optically thin at long wavelengths gt 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
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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?

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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?