Clare Louise Dobbs- The Formation of Molecular Clouds in Spiral Galaxies
Star formation from (local) molecular clouds to spiral arms
description
Transcript of Star formation from (local) molecular clouds to spiral arms
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Star formation from (local) molecular clouds to spiral arms
Lee HartmannUniversity of Michigan
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Structure of molecular clouds
Efficiency of star formation
Feedback
Mechanisms for forming molecular clouds
Implications for:
• observations of distant objects
• star formation rates
• IMFs
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12CO13CO
Taurus: Goldsmith et al. 2009
Fraction of mass in dense regions ≈ 10%Fraction of mass in stars ≈ 2%
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Efficiency of star formation:
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Structure of molecular clouds most of the mass ⇒is at low density
Efficiency of star formation: low, because only high-density regions form stars... and because of
Feedback! (strongly limits cloud lifetimes)
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Orion Nebula regionMegeath et al.
Blow-out by O7 @ ~ 1 Myr
Blow-out by B star(s) @ 2-3 Myr
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10 pc
d=2 kpc
W5
24, 8, 4.5 m
Small Green Circles: IR-ex sources, Big Green/Blue Circles: Protostars
Koenig et al. 2008
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12CO
Taurus: low-density regions show magnetic "striations"
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Low efficiency:
not due to ambipolar diffusion of magnetic fields
• feedback: energy input from massive stars limits cloud lifetimes
• structured clouds: high density regions small mass/volume filling factors due to
• magnetic support of low-density regions (e.g., Price & Bate 2009)• gravitational focusing
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Structure of molecular clouds most of the mass ⇒is at low density
Efficiency of star formation: low, because of feedback and
Gravitational focusing- a natural byproduct of the formation mechanisms for molecular clouds
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Young stars in Orion: most have ages ~ 1-2 Myr
~ 70 pc
crossing time ~ 10-20 Myr ⇒ information not propagated laterally; swept up!
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12CO
Orion: supernova bubble?
140 pc below galactic plane
Star -forming clouds produced by shocked flows
spiral shocks in galaxies
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Finite sheet evolution with gravity
Burkert & Hartmann 04; piece of bubble wall sheet
how does it know to be in virial equilibrium?answer: it doesn't
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“Orion A” model (Hartmann & Burkert 2007); collapse of finite, massive, elliptical, rotating sheet
13CO, Bally et al.
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Short radius of curvature results in extra mass concentrations assemble cluster gas/stars
Global collapse (over ~ 2 Myr) - makes filamentary ridge, Orion Nebula cluster
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Orion Nebula cluster- (optical) kinematics of stars and gas: evidence for infall?
J. Tobin, et al. 2009
V
E. Proszkow, F. Adams
vlsr (km/s))
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gravitational focusing: clusters form preferentially at ends of filaments
can’t escape large-scale focusing by gravity
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Cluster gravitational focusing- any short radius of curvature will do
Gutermuth et al.
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NEED turbulence to generate density fluctuations during cloud formation- must be rapid to compete with global collapse
Thin shell Kelvin-
Helmholtz cooling
(thermal instability)
and gravity...
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F. Heitsch et al. 2007; sheet made by inflows with cooling, gravity
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Orion Nebula regionMegeath et al.
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Structure of molecular clouds most of the mass ⇒is at low density
Efficiency of star formation: low, because of feedback and
Gravitational focusing- a natural byproduct of the formation mechanisms for molecular clouds: make
clustersstars...
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What about the stellar IMF?
log M→
N(log m)
fragmentation
“competitive accretion”(e.g., Bonnell et al.)
not clear evidence of variation, though massive clusters near GC suggest slightly flatter upper IMF (Stolte, Figer, etc.)
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Numerical simulations of competitive accretion in sheets (Hsu et al. 2010);
high-mass IMF depends upon amount of accretion, evolution toward Salpeter slope (or beyond?)
-1.35
⇒as limiting slope
Similar to star cluster IMF (Fall, Chandar) ; gravitational focussing
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Compression
efficiency
dM(tot)/dt
gravitational focusing
feedback (dispersal)
The local story:
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How does this work for the molecular "ring"?
Dame 1993, AIPC 278, 267
Jackson et al.
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Peek 2009, ApJ 698, 1429, from Stahler & Palla 2005
Does star formation follow the H2 content?
Is the SFR low in the molecular "ring"?
Or just more diffuse "shielded" gas?
AV ~0.5 in ~ 320 pc
AV ~0.5 in ~ 160 pc
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2007
12CO can be seen at low AV ; if more shielding because higher average gas density, a solar neighborhood "molecular cloud" may differ from a molecular ring "cloud"
12CO is a column density tracer, not just a density tracer
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Implications for distant objects:
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Age "spreads" and "cloud lifetimes"
~ 10 Myr-old cluster:supernova/winds
50 pc
100 m IRAS dust emission
1 Myr-old stars
~ 4 Myr-old cluster, H II region
Extragalactic view: (100 pc) 10 Myr “age spread”;H II, H I, CO
Cep OB2
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Star formation where gas is compressed by shocks
NGC 6946H
• outer disk spiral shock; not enough material to continue propagating SF
• inner disk; potential well, (Q <~ 1), more gas more continuing SF (local feedback)
Ferguson et al. 1998
d/dt eff [vorb - v(pattern)] /tdyn
efficiency ~ 2% per cloud, few triggered 10%/orbit
one version of the Kennicutt-Schmidt law
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Gas content and SFR?
• molecular gas should trace SFR more closely, as it generally traces the densest gas
• high-density molecular tracers should follow the SFR very closely; but this does not address the question of the RATE of formation of dense molecular gas
• CO is sensitive to column density as well as density; likely to trace different phases in differing regions (and most CO emission comes from non-star forming gas in the solar neighborhood)
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Structure of molecular clouds: most mass at low
Efficiency of star formation ~ few %
Feedback + gravitational focusing limit efficiency
Molecular clouds formed by large-scale flows; creates turbulent structure necessary for fragmentation into stars
distant objects: averaging over cycling of gas
star formation rates set by gas content (not just molecular gas) plus rate of compressions
gravitational focusing may lead to similar massive star and cluster IMFs, but physics of peak unclear
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Typical numerical simulations:start with imposed turbulent velocity ≈
virial ⇒ prevents immediate collapse
How do clouds "know" to have this level of turbulence, given how the clouds form?
Too much: doesn't collapse, expands; too little, collapse; how to hit the sweet spot?
can't in general.
However, turbulence IS needed to make stellar fragments...
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Ages of young associations/clusters
Fast onset after MC formation!
<t> << t(cross); clouds swept-up by large scale flows
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Growth of high-mass power-law tail:•doesn’t require initial cluster environment•dM/dt M∝ 2
dM/dt
log M
Bondi-Hoyle:dM/dt M∝ 2 ρ v-3
M2
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Orion A: 13CO (Bally et al.)
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Ages of young associations/clusters
Fast onset after MC formation!(because cloud is already collapsing globally)
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Orion Nebula cluster- kinematics of stars and gas: evidence for infall?
J. Tobin, et al. 2009
V
E. Proszkow, F. Adams
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NO molecular gas!
Hurray! ONE region not peaking at 1-2 Myr ago...OOPS; NO molecular gas!
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Hsu, Heitsch et al.
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Compression
efficiency
dM(tot)/dt
gravitational focusing
feedback (dispersal)
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Compression (rate of)
gravitational focusing (makes low efficiency by feedback possible)
feedback (dispersal)
dM(tot)/dt efficiency;
Summary: SFR set by
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NO molecular gas!
Hurray! ONE region not peaking at 1-2 Myr ago...OOPS; NO molecular gas (only on periphery)
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A popular cloud model:
Must put in (arbitrary) velocity field with some "large scale" component to make it look like a real cloud
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Accretion of randomly-placed sink particles in a sheet; does competitive accretion still work?
Tina Hsu, LH, Gilberto Gomez, Fabian Heitsch
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Another popular cloud model:periodic box
This model also needs imposed turbulence to make reasonable-looking structure;
also: eliminates large-scale gravity
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Sco OB2: quickly emptied by winds, SN
de Geus 1992; Preibisch & Zinnecker 99, 02
~ 140 pc
age ~ 5 Myr
age ~ 10-15 Myr
Oph MC(~ 1 Myr)
Upper Sco
Ophiuchus molecular cloud
HI shells