Download Flow-Driven Formation of Molecular Clouds

Flow-Driven Formation of Molecular Clouds:
Insights from Numerical Models
Lee Hartmann
Javier Ballesteros-Paredes
Adrianne Slyz
Julien Devriendt
Andreas Burkert
The Cypress Cloud, Spitzer/GLIMPSE, FH et al. 09
The Classical Problem of Star Formation:
If all the molecular gas in the Galaxy collapsed
on its free-fall time, the star formation rate
would be ~20 times higher than observed.
Traditional solution:
Molecular clouds supported against collapse
for many free-fall times
by turbulence and/or magnetic fields.
Star formation: slow equilibrium process.
characteristic timescale:
3Myr @ 100 cm-3
What sets the star formation efficiency?
Cited evidence:
- long cloud lifetimes
- existence of starless clouds
- turbulent linewidths
- magnetic fields
The Points to be Made:
(1) Gravity is a long-range force. Thus, global gravity rules.
Filaments are a natural consequence of global gravity.
(2) Molecular clouds are dynamic ( = not in equilibrium).
They are collapsing during their formation.
(3) “Turbulence” in molecular clouds is driven by gravity.
Turbulent support does not exist.
(4) Star formation is “rapid”, but inefficient.
The SFE is set by rapid fragmentation during the cloud’s
formation (and stellar feedback).
What sets the star formation efficiency?
Why rethinking our picture of star formation?
Molecular clouds have (“turbulent”) structure.
Perseus, 12CO
Ridge et al. 2006
Taurus-Perseus, 12CO
Ungerechts & Thaddeus 1987
Taurus, 12CO
Goldsmith et al. 2008
What sets the star formation efficiency?
The Classical Problem of Star Formation:
“Long” cloud lifetimes? Not in local star-forming regions!
<10 Myr: local star-forming regions
Hartmann et al. 01, Ballesteros-Paredes & Hartmann 07
What sets the star formation efficiency?
Why Look at Molecular Cloud Formation?
Starless clouds?
Most clouds form stars.
Stellar age spreads are small (1-3 Myr).
Thus, star formation is rapid.
stellar ages
Core, 24 & CS; Megeath et al. 2009
Maddalena-Thaddeus Cloud, 13CO
Lee et al. 1994
What sets the star formation efficiency?
Hartmann 03
Cep OB2: supernova, H II region-driven bubbles
1 Myr-old stars
50 pc
100 m IRAS
dust emission
Extragalactic view: (only100 pc)
10 Myr “age spread”;
H II, H I, CO (H2);
is this a single cloud?
~ 10 Myrold cluster:
supernova/
winds
~ 4 Myr-old cluster,
H II region
Physical Constraints
Rapid onset of star formation requires specific cloud properties:
free-fall time of a spherical cloud of uniform density:
3Myr @ 100 cm-3
- Freefall time independent of radius.
- Linear density perturbations will be swept up in global collapse.
Burkert & Hartmann 04
- Massive stars form only rarely (if at all) in “isolation”.
Chu & Gruendl 08, Lamb et al. 09
The rapid onset of star formation requires
non-linear density seeds during cloud formation.
Flow-Driven Cloud Formation
The Physical Regimes of Cloud Formation
warm neutral gas
:T = 5000-8000 K, n = 1…3 cm-3
cold neutral/molecular gas:T<100 K, n >100 cm-3
warm ionized gas
:T = 10000 K, n < 1 cm-3
thermal
instability
Elmegreen 07
Flow-Driven Cloud Formation
Fluid Dynamics of Cloud Formation
Large-scale flows:
- spiral arms
- expanding/colliding shells
- galaxy mergers
Processes & Agents:
FH et al 08b
- shocks & shear flows
Thermal Equilibrium curve: P  n
WIM
fragmentation, turbulence
- radiative losses/thermal instability
fragmentation, strong compression
- gravity
fragmentation, collapse
- magnetic fields
we’ll get them later
Flow-Driven Cloud Formation
≈1
WNM
<0
CNM
1
A Numerical Experiment:
Two uniform, identical flows
no assumption about turbulence
colliding head-on at interface
expanding shells, spiral arms
with large-scale geometric perturbation
mimicking unavoidable shear
in non-periodic domain.
allowing global gravitational modes
Heating and cooling to model WNM  CNM.
No stellar feedback.
Hydro and MHD models.
Fixed-grid simulations.
Methods: Proteus
Athena
FH et al. 04, 07, 08
Stone et al. 08
Flow-Driven Cloud Formation
Cooling, Gravity & Geometry
blue/green : thermal fragmentation;
red/yellow : local collapse;
filament : global collapse
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3D at 256 x 512 x 512
19 < log N [cm-2] < 23
n = 3 cm-3
v = 9 km s-1
Flow-Driven Cloud Formation
FH et al. 08a
The “rapid” formation of molecular clouds and stars
Global gravity increases CO formation.
without self-gravity
but
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N(filament) @ 10 Myr ~ 1022 cm-2
FH & Hartmann 08
contours: HI
color
: CO
Flow-Driven Cloud Formation
22 pc
with self-gravity
Global vs local free-fall time
Thermal fragmentation:
 small filling factors
 short free-fall times (not coherent)
 “presetting” local collapse
Densest regions form stars, while the envelope
might never be relevant for star formation.
Goldsmith et al. 08
FH & Hartmann 08
Flow-Driven Cloud Formation
The Role of Turbulence:
“Turbulence” is (at least partially) driven by gravity.
The bulk of the energy is on the largest scales.
There is no such thing as “turbulent support”.
Turbulence in cold gas is not supersonic hydrodynamically.
Flow-Driven Cloud Formation
Clouds are not in equilibrium:
The virial parameters
are of order unity.
Global gravity starts
dominating the cloud’s
evolution.
Flow-Driven Cloud Formation
Magnetic Fields
Collapse of dense regions, support of diffuse envelope.
collapsing cores
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supported, diffuse envelope
FH et al. 09
Taurus, Goldsmith/Heyer 08
44 pc
field in equipartition with flow energy
Flow-Driven Cloud Formation
FH et al. 09
The Major Points to be Made:
(1) Gravity is a long-range force.
Thus, global gravity rules.
Filaments are a natural
consequence of global gravity.
(2) Molecular clouds are dynamic
( = not in equilibrium).
They are collapsing during their
formation.
(3) Turbulence in molecular clouds is
driven by gravity.Turbulent support
does not exist.
The Points to be Made
The Major Points to be Made:
(4) Star formation is “rapid”,
but inefficient. The SFE is
set by rapid fragmentation
during the cloud’s formation
(and stellar feedback).
QuickTime™ and a
QuickTime™
and a
PNG decompressor
YUV420
codec
decompressor
are
needed
to see
this picture.
are needed to see this picture.
Gritschneder et al. 09
Elmegreen 07
The Points to be Made
The Major Points to be Made:
(1) Gravity is a long-range force. Thus, global gravity rules.
Filaments are a natural consequence of global gravity.
(2) Molecular clouds are dynamic ( = not in equilibrium).
They are collapsing during their formation.
(3) Turbulence in molecular clouds is driven by gravity.
Turbulent support does not exist.
(4) Star formation is “rapid”, but inefficient.
The SFE is set by rapid fragmentation during the cloud’s
formation (and stellar feedback).
The Points to be Made
END