Transcript The Life Cycle of Giant Molecular Clouds

The Life Cycle of
Giant Molecular Clouds
Charlotte Christensen
Observational Constraints on
The Life Cycle of
Giant Molecular Clouds
in Milky Way-like Galaxies
Charlotte Christensen
Coming up
• Physical Background
• Lifecycle
•
•
•
•
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Formation
Core Formation
Protostar Formation
Star Formation
Dispersal
• Nagging Questions
Meet the Molecules
Meet the Molecules
HII
Meet the Molecules
HI
Meet the Molecules
H2
Meet the Molecules
12CO
Meet the Molecules
13CO
Meet the Molecules
NH3
3 Phase Interstellar Media
• Hot Ionized Medium
• Warm Neutral/Ionized Medium
• Cold Neutral Medium
3 Phase Interstellar Media
• Hot Ionized Medium
• HII
• T  106 - 107 K
•   10-4 - 10-2 cm-3
• Warm Neutral/Ionized Medium
• Cold Neutral Medium
Haffner et al, 2003
3 Phase Interstellar Media
• Hot Ionized Media
• Warm Neutral/Ionized Media
• HII & HI
• T  6000 -- 12,000K
•   0.01 cm-3
• Cold Neutral Media
Dickey & Lockman, 1990
MW 21cm radiation
3 Phase Interstellar Media
• Hot Ionized Media
• Warm Neutral/Ionized Media
• Cold Neutral Media
• HI & H2
• T  15 -- 100K
•   100 -- 5000 cm-3
MW CO emission
Dame et al, 2001
Molecular Hydrogen Clouds
• Self-gravitating
(rather than diffuse)
• H2, molecules, and
dust grains
• 30 - 60% of the gas
mass
• Occupy > 1% of the
volume
• Site of star formation
Eagle Nebula
HST
Size Scales
Mass (MO)
Size (pc)  (cm-3)
Superclouds /
107
GMAs
Giant Molecular 104 -- 106
Clouds
Molecular Clouds 103 -- 104
--
--
50
100
10
100
Bok Globules
1 -- 1000
1
104
Cores
1 -- 1000
1
104
Size Scales
Mass (MO)
Size (pc)  (cm-3)
Superclouds /
107
GMAs
Giant Molecular 104 -- 106
Clouds
Molecular Clouds 103 -- 104
--
--
50
100
10
100
Bok Globules
1 -- 1000
1
104
Cores
1 -- 1000
1
104
Some Timescales
• Crossing Time
• Time for a sound wave to propagate
through
• c =  10 Myr
• Dynamical Time
• Time for a particle to free fall to center
• dyn = G-1/2  2 Myr
• “Dynamic” vs “Quasi-Static” Evolution
Support
• Assume Equilibrium
• Virial Theorem
Jeans Mass:
2T +W=0
Kinetic Energy
Potential Energy
Support
• Assume Equilibrium
• Outside Pressure
2(T - T0) + W = 0
Kinetic Energy
KE from External Pressure
Potential Energy
Support
• Assume Equilibrium
• Turbulence vs Thermal KE
2(T + TP - T0) + W = 0
Thermal KE
KE from External Pressure
Potential Energy
Turbulent KE
Support
• Assume Equilibrium
• Magnetic Field
Mag. Enegry
2(T + TP - T0) + W + B = 0
Thermal KE
KE from External Pressure
Potential Energy
Turbulent KE
Support
• Assume Equilibrium
• Magnetic Field
Mag. Enegry
2(T + TP - T0) + W + B = 0
Thermal KE
KE from External Pressure
Potential Energy
Turbulent KE
Turbulent Support -Source
• Internal
• Stellar Winds
• Bipolar Outflows
• HII
• External
•
•
•
•
Density Waves
Differential Rotation
Supernovae
Winds from Massive Stars
Turbulent Support -Decay
• Close to a Kolmogrov Spectrum
• Cascade down to lower energies
• Large eddies form small eddies
• Small eddies dissipated through friction
• Timescale:  1 Myr
Magnetic Field Support -Source
• Galactic Dynamo
NGC 6946
• Seed Magnetic Field
• Differential Rotation
• Convection
• Throughout MW
• Seen in polarization
and Zeeman splitting
MPIfR Bonn
Magnetic Field Support -Decay
• Ambipolar Diffusion -- Decoupling of
charged and neutral particles
• Timescale: 10 Myr
• Depends on:
• Density
• Magnetic Flux
• Ionization Fraction
Life Cycle
Cloud Formation
Protostar Collapse
Cloud Core Formation
Cloud Dispersal
Stars Form
Life Cycle
Cloud Formation
Protostar Collapse
Cloud Core Formation
Cloud Dispersal
Stars Form
Theories
• Collisional build up of molecular clouds
• Growth time  collisional time
• Quiescent growth of ambient H2
• Gravitational/magnetic instability
• Shock compression
• Spiral Arms
• Supernovae
• From HI of H2?
Correlation with HI
• Filaments of HI
around all GMCs
M33
all HI
w/
CO
Density
Engargiola et al, 2003
Correlation with Spiral Arms
•  60% of H2 in spiral arms
• Grand design spirals:
• > 90% (Nieten et al. 2006, Garcia-Burillo et al 1993)
Rosolowsky et al, 2007
M33
Age Limits
•  = 10-20 Myr
• Collisional build
up of molecular
clouds
M33
•  = 2000 Myr
• Quiescent growth
of ambient H2
• H2 = 0.3 MO pc2
•  = 100 Myr
Engargiola et al, 2003
Shocks
• Observation of a
shocked GMA
M31
12C
Tosaki, 2007
13C
GMC Formation -Conclusions
• Formed primarily
from either HI or H2
• Compressed to selfgravitating clouds in
spiral arms
Life Cycle
Cloud Formation
Protostar Collapse
Cloud Core Formation
Cloud Dispersal
Stars Form
Cloud Core Formation
Lagoon Nebula
• GMC is supported by:
• Magnetic flux
• Turbulence
• Support is removed either
• Slowly by Ambipolar diffusion
• Fast by decay of turbulence and
turbulence amplified diffusion
• Cores (regions 2-4 times ambient density)
form at  10% efficiency
Initial Conditions
• Cloud envelope is
• In non-equilibrium
• Magnetically subcritical (Cortes et al, 2005)
• Very inhomogenous
Carina, HST
Observations of Cores
Myers & Fuller, 1991
Observations of Cores
• Cores are:
Oblate
• Non-isotropic
• More prolate than oblate
• Not necessarily aligned
with the magnetic field
(Glenn 1999)
Prolate
Ratio of Clouds without Stars
• One last test of timescale:
• NNS/NT = NS/ T
Cloud Formation
Protostar Collapse
Cloud Core Formation
Cloud Dispersal
Stars Form
Ratio of Clouds without Stars
M33 -- Distance between GMC and HII
• Very few MW
GMCs without
SF
• 25% of GMCs
in other
galaxies have
no associate
HII regions
(Blitz, 2006)
Engargiola, et al 2003
Ratio of Clouds without Stars
• NNS/NT = NS/ T  1/4
• Dynamic Collapse
Protostar Collapse
Cloud Dispersal
Cloud Formation
Cloud Core Formation
Stars Form
Life Cycle
Cloud Formation
Protostar Collapse
Cloud Core Formation
Cloud Dispersal
Stars Form
Core Collapse to Protostar
• Overdensties collapse
• Collapse regulated by
• Turbulence
• Magnetic Field
• Fragmentation
• Protostar formation
when core becomes
opaque
Core Sizes &Densities
Log Density
Enoch et al, 2008
Radius (pc)
Lee et al, 1999
Protostar Formation
Size
Magnetic Support
• Cores are (probably)
supercritical,
i.e. not supported by
the magnetic field
• M/B = c G-1/2
• c  0.12
Crutcher, 1999
Turbulence
• Cores are turbulent
• Motions are
Supersonic
• Turbulence
from
shocks or
MHD waves
Myers & Khersonsky, 1994
MHD Turbulence
• Dependent on Ionization
• Decays by ***
• Decay rate is still comparable to nonmagnetic turbulence
• Speeds close to Alfven speed
Time Scales
• We have flow of
material onto
magneticallyunsupported cores
• Larger, more massive
cores collapse to
protostars
• How fast does this
happen?
Time Scales -Spiral Arm Offset
Time Scales -Spiral Arm Offset
12CO
Tosaki, 2002
M51 13CO
H
Time Scales -Spiral Arm Offset
• Difference between peaks  10 Myr
• Long delay of SF OR staggered SF
Tosaki, 2002
Time Scales -Statistcs
• Ratio of clouds without protostars:
• NNSC/NC = NSC/ C
Protostar Collapse
Cloud Dispersal
Cloud Formation
Cloud Core Formation
Stars Form
Time Scales -Statistics
• Optically Selected
MW Cores:
• NNSC/NC = 306/400
(Lee & Myers, 1999)
• Perseus, Serpens,
& Ophiuchus:
• NNSC/NC = 108/200
(Enoch et al, 2008)
• 25% - 50% of core
life before SF
(Enoch et al, 2008)
Time Scales -Statistics
• Lifetime of a protostar  2 - 5 x 105 Myr
• Lifetime of a core  0.3 - 1 x 106 Myr
Protostar Collapse
Cloud Dispersal
Cloud Formation
0.5 Myr
Cloud Core Formation
Stars Form
Life Cycle
Cloud Formation
Protostar Collapse
Cloud Core Formation
Cloud Dispersal
Stars Form
Stars Form
• Powered by
gravitational
energy
• Envelopes of
accreting material
• T Tauri Stars
Trifid, HST
Size
Starless
Perseus Cores
Younger Protostar
Older Protostar
Hatchel & Fullerl, 2008
Time Scale
• T Tauri
Problem
• Most stars
form within
3 Myr
Palla & Stahler, 2000
Location
Huff & Stahler, 2006
Time Scale
• Star formation lasts  2 - 4 Myr
• Clouds gone after 5 - 10 Myr
Protostar Collapse
Cloud Dispersal
Cloud Formation
2 - 4 Myr
Cloud Core Formation
Stars Form
Lifecycle
Cloud Formation
Protostar Collapse
Cloud Core Formation
Cloud Dispersal
Stars Form
Clouds Dispersing
Leisawitz, 1989
Proximity to New Stars
• Star clusters older
than 10 Myr have no
associated clouds
Leisawitz, 1989
Cascading SF
M51, HST
• Dispersing clouds
may spark SF
elsewhere
Hartmann
Putting it all Together
Cloud Core Formation
Stars Form
Cloud
Formation
0
Protostar
Collapse
Cloud Dispersal
1
4
Cascading SF
10 - 20 Myr
Nagging Questions
• Do clouds form from HI of H2?
• How long before cores form?
• What effect does the magnetic field
have on turbulence?
Thanks
• Tom Quinn, Fabio
Governato, Julianne
Dalcanton, Andrew
Connely, Bruce Hevly
• Adrienne and David
for making me dinner
• Everybody who came
to my practice talk
Gas In-fall Onto Cores
Lee, 2001
Alignment
MHD Turbulence
Padoan, 2004
Core Densities
Enoch, 2008
Location
Huff & Stahler, 2006
More Dispersal
Jorgensen, 2007