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The Life Cycle of Giant Molecular Clouds
Charlotte Christensen
Observational Constraints onThe Life Cycle of
Giant Molecular Clouds in Milky Way-like Galaxies
Charlotte Christensen
Coming up
• Physical Background
• Lifecycle• Formation•Core Formation• Protostar Formation• Star Formation•Dispersal
• Nagging Questions
Meet the Molecules
Meet the Molecules
HIIHII
Meet the Molecules
HIHI
Meet the Molecules
HH22
Meet the Molecules1212COCO
Meet the Molecules
1313COCO
Meet the Molecules
NHNH33
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, 2003Haffner 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
MW 21cm radiationMW 21cm radiation
Dickey & Lockman, 1990Dickey & Lockman, 1990
3 Phase Interstellar Media
• Hot Ionized Media• Warm Neutral/Ionized Media• Cold Neutral Media
•HI & H2
• T 15 -- 100K• 100 -- 5000 cm-3
Dame et al, 2001Dame et al, 2001
MW CO emissionMW CO emission
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 NebulaHST
Size ScalesMass (MO) Size (pc) (cm-3)
Superclouds / GMAs
107 -- --
Giant Molecular Clouds
104 -- 106 50 100
Molecular Clouds 103 -- 104 10 100
Bok Globules 1 -- 1000 1 104
Cores 1 -- 1000 1 104
Size ScalesMass (MO) Size (pc) (cm-3)
Superclouds / GMAs
107 -- --
Giant Molecular Clouds
104 -- 106 50 100
Molecular Clouds 103 -- 104 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
2 T + W = 02 T + W = 0
Kinetic EnergyKinetic Energy
Potential EnergyPotential Energy
Jeans Mass:
Support
• Assume Equilibrium•Outside Pressure
2(T - T2(T - T00) + W = 0) + W = 0
Potential EnergyPotential Energy
KE from External PressureKE from External Pressure
Kinetic EnergyKinetic Energy
Support
• Assume Equilibrium• Turbulence vs Thermal KE
2(T2(T + T + TPP - T - T00) + W = 0) + W = 0
Potential EnergyPotential Energy
KE from External PressureKE from External Pressure
Thermal KEThermal KE
Turbulent KETurbulent KE
Support
• Assume Equilibrium•Magnetic Field
2(T2(T + T + TPP - T - T00) + W + B = 0) + W + B = 0
Potential EnergyPotential Energy
KE from External PressureKE from External Pressure
Thermal KEThermal KE
Turbulent KETurbulent KE
Mag. EnegryMag. Enegry
Support
• Assume Equilibrium•Magnetic Field
2(T2(T + T + TPP - T - T00) + W + B = 0) + W + B = 0
Potential Energy
KE from External Pressure
Thermal KE
Turbulent KE
Mag. EnegryMag. Enegry
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• Seed Magnetic Field• Differential Rotation• Convection
• Throughout MW• Seen in polarization
and Zeeman splitting
MPIfR Bonn
NGC 6946
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
Cloud Core Formation
Protostar Collapse
Stars Form
Cloud Dispersal
Life Cycle
Cloud Formation
Cloud Core Formation
Protostar Collapse
Stars Form
Cloud Dispersal
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?
w/ CO
all HIall HI
Correlation with HI
• Filaments of HI around all GMCs
Engargiola et al, 2003Engargiola et al, 2003
M33M33
DensityDensity
Correlation with Spiral Arms
M33M33
• 60% of H2 in spiral arms
• Grand design spirals: • > 90% (Nieten et al. 2006, Garcia-Burillo et al 1993)
Rosolowsky et al, 2007
Age Limits
• = 10-20 Myr• Collisional build
up of molecular clouds• = 2000 Myr
• Quiescent growth of ambient H2
• H2 = 0.3 MO pc2
• = 100 MyrEngargiola et al, 2003Engargiola et al, 2003
M33M33
Shocks
• Observation of a shocked GMA
Tosaki, 2007Tosaki, 2007
1212CC 1313CC
M31
GMC Formation -- Conclusions
• Formed primarily from either HI or H2
• Compressed to self-gravitating clouds in spiral arms
Life Cycle
Cloud Formation
Cloud Core Formation
Protostar Collapse
Stars Form
Cloud Dispersal
Cloud Core Formation
• 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
Lagoon Nebula
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:•Non-isotropic•More prolate than oblate•Not necessarily aligned
with the magnetic field (Glenn 1999)
Prolate
Oblate
Ratio of Clouds without Stars
• One last test of timescale:•NNS/NT = NS/ T
Cloud Formation
Cloud Core Formation
Protostar Collapse
Stars Form
Cloud Dispersal
Ratio of Clouds without Stars
• Very few MW GMCs without SF
• 25% of GMCs in other galaxies have no associate HII regions (Blitz, 2006)
Engargiola, et al 2003Engargiola, et al 2003
M33 -- Distance between GMC and HII
Ratio of Clouds without Stars
• NNS/NT = NS/ T 1/4
• Dynamic Collapse
Cloud Formation
Cloud Core Formation
Protostar Collapse
Stars Form
Cloud Dispersal
Life Cycle
Cloud Formation
Cloud Core Formation
Protostar Collapse
Stars Form
Cloud Dispersal
Core Collapse to Protostar
• Overdensties collapse
• Collapse regulated by• Turbulence•Magnetic Field
• Fragmentation
• Protostar formation when core becomes opaque
Core Sizes &Densities
Radius (pc)
Lee et al, 1999
Enoch et al, 2008
Log Density
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
Critical
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 non-magnetic turbulence
• Speeds close to Alfven speed
Time Scales
• We have flow of material onto magnetically-unsupported cores
• Larger, more massive cores collapse to protostars
• How fast does this happen?
Time Scales -- Spiral Arm Offset
Time Scales -- Spiral Arm Offset
Tosaki, 2002
M51 13CO12CO 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
Cloud Formation
Cloud Core Formation
Protostar Collapse
Stars Form
Cloud Dispersal
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
Cloud Formation
Cloud Core Formation
Protostar Collapse
Stars Form
Cloud Dispersal
0.5 Myr
Life Cycle
Cloud Formation
Cloud Core Formation
Protostar Collapse
Stars Form
Cloud Dispersal
Stars Form
• Powered by gravitational energy
• Envelopes of accreting material
• T Tauri Stars
Trifid, HST
Size
Hatchel & Fullerl, 2008
Younger Protostar
Older Protostar
Starless
Perseus Cores
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
Cloud Formation
Cloud Core Formation
Protostar Collapse
Stars Form
Cloud Dispersal
2 - 4 Myr
Lifecycle
Cloud Formation
Cloud Core Formation
Protostar Collapse
Stars Form
Cloud Dispersal
Clouds Dispersing
Leisawitz, 1989
Proximity to New Stars
• Star clusters older than 10 Myr have no associated clouds
Leisawitz, 1989
Cascading SF
• Dispersing clouds may spark SF elsewhere
Hartmann
M51, HST
Putting it all TogetherCloud Core Formation
Protostar Collapse
Stars Form
Cloud Dispersal
Cloud Formation
Cascading SF
0 1 4
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