the life cycle of giant molecular clouds charlotte christensen
TRANSCRIPT
![Page 1: The Life Cycle of Giant Molecular Clouds Charlotte Christensen](https://reader030.vdocuments.us/reader030/viewer/2022033107/56649d9f5503460f94a897ad/html5/thumbnails/1.jpg)
The Life Cycle of Giant Molecular Clouds
Charlotte Christensen
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Observational Constraints onThe Life Cycle of
Giant Molecular Clouds in Milky Way-like Galaxies
Charlotte Christensen
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Coming up
• Physical Background
• Lifecycle• Formation•Core Formation• Protostar Formation• Star Formation•Dispersal
• Nagging Questions
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Meet the Molecules
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Meet the Molecules
HIIHII
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Meet the Molecules
HIHI
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Meet the Molecules
HH22
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Meet the Molecules1212COCO
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Meet the Molecules
1313COCO
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Meet the Molecules
NHNH33
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3 Phase Interstellar Media
• Hot Ionized Medium
• Warm Neutral/Ionized Medium
• Cold Neutral Medium
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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
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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
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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
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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
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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
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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
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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
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Support
• Assume Equilibrium• Virial Theorem
2 T + W = 02 T + W = 0
Kinetic EnergyKinetic Energy
Potential EnergyPotential Energy
Jeans Mass:
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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
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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
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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
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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
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Turbulent Support -- Source
• Internal• Stellar Winds• Bipolar Outflows•HII
• External•Density Waves•Differential Rotation• Supernovae•Winds from Massive Stars
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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
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Magnetic Field Support -- Source
• Galactic Dynamo• Seed Magnetic Field• Differential Rotation• Convection
• Throughout MW• Seen in polarization
and Zeeman splitting
MPIfR Bonn
NGC 6946
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Magnetic Field Support -- Decay
• Ambipolar Diffusion -- Decoupling of charged and neutral particles
• Timescale: 10 Myr
• Depends on: •Density•Magnetic Flux• Ionization Fraction
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Life Cycle
Cloud Formation
Cloud Core Formation
Protostar Collapse
Stars Form
Cloud Dispersal
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Life Cycle
Cloud Formation
Cloud Core Formation
Protostar Collapse
Stars Form
Cloud Dispersal
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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?
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w/ CO
all HIall HI
Correlation with HI
• Filaments of HI around all GMCs
Engargiola et al, 2003Engargiola et al, 2003
M33M33
DensityDensity
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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
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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
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Shocks
• Observation of a shocked GMA
Tosaki, 2007Tosaki, 2007
1212CC 1313CC
M31
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GMC Formation -- Conclusions
• Formed primarily from either HI or H2
• Compressed to self-gravitating clouds in spiral arms
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Life Cycle
Cloud Formation
Cloud Core Formation
Protostar Collapse
Stars Form
Cloud Dispersal
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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
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Initial Conditions
• Cloud envelope is• In non-equilibrium•Magnetically subcritical (Cortes et al, 2005)
• Very inhomogenous
Carina, HST
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Observations of Cores
Myers & Fuller, 1991
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Observations of Cores
• Cores are:•Non-isotropic•More prolate than oblate•Not necessarily aligned
with the magnetic field (Glenn 1999)
Prolate
Oblate
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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
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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
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Ratio of Clouds without Stars
• NNS/NT = NS/ T 1/4
• Dynamic Collapse
Cloud Formation
Cloud Core Formation
Protostar Collapse
Stars Form
Cloud Dispersal
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Life Cycle
Cloud Formation
Cloud Core Formation
Protostar Collapse
Stars Form
Cloud Dispersal
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Core Collapse to Protostar
• Overdensties collapse
• Collapse regulated by• Turbulence•Magnetic Field
• Fragmentation
• Protostar formation when core becomes opaque
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Core Sizes &Densities
Radius (pc)
Lee et al, 1999
Enoch et al, 2008
Log Density
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Protostar Formation
Size
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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
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Turbulence
• Cores are turbulent
• Motions are Supersonic
• Turbulence from shocks or MHD waves
Myers & Khersonsky, 1994
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MHD Turbulence
• Dependent on Ionization
• Decays by ***
• Decay rate is still comparable to non-magnetic turbulence
• Speeds close to Alfven speed
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Time Scales
• We have flow of material onto magnetically-unsupported cores
• Larger, more massive cores collapse to protostars
• How fast does this happen?
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Time Scales -- Spiral Arm Offset
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Time Scales -- Spiral Arm Offset
Tosaki, 2002
M51 13CO12CO H
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Time Scales -- Spiral Arm Offset
• Difference between peaks 10 Myr
• Long delay of SF OR staggered SF
Tosaki, 2002
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Time Scales --Statistcs
• Ratio of clouds without protostars:•NNSC/NC = NSC/ C
Cloud Formation
Cloud Core Formation
Protostar Collapse
Stars Form
Cloud Dispersal
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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)
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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
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Life Cycle
Cloud Formation
Cloud Core Formation
Protostar Collapse
Stars Form
Cloud Dispersal
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Stars Form
• Powered by gravitational energy
• Envelopes of accreting material
• T Tauri Stars
Trifid, HST
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Size
Hatchel & Fullerl, 2008
Younger Protostar
Older Protostar
Starless
Perseus Cores
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Time Scale
• T Tauri Problem•Most stars
form within 3 Myr
Palla & Stahler, 2000
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Location
Huff & Stahler, 2006
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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
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Lifecycle
Cloud Formation
Cloud Core Formation
Protostar Collapse
Stars Form
Cloud Dispersal
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Clouds Dispersing
Leisawitz, 1989
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Proximity to New Stars
• Star clusters older than 10 Myr have no associated clouds
Leisawitz, 1989
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Cascading SF
• Dispersing clouds may spark SF elsewhere
Hartmann
M51, HST
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Putting it all TogetherCloud Core Formation
Protostar Collapse
Stars Form
Cloud Dispersal
Cloud Formation
Cascading SF
0 1 4
10 - 20 Myr
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Nagging Questions
• Do clouds form from HI of H2?
• How long before cores form?
• What effect does the magnetic field have on turbulence?
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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
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Gas In-fall Onto Cores
Lee, 2001
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Alignment
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MHD Turbulence
Padoan, 2004
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Core Densities
Enoch, 2008
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Location
Huff & Stahler, 2006
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More Dispersal
Jorgensen, 2007