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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 Formation Core Formation Protostar Formation
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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 12CO
Meet the Molecules 13CO
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 MW 21cm radiation Dickey & Lockman, 1990
Dame et al, 2001 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
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
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: 2 T + 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 NGC 6946 • Galactic Dynamo • 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
Cloud Formation Protostar Collapse Cloud Dispersal Cloud Core Formation Stars Form Life Cycle
Life Cycle Cloud Formation Protostar Collapse Cloud Dispersal Cloud Core Formation 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?
all HI w/ CO Correlation with HI M33 • Filaments of HI around all GMCs 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) M33 Rosolowsky et al, 2007
Age Limits M33 • = 10-20 Myr • Collisional build up of molecular clouds • = 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 Tosaki, 2007 12C 13C
GMC Formation -- Conclusions • Formed primarily from either HI or H2 • Compressed to self-gravitating clouds in spiral arms
Life Cycle Cloud Formation Protostar Collapse Cloud Dispersal Cloud Core Formation 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 Oblate • Cores are: • Non-isotropic • More prolate than oblate • Not necessarily aligned with the magnetic field (Glenn 1999) Prolate
Cloud Formation Protostar Collapse Cloud Dispersal Cloud Core Formation Stars Form Ratio of Clouds without Stars • One last test of timescale: • NNS/NT = NS/ T
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 Dispersal Cloud Core Formation Stars Form
Core Collapse to Protostar • Overdensties collapse • Collapse regulated by • Turbulence • Magnetic Field • Fragmentation • Protostar formation when core becomes opaque
Log Density Enoch et al, 2008 Core Sizes &Densities 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 Critical 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 non-magnetic turbulence • Speeds close to Alfven speed