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The Life Cycle of Giant Molecular Clouds

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  1. The Life Cycle of Giant Molecular Clouds Charlotte Christensen

  2. Observational Constraints onThe Life Cycle of Giant Molecular Clouds in Milky Way-like Galaxies Charlotte Christensen

  3. Coming up • Physical Background • Lifecycle • Formation • Core Formation • Protostar Formation • Star Formation • Dispersal • Nagging Questions

  4. Meet the Molecules

  5. Meet the Molecules HII

  6. Meet the Molecules HI

  7. Meet the Molecules H2

  8. Meet the Molecules 12CO

  9. Meet the Molecules 13CO

  10. Meet the Molecules NH3

  11. 3 Phase Interstellar Media • Hot Ionized Medium • Warm Neutral/Ionized Medium • Cold Neutral Medium

  12. 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

  13. 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

  14. 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

  15. 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

  16. Size Scales

  17. Size Scales

  18. 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

  19. Support • Assume Equilibrium • Virial Theorem Jeans Mass: 2 T + W = 0 Kinetic Energy Potential Energy

  20. Support • Assume Equilibrium • Outside Pressure 2(T - T0) + W = 0 Kinetic Energy KE from External Pressure Potential Energy

  21. Support • Assume Equilibrium • Turbulence vs Thermal KE 2(T + TP - T0) + W = 0 Thermal KE KE from External Pressure Potential Energy Turbulent KE

  22. Support • Assume Equilibrium • Magnetic Field Mag. Enegry 2(T + TP - T0) + W + B = 0 Thermal KE KE from External Pressure Potential Energy Turbulent KE

  23. Support • Assume Equilibrium • Magnetic Field Mag. Enegry 2(T + TP - T0) + W + B = 0 Thermal KE KE from External Pressure Potential Energy Turbulent KE

  24. Turbulent Support -- Source • Internal • Stellar Winds • Bipolar Outflows • HII • External • Density Waves • Differential Rotation • Supernovae • Winds from Massive Stars

  25. 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

  26. Magnetic Field Support -- Source NGC 6946 • Galactic Dynamo • Seed Magnetic Field • Differential Rotation • Convection • Throughout MW • Seen in polarization and Zeeman splitting MPIfR Bonn

  27. Magnetic Field Support -- Decay • Ambipolar Diffusion -- Decoupling of charged and neutral particles • Timescale: 10 Myr • Depends on: • Density • Magnetic Flux • Ionization Fraction

  28. Cloud Formation Protostar Collapse Cloud Dispersal Cloud Core Formation Stars Form Life Cycle

  29. Life Cycle Cloud Formation Protostar Collapse Cloud Dispersal Cloud Core Formation Stars Form

  30. 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?

  31. all HI w/ CO Correlation with HI M33 • Filaments of HI around all GMCs Density Engargiola et al, 2003

  32. 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

  33. 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

  34. Shocks • Observation of a shocked GMA M31 Tosaki, 2007 12C 13C

  35. GMC Formation -- Conclusions • Formed primarily from either HI or H2 • Compressed to self-gravitating clouds in spiral arms

  36. Life Cycle Cloud Formation Protostar Collapse Cloud Dispersal Cloud Core Formation Stars Form

  37. 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

  38. Initial Conditions • Cloud envelope is • In non-equilibrium • Magnetically subcritical (Cortes et al, 2005) • Very inhomogenous Carina, HST

  39. Observations of Cores Myers & Fuller, 1991

  40. Observations of Cores Oblate • Cores are: • Non-isotropic • More prolate than oblate • Not necessarily aligned with the magnetic field (Glenn 1999) Prolate

  41. 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

  42. 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

  43. Ratio of Clouds without Stars • NNS/NT = NS/ T  1/4 • Dynamic Collapse Protostar Collapse Cloud Dispersal Cloud Formation Cloud Core Formation Stars Form

  44. Life Cycle Cloud Formation Protostar Collapse Cloud Dispersal Cloud Core Formation Stars Form

  45. Core Collapse to Protostar • Overdensties collapse • Collapse regulated by • Turbulence • Magnetic Field • Fragmentation • Protostar formation when core becomes opaque

  46. Log Density Enoch et al, 2008 Core Sizes &Densities Radius (pc) Lee et al, 1999

  47. Protostar Formation Size

  48. 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

  49. Turbulence • Cores are turbulent • Motions are Supersonic • Turbulence from shocks or MHD waves Myers & Khersonsky, 1994

  50. MHD Turbulence • Dependent on Ionization • Decays by *** • Decay rate is still comparable to non-magnetic turbulence • Speeds close to Alfven speed