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Spiral Triggering of Star Formation

Spiral Triggering of Star Formation. Ian Bonnell, Clare Dobbs Tom Robitaille, University of St Andrews Jim Pringle IoA, Cambridge. Dynamical Models of Star Formation. Local regions of GMCs Models for the origin of Stellar clusters Massive stars Brown dwarfs Initial Mass Function

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Spiral Triggering of Star Formation

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  1. Spiral Triggering of Star Formation Ian Bonnell, Clare Dobbs Tom Robitaille, University of St AndrewsJim PringleIoA, Cambridge

  2. Dynamical Models of Star Formation • Local regions of GMCs • Models for the origin of • Stellar clusters • Massive stars • Brown dwarfs • Initial Mass Function • But not the initial conditions for star formation

  3. Giant Molecular Clouds • Stars form in molecular clouds • Molecular cloud properties • Mass: 1000’s to >105 Msun • Sizes: ~ 10 pc • Densities: 10-19 to 10-22 g cm-3 • Cold: T ~ 10 K • Located in spiral arms • Lots of structure • Supersonic ‘turbulence’ • Larson relation:

  4. Spiral Shocks and Star Formation • Do spiral shocks control star formation? • Roberts 1971 • Gas dynamics in 2 (4) armed spiral potential • External potential • SPH simulations (4 x 105 to 4 x 106 particles) • Isothermal (100 K) • Clumpy : average 10-3Msun /pc3 ; max 10-1Msun /pc3 • Self gravity • Star formation modeled with sink-particles

  5. Initial Conditions • Test particle simulation in spiral potential • Inside co-rotation • Region of over-density of 100 pc chosen • Proto-GMC traced backwards • Replace by self-gravitating SPH particles • Surface density 0.1 to 1 Msun pc-2

  6. Spiral Triggering of star formation • Follow gas flow through spiral arm • Shocks leaving pot. minimum • Form dense clouds • GMCs • Onset of gravitational collapse and SF • Forms stellar clusters • At r > 103Msun pc-3 • Masses 102 to 104 Msun

  7. Low surface density simulation S = 0.1 Msun pc-2 (105 Msun)

  8. Low surface density simulation S = 0.1 Msun pc-2 (105 Msun)

  9. High surface density simulation S = 1.0 Msun pc-2 (106 Msun) Size ~ 500 pc

  10. Formation of Giant Molecular Clouds • Convergent gas streams • Due to spiral potential • Clumpy shock forms substructure (GMCs?) • Dissipate kinetic energy in shock • Forms bound substructure Star Formation • Structures due to instabilities • Self-gravity ? Probably not • Kelvin-Helmholtz ? Size ~ 50 pc • Edges sharper on upwind side

  11. GMC Kinematics • Convergent gas streams • Clumpy gas • Broadens shock • Post-shock velocity depends on • Density of incoming clump • Mass loading in shock • generates velocity dispersion Velocity dispersion in plane of galaxy

  12. Star Formation and Efficiencies • Star formation requires: • Orbit crowding • shock • Enough gas mass • GMC lifetimes ~ 107 years (few dynamical times) • Star Formation Efficiencies Low • 5 to 30 % of gas mass formed into stars • Without any feedback • Why? • Clouds globally unbound • Majority of mass escapes • Clouds disperse leaving spiral arms

  13. Unbound Clouds and SF Efficiency Clark et al 2004 • Globally unbound GMCs • Local dissipation of turbulence • Star formation • SF involves ~10% of mass

  14. Global disk simulations • Clare Dobbs poster (no. 18) • Goal: explore gas dynamics through multiple spiral arm passages • Non self-gravitating • 4 armed spiral • Gas ring: 5 to 10 kpc (co-rotation 10 kpc) • Mass: 5 x 108 Msun • Isothermal (100 to 104 K) • Distribution: globally uniform, locally clumpy • Post-processed H2 formation • Bergin et al (2004)

  15. T=100 K

  16. Location of H2 gas Size scale: 22kpc, 11kpc, 6kpc, 3kpc

  17. Formation of Molecular Clouds Size ~ 4 kpc

  18. Formation of H2 • Molecular gas formed in spiral arms • Higher density • Higher extinction • Giant Molecular Clouds: • Almost completely in spiral arms • Mass components: • 10 % over full disk • 30-50 % in spiral arms Azimuthal distribution of gas and H2

  19. Spiral shocks and structure generation • Molecular cloud spacing~ 500 pc • Not due to self-gravity • Simulation produces spurs and feathering • Due to clumps in arms • Sheared in the inter-arm region • Disappears at higher gas temperatures

  20. Velocity dispersion • Velocity dispersion driven by spiral shocks • Due to clumpy shocks • Velocity dispersion increases in each spiral arm passage • Lower in interarm regions Azimuthal distribution of velocity dispersion

  21. A local viewpoint of spiral shocks Spot: motion of one gas particle 1kpc region centred on gas particle

  22. Conclusions • Spiral shocks can trigger star formation • Produce realistic GMCs • Structures • Kinematics (not turbulence) • Low star formation efficiencies (clouds unbound) • Global disk simulations • Generates Spurs and feathering (when cold) • Produce GMCs in spiral arms 10% of gas in H2 • Observable signatures • as gas passes through shocks

  23. Modelling Spiral Galaxies • Pass gas through Galactic potential, consisting of 3 components: • Disc: Logarithmic potential (Binney & Tremaine) • - Flat rotation curve, v0=220km/s Spiral: Cox & Gomez (2002) (sum of 3 perturbations) - Milky Way parameters with 4 arms - Pattern speed of 210-8 rad/yr-1 • Halo: Caldwell & Ostriker (1981) • No self-gravity/ magnetic fields

  24. Velocity dispersion in clumpy shocks - Gas through 1D sinusoidal potential. - Velocity dispersion flat and subsonic for uniform shock (-) - Velocity size-scale relation  (v)  r 0.5 for clumpy shock (-)

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