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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 QuickTime™ and a BMP decompressor are needed to see this picture. – – – – Stellar clusters Massive stars Brown dwarfs Initial Mass Function – But not the initial conditions for star formation 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: v R 0 .5 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 10-3 Msun /pc3 ; max 10-1 Msun /pc3 Self gravity Star formation modeled with sink-particles 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 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 > 103 Msun pc-3 • Masses 102 to 104 Msun Low surface density simulation S = 0.1 Msun pc-2 (105 Msun) QuickTime™ and a BMP decompressor are needed to see this picture. Low surface density simulation S = 0.1 Msun pc-2 (105 Msun) QuickTime™ and a FLIC Animation decompressor are needed to see this picture. High surface density simulation S = 1.0 Msun pc-2 (106 Msun) QuickTime™ and a BMP decompressor are needed to see this picture. Size ~ 500 pc 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 QuickTime™ and a FLIC Animation decompressor are needed to see this picture. 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 v R 0 .5 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 Unbound Clouds and SF Efficiency Clark et al 2004 •Globally unbound GMCs •Local dissipation of turbulence •Star formation • SF involves ~10% of mass 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) T=100 K QuickTime™ and a FLIC Animation decompressor are needed to see this picture. Location of H2 gas Size scale: 22kpc, 11kpc, 6kpc, 3kpc Formation of Molecular Clouds QuickTime™ and a FLIC Animation decompressor are needed to see this picture. Size ~ 4 kpc 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 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 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 A local viewpoint of spiral shocks QuickTime™ and a FLIC Animation decompressor are needed to see this picture. Spot: motion of one gas particle QuickTime™ and a FLIC Animation decompressor are needed to see this picture. 1kpc region centred on gas particle 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 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 210 -8 rad/yr -1 Halo: Caldwell & Ostriker (1981) No self-gravity/ magnetic fields r rc 1 r rc 2 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 (-)