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Star Formation at Very Low Metallicity

Star Formation at Very Low Metallicity. Anne-Katharina Jappsen. Collaborators. Simon Glover, Heidelberg, Germany Ralf Klessen , Heidelberg, Germany Mordecai-Mark Mac Low, AMNH, New York Spyridon Kitsionas , AIP, Potsdam, Germany. The Initial Mass Function.

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Star Formation at Very Low Metallicity

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  1. Star Formation at Very Low Metallicity Anne-Katharina Jappsen

  2. Collaborators • Simon Glover, Heidelberg, Germany • Ralf Klessen, Heidelberg, Germany • Mordecai-Mark Mac Low, AMNH, New York • SpyridonKitsionas, AIP, Potsdam, Germany

  3. The Initial Mass Function

  4. From Pop III Stars to the IMF? star formation in the early universe: • 30 Msun < M < 600 Msun(e.g. O’Shea & Norman 07) • Z = 0 (Pop III) ➞ Z < 10-3Zsun (Pop II.5) • Mchar~ 100 - 300 Msun present-day star formation: • 0.01 Msun < M < 100 Msun • Z > 10-5Zsun , Z = Zsun • Mchar ~ 0.2 Msun

  5. Critical Metallicity Bromm et al. 2001: • SPH-simulations of collapsing dark matter mini-halos • no H2 or other molecules • no dust cooling • only C and O atomic cooling 10-4Zsun< Zcr < 10-3Zsun

  6. Dependence on Metallicity Omukai et al. 2005: one-zone model, H2 , HD and other molecules, metal cooling, dust cooling 1Msun 102 Msun 10-2 Msun t = 1

  7. Present-day star formation Omukai et al. 2005: one-zone model, H2 , HD and other molecules, metal cooling, dust cooling t = 1 Z=0

  8. Dependence on Z at low r Omukai et al. 2005: one-zone model, H2 , HD and other molecules, metal cooling, dust cooling t = 1

  9. Numerical Model • Smoothed Particle Hydrodynamics • Gadget-1 & Gadget-2 (Springel et al. 01, Springel 05) • Sink particles (Bate et al. 95) • chemistry and cooling • particle splitting (Kitsionas & Whitworth 02)

  10. Chemical Model

  11. Cooling and Heating • gas-grain energy transfer • H collisional ionization • H+ recombination • H2 rovibrational lines • H2 collisional dissociation • Ly-alpha & Compton cooling • Fine-structure cooling from C, O and Si • photoelectric effect • H2 photodissociation • UV pumping of H2 • H2 formation on dust grains

  12. Dependence on Metallicity at Low Density • gas fully ionized • initial temperature: 10000 K • centrally condensed halo • contained gas mass: 17% of DM Mass • number of gas particles: 105 – 106 • resolution limit: 20 MSUN – 400 MSUN

  13. Dependence on Metallicity at Low Density • halo size: 5 x 104Msun – 107Msun • redshift: 15, 20, 25, 30 • metallicity: zero, 10-4Zsun, 10-3Zsun, 10-2Zsun, 0.1 Zsun • UV background: J21 = 0, 10-2, 10-1 • dust: yes or no (Jappsen et al. 07)

  14. Dependence on Metallicity at Low Density

  15. Influence of Different Initial Conditions example II solid-body rotating top-hat (cf. Bromm et al. 1999) cold initial conditions with dark matter fluctuations top-hat approximation T = 200 K MDM = 2 x 106 Msun Mres,gas= 12 Msun example I centrally condensed halo hot, ionized initial conditions • NFW profile, rs= 29 pc • T = 10000 K • z = 25 • MDM = 8 x 105 Msun • Mres,gas= 1.5 Msun

  16. Example I after 52 Myrs CMB

  17. Example II • Rotating top-hat with dark matter fluctuations and cold gas initially: gas fragments no matter what metallicity, because unstable disk builds up (Jappsen et al. 09) H2 is the dominant coolant! “critical metallicity” only represents point where metal-line cooling dominates molecular cooling

  18. Conclusions – so far • H2 is the dominant and most effective coolant • different initial conditions can help or hinder fragmentation ⇒ we need more accurate initial conditions from observations and modeling of galaxy formation • there is no “critical metallicity” for fragmentation at densities below 105 cm-3 • Transition from Pop III to modern IMF maybe at higher densities due to dust-induced fragmentation:

  19. Dependence on Z at high r Omukai et al. 2005: one-zone model, H2 , HD and other molecules, metal cooling, dust cooling t = 1

  20. Dust-induced Fragmentation • Clark et al. 2008 study dust-induced fragmentation in 3D numerical simulations of star formation in the early universe • dense cluster of low-mass protostars builds up: • mass spectrum peaks below 1 Msun • cluster VERY dense (nstars = 2.5 x 109 pc-3) • fragmentation at density ngas = 1012 - 1013 cm-3

  21. Conclusions • H2 is the dominant and most effective coolant at n < 105 cm-3 • there is no “critical metallicity” for fragmentation at densities below 105 cm-3 • different initial conditions can help or hinder fragmentation ⇒ we need more accurate initial conditions from observations and modeling of galaxy formation • Transition from Pop III to modern IMF maybe at higher densities due to dust-induced fragmentation at Z = 10-5 Zsun

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