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Feedback Effects of the First Stars on Nearby Halos

Feedback Effects of the First Stars on Nearby Halos. Kyungjin Ahn The University of Texas at Austin The End of the Dark Ages STSCI March 13 , 200 6. Outline. Introduction Code Description Initial Setup Result Conclusion. Dark Ages and Reionization.

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Feedback Effects of the First Stars on Nearby Halos

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  1. Feedback Effects of the First Stars on Nearby Halos Kyungjin Ahn The University of Texas at Austin The End of the Dark Ages STSCI March13, 2006

  2. Outline • Introduction • Code Description • Initial Setup • Result • Conclusion

  3. Dark Ages and Reionization • End of dark ages – reionization – is only observed indirectly • WMAP 1st year result : Need for high-redshift reionization sources • Gunn-Peterson Effect • Ly Forest Temperature • First Stars • Prime candidate for early reionization sources • Forms by H2 cooling • Feedback effects may be self-regulating (e.g. Haiman, Abel, Rees 2000)

  4. Feedback effects of the First Stars • Feedback Effects (positive vs. negative for further star formation) • Negative– star formation quenched • H2 is fragile: dissociation by Lyman-Werner band photons (Haiman, Abel, Rees 2000; Machacek, Bryan, Abel 2001) • Positive– star formation promoted • Hard photons partially ionize IGM to create H2 (Haiman, Rees, Loeb 1996; Ricotti, Gnedin, Shull 2002)

  5. Feedback effects of the First Stars • Feedback Effects of the First Stars onto Nearby Collapsed Objects (study by 3-D simulations) • O'shea et al. (2005) • Assume full ionization of nearby halos of M~5*105 Msolar • Quick formation of H2 after source dies • Inner core collapses; Outer region evaporates • Alvarez, Bromm, Shapiro (2005) • Track I-front propagation through nearby halos of M~5*105 Msolar • I-front slows down and being trapped. • I-front fails to reach the center: Center remains neutral • Neutral center: no further formation of H2 after source dies at the center • Negative feedback then??

  6. Feedback effects of the First Stars • Alvarez, Bromm, Shapiro (2005) Fig. 8.— Volume visualization at z = 20 of neutral density field (blue – low density, red – high density) and I-front (translucent whitesurface). Top row panels show a cubic volume ∼ 13.6 kpc (proper) across, middle row ∼ 6.8 kpc, and bottom row ∼ 3.4 kpc. Left columnis at the initial time, middle column shows simulation at t∗ = 3 Myr for the run with stellar mass M∗ = 80M⊙, and the right columnshows simulation at t∗ = 2.2 Myr for the run with stellar mass M∗ = 200M⊙. The empty black region in the lower panels of middle andright columns indicates fully ionized gas around the source, and is fully revealed as the volume visualized shrinks to exclude the I-frontthat obscures this region in the larger volumes above.

  7. Feedback effects of the First Stars(In collaboration with Paul Shapiro) • Feedback Effects of the First Stars onto Nearby Collapsed Objects (study by 1-D simulation) • Use 1-D radiation, hydrodynamics code • Full treatment of primordial chemistry, radiative transfer, cooling/heating, hydrodynamics • 1-D spherical geometry • Ultra high resolution possible • Analysis relatively easier than 3-D • Follow I-front propagation of the radiation from outer source in detail • Is I-front trapped? • What happens to the center? • Any H2 formation/dissociation interesting? • Is it positive or negative feedback effect?

  8. 1-D Spherical, Radiation-Hydro Code • Gravity • Dark matter: use fluid approximation. Better than radial shells. Ocasionally frozen gravity is not a bad approximation. • Baryon: Gravity involved hydrodynamics. • Chemistry • Solve primordial chemistry, neglecting HD and HLi. H, H-, H+, H2, He, He+, He++, e • ionization, dissociation, recombination, radiative transfer • Cooling/Heating • excitation, recombination, free-free, H2 • photoheating • adiabatic compression/rarefaction

  9. Initial Setup • Experiment 1 (Artificial) • Test O'shea et al. result • Fully ionize the target halo without disturbing the structure • Let it evolve without source • Experiment 2 (Realistic) • Start out with a halo profile (TIS profile) • Abundance of electron and H2 molecule with equilibrium value of a given halo: Departs from primodial values xe=10-4, xH2=2x10-6 • Send plane-parallel, black-body radiation from outside 120Msolar, 105K, 106.24Lsolar, tstar=2.5 Myr • Place different-mass target halos at 360pc.

  10. Rule of Thumbs • H atomic cooling : down to ~10000K • collisional ionization : at > ~10000K • photo-ionization front : thickness ~ mean free path of ionizing photons • R-type ionization front: I-front moves supersonically into neutral region; gas doesn’t respond dynamically • D-type ionization front: I-front moves subsonically into neutral region; gas responds, shock-front develops • Primary H2 formation mechanism H + e  H- + g; H- + H  H2 + e • H2 cooling : down to ~100K at H2/H >~ 10-4 • Low temperature, T <~ 1000K, required to have H2/H >~ 10-4 to be safe from collisional dissociation • H2 self-shielding effective at N(H2)>~1014cm-2 in static.

  11. Experiment 1 (O’shea et al. type) • No radiation (after star died) • Fully-ionized gas quickly forms a lot of H2 • Core region quickly cools to ~100K • Outer region evaporates • Again, can this full ionization happen in the first place?

  12. Initial Setup • Experiment 2 (Realistic)

  13. Result: Experiment 2 (collapse fails) • 105Msolar target halo • Movie during the lifetime of star (2.5 Myr) • Enet=kinetic energy + thermal energy + potential energy • Enet<0  collapse • Enet>0  exodus

  14. Result: Experiment 2 (collapse fails) • 105Msolar target halo • Movie after the lifetime of star

  15. Result: Experiment 2 (collapse successful) • 4x105Msolar target halo • Movie during the lifetime of star (2.5 Myr)

  16. Result: Experiment 2 (collapse successful) • 4x105Msolar target halo • Movie after the lifetime of star • Cooling at the center & in H2 precursor shell leads to negative net energy  collapse of neutral region.

  17. Result: Features to note • I-front slows down, finally gets trapped. • Transition to D-type front • Precursor H2 shell formation • Long mean-free-path of ionizing photons • Partial ionization -> H- formation -> H2 formation • Shielding + H2 molecule cooling • Shock-front is driven, with T~1000 - 10000 K • Heating!! • Shock-front accelerates in constant-density core • Accelerates up to >~ 10000K  heated enough to lead to collisional ionization  electron, H2 formation: Basically identical to Shapiro & Kang 1987, with vs~15 km s-1 • H atomic cooling + H2 molecule cooling

  18. H2 Shell • Forms through electrons in the partially ionized region • Gains substantial column density, of order ~1016-1018 cm-2. Self-shields against dissociating photons. (Not completely, though, because of peculiar motion of H2 precursor shell) • Neutral region sees weakned dissociating photons. • Helps cool the gas against shock-heating  fragmentation?

  19. H2 Shell Structure • Ricotti, Gnedin, Shull 2001 • Into static IGM • Our result • Into minihalos

  20. Conclusion • Minihalos (target) nearby the first Stars (source) • I-front trapped; Ionized gas evaporates • H2 formation in evaporating gas doesn’t help, just evaporates • H2 shell forms ahead of I-front: shielding dissociation + cooling • Shock is driven to the neutral region: active heating  collisional ionization. (universal?) • Competition between H2 cooling & shock-heating determines the fate of neutral region. Higher the mass, more efficient the cooling -> critical minihalo mass for hosting 2nd generation stars. • In preparation • Wider parameter search • Jeans mass? • IMF? • Sequential star formation? Photon budget? • See Brian O’shea’s talk too (Another feedback).

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