Population III stars hidden or disappeared.pdf

a r X i v : 0 7 0 7 .1 4 3 3 v 1 [ a s t r o - p h ] 1 0 J u l 2 0 0 7 Mon. Not. R. Astron. Soc. 000, 1–?? (2007) Printed 1 February 2008 (MN LATEX style file v2.2) Population III stars: hidden or disappeared ? Luca Tornatore1, Andrea Ferrara1 Raffaella Schneider2 1 SISSA/Internation School for Advanced Studies, Via Beirut 4, 34100 Trieste, Italy 2 INAF/Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy Feb 01, 2007 ABSTRACT A Pop III/Pop II transition from massive to normal stars is predicted to occur when the metallicity of the star forming gas crosses the critical range Zcr = 10 ?5±1 Z⊙. To investigate the cosmic implications of such process we use numerical simulations which follow the evolution, metal enrichment and energy deposition of both Pop III and Pop II stars. We find that: (i) due to inefficient heavy element transport by outflows and slow ”genetic” transmission during hierarchical growth, large fluctuations around the average metallicity arise; as a result Pop III star formation continues down to z = 2.5, but at a low peak rate of 10?5M⊙yr ?1Mpc?3 occurring at z ≈ 6 (about 10?4 of the Pop II one); (ii) Pop III star formation proceeds in a ”inside-out” mode in which formation sites are progressively confined at the periphery of collapsed structures, where the low gas density and correspondingly long free-fall timescales result in a very inefficient astration. These conclusions strongly encourage deep searches for pristine star formation sites at moderate (2 z 5) redshifts where metal free stars are likely to be hidden. Key words: galaxies: formation - cosmology: theory - cosmology: observations - intergalactic medium 1 INTRODUCTION The physical conditions in primordial star-forming regions appear to systematically favor the formation of very mas- sive stars. This is due to the combined effect of the larger gas fragmentation scale and accretion rate, and the very limited opacity. On the other hand, observations of