The Transition from the First Stars to the Second Stars in the Early Universe.pdf

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The Transition from the First Stars to the Second Stars in the Early Universe.pdf

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The Transition from the First Stars to the Second Stars in the Early Universe

a r X i v : 0 7 0 4 .0 4 7 7 v 1 [ a s t r o - p h ] 4 A p r 2 0 0 7 The Transition from the First Stars to the Second Stars in the Early Universe Britton D. Smith and Steinn Sigurdsson 525 Davey Laboratory, Department of Astronomy Astrophysics, The Pennsylvania State University, University Park, PA, 16802 britton@astro.psu.edu,steinn@astro.psu.edu ABSTRACT We observe a sharp transition from a singular, high-mass mode of star forma- tion, to a low-mass dominated mode, in numerical simulations, at a metallicity of 10?3 Z⊙. We incorporate a new method for including the radiative cooling from metals into adaptive mesh-refinement hydrodynamic simulations. Our re- sults illustrate how metals, produced by the first stars, led to a transition from the high-mass star formation mode of Pop III stars, to the low-mass mode that dominates today. We ran hydrodynamic simulations with cosmological initial conditions in the standard ΛCDM model, with metallicities, from zero to 10?2 Z⊙, beginnning at redshift, z = 99. The simulations were run until a dense core forms at the center of a 5 × 105 M⊙ dark matter halo, at z ～ 18. Analysis of the central 1 M⊙ core reveals that the two simulations with the lowest metallicities, Z = 0 and 10?4 Z⊙, contain one clump with 99% of the mass, while the two with metallicities, Z = 10?3 and 10?2 Z⊙, each contain two clumps that share most of the mass. The Z = 10?3 Z⊙ simulation also produced two low-mass proto-stellar objects with masses between 10?2 and 10?1 M⊙. Gas with Z ≥ 10 ?3 Z⊙ is able to cool to the temperature of the CMB, which sets a lower limit to the minimum fragmentation mass. This suggests that the second generation stars produced a spectrum of lower mass stars, but were still more massive on average than stars formed in the local universe. Subject headings: stars: formation 1. Introduction Numerical simulations have shown that the very first stars invariably formed in isolation and were much more massive than the sun, due ma