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The First Stars and Galaxies: Challenges for the Next Decade Participants
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Talk
Title: The Pop III - II Transition Author(s): Raffaella Schneider Abstract: The Pop III - II Transition Physical conditions in primordial star forming regions systematically favor the formation of very massive stars. This is due to the combined effect of the larger fragmentation scale and accretion rate, and the very limited opacity. On the other hand, observations of present-day stellar populations (Pop II/I stars) show that stars form with a Salpeter-like IMF and with a characteristic mass of ~ 1 Msun. Thus, unless the current picture of primordial star formation is lacking in some fundamental ingredient, a transition between these two modes of star formation must have occurred at some time during cosmic evolution. Following a number of detailed studies, the emerging physical scenario suggests that the fragmentation properties of the collapsing gas clouds change as the mean metallicity of the gas increases above a critical threshold: 10^{-6} Zsun < Zcr < 10^{-4} Zsun, where the lower limit applies when a fraction of metals in condensed in dust grains and the upper limit applies when only gas-phase metals are present. According to this view, the formation of Pop III stars (defined as those with Z < Zcr) is regulated by the rate at which heavy elements are produced and mixed in the gas surrounding the first star formation sites, the so-called chemical feedback. In this review, I will first summarize the results of low-metallicity star formation studies. Then I will present semi-analytic and numerical models which attemp to describe chemical feedback and the Pop III - II transition on cosmic scales. Finally, I will discuss the implications of those models for the observability of Pop III stars through direct detection at moderate redshifts or through the identification of their peculiar nucleosynthetic signatures in local samples of metal-poor stars.
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