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First Stars III
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Talk
Title: The impact of stellar rotation on our understanding of the earliest phases of the enrichment of the Universe Presenter: Cristina Chiappini Abstract: It is in the Milky Way halo that the oldest and most metal-poor stars in the Universe are found, born at times or equivalent redshifts still out of reach for the deepest surveys of primordial galaxies. These stars contain a memory of the unique nucleosynthesis in the First Stars, as illustrated by the new abundance patterns and the striking C, N, O and heavy metals overabundances revealed at very-low metallicities. Such fossil records provide invaluable constraints on stellar models. The effects of stellar axial rotation are numerous and at low metallicity may lead to a drastic revision of current wisdom. For instance we have shown that the first massive star generations probably contained a high proportion of fast rotators. This seems to be required for explaining the high N/O ratio observed at the surface of very metal-poor stars (Chiappini et al. 2006, A&A 449, L27-L30). Yields for zero metallicity fast rotating stars are now being computed and will be included in chemical evolution models, which will guide our search for the First Stars imprints in subsequent stellar generations, potentially leading to a way of estimating the IMF at early phases. We will also check the impact of the new zero-metallicity yields in our previous results (Chiappini et al. 2006 - obtained with stellar yields down to Z=10^-8). We will investigate the resulting evolution of the C isotopic ratio in an ISM polluted mainly by the First Stars. From observations, it appears that the CEMP-no stars (the carbon enhanced metal poor stars where no strong-s or r-process elements are observed) have isotopic ratios lower than the typical value of CEMP-s stars. This might indicate that the former stars were born from an ISM that was mainly polluted by the stellar winds of massive stars. In fact, mass-loss could well be the only way for very metal-poor massive stars to contribute to the galactic enrichment, as probably most of their stellar material was locked into black holes. Chemical evolution models where only the wind contribution of massive stars is considered seem promising in explaining the high CNO enhancements observed in CEMP-no stars. The impact of the First Stars on the evolution of other elements such as nitrogen, helium and lithium will also be discussed.
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