CNO production in first generation stars.pdf

a r X i v : a s t r o - p h / 0 6 1 0 3 1 0 v 1 1 1 O c t 2 0 0 6 CNO production in the first generation stars Sylvia Ekstr?m ? , Georges Meynet André Maeder Geneva Observatory, Switzerland E-mails: sylvia.ekstrom@obs.unige.ch, georges.meynet@obs.unige.ch, andre.maeder@obs.unige.ch Big Bang nucleosynthesis produces only light elements and the very first generation stars are thus formed from metal-free clouds. They start the production of heavy elements during their life, and enrich the interstellar medium through their explosive death. Stellar evolution models show that the treatment of rotation has important effects on the evolution of those metal-free stars: for example, rotating models produce up to five orders of magnitude more primary nitrogen than non rotating models, due to internal mixing. This will have an impact in the composition of the second generation stars, some of which may now be observed in the Galactic halo. In the case Population III stars were very massive and would end up as direct black holes, rotation again have an interesting effect of enhancing mass loss through centrifugal force and surface enrichment. CNO composition patterns observed in ultra metal-poor halo stars may be explained by a ’wind only’ contribution. International Symposium on Nuclear Astrophysics — Nuclei in the Cosmos — IX June 25-30 2006 CERN, Geneva, Switzerland ? Speaker. c? Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/ CNO production in the first generation stars Sylvia Ekstr?m 1. Primordial stars 1.1 Birth After the Big Bang, the early Universe is composed only from H and He, a little Be, B and Li, but no heavier elements. At the end of the so-called ’Dark Age’, halos of dark matter start to collapse and form stars with the gas trapped within. The star-forming clouds are almost pure H and He, so fragmentation cannot occur, leading to a massive or very massive star formati