Georges Meynet (Geneva Observatory, Switzerland)

The first generations of stars in the Universe
When May 29, 2015 from 10:30 AM to 12:00 PM
  • Colloquium
Where SH, Lecture Hall
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After a recall of the main observations of the surface abundances of the most iron-poor stars in the halo, we shall discuss what these observations tell us about the nucleosynthesis occurring in the first stellar generations in the Universe. We shall show that, independently of the stellar models, two points can be deduced just from considerations based on the nuclear reactions active in the H- and He-burning regions: 1.- the need for some partial mixing between the H- and the He-burning zones; 2.- Most of the first stars must have chemically enriched their environment by the loss of their H-rich envelope, keeping the contributions from the regions inside the He-burning zone at a modest level. We shall then discuss what could be the physical causes responsible for the mixing implied by point 1, and for the loss of only the outer layers implied by point 2. Among the physical mechanisms, axial rotation does appear as very promiseful. This would imply relatively fast initial rotations for the first stars. Implications of this fast rotation on other questions as the origin of the anti-correlation in the globular clusters, the expected frequency of long Soft Gamma Ray bursts in the early Universe, or the contribution of stars in ionizing photons at early cosmic epochs will be briefly mentioned.