18/02/2004 – Auditório Principal Bloco G – IAG/USP

Dra. Monique Spite

GEPI – Observatoire de Paris-Meudon/França

Título/Title: The first galactic Stars. Mixing and Abundance of the light elements:
C N O at the very beginning of the Galaxy

Resumo/Abstract:

I will rapidly present the main topics of the project “First Stars” and the first results obtained from the old giant “eXtremey Metal-Poor” stars (XMP).

The abundances in the atmosphere of these stars should provide information concerning the chemical abundances at the earliest stage of galactic evolution.We could determine the ratios of the different elements in the matter at a very early epoch in our Galaxy. BUT, unlike the others ratios, [C/Fe] is very different from star to star. Was this ratio really different from place to place in the early Galaxy or has it been alterated in the atmosphere of the giant stars by mixing with deep layers (although this type of mixing is not predicted at this stage by theory)?

We show that the nitrogen abundance is anticorrelated with the carbon abundance, and that,on the contrary, the lithium abundance is correlated. This strongly suggest that the atmosphere of several stars in our sample has been mixed with the layer where the CNO process occurs. We finally could define two group of stars one of “mixed stars” and one of “unmixed stars”. From the stars of the second group we could determine the abundance of carbon and nitrogen at the very begining of our Galaxy, and the trends of [N/O] vs [O/H]. This relation is compared to the trends obtrained from the DLAs and H II regions in irregular galaxies.

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