To be published in:
Astronomy & Astrophysics
1 Institut d'Astrophysique et de Géophysique - Université de Liège, Avenue de Cointe 5, 4000 Liège, Belgium
* Maître de Recherches FNRS, Belgium
** Chargé de Recherche FNRS, Belgium
+ Based on observations obtained at the European Southern Observatory, La Silla, Chile.
Accurate relative abundances have been obtained for a sample of 21 mildly metal-poor stars from the analysis of high resolution and high signal-to-noise spectra. In order to reach the highest coherence and internal precision, lines with similar dependency on the stellar atmospheric parameters were selected, and the analysis was carried out in a strictly differential way within the sample.
With these accurate results, correlations between relative abundances have been searched for, with a special emphasis on the neutron capture elements.
This analysis shows that the r elements are closely correlated to the elements, which is in agreement with the generally accepted idea that the r-process takes place during the explosion of massive stars.
The situation is more complex as far as the s elements are concerned. Their relation with the elements is not linear. In a first group of stars, the relative abundance of the s elements increases only slightly with the elements overabundance until the latter reaches a maximum value. For the second group, the s elements show a rather large range of enhancement and a constant (and maximum) value of the elements overabundance.
This peculiar behaviour leads us to distinguish between two sub-populations of metal-poor stars, namely Pop IIa (first group) and Pop IIb (second group).
We suggest a scenario of formation of metal-poor stars based on two distinct phases of chemical enrichment, a first phase essentially consisting in supernova explosions of massive stars, and a second phase where the enrichment is provided by stellar winds from intermediate mass stars. More specifically, we assume that all thick disk and field halo stars were born in globular clusters, from which they escaped, either during an early disruption of the cluster (Pop IIa) or, later, through an evaporation process (Pop IIb).
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