S-process in extremely metal-poor stars

Abstract

The atmospheres of low metallicity stars possess clues about the nucleosynthetic processes occurring in the early stages of our Galaxy. Since large sky surveys using low and medium resolution spectroscopy have been produced, a large number of metal-poor stars have been catalogued and analyzed in detail. The abundances of several elements obtained using high resolution spectra have shown that these objects are extremely complex and can be divided into several subclasses depending on their chemical signatures. Among them, the carbon enriched stars have been subject of intensive research in the past decades. The origin of the excess of carbon in such stars is not established yet, however, the most likely scenario involves mass transfer from a more massive companion (during its AGB phase) onto the observed star surface. Concerning modeling, few detailed models of nucleosynthesis during the AGB phase can be found in the literature. Most models for this evolutionary stage focused on metallicities larger than [Fe/H]>-2.5, for which the s-process occurs in a radiative layer and strongly depends on the formation of the ^{13}C pocket (Bisterzo et al. 2011,Lugaro et al. 2012).Another possible scenario, suggested by Suda et al. 2004, to explain the origin of the carbon and s-process elements in the carbon-rich low metallicity stars involves the AGB phase of stars with [Fe/H]<-2.5. In this metallicity regime, extra mixing processes can take place in the beginning of the AGB phase, leading to a convective s-process. In this case, the neutron density is much larger than the density found in the ^{13}C pocket, leading to a huge s-process formation already in the beginning of the AGB phase. However, as explained above, evolution and nucleosynthesis models for the AGB phase of stars with metallicities [Fe/H]<-2.5 are not available in the literature. Therefore, the aim of this project is to produce AGB models for this metallicity regime in which the s-process occurs in a convective way. Thus, it could be possible to quantify the contribution of these stars to the bulk of the s-process in the early Galaxy. In order to do this, we will produce a grid of AGB models using the stellar evolution code GARSTEC (Weiss et al. 2008) and the s-process code written by the candidate during her PhD. We also aim to investigate the influence of the main uncertainties in the final abundances.