S-processing in massive stars as a function of metallicity and interpretation of observational trends

Abstract

In this paper we first analyze the s-process in massive stars as a function of metallicity. The nucleosynthesis occurring in both core helium and shell carbon burning is investigated by numerically modeling nuclear reaction networks, subject to the conditions provided by stellar models. The efficiency of the neutron-capture mechanism depends on the amount of enhancement that is assumed for the alpha-rich nuclei, that are observed to be overabundant with respect to iron in stars of metallicity lower than solar. We find that the s-process in massive stars is "secondary-like" in the disk, the amount of s-process matter ejected being roughly proportional to Fe. In the halo, the s-efficiency drops at low metallicities, below [Fe/H] approximately -2. A critical neutron poison during He burning is O-16, despite its very low 30 keV neutron capture cross section; the importance of this nucleus as a neutron absorber grows as the metallicity decreases. When taking into account the possibility that the cross section of O-16 is up to a factor 10 higher than currently assumed, the efficiency of the s-process in low-metallicity stars is even more reduced, the secondary-like nature, however, being preserved in the disk. Second, the elemental s-contributions from massive stars to the solar abundances from iron to zirconium (the so-called weak component) are presented. We find that the weak component can account for a consistent fraction of the solar Cu, Ga, Ge, and Se. In this atomic mass range, where both the weak and the main component (from thermally pulsing AGB low-mass stars) are involved, as well as the e- and r-processes, our analysis allows us to separate the contributions to the solar system abundances coming from the various nucleosynthetic sources. We discuss the evolutionary trends of elements such as Co, Ni, Cu, Zn, Rb, Sr, Y, and Zr in comparison with observations of stars belonging to the halo, where massive stars give their major imprint.