ar X iv : a st ro - p h / 06 09 78 8 v 1 2 8 Se p 20 06 s - Process Nucleosynthesis in Advanced Burning Phases of Massive Stars

We present a detailed study of s-process nucleosynthesis in massive stars of solar-like initial composition and masses 15, 20, 25, and 30 M⊙. We update our previous results of s-process nucleosynthesis during the core He-burning of these stars and then focus on an analysis of the s-process under the physical conditions encountered during the shell-carbon burning. We show that the recent compilation of the Ne(α, n)Mg rate leads to a remarkable reduction of the efficiency of the s-process during core He-burning. In particular, this rate leads to the lowest overproduction factor of Kr found to date during core He-burning in massive stars. The s-process yields resulting from shell carbon burning turn out to be very sensitive to the structural evolution of the carbon shell. This structure is influenced by the mass fraction of C attained at the end of core helium burning, which in turn is mainly determined by the C(α, γ)O reaction. The still present uncertainty in the rate for this reaction implies that the s-process in massive stars is also subject to this uncertainty. We identify some isotopes like Zn and Rb as the signatures of the s-process during shell carbon burning in massive stars. In determining the relative contribution of our s-only stellar yields to the solar abundances, we find it is important to take into account the neutron exposure of shell carbon burning. When we analyze our yields with a Salpeter Initial Mass Function, we find that massive stars contribute at least 40% to s-only nuclei with mass A ≤ 87. For s-only nuclei with mass A > 90, massive stars contribute on average ∼7%, except for Gd, Os, and Hg which are ∼14%, ∼13%, and ∼11%, respectively. Subject headings: nuclear reactions, nucleosynthesis, abundances—stars:evolution –stars: interiors Department of Physics and Astronomy, Clemson University, Clemson, SC 29634-0978, [email protected]; [email protected] Department of Physics, American University of Beirut, Beirut, Lebanon; [email protected]