s‐Process Nucleosynthesis in Advanced Burning Phases of Massive Stars

Abstract

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 22 Ne(α, n) 25 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 80 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 12 C attained at the end of core helium burning, which in turn is mainly determined by the 12 C(α, γ) 16 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 70 Zn and 87 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 152 Gd, 187 Os, and 198 Hg which are ∼14%, ∼13%, and ∼11%, respectively. compare the production factor distribution of s-only nuclei from our stellar models with the distribution from the solar abundance. In §7 and in 8 we discuss and summarize the main conclusions of this paper. Stellar ModelsThe results of the s-process presented in this paper have been obtained by using stellar models described in our previous paper (EMT04) for stars of masses 15, 20, 25, and 30 M ⊙ with initial solar-like composition. We evolved our models beyond core oxygen burning, and this allows us to investigate the s-process nucleosynthesis not only during core He-burning, but also during the important phase of shell C-burning. The network we have used for the s-process is listed in Table 1 of The et al. (2000, hereafter TEM00). It includes 632 nuclei up to 210 Bi and is sufficiently inclusive that the s-process nucleosynthesis can be studied in detail. The sources of the important nuclear reaction rates for each studied model are summarized in Table 1. The nuclear data have been updated as follows:1. The nuclear masses were taken from the compilation by Audi & Wapstra (1995) 2. The thermonuclear reaction rates were taken from the compilation of the "NACRE" collaboration (Angulo et al. 1999), and the "Non-Smoker" rates according to Rauscher & Thielemann (2000). Most of the electron capture and β-decay rates (ref...

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