S-process Nucleosynthesis in AGB Stars

Astronomy and Astrophysics – Astrophysics

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Scientific paper

A large fraction of the cosmic abundance of nuclei heavier than Fe is produced by slow neutron captures (s process). The main astrophysical site of the s process has been identified to be the deep layers of low to intermediate mass stars at the end of their evolution, when they reach the Asymptotic Giant Branch (AGB). The s-process products are brought to the stellar surface by recurrent episodes of deep mixing and they are carried into the interstellar medium by strong stellar winds. I present the most recent mechanism proposed for the occurrence of the s process in AGB stars and discuss the main uncertainties related to the stellar modelling and to the operation of the two neutron sources: the ^13C(α,n)^16O reaction, which is the main neutron source, and the ^22Ne(α,n)^25Mg reaction. Since the s-process efficiency is proportional to ^13C/Fe and the ^13C amount does not depend on the initial composition, different final abundance distributions are obtained when varying the stellar metallicity. Many components have contributed to the s-process solar abundances of nuclei with 88 < A< 208. In particular, I will discuss the production of some nuclei for which the comparison between model predictions and observational constraints calls for better determinations of neutron capture cross sections and β decay rates.

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