s-Process Nucleosynthesis in AGB Stars: a Test for Stellar Evolution

We study the slow neutron capture process (s process) in Asymptotic Giant Branch (AGB) stars using three different stellar evolutionary models computed for a 3 M⊙ and solar metallicity star. First we investigate the formation and the efficiency of the main neutron source: the C(α,n)O reaction that occurs in radiative conditions. A tiny region rich in C (the C pocket) is created by proton captures on the abundant C in the top layers of the He intershell, the zone between the H shell and the He shell. We parametrically vary the number of protons mixed from the envelope. For high local protons over C number ratio, p/C ∼> 0.3, most of the C nuclei produced are further converted by proton capture to N. Besides, N nuclei represent a major neutron poison. We find