Nucleosynthesis and Mixing on the Asymptotic Giant Branch. I. MS and S Stars with and without TC

The study fits the elemental abundances observed in MS and S giants using self-consistent models of nucleosynthesis and dredge-up for the TP-AGB evolutionary phases of low-mass stars. The initial envelope abundances of C-12, C-13, and N-14 are taken from observations of red giants that experienced the first dredge-up and are not yet on the AGB. It is found that the observed abundances of MS and S stars are readily explained by TP-AGB models of low-mass stars in which neutrons are released by the C-13(alpha, n)O-16 neutron source and in which the effects of mass loss and mixing due to envelope penetration are taken into account. The observed distributions of s-elements alone do not suffice to derive the neutron exposure in the He-shell, because the observed distributions are strongly affected also by how and when the third dredge-up occurs. The relative roles of mixing and of nucleosynthesis may be estimated by requiring the models to reproduce the abundances of C as well as of the s-elements. It is shown that single Tc-poor S star may exist, if the dredge-up phenomenon occurs rarely enough to allow Tc to decay between successive mixing episodes.