Neutron capture nucleosynthesis in AGB stars

This paper is a general review of s-process studies that are of interest for an understanding of the solar abundances of the heavy isotopes above A similar to 85, and for the interpretation of isotopic ratios measured in circumstellar SiC grains embedded in meteorites. We first summarize the developments of nucleosynthesis theory of the s-process up to the advent of detailed stellar models for the final stages of low and intermediate mass stars (the so-called Asymptotic Giant Branch, or AGE, phases). Then we briefly analyze these stellar models, in an essentially historical approach. After showing that stars more massive than about 3-4 M. have to be discarded as a source of the solar system s-process elements, we dedicate more space and emphasis to recent models of lower mass stars, where the dominant neutron source is the reaction C-13(alpha,n)O-16. In this context we discuss how and why the first hypothesis of a release of neutrons in convective layers had to be modified, recognizing that most s-processing in AGE stars occurs in the radiative interpulse phases. We then spend the second part of the paper in analyzing in detail recent results from this scenario, and in showing how they can serve to interpret most isotopic compositions measured in SiC grains. In doing this we also present AGE model predictions for the s-process Zr and Mo isotopic composition, for which first experimental results have been presented.