Heavy-Element Nucleosynthesis in AGB Stars

The production of certain neutron-rich elements heavier than iron occurs during He shell-burning on the asymptotic giant branch (AGB). These neutron captures occur at rather low neutron densities and, thus, the resulting heavy-element nucleosynthesis is characterisitic of the so-called s-process. Abundance analyses of the s-process elements in AGB stars can reveal details of the neutron densities and the stage of AGB evolution at which s-processing occurs, as well as the nature of the neutron source. These details derived from observations can constrain models of stars evolving along the AGB. Recent results concerning the nature of the s-process as a function of metallicity and the nature of the neutron source are reviewed.