The s-process in AGB stars

About half of the solar abundances of elements heavier than iron are made by the slow neutron capture process (the s process) occurring in low and intermediate-mass asymptotic giant branch (AGB) stars. Elements are dredged from the core to the surface and then expelled into the interstellar medium through strong stellar winds. In comparison to the rapid neutron capture process, modelling the s-process has presented fewer difficulties because most of the nuclei involved are near the valley of stability and therefore available for experiments. Also, many giant stars are observed with enrichments of heavy elements produced by the s process, providing unique constraints on stellar nucleosynthesis models. However, many important uncertainties still remain including the mechanism leading to the formation of ¹³C pockets where ¹³C is the main neutron source, and the efficiency of convection leading to the third dredge-up in AGB stellar models. There is an increasing wealth of observational data that is being used to constrain s-process modelling in AGB stars. These include spectroscopic abundances from AGB stars and their progeny (post-AGB stars, planetary nebulae), pre-solar meteoritic grains, globular cluster stars, and very metal-poor halo stars with enrichments of carbon and s-process elements. The current status of AGB s-process models is reviewed, followed by recent efforts to use the heavy-element composition of planetary nebulae as constraints on mixing and nucleosynthesis in AGB stars. Finally, we discuss a new potential site of the s-process: the dual-core He-flash in primordial low-mass giant stars.