Mixing and the s-process in rotating AGB stars

We model the nucleosynthesis during a radiative interpulse phase of a rotating 3M⊙ Asymptotic Giant Branch (AGB) star. We find an enhanced production of the neutron source species C compared to non-rotating models due to shear mixing of protons and C at the core-envelope interface. We estimate that the resulting total production of heavy elements by slow neutron capture (s-process) is too low to account for most observations. This due to the fact that rotationally induced mixing during the interpulse phase causes a pollution of the C pocket layer with the neutron poison N. As a result we find a maximum neutron exposure of τmax = 0.04mbarn −1 in the s-process layer of our solar metallicity model with rotation. This is about a factor of 5 . . . 10 less than required to reproduce the observed stellar s-process abundance patterns. We compare our results with models that include hydrodynamic overshooting mixing, and with simple parametric models including the combined effects of overshooting and mixing in the interpulse. Within the parametric model a range of mixing efficiencies during the interpulse phase correlates with a spread in the s-process-efficiency. Such a spread is observed in AGB and post-AGB stars as well as in pre-solar SiC grains.