Thes‐Process in Rotating Asymptotic Giant Branch Stars

Abstract

We model the nucleosynthesis during the thermal pulse phase of a rotating, solar metallicity, asymptotic giant branch (AGB) star of 3 M☉, which was evolved from a main-sequence model rotating with 250 km s-1 at the stellar equator. Rotationally induced mixing during the thermal pulses produces a layer (~2 × 10-5 M☉) on top of the CO core where large amounts of protons and 12C coexist. With a postprocessing nucleosynthesis and mixing code, we follow the abundance evolution in this layer, in particular that of the neutron source 13C and of the neutron poison 14N. In our AGB model mixing persists during the entire interpulse phase because of the steep angular velocity gradient at the core-envelope interface, thereby spreading 14N over the entire 13C-rich part of the layer. We follow the neutron production during the interpulse phase and find a resulting maximum neutron exposure of τmax = 0.04 mbarn-1, which is too small to produce any significant s-process. In parametric models, we then investigate the combined effects of diffusive overshooting from the convective envelope and rotationally induced mixing. Just adding the overshooting and leaving the rotational mixing unchanged results in a small maximum neutron exposure (0.03 mbarn-1). Models with overshoot and weaker interpulse mixing—as perhaps expected from more slowly rotating stars—yield larger neutron exposures. In a model with overshooting without any interpulse mixing a neutron exposure of up to 0.72 mbarn-1 is obtained, which is larger than required by observations. We conclude that the incorporation of rotationally induced mixing processes has important consequences for the production of heavy elements in AGB stars. While through a distribution of initial rotation rates, it may lead to a natural spread in the neutron exposures obtained in AGB stars of a given mass in general—as appears to be required by observations—it may moderate the large neutron exposures found in models with diffusive overshoot in particular. Our results suggest that both processes, diffusive overshoot and rotational mixing, may be required to obtain a consistent description of the s-process in AGB stars that fulfills all observational constraints. Finally, we find that mixing due to rotation within our current framework does increase the production of 15N in the partial mixing zone. However, this increase is not large enough to boost the production of fluorine to the level required by observations.