2003/05/26 by Falk Herwig, Norbert Langer, N. Langer +1
Physics and Astronomy · #Astro and Planetary Science #Astronomical and nuclear sciences #Stellar, planetary, and galactic studies #astro-ph
paper · pdf · doi:10.1086/376726
published as Astrophys.J. 593 (2003) 1056-1073 · 50 pages, 13 figures, ApJ in press, tentatively scheduled for v593 n2 August 20, 2003
arxiv created 2003/05/26 · openalex publication_date 2003/08/11 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/07/31
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 12 C coexist. With a postprocessing nucleosynthesis and mixing code, we follow the abundance evolution in this layer, in particular that of the neutron source 13 C and of the neutron poison 14 N. 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 14 N over the entire 13 C-rich part of the layer. We follow the neutron production during the interpulse phase and find a resulting maximum neutron exposure of τ max = 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 15 N in the partial mixing zone. However, this increase is not large enough to boost the production of fluorine to the level required by observations.