2000/07/26 by Falk Herwig, Norbert Langer, N. Langer +2
Physics and Astronomy · #Astro and Planetary Science #Astronomy and Astrophysical Research #Astrophysics (astro-ph) #FOS: Physical sciences #Stellar, planetary, and galactic studies #astro-ph
paper · pdf · doi:10.48550/arxiv.astro-ph/0007390
11 pages, 7 figures, Invited talk, Torino-Melbourne-Pasadena Workshop in honour of Prof. Gerald J. Wasserburg, June 21-23, 2000, Torino, Italy, to appear in Mem. Soc. Astron. It
arxiv created 2000/07/26 · openalex publication_date 2000/07/26 · arxiv updated 2009/12/01 · openalex created_date 2025/10/24 · openalex updated_date 2026/07/28
(abridged) Observations clearly show that low-mass AGB stars can provide a\nnucleosynthesis site of the s-process. Recent stellar evolution models indicate\nthat radiative burning of C13 between thermal pulses in low-mass AGB stars may\nindeed provide the needed neutrons. Some mixing between the proton-rich\nenvelope and the carbon-rich core may lead to the production of C13. However,\nthe responsible physical mechanism is not yet unambiguously identified. We\npresent stellar model calculations with overshoot and rotation. Overshoot, with\na time-dependent and exponentially decaying efficiency, leads to a partial\nmixture of protons and C12 during the third dredge-up. According to the\ndepth-dependent ratio of protons and C12, a small C13-pocket forms underneath a\nN14-rich layer. Overshoot does not allow for any mixing during the interpulse\nphase. Rotation introduces mixing driven by large angular velocity gradients\nwhich form at the envelope-core interface in AGB stars, in particular after a\nthermal pulse. This leads to partial mixing after a pulse, as in the case of\novershoot. However, rotation continues to mix the region of the C13-pocket with\na diffusion coefficient of log D ~ 2...3 [cm**2 / s]. This does not only spread\nthe C13-pocket, but also the more massive N14-rich layer, and finally leads to\nmixture of both layers. By the time when the temperature there has risen to\nabout 9*10E7K and neutron production sets in, the N14 abundance exceeds the C13\nabundance by a factor of 5...10. We analyze the role of N14 as a neutron poison\nby considering the recycling of neutrons via N14(n,p)C14 and\nC12(p,gamma)N13(beta+)C13 qualitatively. We find that as long as X(N14) <<\nX(C12), the s-process will still be possible to occur under radiative\nconditions.\n