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Neutron-capture nucleosynthesis in AGB stars
Received 31 January 2000 / Accepted 14 August 2000
Recent AGB models including diffusive overshoot or rotational effects suggest the partial mixing (PM) of protons from the H-rich envelope into the C-rich layers during the third dredge-up. In order to study the impact of such a mixing on the surface abundances, nucleosynthesis calculations based on stellar AGB models are performed for different assumptions of protons (ranging from to 0.7) in the PM zone. For high proton-to-12C abundance ratios, light nuclei such as fluorine and sodium are efficiently produced, while heavier s-process nuclei are synthesized for lower proton-to- ratios.
In the framework of the PM model, assuming a smooth exponentially decreasing proton profile, the surface abundance evolution is correlated with that of s-process nuclei in agreement with observations. However, as a function of the surface C/O abundance ratio, the surface enrichment remains difficult to reconcile with observations in AGB stars. Sodium is predicted to be efficiently produced in a small region of the PM zone with proton-to-12C abundance ratio of about 10, but with large overproduction factors (up to fifty times higher than the sodium left over by the hydrogen burning shell).
The primary pocket formed in the PM zone at low proton-to- ratios is responsible for an efficient production of s-process nuclei. A table of elemental overabundances predicted at the surface of AGB stars at four different metallicities is presented.
All the nucleosynthesis calculations are shown to suffer from major nuclear reaction rate uncertainties, in particular, , and . The major uncertainties associated with the amount of protons mixed into the C-rich zone are found in the extent of the PM zone rather than in the adopted H profile.
Finally, the PM scenario predicts that low-metallicity AGB stars enriched in s-process elements should exhibit a large overproduction of Pb and Bi compared to other s-isotopes. The search of such Pb-stars is highly encouraged.
Key words: nuclear reactions, nucleosynthesis, abundances stars: abundances stars: AGB and post-AGB
Send offprint requests to: S. Goriely
Online publication: October 24, 2000