O. Trippella^1,2, M. Busso^1,2, E. Maiorca^2,3, F. Käppeler⁴ and S. Palmerini⁵

¹Department of Physics, University of Perugia, via A. Pascoli, I-06123 Perugia, Italy
²INFN, Section of Perugia, via A. Pascoli, I-06123 Perugia, Italy
³INAF, Observatory of Arcetri, viale E. Fermi 5, I-50125 Florence, Italy
⁴Karlsruhe Institute of Technology, Campus North, Institute of Nuclear Physics, P.O. Box 3640, D-76021 Karlsruhe, Germany
⁵INFN, Laboratori Nazionali del Sud, via Santa Sofia 62, I-95125 Catania, Italy

Slow neutron captures at A  85 are mainly guaranteed by the reaction ¹³C(α,n)¹⁶O in asymptotic giant branch (AGB) stars, requiring proton injections from the envelope. These were so far assumed to involve a small mass ( 10^-3 M☉), but models with rotation suggest that in such tiny layers excessive ¹⁴N hampers s-processing. Furthermore, s-element abundances in galaxies require ¹³C-rich layers substantially extended in mass ( 4 × 10^-3 M_☉). We therefore present new calculations aimed at clarifying those issues and at understanding whether the solar composition helps to constrain the ¹³C “pocket” extension. We show that: (1) mixing “from bottom to top” (as in magnetic buoyancy or other forced mechanisms) can form a ¹³C reservoir substantially larger than assumed so far, covering most of the He-rich layers; (2) on the basis of this idea, stellar models at a fixed metallicity reproduce the main s-component as accurately as before; and (3) they make nuclear contributions from unknown nucleosynthesis processes (LEPP) unnecessary, against common assumptions. These models also avoid problems of mixing at the envelope border and fulfil requirements from C-star luminosities. They yield a large production of nuclei below A = 100, so that ^86,87Sr may be fully synthesized by AGB stars, while ⁸⁸Sr, ⁸⁹Y, and ⁹⁴Zr are contributed more efficiently than before. Finally, we suggest tests suitable for providing a final answer regarding the extension of the ¹³C pocket.

Reference
Trippella O, Busso M, Maiorca E, Käppeler F and Palmerini S (2014) s-Processing in AGB Stars Revisited. I. Does the Main Component Constrain the Neutron Source in the 13C Pocket?. The Astrophysical Journal 787:41.
[doi:10.1088/0004-637X/787/1/41]

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