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Title: s-processing in AGB stars revisited. I. Does the main component constrain the neutron source in the {sup 13}C pocket?

Slow neutron captures at A ≳ 85 are mainly guaranteed by the reaction {sup 13}C(α,n){sup 16}O in asymptotic giant branch (AGB) stars, requiring proton injections from the envelope. These were so far assumed to involve a small mass (≲ 10{sup –3} M {sub ☉}), but models with rotation suggest that in such tiny layers excessive {sup 14}N hampers s-processing. Furthermore, s-element abundances in galaxies require {sup 13}C-rich layers substantially extended in mass (≳ 4 × 10{sup –3} M {sub ☉}). We therefore present new calculations aimed at clarifying those issues and at understanding whether the solar composition helps to constrain the {sup 13}C 'pocket' extension. We show that: (1) mixing 'from bottom to top' (as in magnetic buoyancy or other forced mechanisms) can form a {sup 13}C 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 nucleimore » below A = 100, so that {sup 86,} {sup 87}Sr may be fully synthesized by AGB stars, while {sup 88}Sr, {sup 89}Y, and {sup 94}Zr are contributed more efficiently than before. Finally, we suggest tests suitable for providing a final answer regarding the extension of the {sup 13}C pocket.« less
Authors:
;  [1] ;  [2] ;  [3] ;  [4]
  1. Department of Physics, University of Perugia, via A. Pascoli, I-06123 Perugia (Italy)
  2. INFN, Section of Perugia, via A. Pascoli, I-06123 Perugia (Italy)
  3. Karlsruhe Institute of Technology, Campus North, Institute of Nuclear Physics, P.O. Box 3640, D-76021 Karlsruhe (Germany)
  4. INFN, Laboratori Nazionali del Sud, via Santa Sofia 62, I-95125 Catania (Italy)
Publication Date:
OSTI Identifier:
22356841
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 787; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ALPHA REACTIONS; CARBON 13 TARGET; ELEMENT ABUNDANCE; GALAXIES; GIANT STARS; LUMINOSITY; MASS; METALLICITY; NEUTRON EMISSION; NITROGEN 14; NUCLEAR REACTIONS; NUCLEOSYNTHESIS; OXYGEN 16; SLOW NEUTRONS; STAR EVOLUTION; STRONTIUM 87; STRONTIUM 88; YTTRIUM 89; ZIRCONIUM 94