S-processing in AGB stars revisited. I. Does the main component constrain the neutron source in the 13C pocket?

Abstract

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. © 2014. The American Astronomical Society. All rights reserved..