Does the main component of the s-process in AGB stars constrain the neutron source in the 13C-pocket?

Abstract

The 13 C(α, n) 16 O reaction is considered as the main responsible for the production of the sprocess main component in low-mass AGB stars. Its activation requires a physical mechanism to ingest protons in the He-rich region forming a 13 C reservoir, or pocket. This was typically assumed to involve a small mass (≤ 10 −3 M ) [7], but recent observations in several astrophysical sites showed enhancements of s-element abundances with respect to the Sun suggesting a more effective s-process nucleosynthesis. This requires a more extended 13 C pocket covering, in mass (≥ 4 × 10 −3 M ), most of the He-rich layers. Recently, we suggested that magnetic buoyancy could promote forced mixing [14,20] to produce a 13 C reservoir larger than assumed so far. Here we investigate if the solar composition of neutron rich elements, from Sr to Bi, can constrain the 13 C-pocket extension. Stellar models at a fixed metallicity, based on a large 13 C reservoir reproduce the distribution of s-only elements within the uncertainty of 10%, also fulfilling C-star luminosity observations. Assuming this new scenario, a large production of nuclei below A = 90 is expected, so that 86, 87 Sr may be fully synthesized by AGB stars, while 88 Sr, 89 Y and 94 Zr are contributed more efficiently than before.