2016ApJ...818..125T

Slow neutron captures are responsible for the production of about 50% of elements heavier than iron, mainly occurring during the asymptotic giant branch phase of low-mass stars (1 ≲ M/M_☉≲ 3), where the main neutron source is the ¹³C(α, n)¹⁶O reaction. This last reaction is activated from locally produced ¹³C, formed by partial mixing of hydrogen into the He-rich layers. We present here the first attempt to describe a physical mechanism for the formation of the ¹³C reservoir, studying the mass circulation induced by magnetic buoyancy without adding new free parameters to those already involved in stellar modeling. Our approach represents the application to the stellar layers relevant for s-processing of recent exact analytical 2D and 3D models for magneto-hydrodynamic processes at the base of convective envelopes in evolved stars in order to promote downflows of envelope material for mass conservation during the occurrence of a dredge-up phenomenon. We find that the proton penetration is characterized by small concentrations, but is extended over a large fractional mass of the He-layers, thus producing ¹³C reservoirs of several 10^–3 M_☉. The ensuing ¹³C-enriched zone has an almost flat profile, while only a limited production of ¹⁴N occurs. In order to verify the effects of our new findings we show how the abundances of the main s-component nuclei can be accounted for in solar proportions and how our large ¹³C-reservoir allows us to solve a few so far unexplained features in the abundance distribution of post-AGB objects.