s-processing in AGB Stars Revisited. III. Neutron Captures from MHD Mixing at Different Metallicities and Observational Constraints

We present post-process neutron-capture computations for Asymptotic Giant Branch (AGB) stars of 1.5-3 M and metallicities -1.3 <= [Fe/H] <= 0.1. The reference stellar models are computed with the FRANEC code, using the Schwarzschild's criterion for convection; our motivations for this choice are outlined. We assume that MHD processes induce the penetration of protons below the convective boundary, when the Third Dredge Up occurs. There, the C-13 n-source can subsequently operate, merging its effects with those of the Ne-22(alpha, n)Mg-25 reaction, activated at the temperature peaks characterizing AGB stages. This work has three main scopes. (i) We provide a grid of abundance yields, as produced through our MHD mixing scheme, which are uniformly sampled in mass and metallicity. From this, we deduce that the solar s-process distribution, as well as the abundances in recent stellar populations, can be accounted for, without the need of the extra primary-like contributions suggested in the past. (ii) We formulate analytic expressions for the mass of the C-13-pockets generated to allow easy verification of our findings. (iii) We compare our results with observations of evolved stars and with isotopic ratios in presolar SiC grains, also noticing how some flux tubes should survive turbulent disruption, carrying C-rich materials into the winds even when the envelope is O-rich. This wind phase is approximated through the G-component of AGB s-processing. We conclude that MHD-induced mixing is adequate to drive slow n-capture phenomena accounting for observations; our prescriptions should permit its inclusion into current stellar evolutionary codes.

Automated summary

The study presents post-process neutron-capture computations for AGB stars of 1.5-3 solar masses and metallicities -1.3 <= [Fe/H] <= 0.1, examining the effects of MHD-induced mixing on abundance yields and isotopic ratios. Analytic expressions for C-13 pockets are formulated for easy verification of findings. Results are compared with observations of evolved stars and presolar SiC grains, showing how MHD-induced mixing can account for slow n-capture phenomena in stellar evolution.