s-Processing from MHD-induced mixing and isotopic abundances in presolar SiC grains

TL;DR

This study investigates how magnetohydrodynamic processes in AGB stars can explain the formation of 13C and s-process element abundances, matching isotopic data from presolar SiC grains and advancing understanding of stellar nucleosynthesis.

Contribution

It introduces a physically-based MHD mixing model for low-mass AGB stars that successfully reproduces isotopic ratios observed in presolar SiC grains, improving upon previous parameterized models.

Findings

MHD-driven mixing models match SiC grain isotopic data.

The 13C distribution from MHD models aligns with observational constraints.

Magnetic buoyancy can effectively induce proton injection in AGB stars.

Abstract

In the past years the observational evidence that s-process elements from Sr to Pb are produced by stars ascending the socalled Asymptotic Giant Branch (or AGB) could not be explained by self-consistent models, forcing researchers to extensive parameterizations. The crucial point is to understand how protons can be injected from the envelope into the He-rich layers, yielding the formation of 13C and then the activation of the 13C(a,n)16O reaction. Only recently, attempts to solve this problem started to consider quantitatively physically-based mixing mechanisms. Among them, MHD processes in the plasma were suggested to yield mass transport through magnetic buoyancy. In this framework, we compare results of nucleosynthesis models for Low Mass AGB Stars (M<=3Mo), developed from the MHD scenario, with the record of isotopic abundance ratios of s-elements in presolar SiC grains, which were shown to offer precise constraints on the 13C reservoir. We find that n-captures driven by magnetically-induced mixing can indeed account for the SiC data quite well and that this is due to the fact that our 13C distribution fulfils the above constraints rather accurately. We suggest that similar tests should be now performed using different physical models for mixing. Such comparisons would indeed improve decisively our understanding of the formation of the neutron source.