Mixing and Magnetic Fields in Asymptotic Giant Branch Stars in the Framework of FRUITY Models

Diego Vescovi

doi:10.3390/universe8010016

Abstract

In the last few years, the modeling of asymptotic giant branch (AGB) stars has been much investigated, both focusing on nucleosynthesis and stellar evolution aspects. Recent advances in the input physics required for stellar computations made it possible to construct more accurate evolutionary models, which are an essential tool to interpret the wealth of available observational and nucleosynthetic data. Motivated by such improvements, the FUNS stellar evolutionary code has been updated. Nonetheless, mixing processes occurring in AGB stars’ interiors are currently not well-understood. This is especially true for the physical mechanism leading to the formation of the 13C pocket, the major neutron source in low-mass AGB stars. In this regard, post-processing s-process models assuming that partial mixing of protons is induced by magneto-hydrodynamics processes were shown to reproduce many observations. Such mixing prescriptions have now been implemented in the FUNS code to compute stellar models with fully coupled nucleosynthesis. Here, we review the new generation of FRUITY models that include the effects of mixing triggered by magnetic fields by comparing theoretical findings with observational constraints available either from the isotopic analysis of trace-heavy elements in presolar grains or from carbon AGB stars and Galactic open clusters.

Highlights

The synthesis of elements heavier than iron mostly takes place through neutroncapture processes
We briefly report the major updates to the equation of state (EOS), the heavy-element admixture, the opacity tables, the mass-loss rate, and the nuclear reaction network
The appearance of carbon and s-process heavy elements on the surface of asymptotic giant branch (AGB) stars is related to the interplay of mixing and nuclear processes occurring in their interiors, but the physical mechanism that drives the creation of a 13C pocket remains uncertain

Summary

Introduction

The synthesis of elements heavier than iron mostly takes place through neutroncapture processes. The abundance of about half of nuclei heavier than strontium is accounted for by slow neutron captures occurring in low- and, to a lesser degree, intermediate-mass stars (M 8 M ). In these stars, once the core He supply has been exhausted after the central He burning phase, and electron degeneracy has been set up in the resulting C-O core, He burning is established in a thin shell. While classical post-process models generally assume an ad hoc 13C pocket [9,10], several physicallybased approaches have recently been devised to model the partial mixing of proton-rich material from the convective envelope

Mixing Processes in AGB Stars

Magnetic-Buoyancy-Induced Mixing and the 13C Pocket Formation

Fluorine Nucleosynthesis

Stellar Yields and the Galactic Evolution of Yttrium Abundance

Findings

Summary and Outlook