Modeling the formation of the 13C neutron source in AGB stars

D Vescovi,S Cristallo

doi:10.1088/1742-6596/1668/1/012047

Abstract

A major source of uncertainty in AGB models is the partial-mixing process of hydrogen, required for the formation of the so-called 13C pocket. Among the attempts to derive a self-consistent treatment of this physical process, there are 2D and 3D simulations of magnetic buoyancy. The 13C pocket resulting from mixing induced by magnetic buoyancy extends over a region larger than those so far assumed, showing an almost flat 13C distribution and a negligible amount of 14N. Recently, it has been proved to be a good candidate to match the records of isotopic abundance ratios of s-elements in presolar SiC grains. However, up to date such a magnetic mixing has been applied in post-process calculations only, being never implemented in a stellar evolutionary code. Here we present new stellar models, performed with the 1-d hydrostatic FUNS evolutionary code, which include magnetic buoyancy. We comment the resulting s-process distributions and show preliminary comparisons to spectroscopic observations and pre-solar grains measurements.

Highlights

The existence of about half of the nuclei heavier than iron can be explained through neutron (n) captures occurring in the Asymptotic Giant Branch (AGB) phase of LowMass Stars (LMS)
Many physically-based approaches have been developed in order to model the penetration of proton-rich material from the convective envelope to the He-intershell, involving an opacity-induced overshoot [3] or a mixing induced by internal gravity waves [4]
First outcomes confirms the recent results from [6, 7], showing that magnetic AGB models can reproduce the majority of isotopic ratios of mainstream grains

Summary

Introduction

The existence of about half of the nuclei heavier than iron can be explained through neutron (n) captures occurring in the Asymptotic Giant Branch (AGB) phase of LowMass Stars (LMS) In this process, because of the longer timescale, n-captures usually occur at a slow (s) rate compared to the β-decay of unstable nuclei. Many physically-based approaches have been developed in order to model the penetration of proton-rich material from the convective envelope to the He-intershell, involving an opacity-induced overshoot [3] or a mixing induced by internal gravity waves [4] Another approach suggests that the magnetic activity of LMS stars could induce the buoyancy of the material of He-intershell to the envelope [5]. We adopted the formalism developed in [5] for describing the magneticallydriven expansion of material in the radiative He-intershell, derived the corresponding radial velocity of the induced proton downflow, and implemented such magnetic mixing for the formation of the 13C-pocket in the FUNS hydrostatic stellar evolutionary code [8]

Models at different metallicities

Conclusions