Magnetic Mixing in Red Giant and Asymptotic Giant Branch Stars

TL;DR

This paper models an alpha-omega dynamo in RGB and AGB stars, showing magnetic flux tubes could enable mixing processes that explain observed isotopic abundances in these stars' atmospheres.

Contribution

It introduces a dynamo model in RGB/AGB stars demonstrating magnetic fields strong enough for mixing, a novel physical mechanism for stellar surface composition changes.

Findings

Peak magnetic fields of 3000-5000 G at the convection zone base.

Magnetic flux tubes could facilitate mixing processes like Cool Bottom Processing.

Dynamo sustainability depends on convection resupplying shear.

Abstract

The available information on isotopic abundances in the atmospheres of low-mass Red Giant Branch (RGB) and Asymptotic Giant Branch (AGB) stars requires that episodes of extensive mixing occur below the convective envelope, reaching down to layers close to the hydrogen burning shell (Cool Bottom Processing). Recently \cite{Busso:2007jw} suggested that dynamo-produced buoyant magnetic flux tubes could provide the necessary physical mechanisms and also supply sufficient transport rates. Here, we present an $\alpha-\Omega$ dynamo in the envelope of an RGB/AGB star in which shear and rotation drain via turbulent dissipation and Poynting flux. In this context, if the dynamo is to sustain throughout either phase, convection must resupply shear. Under this condition, volume-averaged, peak toroidal field strengths of $<B_\phi>\simeq3\times10^3$ G (RGB) and $<B_\phi>\simeq5\times10^3$ G (AGB) are possible at the base of the convection zone. If the magnetic fields are concentrated in flux tubes, the corresponding field strengths are comparable to those required by Cool Bottom Processing.