Dynamos and Chemical Mixing in Evolved Stars

TL;DR

This paper explores how an alpha-omega dynamo in evolved stars can sustain magnetic fields capable of driving chemical mixing, addressing the transport of material in low-mass RGB and AGB stars.

Contribution

It demonstrates the conditions under which an alpha-omega dynamo can maintain strong magnetic fields in AGB stars, facilitating chemical mixing processes.

Findings

Large-scale toroidal fields of ~3×10^4 G can be sustained.

Convection can resupply shear, maintaining the dynamo.

Magnetic flux tubes could enable anomalous mixing in evolved stars.

Abstract

In low-mass Red Giant Branch (RGB) and Asymptotic Giant Branch (AGB) stars, anomalous mixing must transport material near the hydrogen-burning shell to the convective envelope. Recently, it was suggested that buoyant magnetic flux tubes could supply the necessary transport rate (Busso et al. 2007). The fields are assumed to originate from a dynamo operating in the stellar interior. Here, we show what is required of an $\alpha-\Omega$ dynamo in the envelope of an AGB star to maintain these fields. Differential rotation and rotation drain via turbulent dissipation and Poynting flux, so if shear can be resupplied by convection, then large-scale toroidal field strengths of $\left<B_\phi\right>\simeq3\times10^4$ G can be sustained at the base of the convection zone.