Magneto-Thermohaline Mixing in Red Giants

TL;DR

This paper revises a magnetic buoyancy model for mixing in red giants, emphasizing heat exchange effects, magnetic field strength, and the role of differential rotation in generating magnetic flux rings.

Contribution

It introduces a revised model accounting for heat exchange, magnetic field effects, and the formation of flux rings via differential rotation, advancing understanding of stellar mixing mechanisms.

Findings

Heat exchange increases buoyant rise time by five orders of magnitude.

Magnetic flux rings are equivalent to pure magnetic buoyancy with B_phi ~ 10 MG.

Toroidal magnetic fields are generated by differential rotation, facilitating flux ring formation.

Abstract

We revise a magnetic buoyancy model that has recently been proposed as a mechanism for extra mixing in the radiative zones of low-mass red giants. The most important revision is our accounting of the heat exchange between rising magnetic flux rings and their surrounding medium. This increases the buoyant rising time by five orders of magnitude, therefore the number of magnetic flux rings participating in the mixing has to be increased correspondingly. On the other hand, our revised model takes advantage of the fact that the mean molecular weight of the rings formed in the vicinity of the hydrogen burning shell has been reduced by 3He burning. This increases their thermohaline buoyancy (hence, decreases the total ring number) considerably, making it equivalent to the pure magnetic buoyancy produced by a frozen-in toroidal field with B_phi ~ 10 MG. We emphasize that some toroidal field is still needed for the rings to remain cohesive while rising. Besides, this field prevents the horizontal turbulent diffusion from eroding the mu contrast between the rings and their surrounding medium. We propose that the necessary toroidal magnetic field is generated by differential rotation of the radiative zone, that stretches a pre-existing poloidal field around the rotation axis, and that magnetic flux rings are formed as a result of its buoyancy-related instability.