Thermohaline Mixing: Does It Really Govern the Atmospheric Chemical Composition of Low-Mass Red Giants?

TL;DR

This study uses 3D simulations to evaluate thermohaline mixing in low-mass red giants, finding it insufficient to explain observed chemical compositions and suggesting magnetic fields as a potential alternative mechanism.

Contribution

First 3D numerical simulations of thermohaline convection in RGB stars showing lower mixing rates than observations and exploring magnetic effects as an alternative.

Findings

Thermohaline mixing rate is 50 times lower than observed.

Large-scale salt-fingering instabilities do not enhance mixing.

Magnetic fields may shift salt-finger sizes, potentially increasing mixing.

Abstract

First results of our 3D numerical simulations of thermohaline convection driven by 3He burning in a low-mass RGB star at the bump luminosity are presented. They confirm our previous conclusion that this convection has a mixing rate which is a factor of 50 lower than the observationally constrained rate of RGB extra-mixing. It is also shown that the large-scale instabilities of salt-fingering mean field (those of the Boussinesq and advection-diffusion equations averaged over length and time scales of many salt fingers), which have been observed to increase the rate of oceanic thermohaline mixing up to one order of magnitude, do not enhance the RGB thermohaline mixing. We speculate on possible alternative solutions of the problem of RGB extra-mixing, among which the most promising one that is related to thermohaline mixing is going to take advantage of the shifting of salt-finger spectrum towards larger diameters by toroidal magnetic field.