Numerical Simulations of Thermohaline Convection: Implications for Extra-Mixing in Low-Mass RGB Stars

TL;DR

This study uses 2D numerical simulations to evaluate thermohaline convection as a mechanism for extra-mixing in low-mass RGB stars, finding that current salt-fingering models are insufficient but linear theory may still explain observations.

Contribution

It provides the first detailed 2D simulations of thermohaline convection in RGB star conditions, highlighting discrepancies with observed mixing and suggesting linear theory may be more accurate.

Findings

2D simulations show salt-finger aspect ratios are too small to match observations

Thermohaline diffusion coefficient from linear stability analysis can explain RGB extra-mixing

Follow-up 3D simulations are necessary to confirm results

Abstract

Low-mass stars are known to experience extra-mixing in their radiative zones on the red-giant branch (RGB) above the bump luminosity. To determine if the salt-fingering transport of chemical composition driven by 3He burning is efficient enough to produce RGB extra-mixing, 2D numerical simulations of thermohaline convection for physical conditions corresponding to the RGB case have been carried out. We have found that the effective ratio of a salt-finger's length to its diameter a_eff < 0.5 is more than ten times smaller than the value needed to reproduce observations (a_obs > 7). On the other hand, using the thermohaline diffusion coefficient from linear stability analysis together with a=a_obs is able to describe the RGB extra-mixing at all metallicities so well that it is tempting to believe that it may represent the true mechanism. In view of these results, follow-up 3D numerical simulations of thermohaline convection for the RGB case are clearly needed.