Thermohaline mixing in evolved low-mass stars

TL;DR

This study investigates thermohaline mixing in low-mass stars, confirming its role in altering surface abundances during various evolutionary stages, but highlights uncertainties in its efficiency and interaction with other processes.

Contribution

The paper provides detailed modeling of thermohaline mixing in stars from 1 to 3 solar masses, including its effects during different evolutionary phases and its interaction with rotation and magnetic fields.

Findings

Thermohaline mixing can reduce ^3He abundance in stars with initial mass ≤ 1.5Msun.

It operates during the RGB, core helium burning, and AGB phases.

Current models underestimate the mixing speed needed to match observed surface abundances.

Abstract

Thermohaline mixing has recently been proposed to occur in low-mass red giants, with large consequence for the chemical yields of low-mass stars. We investigate the role of thermohaline mixing during the evolution of stars between 1Msun and 3Msun, in comparison to other mixing processes acting in these stars. We use a stellar evolution code which includes rotational mixing, internal magnetic fields and thermohaline mixing. We confirm that during the red giant stage, thermohaline mixing has the potential to decrease the abundance of ^3He which is produced earlier on the main sequence. In our models we find that this process is working on the RGB only in stars with initial mass M \simle 1.5Msun. Moreover we report that thermohaline mixing is present also during core helium burning and beyond, and has the potential to change the surface abundances of AGB stars. While we find rotational and magnetic mixing to be negligible compared to the thermohaline mixing in the relevant layers, the interaction of thermohaline motions with the differential rotation may be essential to establish the time scale of thermohaline mixing in red giants. To explain the surface abundances observed at the bump in the luminosity function, the speed of the mixing process needs to be more than two orders of magnitude higher than in our models. However it is not clear if thermohaline mixing is the only physical process responsible for these surface abundance anomalies. Therefore, at this stage, it is not possible to calibrate the efficiency of thermohaline mixing against the observations.