Thermohaline instabilities inside stars: a synthetic study including external turbulence and radiative levitation

TL;DR

This paper derives a new expression for thermohaline mixing in stars, accounting for radiative levitation and turbulence, revealing their limited impact on instability and emphasizing the importance of including these effects in stellar models.

Contribution

It introduces a new thermohaline mixing coefficient that incorporates radiative levitation and turbulence effects, advancing the understanding of element accumulation in stellar layers.

Findings

Radiative accelerations have a small effect on thermohaline instability.

Horizontal turbulence can suppress thermohaline convection.

Inclusion of these effects alters predictions of stellar surface abundances.

Abstract

We have derived a new expression for the thermohaline mixing coefficient in stars, including the effects of radiative levitation and external turbulence, by solving Boussinesq equations in a quasi-incompressible fluid with a linear approximation. It is well known that radiative levitation of individual elements can lead to their accumulation in specific stellar layers. In some cases, it can induce important effects on the stellar structure. Here we confirm that this accumulation is moderated by thermohaline convection due to the resulting inverse $\mu$-gradient. The new coefficient that we have derived shows that the effect of radiative accelerations on the thermohaline instability itself is small. This effect must however be checked in all computations. We also confirm that the presence of large horizontal turbulence can reduce or even suppress the thermohaline convection. These results are important as they concern all the cases of heavy element accumulation in stars. The computations of radiative diffusion have to be revisited including thermohaline convection and its consequences. It may be one of the basic reasons for the fact that the observed abundances are always smaller than those predicted by pure atomic diffusion. In any case, these processes have to compete with rotation-induced mixing, but this competition is more complex than previously thought due to their mutual interaction.