Double-diffusive mixing in stellar interiors in the presence of horizontal gradients

TL;DR

This paper investigates a new mixing mechanism in stellar interiors caused by double-diffusive instabilities with horizontal gradients, showing that it can significantly enhance material transport beyond pure diffusion.

Contribution

It identifies and characterizes a novel double-diffusive mixing process in stars involving horizontal gradients, supported by 3D simulations, extending oceanographic instability concepts to astrophysics.

Findings

Material transport can be highly enhanced in the intrusive regime.

The instability leads to stacking of fingering layers similar to oceanic intrusions.

Enhanced mixing occurs even in systems stable to thermohaline convection.

Abstract

We have identified an important source of mixing in stellar radiation zones, that would arise whenever two conditions are satisfied: (1) the presence of an inverse vertical compositional gradient, and (2) the presence of density-compensating horizontal gradients of temperature (alternatively, entropy) and composition. The former can be caused naturally by any off-center burning process, by atomic diffusion, or by surface accretion. The latter could be caused by rotation, tides, meridional flows, etc. The linear instability and its nonlinear development have been well-studied in the oceanographic context. It is known to drive the formation of stacks of fingering layers separated by diffusive interfaces, called intrusions. Using 3D numerical simulations of the process in the astrophysically-relevant region of parameter space, we find similar results, and demonstrate that the material transport in the intrusive regime can be highly enhanced compared with pure diffusion, even in systems which would otherwise be stable to fingering (thermohaline) convection.