The Interaction between Shear and Fingering (Thermohaline) Convection

Abstract

Fingering convection is a turbulent mixing process that can occur in stellar radiative regions whenever the mean molecular weight increases with radius. In some cases, it can have a significant observable impact on stellar structure and evolution. The efficiency of mixing by fingering convection as a standalone process has been studied by Brown et al., but other processes such as rotation, magnetic fields, and shear can affect it. In this paper, we present a first study of the effect of shear on fingering (thermohaline) convection in astrophysics. Using direct numerical simulations, we find that a moderate amount of shear (that is not intrinsically shear unstable) always decreases the mixing efficiency of fingering convection, as a result of the tilt it imparts to the fingering structures. We propose a simple analytical extension of the Brown et al. model in the presence of shear that satisfactorily explains the numerically derived turbulent compositional mixing coefficient for moderate shearing rates and can trivially be implemented in stellar evolution codes. We also measure from the numerical simulations a turbulent viscosity and find that the latter is strongly tied to the turbulent compositional mixing coefficient. Observational implications and caveats of the model are discussed.