Vertical shear instabilities in rotating stellar radiation zones: effects of the full Coriolis acceleration and thermal diffusion

TL;DR

This study analyzes vertical shear instabilities in stellar radiation zones, incorporating the full Coriolis acceleration and thermal diffusion, to better understand turbulent transport and its dependence on stellar rotation and stratification.

Contribution

It provides a comprehensive linear stability analysis of shear instabilities with the full Coriolis effect, deriving new turbulence prescriptions for stellar models.

Findings

Inflectional instability dominates in equatorial regions of slow rotators.

Inertial instability is prevalent in polar regions of fast rotators.

Thermal diffusion enhances both types of instabilities.

Abstract

Rotation deeply impacts the structure and the evolution of stars. To build coherent 1D or multi-D stellar structure and evolution models, we must systematically evaluate the turbulent transport of momentum and matter induced by hydrodynamical instabilities of radial and latitudinal differential rotation in stably stratified thermally diffusive stellar radiation zones. In this work, we investigate vertical shear instabilities in these regions. The full Coriolis acceleration with the complete rotation vector at a general latitude is taken into account. We formulate the problem by considering a canonical shear flow with a hyperbolic-tangent profile. We perform linear stability analysis on this base flow using both numerical and asymptotic Wentzel-Kramers-Brillouin-Jeffreys (WKBJ) methods. Two types of instabilities are identified and explored: inflectional instability, which occurs in the presence of an inflection point in shear flow, and inertial instability due to an imbalance between the centrifugal acceleration and pressure gradient. Both instabilities are promoted as thermal diffusion becomes stronger or stratification becomes weaker. Effects of the full Coriolis acceleration are found to be more complex according to parametric investigations in wide ranges of colatitudes and rotation-to-shear and rotation-to-stratification ratios. Also, new prescriptions for the vertical eddy viscosity are derived to model the turbulent transport triggered by each instability. We foresee that the inflectional instability will be responsible for turbulent transport in the equatorial region of strongly-stratified radiative zones in slowly rotating stars while the inertial instability triggers turbulence in the polar regions of weakly-stratified radiative zones in fast-rotating stars.