Transport of angular momentum and chemical species by anisotropic mixing in stellar radiative interiors

TL;DR

This paper investigates how anisotropic turbulence in stellar radiative zones transports angular momentum and chemical species, revealing that angular momentum transport is mainly non-diffusive and more efficient than chemical mixing, impacting stellar rotational evolution.

Contribution

It provides new estimates of turbulent transport coefficients in stratified rotating stellar interiors, highlighting the dominance of the Lambda-effect in angular momentum transport.

Findings

Angular momentum transport is mainly non-diffusive and outward.

Chemical species are transported primarily by small radial diffusion.

Rotational coupling timescale is estimated to be less than 100 Myr.

Abstract

Small levels of turbulence can be present in stellar radiative interiors due to, e.g., instability of rotational shear. In this paper we estimate turbulent transport coefficients for stably stratified rotating stellar radiation zones. Stable stratification induces strong anisotropy with a very small ratio of radial-to-horizontal turbulence intensities. Angular momentum is transported mainly due to the correlation between azimuthal and radial turbulent motions induced by the Coriolis force. This non-diffusive transport known as the Lambda-effect has outward direction in radius and is much more efficient compared to the effect of radial eddy viscosity. Chemical species are transported by small radial diffusion only. This result is confirmed using direct numerical simulations combined with the test-scalar method. As a consequence of the non-diffusive transport of angular momentum, the estimated characteristic time of rotational coupling (< 100 Myr) between radiative core and convective envelope in young solar-type stars is much shorter compared to the time-scale of Lithium depletion (~1 Gyr).