A two-dimensional mixing length theory of convective transport

TL;DR

This paper develops a 2D convective transport model for stellar evolution, incorporating anisotropic angular momentum fluxes and a self-consistent approach to angular momentum and heat transfer in stars.

Contribution

It introduces a novel 2D convective prescription based on linear mode spectra and saturation, providing closed-form expressions for fluxes with minimal free parameters.

Findings

Provides a self-consistent 2D model for angular momentum transport in stars.

Derives linear coefficients linking fluxes to rotation and its gradient.

Shows agreement with previous numerical studies.

Abstract

The helioseismic observations of the internal rotation profile of the Sun raise questions about the two-dimensional (2D) nature of the transport of angular momentum in stars. Here we derive a convective prescription for axisymmetric (2D) stellar evolution models. We describe the small scale motions by a spectrum of unstable linear modes in a Boussinesq fluid. Our saturation prescription makes use of the angular dependence of the linear dispersion relation to estimate the anisotropy of convective velocities. We are then able to provide closed form expressions for the thermal and angular momentum fluxes with only one free parameter, the mixing length. We illustrate our prescription for slow rotation, to first order in the rotation rate. In this limit, the thermodynamical variables are spherically symetric, while the angular momentum depends both on radius and latitude. We obtain a closed set of equations for stellar evolution, with a self-consistent description for the transport of angular momentum in convective regions. We derive the linear coefficients which link the angular momentum flux to the rotation rate ($\Lambda$- effect) and its gradient ($\alpha$-effect). We compare our results to former relevant numerical work.