Transport and mixing in the radiation zones of rotating stars: I-Hydrodynamical processes

TL;DR

This paper advances the modeling of rotational mixing in stellar radiation zones by incorporating higher-order asymmetries, differential rotation, and non-stationary effects, improving the understanding of angular momentum, heat, and chemical transport.

Contribution

It extends previous models by including higher-order departures from spherical symmetry and explicit latitude-dependent differential rotation, enabling more comprehensive simulations of stellar radiation zones.

Findings

Enhanced model of rotational mixing incorporating non-spherical effects.

Explicit treatment of differential rotation in latitude.

Framework ready for implementation in stellar evolution codes.

Abstract

The purpose of this paper is to improve the modelization of the rotational mixing which occurs in stellar radiation zones, through the combined action of the thermally driven meridional circulation and of the turbulence generated by the shear of differential rotation. The turbulence is assumed to be anisotropic, due to the stratification, with stronger transport in the horizontal directions than in the vertical. The main difference with the former treatments by Zahn (1992) and Maeder & Zahn (1998) is that we expand here the departures from spherical symmetry to higher order, and include explicitly the differential rotation in latitude, to first order. This allows us to treat simultaneously the bulk of a radiation zone and its tachocline(s). Moreover, we take fully into account the non-stationarity of the problem, which will enable us to tackle the rapid phases of evolution. The system of partial differential equations, which govern the transport of angular momentum, heat and chemical elements, is written in a form which makes it ready to implement in a stellar evolution code. Here the effect of a magnetic field is deliberately ignored; it will be included in forthcoming papers.