Stellar evolution with rotation and magnetic fields II: General equations for the transport by Tayler--Spruit dynamo

TL;DR

This paper advances the Tayler--Spruit dynamo theory by developing general equations that include nonadiabatic effects and complex feedback mechanisms, improving understanding of magnetic field transport in differentially rotating stars.

Contribution

It introduces a more comprehensive set of equations for magnetic transport in stars, accounting for nonadiabatic effects and complex feedback, without simplifying assumptions about gradients.

Findings

Stars with magnetic fields rotate nearly as solid bodies.

Magnetic models better match some stellar properties but do not explain nitrogen excesses.

Feedback between magnetic and thermal instabilities affects stellar evolution.

Abstract

We further develop the Tayler--Spruit dynamo theory, based on the most efficient instability for generating magnetic fields in radiative layers of differentially rotating stars. We avoid the simplifying assumptions that either the $\mu$-- or the $T$--gradient dominates, but we treat the general case and we also account for the nonadiabatic effects, which favour the growth of the magnetic field. Stars with a magnetic field rotate almost as a solid body. Several of their properties (size of the core, MS lifetimes, tracks, abundances) are closer to those of models without rotation than with rotation only. In particular, the observed N/C or N/H excesses in OB stars are better explained by our previous models with rotation only than by the present models with magnetic fields that predict no nitrogen excesses. We show that there is a complex feedback loop between the magnetic instability and the thermal instability driving meridional circulation. This opens the possibility for further magnetic models, but at this stage we do not know the relative importance of the magnetic fields due to the Tayler instability in stellar interiors.