Stellar evolution of massive stars with a radiative alpha-omega dynamo

TL;DR

This paper develops a detailed one-dimensional model of magnetic field evolution in massive stars with a radiative alpha-omega dynamo, explaining observed nitrogen enrichment patterns and predicting a mass-dependent dynamo strength.

Contribution

It introduces a coupled advection-diffusion model for magnetic fields in stellar rotation codes, enhancing understanding of magnetic effects in massive star evolution.

Findings

Reproduces observed nitrogen enrichment in massive stars.

Explains slowly-rotating nitrogen-enriched stars.

Predicts a sharp decrease in dynamo strength above a certain stellar mass.

Abstract

Models of rotationally-driven dynamos in stellar radiative zones have suggested that magnetohydrodynamic transport of angular momentum and chemical composition can dominate over the otherwise purely hydrodynamic processes. A proper consideration of the interaction between rotation and magnetic fields is therefore essential. Previous studies have focused on a magnetic model where the magnetic field strength is derived as a function of the stellar structure and angular momentum distribution. We have adapted our one-dimensional stellar rotation code, RoSE, to model the poloidal and toroidal magnetic field strengths with a pair of time-dependent advection-diffusion equations coupled to the equations for the evolution of the angular momentum distribution and stellar structure. This produces a much more complete, though still reasonably simple, model for the magnetic field evolution. Our model reproduces well observed surface nitrogen enrichment of massive stars in the Large Magellanic Cloud. In particular it reproduces a population of slowly-rotating nitrogen-enriched stars that cannot be explained by rotational mixing alone alongside the traditional rotationlly-enriched stars. The model further predicts a strong mass-dependency for the dynamo-driven field. Above a threshold mass, the strength of the magnetic dynamo decreases abruptly and so we predict that more massive stars are much less likely to support a dynamo-driven field than less massive stars.