Rotation and magnetism in intermediate mass stars

TL;DR

This paper models the evolution of large-scale magnetic fields in intermediate-mass stars, linking magnetic flux conservation to observed stellar and white dwarf magnetic fields, and incorporating these effects into stellar evolution simulations.

Contribution

It introduces a coupled advection-diffusion model for magnetic fields and angular momentum in stellar evolution, emphasizing the role of magnetic flux conservation in late-stage stellar magnetism.

Findings

Large-scale magnetic fields can be sustained until late stellar stages.

Magnetic flux conservation explains high magnetic fields in white dwarfs.

Magnetic fields concentrate in radiative zones and the core during evolution.

Abstract

Rotation and magnetism are increasingly recognized as important phenomena in stellar evolution. Surface magnetic fields from a few to $20{,}000\,$G have been observed and models have suggested that magnetohydrodynamic transport of angular momentum and chemical composition could explain the peculiar composition of some stars. Stellar remnants such as white dwarfs have been observed with fields from a few to more than $10^{9}\,$G. We investigate the origin of and the evolution, on thermal and nuclear rather than dynamical time-scales, of an averaged large-scale magnetic field throughout a star's life and its coupling to stellar rotation. Large-scale magnetic fields sustained until late stages of stellar evolution with conservation of magnetic flux could explain the very high fields observed in white dwarfs. We include these effects in the Cambridge stellar evolution code using three time-dependant advection-diffusion equations coupled to the structural and composition equations of stars to model the evolution of angular momentum and the two components of the magnetic field. We present the evolution in various cases for a $3\rm\,M_{\odot}$ star from the beginning to the late stages of its life. Our particular model assumes that turbulent motions, including convection, favour small-scale field at the expense of large-scale field. As a result the large-scale field concentrates in radiative zones of the star and so is exchanged between the core and the envelope of the star as it evolves. The field is sustained until the end of the asymptotic giant branch, when it concentrates in the degenerate core.