Stellar evolution with rotation and magnetic fields:I. The relative importance of rotational and magnetic effects

TL;DR

This paper compares the effects of rotation and magnetic fields generated by the Tayler instability in stellar evolution, finding magnetic effects dominate angular momentum and chemical transport, with implications for stellar modeling.

Contribution

It introduces a criterion for the existence of magnetic fields in stellar interiors based on energy considerations and compares magnetic and rotational effects in stellar evolution models.

Findings

Magnetic fields dominate angular momentum and chemical transport over meridional circulation.

Magnetic coupling is more significant than horizontal turbulence in stellar regions.

Magnetic fields do not significantly distort stellar equipotential shapes.

Abstract

We compare the current effects of rotation in stellar evolution to those of the magnetic field created by the Tayler instability. In stellar regions, where magnetic field can be generated by the dynamo due to differential rotation (Spruit 2002), we find that the growth rate of the magnetic instability is much faster than for the thermal instability. Thus, meridional circulation is negligible with respect to the magnetic fields, both for the transport of angular momentum and of chemical elements. Also, the horizontal coupling by the magnetic field, which reaches values of a few $10^5$ G, is much more important than the effects of the horizontal turbulence. The field, however, is not sufficient to distort the shape of the equipotentials. We impose the condition that the energy of the magnetic field created by the Tayler--Spruit dynamo cannot be larger than the energy excess present in the differential rotation. This leads to a criterion for the existence of the magnetic field in stellar interiors. Numerical tests are made in a rotating star model of 15 M$_{\odot}$ rotating with an initial velocity of 300 km$\cdot$s$^{-1}$.