3D evolution of magnetic fields in a differentially rotating stellar radiative zone

TL;DR

This study numerically investigates how magnetic fields evolve in differentially rotating stellar radiative zones, revealing the dominance of magneto-rotational instability and its potential link to magnetic field dichotomies in A stars.

Contribution

It demonstrates that magneto-rotational instability governs magnetic field evolution in unstratified stellar shells with differential rotation, highlighting its role over Tayler instability in such contexts.

Findings

Magneto-rotational instability is triggered in differentially rotating stellar shells.

Enhanced angular momentum transport leads to faster rotation profile flattening.

The instability threshold may explain the magnetic dichotomy in A stars.

Abstract

The question of the origin and evolution of magnetic fields in stars possessing a radiative envelope, like the A-type stars, is still regarded as a challenge for stellar physics. Those zones are likely to be differentially rotating, which suggests that strong interactions between differential rotation and magnetic fields could be at play. We numerically compute the joint evolution of the magnetic and velocity fields in a 3D spherical shell starting from an initial profile for the poloidal magnetic field and differential rotation. The poloidal magnetic field is initially wound-up by the differential rotation to produce a toroidal field which becomes unstable. In the particular setup studied here where the differential rotation is dominant, the magneto-rotational instability is triggered. The growth rate of the instability depends mainly on the initial rotation rate, while the background state typically oscillates over a poloidal Alfv\'en time. We thus find that the axisymmetric magnetic configuration is strongly modified by the instability only if the ratio between the poloidal Alfv\'en frequency and the rotation rate is sufficiently small. An enhanced transport of angular momentum is found in the most unstable cases: the typical time to flatten the rotation profile is then much faster than the diffusion time scale. We conclude that the magneto-rotational instability is always favored (over the Tayler instability) in unstratified spherical shells when an initial poloidal field is sheared by a sufficiently strong cylindrical differential rotation. A possible application to the magnetic desert observed among A stars is given. We argue that the dichotomy between stars exhibiting strong axisymmetric fields (Ap stars) and those harboring a sub-Gauss magnetism could be linked to the threshold for the instability.