Radial differential rotation leading to dipole collapse in pre-main-sequence stars

TL;DR

This study uses 3D simulations to show how radial differential rotation in pre-main-sequence stars can weaken or disrupt magnetic dipoles, linking internal angular momentum evolution to observed stellar magnetism.

Contribution

The paper introduces a detailed simulation-based analysis of how differential rotation affects magnetic field stability in PMS stars, revealing a mechanism for dipole collapse.

Findings

Radial differential rotation can weaken or oscillate magnetic dipoles.

Shearing of poloidal fields perturbs the dynamo mechanism.

Stability depends on shear strength, field strength, and radiative core size.

Abstract

Despite progress in the observations of stellar magnetic fields, their physical mechanisms remain poorly understood. During the pre-main sequence (PMS) phase, the inner layers of stars contract and a radiative core gradually develops. In contrast, the convective envelope is gradually braked through magnetic interactions with the accretion disk and winds. With developing differential rotation inside the star, PMS phase is thus a critical period for magnetic properties of stars when strong initial dipoles can get perturbed, leading to the observed diversity in the magnetism on the main sequence (MS). In this work, we study the impact of differential rotation on such fields. We perform three-dimensional anelastic convective dynamo simulations of rotating spherical shells with an imposed differential rotation (shear) between the boundaries. Density, gravity profiles and convective zone thicknesses were set close to those predicted in PMS low-mass stars by one-dimensional stellar evolution code Cesam2k20. Our results show that radial differential rotation can induce dipole collapse leading to weaker, oscillatory magnetic fields. Differential rotation seems to perturb $\alpha^2$ dynamo mechanism, responsible for dipolar magnetic fields, by shearing poloidal field lines and by affecting turbulent magnetic transport processes. This collapse is moderated by the relative importance of shear compared to the vigor of convective motions, with exact stability criterion depending on the field strength and the size of the radiative core. Applying DNS-based stability criterion in PMS stellar evolution models, we qualitatively reproduce the trends observed in the magnetic topologies of low-mass stars when assuming an efficient internal angular momentum redistribution. This suggests that stellar magnetic properties are intimately related to the PMS angular momentum evolution.