Differential Rotation and Magnetism in Simulations of Fully Convective Stars

TL;DR

This paper uses 3D MHD simulations to explore how fully convective stars generate magnetic fields and differential rotation, revealing complex behaviors that challenge previous expectations based on solar models.

Contribution

It presents novel 3D MHD simulation results showing differential rotation and magnetic field behaviors in fully convective stars across different rotation rates.

Findings

Rapid rotation leads to solar-like differential rotation.

Slower rotation results in anti-solar differential rotation.

Magnetic feedback can significantly reduce differential rotation.

Abstract

Stars of sufficiently low mass are convective throughout their interiors, and so do not possess an internal boundary layer akin to the solar tachocline. Because that interface figures so prominently in many theories of the solar magnetic dynamo, a widespread expectation had been that fully convective stars would exhibit surface magnetic behavior very different from that realized in more massive stars. Here I describe how recent observations and theoretical models of dynamo action in low-mass stars are partly confirming, and partly confounding, this basic expectation. In particular, I present the results of 3--D MHD simulations of dynamo action by convection in rotating spherical shells that approximate the interiors of 0.3 solar-mass stars at a range of rotation rates. The simulated stars can establish latitudinal differential rotation at their surfaces which is solar-like at ``rapid'' rotation rates (defined within) and anti-solar at slower rotation rates; the differential rotation is greatly reduced by feedback from strong dynamo-generated magnetic fields in some parameter regimes. I argue that this ``flip'' in the sense of differential rotation may be observable in the near future. I also briefly describe how the strength and morphology of the magnetic fields varies with the rotation rate of the simulated star, and show that the maximum magnetic energies attained are compatible with simple scaling arguments.