Dynamo Action in Fully Convective Low-Mass Stars

TL;DR

This paper presents 3D simulations demonstrating that fully convective low-mass stars can generate strong, large-scale magnetic fields influenced by rotation, even without a tachocline, challenging previous assumptions.

Contribution

It provides the first detailed global simulations showing magnetic field generation and structure in fully convective stars without a tachocline.

Findings

Magnetic fields of several kG are generated, roughly in equipartition with convection.

Differential rotation is suppressed by magnetic stresses in MHD simulations.

Strong mean magnetic fields are present despite lack of interior shear layers.

Abstract

Recent observations indicate that fully convective stars can effectively build magnetic fields without the aid of a tachocline of shear, that those fields can possess large-scale components, and that they may sense the effects of rotation. Motivated by these puzzles, we present global three-dimensional simulations of convection and dynamo action in the interiors of fully convective M-dwarfs of 0.3 solar masses. We use the Anelastic Spherical Harmonic (ASH) code, adopting a spherical computational domain that extends from 0.08-0.96 times the overall stellar radius. We find that such fully convective stars can generate magnetic fields of several kG strength, roughly in equipartition with the convective flows. Differential rotation is established in hydrodynamic progenitor calculations, but strongly quenched in MHD simulations because of strong Maxwell stresses exerted by the magnetic fields. Despite the absence of interior angular velocity contrasts, the magnetic fields possess strong mean (axisymmetric) components, which we attribute partly to the very strong influence of rotation upon the slowly overturning flows.