The Magnetic Furnace: Intense Core Dynamos in B-stars

TL;DR

This study uses 3D simulations to explore how intense magnetic fields generated by core dynamos in massive B-type stars influence their internal dynamics across different rotation rates.

Contribution

It provides the first detailed 3D simulation analysis of core dynamo action in young B-type stars, revealing magnetic field strengths and flow interactions at various rotation speeds.

Findings

Magnetic fields exceed a megagauss in strength within the core.

Core convection involves turbulent columns aligned with the rotation axis.

Strong magnetic fields coexist with flows without quenching them.

Abstract

The dynamo action achieved in the convective cores of main-sequence massive stars is explored here through 3D global simulations of convective core dynamos operating within a young $\Msun{10}$ B-type star, using the anelastic spherical harmonic (ASH) code. These simulations capture the inner 65\% of this star by radius, encompassing the convective nuclear-burning core (about 23\% by radius) and a portion of the overlying radiative envelope. Eight rotation rates are considered, ranging from 0.05\% to 16\% of the surface breakup velocity, thereby capturing both convection that barely senses the effects of rotation and other situations in which the Coriolis forces are prominent. The vigorous dynamo action realized within all of these turbulent convective cores builds magnetic fields with peak strengths exceeding a megagauss, with the overall magnetic energy (ME) in the faster rotators reaching super-equipartition levels compared to the convective kinetic energy (KE). The core convection typically involves turbulent columnar velocity structures roughly aligned with the rotation axis, with magnetic fields threading through these rolls and possessing complex linkages throughout the core. The very strong fields are able to coexist with the flows without quenching them through Lorentz forces. The velocity and magnetic fields achieve such a state by being nearly co-aligned, and with peak magnetic islands being somewhat displaced from the fastest flows as the intricate evolution proceeds. As the rotation rate is increased, the primary force balance shifts from nonlinear advection balancing Lorentz forces to a magnetostrophic balance between Coriolis and Lorentz forces.