On the Geometry of the Near-Core Magnetic Field in Massive Stars

TL;DR

This study uses 3D MHD simulations to analyze the magnetic field geometry at the near-core boundary of massive stars, revealing a stronger toroidal component than previously assumed, with implications for asteroseismic analysis.

Contribution

It provides new insights into the magnetic field structure at the convective-radiative boundary in massive stars through detailed 3D simulations, challenging prior assumptions.

Findings

Toroidal magnetic field is stronger than poloidal at the boundary.

The shear layer is confined within the buoyancy frequency peak.

Results have implications for asteroseismic inference of stellar rotation and magnetism.

Abstract

It is well-known that the cores of massive stars sustain a stellar dynamo with a complex magnetic field configuration. However, the same cannot be said for the field's strength and geometry at the convective-radiative boundary, which are crucial when performing asteroseismic inference. In this Letter, we present three-dimensional (3D) magnetohydrodynamic (MHD) simulations of a 7 solar mass mid-main sequence star, with particular attention given to the convective-radiative boundary in the near-core region. Our simulations reveal that the toroidal magnetic field is significantly stronger than the poloidal field in this region, contrary to recent assumptions. Moreover, the rotational shear layer, also important for asteroseismic inference, is specifically confined within the extent of the buoyancy frequency peak. These results, which are based on the inferred properties of HD 43317, have widespread implications for asteroseismic studies of rotation, mixing and magnetism in stars. While we expect our results to be broadly applicable across stars with similar buoyancy frequency profiles and stellar masses, we also expect the MHD parameters and the initial stellar rotation rate to impact the geometry of the field and differential rotation at the convective-radiative interface.