The magnetic and spin-down properties of slowly rotating fully convective M dwarfs

TL;DR

This study investigates the magnetic field structures and spin-down behaviors of slowly rotating fully convective M dwarfs using Zeeman-Doppler imaging, revealing distinct magnetic properties and potential underestimations in torque calculations.

Contribution

It provides the first detailed analysis of magnetic field geometries in slowly rotating fully convective M dwarfs, highlighting differences from partially convective stars and implications for stellar spin-down models.

Findings

Magnetic field strengths do not follow the same Rossby number scaling as partially convective stars.

Previous spin-down torque estimates may be underestimated by an order of magnitude.

Fully convective and partially convective stars occupy distinct magnetic energy sequences.

Abstract

The evolution of the magnetism, winds and rotation of low-mass stars are all linked. One of the most common ways to probe the magnetic properties of low-mass stars is with the Zeeman-Doppler imaging (ZDI) technique. The magnetic properties of partially convective stars has been relatively well explored with the ZDI technique, but the same is not true of fully convective stars. In this work, we analyse a sample of stars that have been mapped with ZDI. Notably, this sample contains a number of slowly rotating fully convective M dwarfs whose magnetic fields were recently reconstructed with ZDI. We find that the dipolar, quadrupolar and octupolar field strengths of the slowly rotating fully convective stars do not follow the same Rossby number scaling in the unsaturated regime as partially convective stars. Based on these field strengths, we demonstrate that previous estimates of spin-down torques for slowly rotating fully convective stars could have been underestimated by an order of magnitude or more. Additionally, we also find that fully convective and partially convective stars fall into distinct sequences when comparing their poloidal and toroidal magnetic energies.