On the magnetic topology of partially and fully convective stars

TL;DR

This study compares magnetic flux measurements in early- and mid-M stars, revealing that fully convective stars have a higher proportion of their magnetic flux detectable via large-scale magnetic field measurements, indicating a change in magnetic topology at the convection boundary.

Contribution

It provides new insights into the magnetic topology differences between partially and fully convective M stars by comparing flux measurements from Stokes V and I.

Findings

Fully convective M stars have a higher fraction of magnetic flux detectable in Stokes V.

Most magnetic flux in M dwarfs is stored in small-scale fields not seen in Stokes V.

A change in magnetic topology occurs at the boundary to full convection.

Abstract

We compare the amount of magnetic flux measured in Stokes V and Stokes I in a sample of early- and mid-M stars around the boundary to full convection (~M3.5). Early-M stars possess a radiative core, mid-M stars are fully convective. While Stokes V is sensitive to the net polarity of magnetic flux arising mainly from large-scale configurations, Stokes I measurements can see the total mean flux. We find that in early-M dwarfs, only ~6% of the total magnetic flux is detected in Stokes V. This ratio is more than twice as large, ~14%, in fully convective mid-M dwarfs. The bulk of the magnetic flux on M-dwarfs is not seen in Stokes V. This is presumably because magnetic flux is mainly stored in small scale components. There is also more to learn about the effect of the weak-field approximation on the accuracy of strong field detections. In our limited sample, we see evidence for a change in magnetic topology at the boundary to full convection. Fully convective stars store a 2-3 times higher fraction of their flux in fields visible to Stokes V. We estimate the total magnetic energy detected in Stokes I and compare it to results from Stokes V. We find that in early-M dwarfs only ~0.5% of the total magnetic energy is detected in Stokes V while this fraction is ~2.5% in mid-M dwarfs.