Large-scale magnetic topologies of early M dwarfs

TL;DR

This study investigates the magnetic field topologies of early M dwarfs (spectral types M0-M3) using spectropolarimetry, revealing a significant change in magnetic configurations at spectral type M3 linked to stellar internal structure.

Contribution

It provides new observational insights into how magnetic topologies differ across M dwarf spectral types, especially around the full convection threshold at M3.

Findings

Early M dwarfs host large-scale magnetic fields that are mostly toroidal and non-axisymmetric.

A sharp change in magnetic topology occurs at spectral type M3, with later types showing more poloidal, axisymmetric fields.

The magnetic topology change is not reflected in X-ray luminosities, indicating different dynamo efficiencies.

Abstract

We present here additional results of a spectropolarimetric survey of a small sample of stars ranging from spectral type M0 to M8 aimed at investigating observationally how dynamo processes operate in stars on both sides of the full convection threshold (spectral type M4). The present paper focuses on early M stars (M0--M3), i.e. above the full convection threshold. Applying tomographic imaging techniques to time series of rotationally modulated circularly polarised profiles collected with the NARVAL spectropolarimeter, we determine the rotation period and reconstruct the large-scale magnetic topologies of 6 early M dwarfs. We find that early-M stars preferentially host large-scale fields with dominantly toroidal and non-axisymmetric poloidal configurations, along with significant differential rotation (and long-term variability); only the lowest-mass star of our subsample is found to host an almost fully poloidal, mainly axisymmetric large-scale field ressembling those found in mid-M dwarfs. This abrupt change in the large-scale magnetic topologies of M dwarfs (occuring at spectral type M3) has no related signature on X-ray luminosities (measuring the total amount of magnetic flux); it thus suggests that underlying dynamo processes become more efficient at producing large-scale fields (despite producing the same flux) at spectral types later than M3. We suspect that this change relates to the rapid decrease in the radiative cores of low-mass stars and to the simultaneous sharp increase of the convective turnover times (with decreasing stellar mass) that models predict to occur at M3; it may also be (at least partly) responsible for the reduced magnetic braking reported for fully-convective stars.