Large-scale magnetic topologies of mid-M dwarfs

TL;DR

This study uses spectropolarimetric techniques to analyze magnetic field structures in a small sample of active mid-M dwarfs, revealing predominantly axisymmetric poloidal fields and their stability over a year.

Contribution

It provides new observational constraints on magnetic topologies and stability in fully-convective stars near the M4 spectral type, advancing understanding of stellar dynamos.

Findings

All stars host mainly axisymmetric large-scale poloidal fields.

Magnetic topologies are stable over approximately one year.

Accurate rotational periods enable future studies on rotation-magnetic field relationships.

Abstract

We present in this paper the first results of a spectropolarimetric analysis of a small sample (~ 20) of active stars ranging from spectral type M0 to M8, which are either fully-convective or possess a very small radiative core. This study aims at providing new constraints on dynamo processes in fully-convective stars. The present paper focuses on 5 stars of spectral type ~M4, i.e. with masses close to the full convection threshold (~ 0.35 Msun), and with short rotational periods. Tomographic imaging techniques allow us to reconstruct the surface magnetic topologies from the rotationally modulated time-series of circularly polarised profiles. We fnd that all stars host mainly axisymmetric large-scale poloidal fields. Three stars were observed at two different epochs separated by ~1 yr; we find the magnetic topologies to be globally stable on this timescale. We also provide an accurate estimation of the rotational period of all stars, thus allowing us to start studying how rotation impacts the large-scale magnetic field.