Magnetic geometry of M dwarfs in the southern PLATO field

¹ Laboratoire Univers et Particules de Montpellier, Université de Montpellier, CNRS, LUPM/UMR 5299, 34095 Montpellier, France
² Leiden Observatory, Leiden University, PO Box 9513, 2300 RA, Leiden, The Netherlands
³ Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse, CNRS, IRAP/UMR 5277, 14 Avenue Edouard Belin, 31400 Toulouse, France
⁴ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁵ Tartu Observatory, University of Tartu, Observatooriumi 1, Tõravere 61602, Estonia
⁶ Science Division, Directorate of Science, European Space Research and Technology Centre (ESA/ESTEC), Keplerlaan 1, 2201 AZ, Noordwijk, The Netherlands
⁷ INAF-Catania Astrophysical Observatory, Via S. Sofia 78, I-95127 Catania, Italy

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 16 March 2026
Accepted: 6 April 2026

Abstract

Context. M dwarfs are the most abundant stars in the Galaxy and exhibit diverse magnetic behaviours. While understanding their large-scale magnetic fields is essential for investigating stellar dynamos and assessing the impact of magnetic activity on planetary environments, their magnetic properties and long-term variability remain poorly characterised.

Aims. Our aim was to characterise the large-scale magnetic fields of six M dwarfs in the southern PLATO field, with rotation periods ranging from approximately 1 to 17 days and masses between 0.26 and 0.64 M_⊙. Five of these stars are partially convective, while one is fully convective. These targets extend the mass–rotation diagram into previously unsampled regions.

Methods. We analysed TESS light curves to determine accurate rotation periods and optimise phase coverage for our spectropolarimetric observations. SPIRou data were reduced to obtain least-squares deconvolution (LSD) profiles and longitudinal field measurements, while synthetic spectra fitting yielded small-scale field strengths. We then applied ZDI to reconstruct the large-scale magnetic topologies of the six targets.

Results. We report a wide diversity of magnetic topologies among the six M dwarfs, with three main results: (1) Rapidly rotating (P_rot < 2 d) early M dwarfs can generate dipole-dominated magnetic fields of moderate intensity, similar to those of less massive mid-M dwarfs; (2) Rapidly rotating mid-M dwarfs can generate non-axisymmetric large-scale magnetic fields featuring a significant toroidal component; (3) We report a moderately rotating (P_rot ∼ 17 d) early M dwarf featuring a surprisingly weak large-scale magnetic field.

Conclusions. Our findings further highlight the diversity of magnetic field configurations among M dwarfs, including in previously unexplored regions of parameter space. Long-term monitoring of our sample is crucial in order to distinguish persistent features from variability-driven excursions and to characterise the long-term evolution of their surface magnetic fields. Complementary PLATO photometry, including flare and spot-induced variability analyses, will be essential to link surface activity with magnetic properties.