Magnetic geometry of M dwarfs in the southern PLATO field

Abstract

M dwarfs are the most abundant stars in the Galaxy and exhibit diverse magnetic behaviours. Understanding their large-scale magnetic fields is essential to study stellar dynamos and assess the impact of magnetic activity on planetary environments, yet their magnetic properties and long-term variability remain poorly characterised. We aim to characterise the large-scale magnetic fields of 6 M dwarfs in the southern PLATO field, with rotation periods from ~1 to 17 days and masses between 0.26 and 0.64 Msun. Five stars are partially convective, one fully convective, extending the mass-rotation diagram to previously unsampled regions. We analysed TESS light curves to determine accurate rotation periods and optimise phase coverage for spectropolarimetric observations. SPIRou data were reduced to obtain LSD profiles and longitudinal field measurements, while synthetic spectra fitting yielded small-scale field strengths. ZDI was applied to reconstruct large-scale magnetic topologies. We report a wide diversity of magnetic topologies among the 6 M dwarfs, with 3 main results: (1) Rapidly-rotating (Prot < 2 d) early M dwarfs can generate dipole-dominated fields of moderate intensity, similar to less massive mid-M dwarfs; (2) rapidly-rotating mid-M dwarfs can generate non-axisymmetric large-scale fields with a significant toroidal component; (3) a moderately-rotating (Prot ~ 17 d) early M dwarf shows a surprisingly weak large-scale field. Our findings highlight the diversity of magnetic configurations, including in previously unexplored regions. Long-term monitoring is crucial to distinguish persistent features from variability-driven excursions and to characterise the evolution of surface magnetic fields. Complementary PLATO photometry, including flare and spot-induced variability analyses, will be essential to link surface activity with magnetic properties.

Accepted in Astronomy & Astrophysics. 13 pages, 10 figures, 3 tables, 9 appendices. v2: corrected ncyc values in Table E1 (Appendix E)

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