2024A&A...686A..66B

Context. Dynamo models describing the generation of stellar magnetic fields for partly and fully convective stars are guided by observational constraints. Zeeman-Doppler imaging has revealed a variety of magnetic field geometries and, for fully convective stars in particular, a dichotomy: either strong, mostly axisymmetric, and dipole-dominated or weak, non-axisymmetric, and multipole-dominated. This dichotomy is explained either by dynamo bistability (i.e., two coexisting and stable dynamo branches) or by long-term magnetic cycles with polarity reversals, but there is no definite conclusion on the matter.
Aims. Our aim is to monitor the evolution of the large-scale field for a sample of nearby M dwarfs with masses between 0.1 and 0.6 M_⊙, which is of prime interest to inform distinct dynamo theories and explain the variety of magnetic field geometries studied in previous works. This also has the potential to put long-term cyclic variations of the Sun's magnetic field into a broader context.
Methods. We analysed optical spectropolarimetric data sets collected with ESPaDOnS and Narval between 2005 and 2016, and near-infrared SPIRou data obtained between 2019 and 2022 for three well-studied, active M dwarfs: EV Lac, DS Leo, and CN Leo. We looked for secular changes in time series of longitudinal magnetic field, width of unpolarised mean-line profiles, and large-scale field topology as retrieved with principal component analysis and Zeeman-Doppler imaging.
Results. We retrieved pulsating (EV Lac), stable (DS Leo), and sine-like (CN Leo) long-term trends in longitudinal field. The width of near-infrared mean-line profiles exhibits rotational modulation only for DS Leo, whereas in the optical it is evident for both EV Lac and DS Leo. The line width variations are not necessarily correlated to those of the longitudinal field, suggesting complex relations between small- and large-scale field. We also recorded topological changes in the form of a reduced axisymmetry for EV Lac and transition from a toroidal-dominated to poloidal-dominated regime for DS Leo. For CN Leo, the topology remained predominantly poloidal, dipolar, and axisymmetric, with only an oscillation in field strength.
Conclusions. Our results show a peculiar evolution of the magnetic field for each M dwarf individually, with DS Leo and EV Lac manifesting more evident variations than CN Leo. These findings confirm that M dwarfs with distinct masses and rotation periods can undergo magnetic long-term variations and suggest an underlying variety of cyclic behaviours of their magnetic fields.