Exploring stellar activity in a sample of active M dwarfs

¹ Astronomy & Astrophysics Division, Physical Research Laboratory, Navrangapura, Ahmedabad 380009, India
² I. Physikalisches Institut, Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany
³ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
⁴ Instituto de Astronomía, Universidad Católica del Norte, Av. Angamos 0610, Antofagasta, Chile,

^★ Corresponding authors: arvindr@prl.res.in; jose.fernandez@ucn.cl

Received: 28 March 2025
Accepted: 6 October 2025

Abstract

Context. Most M dwarfs present high chromospheric activity that can exceed the solar magnetic activity. This can substantially influence planetary, atmospheric, and biological processes and might affect the habitability of orbiting planets. It is therefore important to characterize the magnetic activity of M dwarfs to understand the physical mechanisms that cause it. M dwarfs are the primary targets in the search for exoplanets within the habitable zone.

Aims. We characterize the stellar activity of active M dwarfs by understanding the relations between magnetic activities, stellar parameters, and flare properties.

Methods. We analyzed TESS photometric data combined with spectroscopic observations of active M dwarfs. We examined the relation between the flare occurrence rate, flare energies, rotation period, filling factor, and chromospheric activity indicators. Furthermore, we investigated the correlation between the flare amplitude and duration and the cumulative flare energy frequency distributions to probe the underlying mechanisms driving magnetic activity in flaring M dwarfs.

Results. The distribution of the flare occurrence rate is flat for spectral types M0–M4 (T_eff ~ 3900–3200 K) and declines for later types. The flare occurrence rate and flare activity of faster rotators with P_rot < 1 day is higher. M dwarfs with a higher flare occurrence rate tend to exhibit lower flare amplitudes, which indicates that the frequent flares in these M dwarfs are generally less energetic. Within the mass range of 0.15–0.76 M_⊙, the median of L_Hα/L_bol in evenly divided mass bins of ~0.1 M_⊙ varies by a factor of ~2.5, while ΔEW decreases by 92% across the sample. We derived power-law indexes of the cumulative flare frequency distribution for M dwarf subgroups that indicated a decreasing trend from M0 to M5 dwarfs with a value of α from 1.68 to 1.95, respectively.

Conclusions. We characterized the stellar activity in M dwarfs through chromospheric indicators such as the Hα emission, the starspot coverage, the flare occurrence rates, flare energies, and flare duration. Our results suggest a stellar activity transition near M4. Stronger Hα emission is linked to a higher flare occurrence. The flare occurrence rate of rapid rotators (P_rot < 1 day) is significantly higher, which supports the idea that strong magnetic dynamos in fast-rotating M dwarfs sustain a frequent flaring activity. Our analysis confirms that highly active stars dissipate magnetic energy through numerous low-energy flares and not through a few high-energy events. We also show that chromospheric activity and flare activity follow a power-law relation.