2023MNRAS.524.6085K

The time-scales of variability in active M dwarfs can be related to their various physical parameters. Thus, it is important to understand such variability to decipher the physics of these objects. In this study, we have performed the low resolution (∼5.7 Å) spectroscopic monitoring of 83 M dwarfs (M0-M6.5) to study the variability of H α/H β emissions; over the time-scales from ∼0.7 to 2.3 h with a cadence of ∼3-10 min. Data of a sample of another 43 late-type M dwarfs (M3.5-M8.5) from the literature are also included to explore the entire spectral sequence. 53 of the objects in our sample (∼64 per cent) show statistically significant short-term variability in H α. We show that this variability in 38 of them are most likely to be related to the flaring events. We find that the early M dwarfs are less variable despite showing higher activity strengths (L_H α_/Lbol_ and L_H β_/Lbol_), which saturates around ∼10^–3.8 for M0-M4 types. Using archival photometric light curves from TESS and Kepler/K2 missions, the derived chromospheric emission (H α and H β emission) variability is then explored for any plausible systematics with respect to their rotation phase. The variability indicators clearly show higher variability in late-type M dwarfs (M5-M8.5) with shorter rotation periods (<2 d). For 44 sources, their age has been estimated using StarHorse project and possible correlations with variability have been explored. The possible causes and implications for these behaviours are discussed.