Context. Magnetic activity is a result of the interaction between rotation and dynamo processes in cool stars with outer convection zones. Since open cluster members share a common age, distance, and chemical composition, they provide an opportunity to study the magnetic activity of cool stars in a controlled astrophysical context.
Aims. We used 120-s cadence photometric observations from the Transiting Exoplanet Survey Satellite (TESS) to study the flare activity of stars in three open clusters with different ages: the Pleiades (100 Myr), the Hyades (700 Myr) and Praesepe (700 Myr). We used the Ohio State Multi-Object Spectrograph (OSMOS) at MDM Observatory with the goal of identifying coronal mass ejections (CMEs) on stars in the Pleiades and Praesepe.
Methods. We investigated the flaring characteristics of the cluster members using the TESS light curves: we identified the flares, then divided the dataset into different categories based on the effective temperature of the flaring stars and the amplitude of their light curves after flare removal. We calculated the flare energies and investigated the flare frequency distributions of each previously defined group. We studied the dynamic spectra of the Hα line and its environment for the stars of the Pleiades and Praesepe to search for indication of CMEs. In order to quantify temporal variations in the line strength we measured the Hα chromospheric activity indices for each spectrum.
Results. The power-law indices of the flare frequency distributions obtained in our analysis span the 1.8−2.8 range. We found the highest flare rates in the Pleiades for all investigated spectral types. Our results show that the flare rate of M dwarfs is similar in the Pleiades and the Hyades while for K-type stars the flare rate in the Pleiades is an order of magnitude higher. Our analysis reveals that the flare rate in M stars changes with their effective temperature: the rate is highest for Teff ∼3200−3300 K —coinciding with the transition between partially and fully convective stars— with smooth decrease in flare rate towards cooler and hotter M dwarfs. We detected no CME signals in the MDM/OSMOS spectra, and we used this information to place an upper limit on the CME rate.