TESS flares and chromospheric variability in the Pleiades, the Hyades, and Praesepe

Abstract

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. We used 120s cadence 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) in the Pleiades and Praesepe. 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 and light curve amplitude. We calculated the flare energies and investigated the flare frequency distributions. We studied the dynamic spectra of the H$α$ line and its environment to search for indication of CMEs. 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 T$_{eff}$ $\sim$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 used the MDM/OSMOS spectra to place an upper limit on the CME rate.