Speaker
Description
Magnetic activity in cool stars is closely linked to stellar rotation and dynamo processes. Open clusters offer a controlled environment for investigating stellar activity because their members share ages, distances, and chemical compositions. We investigate how stellar age and effective temperature influence flare activity in cool stars, and search for signatures of coronal mass ejections (CMEs) in young and intermediate-age cluster stars. We analyze 120-s cadence TESS photometry of members of the Pleiades (100 Myr), Hyades (700 Myr), and Praesepe (700 Myr). We identify and characterize flares, calculate their energies, and derive flare frequency distributions for different stellar temperature and activity groups. We also analyze time-resolved hydrogen alpha spectra of Pleiades and Praesepe stars obtained with OSMOS at MDM Observatory to search for CME signatures and measure chromospheric activity. The flare frequency distributions have power-law indices of 1.8–2.8. The Pleiades exhibit the highest flare rates across the investigated spectral types. K dwarfs in the Pleiades flare about an order of magnitude more frequently than those in the Hyades, while M-dwarf flare rates are comparable between the two clusters. The M-dwarf flare rate peaks at ~3200–3300 K effective surface temperature, near the transition between partially and fully convective stars. No CME signatures are detected in the OSMOS spectra. Our main conclusions are the following: Stellar age and effective temperature strongly influence flare activity, with particularly enhanced activity in young K dwarfs and near the fully convective boundary in M dwarfs. The absence of detected CMEs provides an upper limit on their occurrence rates in these clusters. These results provide observational constraints on the evolution of magnetic activity and flaring in cool stars and offer insights into how stellar structure and age shape magnetic phenomena that may influence the environments of orbiting planets.