Work

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

Year: 2026

Type: article

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 for studying the magnetic activity of cool stars in a controlled astrophysical context. We used photometric observations with a cadence of 120 s 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. We investigated the flaring characteristics of the cluster members using the TESS light curves: we identified the flares and 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 an indication of CMEs. In order to quantify temporal variations in the line strength, we measured the Hα chromospheric activity indices for each spectrum. 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 the flare rate for K-type stars in the Pleiades is higher by an order of magnitude. Our analysis reveals that the flare rate in M stars changes with their effective temperature: the rate is highest for T_ ̊m eff ∼3200-3300,K (which coincides with the transition between partially and fully convective stars), with a smooth decrease in flare rate toward cooler and hotter M dwarfs. We detected no CME signals in the OSMOS spectra, and we used this information to place an upper limit on the CME rate.

Source: Astronomy and Astrophysics

DOI: 10.1051/0004-6361/202661417