Flares in open clusters with K2: I. M 45 (Pleiades), M 44 (Praesepe), and M 67

Context. The presence and strength of a stellar magnetic field and activity is rooted in a star’s fundamental parameters such as mass and age. Can flares serve as an accurate stellar “clock”?Aims. To explore if we can quantify an activity-age relation in the form of a flaring-age relation, we measured trends in the flaring rates and energies for stars with different masses and ages.Methods. We investigated the time-domain photometry provided by Kepler’s follow-up mission K2 and searched for flares in three solar metallicity open clusters with well-known ages, M 45 (0.125 Gyr), M 44 (0.63 Gyr), and M 67 (4.3 Gyr). We updated and employed the automated flare finding and analysis pipeline Appaloosa, originally designed for Kepler . We introduced a synthetic flare injection and recovery sub-routine to ascribe detection and energy recovery rates for flares in a broad energy range for each light curve.Results. We collect a sample of 1761 stars, mostly late-K to mid-M dwarfs and found 751 flare candidates with energies ranging from 4 × 1032 erg to 6 × 1034 erg, of which 596 belong to M 45, 155 to M 44, and none to M 67. We find that flaring activity depends both on T eff , and age. But all flare frequency distributions have similar slopes with α  ≈ 2.0−2.4, supporting a universal flare generation process. We discuss implications for the physical conditions under which flares occur, and how the sample’s metallicity and multiplicity affect our results.