2025ApJ...993..144A

Despite being only 85 pc away, the open cluster Coma Berenices (Coma Ber) has not been extensively studied. This is due in part to its sparseness and low proper motion, which together made Coma Ber's membership challenging to establish. Gaia data for ≈400 previously cataloged candidate cluster stars allowed us to identify ≈300 as members. With [Fe/H] measurements for nine members, we found that Coma Ber has a solar metallicity, and then fit isochrones to its color-magnitude diagram to determine that it is 675 ± 100 Myr old. With photometry obtained by the Transiting Exoplanet Survey Satellite and Zwicky Transient Facility, we measured rotation periods (P_rot) for 137 of Coma Ber's low-mass stars, increasing the sample of members with measured P_rot by a factor of six, and extending the rotational census for the cluster from its late F stars through to its fully convective M dwarfs. By measuring the equivalent width of the Hα line for ≈250 stars and collecting X-ray detections for ≈100 (≈85% and ≈33% of the cluster's members, respectively), we characterized magnetic activity in Coma Ber and examined the dependence of chromospheric and of coronal activity on rotation in these stars. Despite having a metallicity that is 0.2 dex below that of their coeval cousins in Praesepe and the Hyades, low-mass stars in Coma Ber seem to follow a similar rotation-activity relation. In detail, however, there are differences that may provide further insight into the impact of metallicity on this still poorly understood relation.