The Stellar Populations of Praesepe and Coma Berenices

Abstract

We present the results of a stellar membership survey of the nearby open clusters Praesepe and Coma Berenices. We have combined archival survey data from the SDSS, 2MASS, USNOB1.0, and UCAC-2.0 surveys to compile proper motions and photometry for ~5 million sources over 300 deg^2. Of these sources, 1010 stars in Praesepe and 98 stars in Coma Ber are identified as candidate members with probability >80%; 442 and 61 are identified as high-probability candidates for the first time. We estimate that this survey is >90% complete across a wide range of spectral types (F0 to M5 in Praesepe, F5 to M6 in Coma Ber). We have also investigated the stellar mass dependence of each cluster's mass and radius in order to quantify the role of mass segregation and tidal stripping in shaping the present-day mass function and spatial distribution of stars. Praesepe shows clear evidence of mass segregation across the full stellar mass range; Coma Ber does not show any clear trend, but low number statistics would mask a trend of the same magnitude as in Praesepe. The mass function for Praesepe (t~600 Myr; M~500 Msun) follows a power law consistent with that of the field present-day mass function, suggesting that any mass-dependent tidal stripping could have removed only the lowest-mass members (<0.15 Msun). Coma Ber, which is younger but much less massive (t~400 Myr; M~100 Msun), follows a significantly shallower power law. This suggests that some tidal stripping has occurred, but the low-mass stellar population has not been strongly depleted down to the survey completeness limit (~0.12 Msun).