The Mass and Structure of the Pleiades Star Cluster from 2MASS

We present the results of a large-scale search for new members of the Pleiades star cluster using 2MASS near-infrared photometry and proper motions derived from POSS plates digitized by the USNO PMM program. The search extends to a 10° radius around the cluster, well beyond the presumed tidal radius, to a limiting magnitude of R~20, corresponding to ~0.07 M_solar at the distance and age of the Pleiades. Multiobject spectroscopy for 528 candidates verifies that the search was extremely effective at detecting cluster stars in the 1-0.1 M_solar mass range using the distribution of Hα emission strengths as an estimate of sample contamination by field stars. When combined with previously identified, higher mass stars, this search provides a sensitive measurement of the stellar mass function and dynamical structure of the Pleiades. The degree of tidal elongation of the halo agrees well with current N-body simulation results. Tidal truncation affects masses below ~1 M_solar. The cluster contains a total mass ~800 M_solar. Evidence for a flatter mass function in the core than in the halo indicates the depletion of stars in the core with mass less than ~0.5 M_solar, relative to stars with mass ~1-0.5 M_solar, and implies a preference for very low-mass objects to populate the halo or escape. The overall mass function is best fitted with a lognormal form that becomes flat at ~0.1 M_solar. Whether sufficient dynamical evaporation has occurred to detectably flatten the initial mass function, via preferential escape of very low-mass stars and brown dwarfs, is undetermined, pending better membership information for stars at large radial distances.