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 degrees radius around the cluster, well beyond the presumed tidal radius, to a limiting magnitude of R similar to 20, corresponding to similar to0.07 M-. 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-. mass range using the distribution of H alpha 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 similar to1 M-.. The cluster contains a total mass similar to 800 M-.. 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 similar to0.5 M-., relative to stars with mass similar to1-0.5 M-., 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 similar to0.1 M-.. 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.