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W. H. Sherry, F. M. Walter (SUNY Stony Brook), S. J. Wolk (Harvard-Smithsonian Center for Astrophysics)
At least 3 of the 4 O stars of the Orion OB1b sub-association, \zeta, \delta, and \sigma Orionis, are associated with clusters of low-mass PMS stars. These clusters appear similar to the Orion nebular cluster (ONC); each consists of several hundred to a few thousand low-mass PMS stars centered on an O star system. These clusters are roughly 4 pc in radius. These PMS clusters in seem to be evolved analogs of the ONC. Like the ONC they should be excellent places to search for very low-mass stars and brown dwarfs because such low mass objects are at their brightest when they are very young.
Recent observational work has shown that the \sigma Orionis cluster is a rich hunting ground for young brown dwarfs. We have used our moderately deep VRI photomery combined with 2MASS JHK photometry to derive mass functions for the \zeta and \delta Orionis clusters and for PMS stars around \epsilon Orionis by fitting our data to the tracks of Baraffe et al. (1998). Our mass function reaches the base of the main sequence where we find several candidate brown dwarf in the \delta Orionis and \zeta Orionis clusters, and around \epsilon Orionis. We expect that these regions, like the \sigma Orionis cluster, will also be fertile hunting grounds for very-low mass stars and brown dwarfs.