Meeting the Cool Neighbors. V. A 2MASS-Selected Sample of Ultracool Dwarfs

Abstract

We present initial results of our effort to create a statistically robust,\nvolume-limited sample of ultracool dwarfs from the 2MASS Second Incremental\nData Release. We are engaged in a multifaceted search for nearby late-type\ndwarfs and this is the first installment of our search using purely photometric\nselection. The goal of this work is a determination of the low-mass star and\nbrown dwarf luminosity function in the infrared. Here, we outline the\nconstruction of the sample, dubbed 2MU2, and present our first results,\nincluding the discovery of 186 M7--L6 dwarfs--47 of these are likely to be\nwithin 20 pc of the Sun. These results represent 66% of the ultracool\ncandidates in our sample yet constitute an 127% increase in the number of\nultracool dwarfs known within the volume searched (covering 40% of the sky out\nto 20 pc). In addition, we have identified 10 M4--M6.5 objects that are likely\nto be within 20 pc (or within 1$\\sigma$). Finally, based on these initial data,\nwe present a preliminary luminosity function and discuss several interesting\nfeatures of the partial sample presented here. Once our sample is complete, we\nwill use our measured luminosity function to constrain the mass function of\nlow-mass stars and brown dwarfs.\n