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D. A. Golimowski (Johns Hopkins University), SDSS Collaboration
Since the inceptions of the DEep Near-Infrared Survey (DENIS), the Two Micron All Sky Survey (2MASS), and the Sloan Digital Sky Survey (SDSS), the known population of dwarfs with spectral types later than M has increased from 2 to over 100. These very cool dwarfs fall into two categories: ``L dwarfs,'' whose near-IR spectra feature carbon exclusively in the form of CO, and even cooler ``T dwarfs,'' whose near-IR spectra show carbon predominantly in CH4. Although L and T dwarfs are intrinsically very faint and very red at optical wavelengths, the excellent sensitivity and color sampling of SDSS have made this survey a powerful tool for identifying in large numbers these formerly elusive substellar objects. In early 1999, SDSS commissioning data revealed the first methane (or T) brown dwarfs known since the discovery of Gliese 229B in 1995, and the first two such objects found isolated in interstellar space. Dozens of L dwarfs have also been discovered from SDSS image and spectroscopic data since commissioning began. In mid-2000, SDSS again broke new ground by producing the first three examples of the ``missing link'' between the CO-dominated L dwarfs and the CH4-dominated T dwarfs. These transition objects thus completed a continuous spectral sequence from the hottest O stars (Teff ~ 40,000 K) to the coolest known brown dwarfs (Teff ~ 750 K).
Preliminary estimates of the space density of SDSS L and T dwarfs show that these objects are at least as abundant as stars in the solar neighborhood (~ 0.10 pc-3). The discoveries of large, nearby populations of L and T dwarfs by SDSS and other sky surveys have spawned feverish efforts to understand not only their atmospheres and interiors, their formation mechanisms, and their mass functions, but also their contributions to the dynamics of solar neighborhood and the Galaxy at large.