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J. D. Kirkpatrick (IPAC, Caltech)
Before the ``Rare Object'' Key Project Team began its search for brown dwarfs in the 2-Micron All-Sky Survey (2MASS), only six objects were known with spectral types later than M9.5 V, the end of the M-dwarf sequence: GD 165B, Gl 229B, Kelu-1, and three cool objects discovered by the DEep Near-Infrared Survey (DENIS). As a result of follow-up using the Low Resolution Imaging Spectrograph (LRIS) at the 10m W. M. Keck Observatory and the Double Spectrograph at the Palomar 5m, we have identified another 37 such objects selected in the first ~1500 sq.\ deg.\ of actual 2MASS survey data. Specifically, we have searched for objects having no or very faint optical counterparts in the POSS-II survey and having an IR detection in 2MASS with Ks < 15.0 or J < 16.0.
Because the TiO and VO bands which dominate the far-optical and near-infrared portions of late-M spectra disappear in these cooler dwarfs, we have defined a new spectral class ``L''--- spanning roughly 2000-1500K --- where metallic oxides are replaced by metallic hydrides as the major spectroscopic signatures.
At least six of the 2MASS L dwarfs show the 6708-Å\ lithium doublet at low resolution. For objects this cool, the presence of lithium proves that they are substellar. Several other objects appear to have lithium lines at the limit of our detectability (~1 Å\ equivalent width) which if verified means that at least a third of the 2MASS L dwarfs are bona fide brown dwarfs. It is not yet known whether any of the L dwarfs are actually hydrogen burning stars.
All of the 2MASS brown dwarfs discovered so far have J-Ks > 1.30. We have not yet, despite deliberately searching for them, found any brown dwarfs with colors resembling the methane brown dwarf Gl 229B (J-Ks \approx -0.1). Implications for the substellar mass function will be given in the companion talk by Reid, and initial results from trigonometric parallax determinations will be given in the companion talk by Dahn.