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Session 55 - New Digital Sky Surveys.
Display session, Wednesday, June 10
Atlas Ballroom,
Before the 2-Micron All-Sky Survey (2MASS) began, 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). In the first 421 sq.\ deg.\ of actual 2MASS survey data, we have identified another twenty such objects spectroscopically confirmed using the Low Resolution Imaging Spectrograph (LRIS) at the W. M. Keck Observatory. Specifically, we searched for 2MASS objects having no optical counterparts in the POSS-I survey, an IR detection with K_s < 14.5, and colors of either J-K_s > 1.30 (like GD 165B) or J-K_s < 0.4 (like Gl 229B). At least six of these new ``post-M'' dwarfs show the 6708-Å\ lithium doublet at low resolution. For objects this cool, the presence of lithium proves that they are substellar. Another four objects appear to have lithium lines at the limit of our detectability (\sim1 Å\ equivalent width) which if verified means that at least half of the 2MASS ``post-M'' objects are bona fide brown dwarfs. 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 will define a new spectral class --- spanning roughly 2000-1500K --- where metallic oxides are replaced by metallic hydrides as the major spectroscopic signatures. These very cool objects will most likely be christened as either spectral class ``H'', ``L'', ``T'', or ``Y''. The choice of letter will be made in advance of the meeting after a thorough discussion with experts on the MK classification system. All of the 2MASS brown dwarfs discovered so far have J-K_s > 1.30. We have not yet, despite deliberately searching for them, found any brown dwarfs with colors resembling the methane brown dwarf Gl 229B. A discussion of the lithium signatures and implications for the substellar mass function will be given in a companion paper by Reid et al.
