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K. Noll (STScI), M. Marley (NMSU), R. Freedman (NASA/Ames), S. Leggett (JAC)
Using the CGS4 Spectrometer on the United Kingdom Infrared Telescope (UKIRT) on Mauna Kea, we have measured the spectra of four brown dwarfs, Kelu 1 (Ruiz et al. 1997), and DENIS-P~J1228.2-1547, DENIS-P~J1058.7-1548, and DENIS-P~J0205.4-1159 (Delfosse et al. 1997) in the previously unobserved interval between 3 and 4 microns. We have also measured the spectra of several late M dwarfs. Our primary motive for these observations was to search for the strong methane fundamental at 3.3 \mum. Because methane is predicted to become thermochemically favored only at temperatures below about 1600~K (Fegley and Lodders, 1997) and because any object this cool is almost certainly substellar, the presence of methane is a powerful proxy indicator of brown dwarf status. None of the objects measured show obvious signs of CH4 absorption similar to that seen in Gl~229B and the recently detected field T-dwarfs suggesting that they have effective temperatures somewhat higher than currently estimated. We have also searched for HDO, H2S and HCl in the spectra and will report on the results of this search.