The 2MASS Wide-Field T Dwarf Search. IV. Hunting Out T Dwarfs with Methane Imaging

We present first results from a major program of methane filter photometryfor low-mass stars and brown dwarfs. The definition of a new methane filterphotometric system is described. A recipe is provided for the differentialcalibration of methane imaging data using existing 2MASS photometry. We showthat these filters are effective in discriminating T dwarfs from other types ofstars, and demonstrate this with Anglo-Australian Telescope observations usingthe IRIS2 imager. Methane imaging data and proper motions are presented for tenT dwarfs identified as part of the 2MASS "Wide Field T Dwarf Search" -- sevenof them initially identified as T dwarfs using methane imaging. We also present near-infrared moderate resolution spectra for five T dwarfs,newly discovered by this technique. Spectral types obtained from these spectraare compared to those derived from both our methane filter observations, andspectral types derived by other observers. Finally, we suggest a range offuture programs to which these filters are clearly well suited: the winnowingof T dwarf and Y dwarf candidate objects coming from the next generation ofnear-infrared sky surveys; the robust detection of candidate planetary-massbrown dwarfs in clusters; the detection of T dwarf companions to known L and Tdwarfs via deep methane imaging; and the search for rotationally-modulatedtime-variable surface features on cool brown dwarfs.Comment: 20 pages. To appear in The Astronomical Journal, Nov. 200