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I.S. McLean, L. Prato, S.S. Kim (UCLA), A.J. Burgasser (Caltech), J.D. Kirkpatrick (IPAC/Caltech)
Since commissioning of the Near-Infrared Spectrometer (NIRSPEC) on the Keck II 10-m Telescope in Hawaii in April 1999 (McLean et al. 2000, ApJ 533, L45), we have been undertaking a large infrared spectroscopic survey of low-mass and sub-stellar mass objects. Most of the sources are selected from the 2MASS. The goals of the survey are as follows. A complete sample of all spectral types at low resolution (R~2500) will be obtained to extend spectral classification schemes to near-infrared wavebands and to give the overall spectral energy distribution of all L- and T-dwarf classes for comparison with models. In addition, we will obtain higher resolution spectra (R~25,000) of a sub-sample of the survey for detailed comparison with models and to search for radial velocity variations. To initiate the program, attention was focussed on the J-band since this region contains several useful atomic and molecular diagnostics. After two objects per sub-class had been observed in the J-band at low resolution and several of the brighter sources had been observed at high resolution in the same band, the program was extended to obtain the complete flux-calibrated spectrum from 0.95-2.5 microns of selected sources. The regime from 0.95-1.0 microns provides an overlap with Keck LRIS spectra from 0.5-1.0 microns already obtained. Over 20 L-dwarfs and 10 T-dwarfs have been observed to date. Several easily measured flux ratios have been identified which enable spectral classification to be based solely on infrared spectra. For example, the strength of the steam band at 1.34 microns to the continuum at 1.29 microns appears to change smoothly and continuously from mid-M all the way through the L-dwarfs and into the T-dwarf range. We also report the first high-resolution infrared spectra and discuss searches for velocity variations at the 1-2 km/s level.