Near-Infrared Spectral Classification of Late M and L Dwarfs

We present near-infrared (1 to 2.5 micron) low-resolution spectroscopy ofeleven ultracool dwarfs with spectral types ranging from M7 to L8. Combiningour observations with data published previously by Leggett et al. (2001), wehave measured equivalent widths for the strongest atomic features andconstructed narrowband indices to gauge the strength of the strongest molecularfeatures. Those measurements show that the behaviourr at near-infraredwavelengths is well correlated with spectral type, where the latter is definedfrom observations between 6300 and 10000 Angstroms. In particular, four indicesdesigned to measure absorption in the wings of the 1.4 and 1.85 micron steambands exhibit a linear, monotonic correlation with spectral type on theKirkpatrick et al. (1999) system, allowing consistent calibration from bothoptical and near-infrared spectroscopy.Comment: Accepted for the Astronomical Journal, March 200