Near-Infrared Spectral Classification of Late M and L Dwarfs

We present near-infrared (1–2.5 μm), low-resolution spectroscopy of 11 ultracool dwarfs, with spectral types ranging from M7 to L8. Combining our observations with data published by Leggett et al. in 2001, we have measured equivalent widths for the strongest atomic features and constructed narrowband indices to gauge the strength of the strongest molecular features. Those measurements show that the behavior at near-infrared wavelengths is well correlated with spectral type, where the latter is defined from observations between 6300 and 10000 Å. In particular, four indices designed to measure absorption in the wings of the 1.4 and 1.85 μm steam bands exhibit a linear, monotonic correlation with spectral type on the Kirkpatrick et al. system (published in 1999), allowing consistent calibration from both optical and near-infrared spectroscopy.