Near-infrared spectral classification of late M and L dwarfs

We present near-infrared (1-2.5 mum), low-resolution spectroscopy of 11 ult racool dwarfs, with spectral types ranging from M7 to L8. Combining our obs ervations with data published by Leggett et al. in 2001, we have measured e quivalent widths for the strongest atomic features and constructed narrowba nd indices to gauge the strength of the strongest molecular features. Those measurements show that the behavior at near-infrared wavelengths is well c orrelated with spectral type, where the latter is defined from observations between 6300 and 10000 Angstrom. In particular, four indices designed to m easure absorption AZ. in the wings of the 1.4 and 1.85 mum steam bands exhi bit a linear, monotonic correlation with spectral type on the Kirkpatrick e t al. system (published in 1999), allowing consistent calibration from both optical and near-infrared spectroscopy.