Infrared Classification of L and T Dwarfs

We use new and published medium resolution 0.6--2.5 $\mu$m spectra of L and T dwarfs to develop a unified classification system for both of these new spectral classes. Two indices of the system at 1.2 $\mu$m and 1.5 $\mu$m are based on nearby absorption bands of water vapor and two are associated with methane bands near 1.6 $\mu$m and 2.2 $\mu$m. The 1.5 $\mu$m index is monotonic through the L and T sequences, and forms the backbone of the system; the indices for the other bands provide extensive, but only partial, coverage. We correlate the 1.5 $\mu$m index with continuum indices shortward of 1 $\mu$m devised by others for classifying L dwarfs, in order to obtain a tight link between optical and infrared classifications. Our proposed system defines ten spectral subclasses for L (L0--L9) and nine for T (T0--T8). The boundary between L and T is defined to be the onset of absorption by methane in the H band. Methane absorption in the K band near 2.2 $\mu$m is found to begin approximately at L8.