Towards Spectral Classification of L and T Dwarfs: Infrared and Optical Spectroscopy and Analysis
Abstract
We present 0.6--2.5 $\mu$m, R \gsim 400 spectra of twenty-seven cool, low luminosity stars and substellar objects. Based on these and previously published spectra we develop a preliminary spectral classification system for L and T dwarfs. For late L and T types the classification system is based entirely on four spectral indices in the 1-2.5 $\mu$m interval. Two of these indices are derived from water absorption bands at 1.15 $\mu$m and 1.4 $\mu$m, the latter of which shows a smooth increase in depth through the L and T sequences and can be used to classify both spectral types. The other two indices make use of methane absorption features in the H and K bands, with the K band index also applicable to mid to late L dwarfs. Continuum indices shortward of 1 $\mu$m used by previous authors to classify L dwarfs are found to be useful only through mid L subclasses. We employ the 1.5 $\mu$m water index and the 2.2 $\mu$m methane index to complete the L classification through L9.5 and to link the new system with a modified version of the 2MASS ``Color-d'' index. By correlating the depths of the methane and water absorption features, we establish a T spectral sequence from types T0 to T8, based on all four indices, which is a smooth continuation of the L sequence. We reclassify two 2MASS L8 dwarfs as L9 and L9.5 and identify one SDSS object as L9. In the proposed system methane absorption appears in the K band approximately at L8, two subclasses earlier than its appearance in the H band. The L and T spectral classes are distinguished by the absence and presence, respectively, of H band methane absorption.Keywords
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