Toward Spectral Classification of L and T Dwarfs: Infrared and Optical Spectroscopy and Analysis

We present 0.6-2.5um, R~400 spectra of twenty-seven cool, low luminositystars and substellar objects. Based on these and previously published spectrawe develop a preliminary spectral classification system for L and T dwarfs. Forlate L and T types the classification system is based entirely on four spectralindices in the 1-2.5um interval. Two of these indices are derived from waterabsorption bands at 1.15um and 1.4um, the latter of which shows a smoothincrease in depth through the L and T sequences and can be used to classifyboth spectral types. The other two indices make use of methane absorptionfeatures in the H and K bands, with the K band index also applicable to mid tolate L dwarfs. Continuum indices shortward of 1um used by previous authors toclassify L dwarfs are found to be useful only through mid L subclasses. Weemploy the 1.5um water index and the 2.2um methane index to complete the Lclassification through L9.5 and to link the new system with a modified versionof the 2MASS ``Color-d'' index. By correlating the depths of the methane andwater absorption features, we establish a T spectral sequence from types T0 toT8, based on all four indices, which is a smooth continuation of the Lsequence. We reclassify two 2MASS L8 dwarfs as L9 and L9.5 and identify oneSDSS object as L9. In the proposed system methane absorption appears in the Kband approximately at L8, two subclasses earlier than its appearance in the Hband. The L and T spectral classes are distinguished by the absence andpresence, respectively, of H band methane absorption.Comment: 40 pages, 14 figures, to be published in Ap.J., Jan 1, 200