The Spectra of T Dwarfs. I. Near‐Infrared Data and Spectral Classification

We present near-infrared spectra for a sample of T dwarfs, including elevennew discoveries made using the Two Micron All Sky Survey. These objects aredistinguished from warmer (L-type) brown dwarfs by the presence of methaneabsorption bands in the 1--2.5 $\micron$ spectral region. A first attempt at anear-infrared classification scheme for T dwarfs is made, based on thestrengths of CH$_4$ and H$_2$O bands and the shapes of the 1.25, 1.6, and 2.1$\micron$ flux peaks. Subtypes T1 V through T8 V are defined, and spectralindices useful for classification are presented. The subclasses appear tofollow a decreasing T$_{eff}$ scale, based on the evolution of CH$_4$ andH$_2$O bands and the properties of L and T dwarfs with known distances.However, we speculate that this scale is not linear with spectral type for cooldwarfs, due to the settling of dust layers below the photosphere and subsequentrapid evolution of spectral morphology around T$_{eff}$ $\sim$ 1300--1500 K.Similarities in near-infrared colors and continuity of spectral featuressuggest that the gap between the latest L dwarfs and earliest T dwarfs has beennearly bridged. This argument is strengthened by the possible role of CH$_4$ asa minor absorber shaping the K-band spectra of the latest L dwarfs. Finally, wediscuss one peculiar T dwarf, 2MASS 0937+2931, which has very bluenear-infrared colors (J-K$_s$ = $-0.89\pm$0.24) due to suppression of the 2.1$\micron$ peak. The feature is likely caused by enhanced collision-inducedH$_2$ absorption in a high pressure or low metallicity photosphere.