The spectra of T dwarfs. I. Near-infrared data and spectral classification

We present near-infrared spectra for a sample of T dwarfs, including 11 new discoveries made using the 2 Micron All Sky Survey. These objects are distinguished from warmer (L-type) brown dwarfs by the presence of methane absorption bands in the 1-5 mum spectral region. A first attempt at a near-infrared classification scheme for T dwarfs is made, based on the strengths of CH4 and H2O bands and the shapes of the 1.25, 1.6, and 2.1 mum flux peaks. Subtypes T1 V-T8 V are defined, and spectral indices useful for classification are presented. The subclasses appear to follow a decreasing T-eff scale, based on the evolution of CH4 and H2O 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 cool dwarfs, due to the settling of dust layers below the photosphere and subsequent rapid evolution of spectral morphology around T-eff similar to 1300-1500K. Similarities in near-infrared colors and continuity of spectral features suggest that the gap between the latest L dwarfs and earliest T dwarfs has been nearly bridged. This argument is strengthened by the possible role of as a minor absorber, shaping the K-band spectra of the latest CH 4 L dwarfs. Finally, we discuss one peculiar T dwarf, 2MASS 0937+2931, which has very blue near-infrared colors (J-K-s = -0.89+/-0.24)due to suppression of the 2.1 mum peak. The feature is likely caused by enhanced collision-induced absorption in a high-pressure or low-metallicity photosphere.