2MASSI J0059538-003730 , the SIMBAD biblio

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 1-2.5 µm 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 µm 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∼1300-1500 K. 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 CH₄ as a minor absorber, shaping the K-band spectra of the latest 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 µm peak. The feature is likely caused by enhanced collision-induced H₂ absorption in a high-pressure or low-metallicity photosphere.