We present near-infrared spectra for a sample of T dwarfs , including eleven new discoveries made using the Two 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 \micron spectral region .
A first attempt at a near-infrared classification scheme for T dwarfs is made , based on the strengths 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 spectral indices useful for classification are presented .
The subclasses appear to follow a decreasing T _ { eff } scale , based on the evolution of CH _ { 4 } and H _ { 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 cool dwarfs , due to the settling of dust layers below the photosphere and subsequent rapid evolution of spectral morphology around T _ { eff } \sim 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 CH _ { 4 } 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 \pm 0.24 ) due to suppression of the 2.1 \micron peak .
The feature is likely caused by enhanced collision-induced H _ { 2 } absorption in a high pressure or low metallicity photosphere .