The spectra of T dwarfs. II. Red optical data

We present 6300-10100 Angstrom spectra for a sample of 13 T dwarfs observed using LRIS mounted on the Keck I 10 m Telescope. A variety of features are identified and analyzed, including pressure-broadened K I and Na I doublets; narrow Cs I and Rb I lines; weak CaH, CrH, and FeH bands; strong H2O absorption; and a possible weak CH4 band. Halpha emission is detected in three of the T dwarfs, strong in the previously reported active T dwarf 2MASS 1237+6526 and weak in SDSS 1254-0122 and 2MASS 1047+2124. None of the T dwarfs exhibit Li I absorption. Guided by the evolution of optical spectral features with near-infrared spectral type, we derive a parallel optical classification scheme, focusing on spectral types T5 to T8, anchored to select spectral standards. We find general agreement between optical and near-infrared types for nearly all the T dwarfs so far observed, including two earlier type T dwarfs, within our classification uncertainties (similar to1 subtype). These results suggest that competing gravity and temperature effects compensate for each other over the 0.6-2.5 mum spectral region. We identify one possible means of disentangling these effects by comparing the strength of the K I red wing to the 9250 Angstrom H2O band. One of our objects, 2MASS 0937+2931, exhibits a peculiar spectrum with a substantial red slope and relatively strong FeH absorption, both consequences of a metal-deficient atmosphere. On the basis of its near-infrared properties and substantial space motion, this object may be a thick disk or halo brown dwarf.