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The Spectra of T Dwarfs. II. Red Optical Data
We present 6300-10100 Å spectra for a sample of 13 T dwarfsobserved using LRIS mountedtpdel 99 on the Keck I 10 m Telescope. Avariety of features are identified and analyzed, includingpressure-broadened K I and Na I doublets; narrow Cs I and Rb I lines;weak CaH, CrH, and FeH bands; strong H2O absorption; and apossible weak CH4 band. Hα emission is detected inthree of the T dwarfs, strong in the previously reported active T dwarf2MASS 1237+6526 and weak in SDSS 1254-0122 and 2MASS 1047+2124. None ofthe T dwarfs exhibit Li I absorption. Guided by the evolution of opticalspectral features with near-infrared spectral type, we derive a paralleloptical classification scheme, focusing on spectral types T5 to T8,anchored to select spectral standards. We find general agreement betweenoptical and near-infrared types for nearly all the T dwarfs so farobserved, including two earlier type T dwarfs, within our classificationuncertainties (~1 subtype). These results suggest that competing gravityand temperature effects compensate for each other over the 0.6-2.5 μmspectral region. We identify one possible means of disentangling theseeffects by comparing the strength of the K I red wing to the 9250Å H2O band. One of our objects, 2MASS 0937+2931,exhibits a peculiar spectrum with a substantial red slope and relativelystrong FeH absorption, both consequences of a metal-deficientatmosphere. On the basis of its near-infrared properties and substantialspace motion, this object may be a thick disk or halo brown dwarf.
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