J. Liebert et al., An improved red spectrum of the methane or T dwarf SDSS 1624+0029: The role of the alkali metals, ASTROPHYS J, 533(2), 2000, pp. L155-L158
A Keek II low-resolution spectrum shortward of 1 mu m is presented for SDSS
1624+0029, the first field methane or T dwarf discovered in the Sloan Digi
tal Sky Survey. Significant flux is detected down to the spectrum's short-w
avelength limit of 6200 Angstrom. The spectrum exhibits a broad absorption
feature centered at 7700 Angstrom, which we interpret as the K I lambda lam
bda 7665, 7699 resonance doublet. The observed flux declines shortward of 7
000 Angstrom, most likely owing to the red wing of the Na I doublet. Both C
s I doublet lines are detected more strongly than in an earlier red spectru
m. Neither Li I absorption nor H alpha emission are detected. An explorator
y model fit to the spectrum suggests that the shape of the red spectrum can
be primarily accounted for by the broad wings of the K I and Na I doublets
. This behavior is consistent with the argument proffered by Burrows, Marle
y, & Sharp that strong alkali absorption is principally responsible for dep
ressing T dwarf spectra shortward of 1 mu m. In particular, there seems no
compelling reason at this time to introduce dust or an additional opacity s
ource in the atmosphere of the Sloan object. The width of the K I and stren
gths of the Cs I lines also indicate that the Sloan object is warmer than G
1 229B.