An improved red spectrum of the methane or T dwarf SDSS 1624+0029: The role of the alkali metals

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 Digital Sky Survey. Significant flux is detected down to the spectrum's short-wavelength limit of 6200 Angstrom. The spectrum exhibits a broad absorption feature centered at 7700 Angstrom, which we interpret as the K I lambda lambda 7665, 7699 resonance doublet. The observed flux declines shortward of 7000 Angstrom, most likely owing to the red wing of the Na I doublet. Both Cs I doublet lines are detected more strongly than in an earlier red spectrum. Neither Li I absorption nor H alpha emission are detected. An exploratory 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, Marley, & Sharp that strong alkali absorption is principally responsible for depressing 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 source in the atmosphere of the Sloan object. The width of the K I and strengths of the Cs I lines also indicate that the Sloan object is warmer than G1 229B.