An Improved Red Spectrum of the Methane or T-dwarf Sdss 1624+0029: Role of the Alkali Metals

A Keck II low resolution spectrum shortward of one micron 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 6200Å. The spectrum exhibits a broad absorption feature centered at 7700Å, which we interpret as the K I 7665/7699 resonance doublet. The observed flux declines shortward of 7000Å, due most likely 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 Hα 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 and Sharp that strong alkali absorption is principally responsible for depressing T dwarf spectra shortward of 1μm. In particular, there seems no compelling reason at this time to introduce dust or an additional opacity source in the atmosphere of the SDSS object. The width of the K I and strengths of the Cs I lines also indicate that the Sloan object is warmer than Gl 229B. Subject headings: stars: individual (SDSS1624+0029 – stars: brown dwarfs – stars: atmospheres Department of Astronomy and Steward Observatory, The University of Arizona, Tucson AZ 85721, [email protected], [email protected] Department of Physics and Astronomy, University of Pennsylvania, 209 South 33rd St., Philadelphia, PA 19104-6396, [email protected] Division of Physics, M/S 103-33, California Institute of Technology, Pasadena CA 91125, [email protected] Infrared Processing and Analysis Center, California Institute of Technology, Pasadena, CA 91125, [email protected], [email protected]