FIRST RESULTS FROM THE SPACE TELESCOPE IMAGING SPECTROGRAPH - OPTICAL-SPECTRA OF GLIESE 229B

We report the first Hubble Space Telescope Space Telescope Imaging Spe ctrograph (STIS) CCD spectroscopy of the bona fide brown dwarf Gliese 229B. The optical spectrum shows absorptions of Cs I at 8944 Angstrom and water vapor bands at 9300-9600 Angstrom. Strong CaH, FeH, TiO, and VO bands observed in late M dwarfs are absent from the spectrum of Gl iese 229B. The formation of dust grains may explain the absence of str ong atomic lines and molecular bands of these refractory elements. The broad spectral coverage obtained helps resolve current speculations a bout the presence of dust clouds in the atmosphere of cool brown dwarf s. We find the slope of the STIS/CCD spectrum and the lack of flux det ected shortward of 8000 Angstrom strongly supports the presence of dus t hazes suspended in the photosphere of GI 229B rather than a complete settling of the grains to regions below the photosphere.