First Ultraviolet Spectrum of a Brown Dwarf: Evidence for H2 Fluorescence and Accretion

We analyze an HST STIS ultraviolet spectrum of the young brown dwarf 2MASSW J1207334-393254, a member of the 10 million year old TW Hya association that has a planetary-mass companion. We detect and identify numerous emission lines. C IV and other ions are seen that arise in hot gas. We identify a series of lines with Lyman-pumped H₂ molecular lines, indicating that cool gas is also present. Overall, this substellar object shows many of the same characteristics as classical T Tauri stars. We interpret our results as direct evidence of accretion from a circumstellar gas disk, consistent with previous claims. The lack of Si IV emission from the accreting gas indicates that silicon has been depleted into grains.

Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with program No. 9841.