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.

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