The Far-Ultraviolet Spectra of TW Hydrae. II. Models of H2 Fluorescence in a Disk

We measure the temperature of warm gas at planet-forming radii in the disk around the classical T Tauri star (CTTS) TW Hya by modeling the H₂ fluorescence observed in Hubble Space Telescope Space Telescope Imaging Spectrograph and Far Ultraviolet Spectroscopic Explorer spectra. Strong Lyα emission irradiates a warm disk surface within 2 AU of the central star and pumps certain excited levels of H₂. We simulate a one-dimensional plane-parallel atmosphere to estimate fluxes for the 140 observed H₂ emission lines and to reconstruct the Lyα emission profile incident upon the warm H₂. The excitation of H₂ can be determined from relative line strengths by measuring self-absorption in lines with low-energy lower levels, or by reconstructing the Lyα profile incident upon the warm H₂ using the total flux from a single upper level and the opacity in the pumping transition. Based on those diagnostics, we estimate that the warm disk surface has a column density of logN(H₂)=18.5^+1.2_-0.8, a temperature T=2500⁺⁷⁰⁰_-500 K, and a filling factor of H₂, as seen by the source of Lyα emission, of 0.25+/-0.08 (all 2 σ error bars). TW Hya produces approximately 10^-3 L_solar in the FUV, about 85% of which is in the Lyα emission line. From the H I absorption observed in the Lyα emission, we infer that dust extinction in our line of sight to TW Hya is negligible.

Based on observations 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 NAS5-26555. This work is also based on data obtained for the FUSE Science Team by the NASA-CNES-CSA FUSE mission, operated by Johns Hopkins University.