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 H-2 fluorescence observed in Hubble Space Telescope Space Telescope Imaging Spectrograph and Far Ultraviolet Spectroscopic Explorer spectra. Strong Lyalpha emission irradiates a warm disk surface within 2 AU of the central star and pumps certain excited levels of H-2. We simulate a one-dimensional plane-parallel atmosphere to estimate fluxes for the 140 observed H-2 emission lines and to reconstruct the Lyalpha emission profile incident upon the warm H-2. The excitation of H-2 can be determined from relative line strengths by measuring self-absorption in lines with low-energy lower levels, or by reconstructing the Lyalpha 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 log N(H-2) = 18: 5(-0.8)(+1.2), a temperature T 2500 = 700 +/- 500 K, and a filling factor of H-2, as seen by the source of Lyalpha emission, of 0: 25 +/- 0: 08 (all 2 sigma error bars). TW Hya produces approximately 10(-3) L. in the FUV, about 85% of which is in the Lyalpha emission line. From the H I absorption observed in the Lyalpha emission, we infer that dust extinction in our line of sight to TW Hya is negligible.