2004ApJ...607..369H

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 H₂emission lines and to reconstruct the Lyα emission profile incident upon the warm H₂. The excitation of H₂can 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 H₂using 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_☉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.