Line emission from gas in optically thick dust disks around young stars

We present self-consistent models of gas in optically thick dusty disks and calculate its thermal, density, and chemical structure. The models focus on an accurate treatment of the upper layers where line emission originates and at radii k0.7 AU. Although our models are applicable to stars of any mass, we present here only results around 1 M stars where we have varied dust properties, X- ray luminosities, and UV luminosities. We separately treat gas and dust thermal balance and calculate line luminosities at infrared and submillimeter wavelengths from all transitions originating in the predominantly neutral gas that lies below the very tenuous and completely ionized surface of the disk. We find that the [ Ar ii] 7 m, [ Ne ii] 12.8 m, [ Fe i] 24 m, [ S i] 25 m, [ Fe ii] 26 m, [ Si ii] 35 m, [ O i] 63 m, and pure rotational lines of H2 and CO can be quite strong and are good indicators of the presence and distribution of gas in disks. Water is an important coolant in the disk, and many water emission lines can be moderately strong. Current and future observational facilities such as the Spitzer Space Telescope, Herschel Space Observatory, and SOFIA are capable of detecting gas emission from young disks. We apply our models to the disk around the nearby young star, TW Hya, and find good agreement between our model calculations and observations. We also predict strong emission lines from the TW Hya disk that are likely to be detected by future facilities. We suggest that the gas disk around TW Hya may be truncated to 120 AU, compared to its dust disk of 174 AU. We speculate that photoevaporation due to the strong stellar FUV field from TW Hya is responsible for the gas disk truncation.