Line Emission from Gas in Optically Thick Dust Disks around Young Stars

Abstract

Wepresent self-consistent models of gas in optically thick dustydisks 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 dustproperties, X-ray luminosities, and UV luminosities. Weseparately 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 ]2 4� m, [S i ]2 5� m, [Fe ii ]2 6� m, [Si ii ]3 5� m, [O i ]6 3� m, and purerotationallinesof H2and CO canbequitestrongandaregoodindicatorsof thepresenceanddistributionof gasin disks. Water is an important coolant in the disk, and many water emission lines can be moderately strong. Current and futureobservationalfacilitiessuchastheSpitzerSpaceTelescope,HerschelSpaceObservatory,andSOFIAarecapable 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. Subject headingg infrared: ISM — line: formation — planetary systems: protoplanetary disks — radiative transfer