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Star formation often occurs within or nearby stellar clusters. Irradiation bynearby massive stars can photoevaporate protoplanetary disks around young stars(so-called proplyds) which raises questions regarding the ability of planetformation to take place in these environments. We investigate thetwo-dimensional physical and chemical structure of a protoplanetary disksurrounding a low-mass (T Tauri) star which is irradiated by a nearby massiveO-type star to determine the survivability and observability of molecules inproplyds. Compared with an isolated star-disk system, the gas temperatureranges from a factor of a few (in the disk midplane) to around two orders ofmagnitude (in the disk surface) higher in the irradiated disk. Although the UVflux in the outer disk, in particular, is several orders of magnitude higher,the surface density of the disk is sufficient for effective shielding of thedisk midplane so that the disk remains predominantly molecular in nature. Wealso find that non-volatile molecules, such as HCN and H2O, are able to freezeout onto dust grains in the disk midplane so that the formation of icyplanetesimals, e.g., comets, may also be possible in proplyds. We havecalculated the molecular line emission from the disk assuming LTE anddetermined that multiple transitions of atomic carbon, CO (and isotopologues,13CO and C18O), HCO+, CN, and HCN may be observable with ALMA, allowingcharacterization of the gas column density, temperature, and optical depth inproplyds at the distance of Orion (~400 pc).

Molecular line emission from a protoplanetary disk irradiated externally by a nearby massive star