Disks around massive young stellar objects, and disks around low-mass stars irradiated by nearby OB associations, are eroded by photo-evaporation. In the latter case, this erosion may be an important factor in planet formation. As Johnstone et al. (1998ApJ...499..758J
) have shown, photo-evaporating material is gravitationally retained within a critical radius from the star, and constitutes an envelope similar to a Photon-Dominated Region (PDR) that normally arises at the edge of a molecular cloud irradiated by a massive star. We explore the chemistry of such a PDR/disk system to examine the contribution that it may make to the molecular species that may be observed. The model is in two phases; firstly, a collapse from low density to a high density appropriate for a disk; and, secondly, a 2D calculation of the irradiation of disk material by the radiation field of the central massive star or nearby OB association. The model follows the chemistry self-consistently through both phases. We compute the column densities of species through the PDR/disk system, averaged over the disk. We validate our model by comparing predicted averaged molecular column densities with those of several species detected in the disk around the 10 solar mass star GL 2591, currently the sole example known of this kind of object. Result
. are in good agreement for a model in which the outer part of the PDR is hot while the inner part is cool, and in which the local ionization rate is comparable with that caused by cosmic rays in the local interstellar medium. We show that in addition to the four detected species, there should be many others also detectable in this system, including HCN, NH3
and CS. Similar conclusions should apply to other disks around massive stars. Disks around low-mass stars are much more common; our models show that when irradiated by a nearby OB association such disks with their attendant PDRs also generate a rich chemistry. No detections of molecules in such objects have yet been reported. However, the models suggest that averaged molecular column densities should be comparable to those detected in disks around massive stars (see references listed in Table 4 for molecules in GL 2591). Potential tracers of irradiated disks around low-mass stars include OH, CH3
H. We note that the detection in a disk of PDR-type chemistry is a clear signature that the disk is undergoing erosion. Its duration is therefore limited, with consequences for planet formation.