1998ApJ...500..329G

We present new data on the photodissociation regions associated with the reflection nebula NGC 7023, particularly the three bright rims to the north, south, and east of the illuminating star HD 200775. ¹³CO (3-2) emission, mapped at 20" resolution at the Caltech Submillimeter Observatory (CSO), delineates a molecular cloud containing a cavity largely devoid of molecular gas around this star. Neutral carbon is closely associated with the ¹³CO emission while ionized carbon is found inside and at the edges of the cavity. The ionized carbon appears to be, at least in part, associated with H I. We have mapped the northern and southern rims in ¹²CO (6-5) emission and found a good association with the H₂ rovibrational emission, although the warm CO gas permeates a larger fraction of the molecular cloud than the vibrationally excited H₂. The column density contrast between the bright rims and the diffuse region inside and in front of the cavity is about 10. Despite the fact that the edges of the cavity are viewed edge-on, the carbon emission extends much farther into the molecular gas than does the photodissociation region, as defined by the H₂ emission region. Geometrically, NGC 7023 consists of a sheet of dense molecular gas in which the star was born, subsequently blowing away much of the surrounding gas. The three bright rims are located at the edges of the remaining molecular cloud and are viewed approximately edge-on. The results are compared to photon-dominated region (PDR) models, invoking direct illumination from the star, which are largely successful, except in explaining the presence of neutral carbon deep in the molecular cloud. We suggest that, in the particular case of NGC 7023, a second PDR has been created at the surface of the molecular cloud by the scattered radiation from HD 200775. This second PDR produces a layer of atomic carbon at the surface of the sheet, which increases the predicted [C]/[CO] abundance ratio to 10%, close to the observed value. Further tests for the applicability of PDR models in such regions are suggested.