Probing the Photodissociation Region toward HD 200775

The illuminating source of the photodissociation region associated with the reflection nebula NGC 7023 is HD 200775. We probed the foreground atomic and molecular material through diagnostics observed in absorption against the background star. Ground-based measurements of Na I, K I, and Ca II and of the molecules CH, CH + , C 2 , and CN were analyzed in order to extract the physical conditions for this material. In particular, estimates for gas density, gas temperature, and the flux of ultraviolet radiation were derived and compared with information obtained from maps of radio emission. The foreground material has lower density than the other portions of the photodissociation region; the observed excitation of C 2 , the observed column of C 2 , and the columns of neutral sodium and potassium are reproduced when the extinction curve derived for this line of sight is adopted. Future models of the NGC 7023 nebula should include the effects associated with this extinction curve. The observed CN column density is larger than our predicted amount; an additional contribution from the background molecular cloud is inferred.