PROBING THE PHOTODISSOCIATION REGION TOWARD HD-200775

The illuminating source of the photodissociation region associated wit h the reflection nebula NGC 7023 is HD 200775. We probed the foregroun d atomic and molecular material through diagnostics observed in absorp tion 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 part icular, estimates for gas density, gas temperature, and the flux of ul traviolet radiation were derived and compared with information obtaine d from maps of radio emission. The foreground material has lower densi ty than the other portions of the photodissociation region; the observ ed excitation of C-2, the observed column of C-2, and the columns of n eutral sodium and potassium are reproduced when the extinction curve d erived for this line of sight is adopted. Future models of the NGC 702 3 nebula should include the effects associated with this extinction cu rve. The observed CN column density is larger than our predicted amoun t; an additional contribution from the background molecular cloud is i nferred.