PHYSICAL CONDITIONS IN THE PHOTODISSOCIATION REGION OF NGC-2023

Observations of the far-infrared fine-structure emission lines of [O I ] (63, 146 mu m), [C II] (158 mu m), and [Si II] (35 mu m) and the adj acent continua were obtained for the reflection nebula NGC 2023 with t he Cryogenic Grating Spectrometer aboard the Kuiper Airborne Observato ry. These fine-structure lines originate from a warm, atomic photodiss ociation region (PDR), where gas has been heated by the illuminating s tar HD 37903. The measured line and continuum intensities were combine d with [C II] and CO observations from the literature to estimate the density and temperature of the PDR. Simple analysis indicates densitie s of similar to 2 x 10(4) cm(-3) and temperatures around 400 K for the emitting gas. Detailed models of the PDR in NGC 2023 have also been c onstructed. Comparisons between observed and predicted line strengths demonstrate that considerable density enhancements (clumps) exist thro ughout the PDR. A two-component model of dense (total hydrogen density = 10(5) cm(-3)) warm (approximate to 750 K) clumps with a beam area f illing factor of 0.11 embedded in a diffuse (750 cm(-3)) cooler (appro ximate to 250 K) halo is in good agreement with the observations.