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Observations of far-infrared fine-structure emission lines and the adjacent continua were obtained with the Cooled Grating Spectrometer (CGS) aboard the Kuiper Airborne Observatory (KAO) for the reflection nebula NGC 2023. The fine-structure lines are interpreted as coming from a warm, atomic photodissociation region (PDR), where gas has been heated by photoelectrons ejected from gas grains following the absorption of UV photons coming from the exciting star HD 37903. Measured intensities in the lines [O{\sc i}] (63, 146 $\mu{\rm m}$), [C{\sc ii}] (158 $\mu{\rm m}$), and [Si{\sc ii}] (35 $\mu{\rm m}$) and in the continuum were combined with CO observations from the literature to estimate the density and temperature of the photodissociation region and to construct a detailed model of NGC 2023. The model predicts temperatures of a few hundreds degrees and $n_o \approx 4000$ ${\rm cm}^{-3}$. This PDR therefore has lower densities and temperatures than most other photodissociation regions which have undergone detailed modeling (e.g. , NGC~7027, Orion Bar, Sgr~A, M82). Comparisons between observed and predicted line strengths suggest that considerable density enhancements (clumps) exist throughout the photodissociation region.
