Physical Conditions in the Photodissociation Region of NGC 2023

Abstract

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 adjacent continua were obtained for the reflection nebula NGC 2023 with the Cryogenic Grating Spectrometer aboard the Kuiper Airborne Observatory. These fine-structure lines originate from a warm, atomic photodissociation region (PDR), where gas has been heated by the illuminating star HD 37903. The measured line and continuum intensities were combined with [C II] and CO observations from the literature to estimate the density and temperature of the PDR. Simple analysis indicates densities of ~2 x 104 cm-3 and temperatures around 400 K for the emitting gas. Detailed models of the PDR in NGC 2023 have also been constructed. Comparisons between observed and predicted line strengths demonstrate that considerable density enhancements (clumps) exist throughout the PDR. A two-component model of dense (total hydrogen density = 105 cm-3) warm (~750 K) clumps with a beam area filling factor of 0.11 embedded in a diffuse (750 cm-3) cooler (~250 K) halo is in good agreement with the observations.