2000ApJ...543..245W

The VLA and the BIMA array were used to obtain high-resolution (10"-20") observations of C⁺, traced by the C91α recombination line at 8.6 GHz, and the dense molecular gas, traced by HCN and HCO⁺(1-0), of the photon-dominated region (PDR) associated with the reflection nebula NGC 2023. Using the VLA, continuum emission is detected at 8.6 GHz from a faint H II region associated with HD 37903. The C91α emission originates from a 0.4 pc long filament, extending from the east to the south of the exciting star HD 37903. Within the filament three C91α clumps can be distinguished, each associated with filamentary vibrationally excited H₂ emission in the direction toward HD 37903. The HCO⁺ emission has a clumpy appearance superimposed on a more extended component. C91α is, in general, closer to the exciting star than HCO⁺ emission as expected from PDR models. The morphologies of HCO⁺ and HCN are quite similar. Based on the C91α line width toward one of the clumps, a limit of 170 K on the kinetic temperature in the ionized carbon layer can be derived. This value is consistent with PDR models with H₂ densities of about 10⁵ cm^–3. However, this result suggests surprisingly low limits on the turbulence in the PDR. We detected a compact 3 mm continuum source in the PDR, which appears to be a cold ``core'' of density 10⁷ cm^–3, 0.03 pc diameter, and 6 M_☉. We conclude that it may have formed within the PDR. In an appendix, observations of the C91α recombination line toward five additional PDRs using the Effelsberg 100 m telescope are described.