The location of the dense and ionized gas in the NGC 2023 photon-dominatedregion

The VLA and the BIMA array were used to obtain high-resolution (10"-20") ob servations of C+, traced by the C91 alpha recombination line at 8.6 GHz, an d the dense molecular gas, traced by HCN and HCO+(1-0), of the photon-domin ated 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 as sociated with HD 37903. The C91 alpha emission originates from a 0.4 pc lon g filament, extending from the east to the south of the exciting star HD 37 903. Within the filament three C91 alpha clumps can be distinguished, each associated with filamentary vibrationally excited H-2 emission in the direc tion toward HD 37903. The HCO+ emission has a clumpy appearance superimpose d on a more extended component. C91 alpha is, in general, closer to the exc iting star than HCO+ emission as expected from PDR models. The morphologies of HCO+ and HCN are quite similar. Based on the C91 alpha line width towar d one of the clumps, a limit of 170 K on the kinetic temperature in the ion ized carbon layer can be derived. This value is consistent with PDR models with H, densities of about 10(5) cm(-3). However, this result suggests surp risingly low limits on the turbulence in the PDR. We detected a compact 3 m m continuum source in the PDR, which appears to be a cold "core" of density 10(7) cm(-3), 0.03 pc diameter, and 6 M-circle dot. We conclude that it ma y have formed within the PDR. In an appendix, observations of the C91 alpha recombination line toward five additional PDRs using the Efflelsberg 100 m telescope are described.