The location of the dense and ionized gas in the NGC 2023 PDR
  The VLA and the BIMA array were used to obtain high resolution (10-20") observations of C+, traced by the C91a 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 HII region associated with HD 37903. The C91a 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 C91a 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. C91a 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 C91a linewidth towards 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 10^5/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 10^7/cm^3, 0.03 parsec diameter, and 6 solar masses. We conclude that it may have formed within the PDR. In an appendix, observations of the C91a recombination line toward five additional PDRs using the Effelsberg 100m telescope are described. 
F. Wyrowski, C. M. Walmsley, W. M. Goss, A. G. G. M. Tielens