  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.
