1997A&A...324.1135W

We have used the Effelsberg 100-m telescope to observe the C91α radio recombination line towards the photon-dominated regions associated with NGC 2023 and S 140. This has been supplemented with observations of the C¹⁸O (2-1) and (3-2) lines using the 3-m KOSMA telescope. We have analyzed the C91α observations using 1-dimensional homogeneous models in order to derive estimates of the density and ultraviolet radiation field. Towards both sources, our radio line data suggest densities of 10⁵cm^–3 although we have evidence for a drop-off to a value of around 10⁴cm^–3 at offsets of 0.4 pc from the exciting star in NGC 2023. The molecular line data towards NGC 2023 are also consistent with high densities (certainly above 10⁴ and probably of order 10⁵cm^–3). We discuss the influence of clumping on these models and conclude that densities as high as 10⁶cm^–3 are possible in both the molecular and carbon line emitting gas.