THE PHOTON-DOMINATED REGIONS ASSOCIATED WITH NGC-2023 AND S-140

We have used the Effelsberg 100-m telescope to observe the C91 alpha r adio recombination line towards the photon-dominated regions associate d with NGC 2023 and S 140. This has been supplemented with observation s of the (CO)-O-18 (2-1) and (3-2) lines using the 3-m KOSMA telescope . We have analyzed the C91 alpha observations using 1-dimensional homo geneous models in order to derive estimates of the density and ultravi olet radiation field. Towards both sources, our radio line data sugges t densities of 10(5) cm(-3) although we have evidence for a drop-off t o a value of around 10(4) cm(-3) at offsets of 0.4 pc from the excitin g star in NGC 2023. The molecular line data towards NGC 2023 are also consistent with high densities (certainly above 10(4) and probably of order 10(5) cm(-3)). We discuss the influence of clumping on these mod els and conclude that densities as high as 10(6) cm(-3) are possible i n both the molecular and carbon line emitting gas.