Astronomy and Astrophysics, volume 324, 1135-1144 (1997/8-3)
The photon dominated regions associated with NGC 2023 and S 140.
WYROWSKI F., WALMSLEY C.M., NATTA A. and TIELENS A.G.G.M.
Abstract (from CDS):
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.
ISM: clouds - ISM: molecules - radio lines: ISM - ISM: individual: NGC 2023 - S 140 (Sh 2-140)