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Astron. Astrophys. 324, 1135-1144

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The photon dominated regions associated with NGC 2023 and S 140

F. Wyrowski1, C.M. Walmsley1,2, A. Natta2, and A.G.G.M. Tielens3

1I. Physikalisches Institut, Univ. zu Köln, Zülpicherstr. 77, D-50937 Köln, Germany
2Osservatorio Astrofisico di Arcetri, Largo E.Fermi 5, I-50125 Firenze, Italy
3NASA Ames Research Center, MS 245-3, Moffet Field, CA 94035, USA

Received 24 September 1996 / Accepted 21 February 1996


We have used the Effelsberg 100-m telescope to observe the C91FORMULA 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 C91 FORMULA 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 105 cm-3 although we have evidence for a drop-off to a value of around 104 cm-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 105 cm-3 ). We discuss the influence of clumping on these models and conclude that densities as high as 106 cm-3 are possible in both the molecular and carbon line emitting gas.

Key words: ISM: clouds - ISM: molecules - radio lines: ISM - ISM: NGC 2023; S 140

Send offprint requests to: F. Wyrowski

© European Southern Observatory (ESO) 1997