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Astron. Astrophys. 324, 1135-1144 (1997)

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The photon dominated regions associated with NGC 2023 and S 140

F. Wyrowski 1, C.M. Walmsley 1, 2, A. Natta 2 and A.G.G.M. Tielens 3

1 I. Physikalisches Institut, Univ. zu Köln, Zülpicherstr. 77, D-50937 Köln, Germany
2 Osservatorio Astrofisico di Arcetri, Largo E.Fermi 5, I-50125 Firenze, Italy
3 NASA 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 C91 [FORMULA] radio recombination line towards the photon-dominated regions associated with NGC 2023 and S 140. This has been supplemented with observations of the C18 O (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 [FORMULA]  cm-3 although we have evidence for a drop-off to a value of around [FORMULA]  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 [FORMULA] and probably of order [FORMULA]  cm-3). We discuss the influence of clumping on these models and conclude that densities as high as [FORMULA]  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

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© European Southern Observatory (ESO) 1997

Online publication: May 5, 1998