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Astron. Astrophys. 318, 579-594 (1997)

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Chemical inhomogeneities in interstellar clouds: the high latitude cloud MCLD 123.5+24.9

M. Gerin 1, 2, E. Falgarone 2, K. Joulain 2, M. Kopp 3, J. Le Bourlot 3, G. Pineau des Forêts 3, E. Roueff 3 and P. Schilke 4

1 Downs Laboratory of Physics, Caltech, Physics Department, 320-47, Pasadena, CA 91125, USA
2 Radioastronomie Millimétrique, Laboratoire de Physique de l'ENS, 24 rue Lhomond, F-75231 Paris Cedex 05, France. URA336 du CNRS
3 DAEC, Observatoire de Paris-Meudon, Place J. Janssen, F-92195 Meudon cedex, France. URA 0173 du CNRS
4 I. Physikalisches Institut der Universität zu Köln, Zülpicher Str. 77, D-50937 Köln, Germany.

Received 26 February 1996 / Accepted 21 May 1996


We report on observations designed to disclose the presence of the High Ionization Phase predicted to exist in dark clouds by chemical models (Le Bourlot et al. 1993). In addition to a high neutral carbon abundance, a high CS/SO abundance ratio and a low abundance of molecular ions, such a phase might be characterized by a low D/H fractionation ratio.
In this paper, we present both observational and model results on the sulfur bearing molecules CS and SO, and their most abundant isotopomers, on the molecular ions HCO [FORMULA], H13 CO [FORMULA], HC18 O [FORMULA] and DCO [FORMULA]. We have also taken new 13 CO and C18 O data to improve the determination of the physical conditions at the observed positions. One of the two dark clouds investigated, a core in the high latitude cloud MCLD 123.5+24.9, presents surprinsingly strong CS and 34 CS lines relative to its column density, together with weak DCO [FORMULA] lines. It has also a cold temperature and rather strong CCH and C3 H2 lines. These observed features are characteristic of the presence of the High Ionization Phase along the line of sight. This cloud core might therefore be a good candidate for further investigation of this new phase of dark interstellar clouds.

Key words: molecular processes – ISM: MCLD 123.5+24.9; abundances; clouds; molecules – radio lines: molecules

Send offprint requests to: M. Gerin, French address

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998