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

M. Gerin , E. Falgarone , K. Joulain , M. Kopp , J. Le Bourlot , G. Pineau des Forêts , E. Roueff , P. Schilke
Downs Laboratory of Physics, Caltech, Physics Department, 320-47, Pasadena CA91125, USA.
Radioastronomie Millimétrique, Laboratoire de Physique de l'ENS, 24 rue Lhomond, 75231 Paris Cedex 05, France
DAEC, Observatoire de Paris-Meudon, Place J. Janssen, 92195 Meudon cedex, France
I. Physikalisches Institut der Universität zu Köln, Zülpicher Str. 77, D-50937, Köln, Germany
Abstract: 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 , , and . We have also taken new and 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 lines relative to its column density, together with weak lines. It has also a cold temperature and rather strong CCH and C H 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.

 
Figure 3: Column density ratio N(CS)/N(SO) as a function of the column density for Polaris (black squares), Taurus (black triangles), TMC1 (asterisks), L134N (crosses), B335 (white square), clouds seen in absorption (white triangles), dark clouds cores (white hexagons), high latitude clouds (starred squares) and globules (stars).