1997A&A...318..579G

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. 1993A&A...267..233L and 1993ApJ...416L..87L). 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⁺, H¹³CO⁺, HC¹⁸O⁺ and DCO⁺. We have also taken new ¹³CO and C¹⁸O 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 ³⁴CS lines relative to its column density, together with weak DCO⁺ lines. It has also a cold temperature and rather strong CCH and C₃H₂ 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.