Optical and UV spectroscopy of stars in the direction of the Riegel & Crutcher cold cloud

Abstract

Optical and UV spectral data are presented for the stars HD 165202, 168708 which lie in the direction of the Riegel & Crutcher cold cloud. These observations complement our earlier studies in which high-resolution HI profiles mapped the cold cloud in self-absorption (Montgomery et al.). HD 165202 lies in a direction close to where we observe a maximum HI self-absorption column density and HD 168708 lies towards the edge of the cold cloud; from their spectra we determine the stellar distances to be ~=200 and 430 pc, respectively. We show that the HI column densities associated with the cold core represent some 3-10 per cent of the total HI gas column observed towards the two stars. We suggest that the cloud core is embedded in diffuse, cold HI gas and that the sightlines contain a warm, intercloud medium; the gas observed in CH^+ may be produced in a thin interface between molecular gas of the cold cloud and the warm medium. Whilst it is not possible to obtain a detailed description of these sightlines, the evidence points to a relatively dense cloud core of space density ~=100-200 cm^-3 and thickness <=1 pc.