RADIO AND OPTICAL OBSERVATIONS OF HIGH-VELOCITY AND INTERMEDIATE-VELOCITY GAS IN THE NEARBY INTERSTELLAR-MEDIUM

Using observations at the 21-cm wavelength obtained with the Lovell Telescope we report on the detection of a weak H I emission feature at a velocity + 100 km s-1 LSR and of column density log N (H I) = 18.5 +/- 0.2 towards the star 4 Lac. The feature corresponds in velocity with a high-velocity gas component which we have observed in several UV resonance lines in IUE spectra of this star. The component gas temperature which is derived from radio, optical and UV spectra lies in the range 5000 to 9000 K and the line-of-sight column thickness is estimated to be less than 10 pc. The relationship between this observed feature and the widely distributed High-Velocity Clouds (HVCs) is considered briefly. H I components are detected also at intermediate velocities (between - 20 and + 25 km s-1) which vary significantly on an angular scale almost-equal-to 0-degrees-1. Towards the star 4 Lac, components are observed in Na I within the same velocity range. If, as we suggest, the H I and Na I components correspond, then the intermediate-velocity gas lies in the foreground to the star and the observed spatial variations occur on a scale approximately < 2 pc.