OPTICAL AND UV SPECTROSCOPY TOWARDS STARS IN THE DIRECTION OF THE COHEN HIGH-VELOCITY H-I-STREAM

We present high-resolution, high signal-to-noise ratio optical spectra and UV spectra obtained with IUE for several stars that lie within and adjacent to the Cohen high-velocity (HV) stream, which is considered by Cohen to lie within 300 pc. From spectral classifications, published photometry and LTE model atmosphere analysis of our stellar spectra, the programme stars are shown to lie at distances from 50 to at feast 600 pc. On the basis of the HI column densities for the HV stream, it is expected that gas at corresponding velocity should be observed in Na I and Ca II in our high-quality spectra, but no such detections are made. Low-velocity (LV) filament gas, which is spatially coincident with the HV stream in the directions studied, is detected, and is shown to lie within similar to 160 pc. The observations suggest that the HV stream lies beyond our programme stars. Alternatively, if the stream is closer and lies at a similar distance to the LV filament, then possible explanations for the non-detection ate (i) the HV gas is depleted in the species observed (for example, for Ca II the depletion would be significantly greater than the average Ca II depletion observed for HVCs), (ii) the Na and Ca in the gas is mostly in the form of Na II and Ca III and (iii) the gas is very clumpy on a fine scale. In the last context, our observations of the LV filament also indicate a clumpy distribution for this gas.