High-velocity gas components towards 4 Lac⋆

A comparison of IUE ultraviolet spectra of the star 4 Lac with observations of a suitable comparison star have confirmed the presence of ‘high-velocity gas components’ in the former which are redshifted by a velocity $$\sim 100\,\text{km}\,{\text{s}}^{-1}$$ from the low-velocity interstellar gas. In addition to providing an unambiguous identification of the species which show high-velocity components, the new data yield improved values for equivalent widths. The derived column densities, element depletions and ionization balances all lie within the range of values obtained from the UV spectroscopy of old supernova remnants. Firmer predictions of the gas electron and hydrogen densities may be possible from observations of fine structure levels in species such as C II and Si II although, as we show, these will require substantial IUE exposure times even for the brighter stars.