An analysis of the shapes of ultraviolet extinction curves. II - The far-UV extinction

In this paper the properties of interstellar extinction in the far-ultraviolet region are examined utilizing IUE extinction curves for a primary data sample of 45 reddened Milky Way OB stars. These results are combined with those derived for the 2175 A bump in a previous study. It is found that IUE extinction curves can be represented by linear combinations of a Lorentzian-like 2175 A bump profile, a well-determined FUV curvature term, and an underlying linear component. The parameters of the linear component are strongly correlated and therefore only five free parameters are required to fit all of the curves in our sample. Three parameters describe the 2175 A bump, one parameter describes the linear background, and one parameter describes the strength of the FUV curvature term. The shape of the FUV curvature is found to be identical, to within the observational errors, for all the curves in the sample - which spans virtually the entire range of extinction curve morphologies observed in the Milky Way.