An Analysis of the Shapes of Ultraviolet Extinction Curves. II. The Far-UV Extinction

Abstract

In this paper we examine the properties of interstellar extinction in the far-ultraviolet region (FUV; λ < 1700 A) 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. We find 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 (i.e., slope and intercept) 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 (centroid, width, and strength), one parameter describes the linear background (either slope or intercept), and one parameter describes the strength of the FUV curvature term. The shape of the FUV curvature is found to be identical, within the observational errors, for all the curves in our sample-which spans virtually the entire range extinction curve morphologies observed in the Milky Way. This implies that the curvature component is not an artifact of the size distribution or relative combinations of the absorbing particles, but is rather a distinct optical property of some physical dust component. We suggest that the FUV curvature seen in the IUE curves represents the long-wavelength tail of a FUV resonance, analogous to the 2175 A bump.