Fuse measurements of far-ultraviolet extinction. III. the dependence on R(V) and discrete feature limits from 75 galactic sightlines

Abstract

We present a sample of 75 extinction curves derived from Far Ultraviolet Spectroscopic Explorer (FUSE) far-ultraviolet spectra supplemented by existing International Ultraviolet Explorer (IUE) spectra. The extinction curves were created using the standard pair method based on a new set of dereddened FUSE+IUE comparison stars. Molecular hydrogen absorption features were removed using individualized H2 models for each sightline. The general shape of the FUSE extinction (8.4 μm-1 < λ-1 < 11 μm-1) was found to be broadly consistent with extrapolations from the IUE extinction (3.3 μm-1 < λ-1 < 8.6 μm-1) curve. Significant differences were seen in the strength of the far-UV rise and the width of the 2175 bump. All the FUSE+IUE extinction curves had positive far-UV slopes giving no indication that the far-UV rise was turning over at the shortest wavelengths. The dependence of A(λ)/A(V) versus R(V)-1 in the far-UV using the sightlines in our sample was found to be stronger than tentatively indicated by previous work. We present an updated R(V)-dependent relationship for the full UV wavelength range (3.3 μm-1 ≤ λ-1 ≤ 11 μm-1). Finally, we searched for discrete absorption features in the far-ultraviolet. We found a 3σ upper limit of 0.12A(V) on features with a resolution of 250 (4 width) and 3σ upper limits of 0.15A(V) for λ-1 < 9.6 μm-1 and 0.68A(V) for λ-1>9.6 μm -1 on features with a resolution of 104 (0.1 width). © 2009. The American Astronomical Society. All rights reserved..