FUSE Measurements of Far-Ultraviolet Extinction. III. The Dependence on R(V) and Discrete Feature Limits from 75 Galactic Sightlines
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 Å 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 Å 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 Å width).