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 FUSE far-ultraviolet spectra supplemented by existing 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 < λ < 11 μm) was found to be broadly consistent with extrapolations from the IUE extinction (3.3 μm < λ < 8.6 μm) 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 ) 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 ≤ λ ≤ 11 μm). 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 < 9.6 μm and ∼0.68A(V ) for λ > 9.6 μm on features with a resolution of 10 (∼0.1 Å width). Subject headings: dust, extinction