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 H-2 models for each sightline. The general shape of the FUSE extinction (8.4 mu m(-1) < lambda(-1) < 11 mu m(-1)) was found to be broadly consistent with extrapolations from the IUE extinction (3.3 mu m(-1) < lambda(-1) < 8.6 mu m(-1)) curve. Significant differences were seen in the strength of the far-UV rise and the width of the 2175 angstrom 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(lambda)/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 mu m(-1) <= lambda(-1) <= 11 mu m(-1)). Finally, we searched for discrete absorption features in the far-ultraviolet. We found a 3 sigma upper limit of similar to 0.12A(V) on features with a resolution of 250 (similar to 4 angstrom width) and 3s upper limits of similar to 0.15A(V) for lambda(-1) < 9.6 mu m(-1) and similar to 0.68A(V) for lambda(-1) > 9.6 mu m(-1) on features with a resolution of 10(4) (similar to 0.1 angstrom width).