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 H_2 models for each sightline. The general shape of the
FUSE extinction (8.4 micron^-1 < lambda^-1 < 11 micron^-1) was found to be
broadly consistent with extrapolations from the IUE extinction (3.3 micron-1 <
lambda^-1 < 8.6 micron^-1) curve. Significant differences were seen in the
strength of the far-UV rise and the width of the 2175 A 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 micron^-1 <= lambda^-1 <= 11 micron^-1). Finally, we searched for discrete
absorption features in the far-ultraviolet. We found a 3 sigma upper limit of
~0.12 A(V) on features with a resolution of 250 (~4 A width) and 3 sigma upper
limits of ~0.15 A(V) for lambda^-1 < 9.6 micron^-1 and ~0.68 A(V) for lambda^-1
> 9.6 micron^-1 on features with a resolution of 10^4 (~0.1 A width).
