THE ANOMALOUS EXTINCTION CURVE IN THE DIRECTION OF RHO OPHIUCHI FROM 950-A TO 1180-A

We report the interstellar extinction from 950 to 1180 angstrom in the direction of rho-Oph. These data were obtained with a moderate-resolution sounding rocket observation of rho-Oph, and two lightly reddened B stars of identical spectral type. Unlike Voyager observations in this wavelength region, our spectral resolution is sufficient to directly measure the contribution of molecular hydrogen along the line of sight and unambiguously identify the dust contribution. The derived curves compare favorably with the extrapolation of functional fits to the rho-Oph derived from longer wavelengths. In addition, we have fitted a model of dust extinction to the derived curve and find that the best fits indicate that the line of sight exhibits a larger than average grain size and a preferential concentration of graphite relative to silicates. The sharp rise seen at the shortest wavelengths (below 1000 angstrom) cannot be explained by standard silicate-graphite dust models and may be indicative of a nonstandard grain shape or a missing component in the extinction models.