CONSTRAINTS ON THE OPTICAL-PROPERTIES OF INTERSTELLAR DUST IN THE FAR-ULTRAVIOLET - VOYAGER OBSERVATIONS OF THE DIFFUSE SKY BACKGROUND

We have observed four regions of the sky, at various Galactic latitudes and H I column densities, with the ultraviolet spectrometer aboard Voyager 2 finding no evidence for the presence of a diffuse radiation field between 912 and 1100 angstrom at a level of 100-200 photons cm-2 s-1 sr-1 angstrom-1. As starlight scattered by dust grains may be expected to be a major contributor to the diffuse background, especially at low Galactic latitudes, we have been able to place a very stringent upper limit of less than 0.1 on the reflectivity of the dust in this spectral range (or, alternatively, a lower limit of 0.95 on <cos theata>). This albedo is much lower than the canonical value of about 0.5 for standard graphite-silicate dust models such as that of Draine and Lee.