ULTRAVIOLET INTERSTELLAR POLARIZATION OBSERVED WITH THE HUBBLE-SPACE-TELESCOPE

We have used the Faint Object Spectrograph of the Hubble Space Telescope to observe interstellar linear polarization from 1300 to 3300 angstrom in two stars with well-studied interstellar polarization at visible wavelengths. The wavelength dependence of linear polarization declines smoothly with decreasing wavelength and is devoid of structure associated with the prominent 2175 angstrom absorption bump in the interstellar extinction curve. The data for one star (HD 161056) are consistent with an extrapolation based on the Serkowski formula of a fit to the ground-based polarization; the other star (HD 7252) shows excess (super-Serkowski) polarization relative to this extrapolation. Out of a total of 10 stars now studied by means of spectropolarimetry in the satellite ultraviolet, including eight observed with the Wisconsin Ultraviolet Photopolarimeter, five (those of longest lambda(max)) show Serkowski behavior, and four others show super-Serkowski behavior; only one (HD 197770) shows evidence for polarization associated with the 2175 angstrom bump. These results place important constraints on the nature of the bump feature.