ULTRAVIOLET INTERSTELLAR POLARIZATION OF GALACTIC STARLIGHT .1. OBSERVATIONS BY THE WISCONSIN ULTRAVIOLET PHOTO POLARIMETER EXPERIMENT

The Wisconsin Ultraviolet Photo Polarimeter Experiment (WUPPE) flew tw ice as part of NASA's Astro Spacelab missions in 1990 December and 199 5 March. A systematic survey of the interstellar polarization in the u ltraviolet was one of the main projects on both flights. The program w as carefully crafted to (1) sample the galactic plane as uniformly as practicable, (2) explore sight lines of diverse chemical composition a nd morphology, and (3) measure the shape of the UV polarization throug h the full range of known wavelengths of peak polarization in the opti cal. We present here Astro-2 data for 20 previously unobserved sight l ines and combine these with previously published UV data and with opti cal observations from the University of Wisconsin's ground based facil ities and elsewhere. We thus have spectropolarimetry from 1500 to 10,0 00 Angstrom for 35 galactic objects in which the polarization appears to be dominated by the interstellar component. The extrapolation of th e empirical Serkowski formula based only on visual data does not provi de a reliable representation of the UV polarization. We find that ther e are substantial differences in the amount of UV polarization relativ e to that in the visual. This may indicate that the small and large al igned grain populations are somewhat independent and this may in turn provide a new diagnostic of varying conditions in the interstellar med ium. Finally, there are several cases in which the UV polarization is enhanced in the 2000-3000 Angstrom range. (C) 1996 American Astronomic al Society.