Ultraviolet interstellar polarization of galactic starlight .1. Observations by the Wisconsin Ultraviolet Photo Polarimeter Experiment

The Wisconsin Ultraviolet Photo Polarimeter Experiment (WUPPE) flew twice as part of NASA's Astro Spacelab missions in 1990 December and 1995 March. A systematic survey of the interstellar polarization in the ultraviolet was one of the main projects on both flights. The program was carefully crafted to (1) sample the galactic plane as uniformly as practicable, (2) explore sight lines of diverse chemical composition and morphology, and (3) measure the shape of the UV polarization through the full range of known wavelengths of peak polarization in the optical. We present here Astro-2 data for 20 previously unobserved sight lines and combine these with previously published UV data and with optical observations from the University of Wisconsin's ground based facilities and elsewhere. We thus have spectropolarimetry from 1500 to 10,000 Angstrom for 35 galactic objects in which the polarization appears to be dominated by the interstellar component. The extrapolation of the empirical Serkowski formula based only on visual data does not provide a reliable representation of the UV polarization. We find that there are substantial differences in the amount of UV polarization relative to that in the visual. This may indicate that the small and large aligned grain populations are somewhat independent and this may in turn provide a new diagnostic of varying conditions in the interstellar medium. Finally, there are several cases in which the UV polarization is enhanced in the 2000-3000 Angstrom range. (C) 1996 American Astronomical Society.