Ultraviolet interstellar linear polarization. I. Applicability of current dust grain models
Abstract
Ultraviolet spectropolarimetric observations obtained with the Wisconsin Ultraviolet Photo-Polarimeter Experiment have provided data on the wavelength dependence of interstellar polarization along eight lines of sight. These data allow us to take a new look at dust grain models which have been used to fit the extinction and polarization in the visible and infrared. The models considered are bare silicate/graphite grains, silicate cores with organic refractory mantles, silicate cores with amorphous carbon mantles, and composite grains. The eight lines of sight show three different interstellar polarization wavelength dependences: Serkowski, super-Serkowski, and a bump at 0.2175 μm. The MRN bare silicate grain model is the most successful at fitting the UV wavelength dependence. The Serkowski and super-Serkowski curves can be fitted by changing the size distribution in a particular way. These lits imply that there should be a correlation between curve type and λmax. Such a relationship is seen in the observed lines of sight where Serkowski curves are associated with larger λmax, and super-Serkowski curves with smaller λmax. The polarization bump seen toward HD 197770 can be well fit with small aligned graphite disks. The apparent necessity of aligning both small silicate and graphite grains seems to raise questions about theories of grain alignment.