Ultraviolet interstellar linear polarization. V. Analysis of the final data set
Using recent measurements of ultraviolet interstellar polarization, we have examined its relationship to ultraviolet extinction and to polarization and extinction measurements in the visible and infrared. The relationship between the relative amount of ultraviolet polarization and the parameter λmax, determined using only visible data, is confirmed and strengthened, for example, by a tight correlation between p(6 μm-1)/pmax and λ-1max. A good fit to the wavelength dependence of the polarization from the infrared to the ultraviolet can be achieved with a five-parameter function combining a power law in the infrared and a Serkowski-like function in the ultraviolet. The polarization efficiency (ratio of polarization to extinction) is less in the ultraviolet than in the visual, and the ratio of these efficiencies increases systematically with λ-1max. We relate these effects to systematic changes in the underlying aligned grain size distribution. The polarization efficiency of the grains causing the 2175 Å extinction bump along most sight lines is so (unusually) small that no statistically significant polarization feature is detectable in that wavelength region. Only two of 28 sight lines show a definite polarization feature. The environments of the two bump sight lines are similar but not unique, and the mechanism for producing the polarization feature along only these two sight lines is still not known.