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 determined j max , using only visible data, is conÐrmed and strengthened, for example, by a tight correlation between p(6 and A good Ðt to the wavelength dependence of the polarization from the infrared km~1)/p max j max ~1 . to the ultraviolet can be achieved with a Ðve-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 We relate these e†ects to systematic changes in the underlying aligned grain size disj max ~1 . tribution. The polarization efficiency of the grains causing the 2175 extinction bump along most sight AŽ lines is so (unusually) small that no statistically signiÐcant polarization feature is detectable in that wavelength region. Only two of 28 sight lines show a deÐnite 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. Subject heading : dust, extinction È polarization È ultraviolet : ISM