Ultraviolet interstellar linear polarization .3. Features

Astro-2 has revealed a broad, weak spectral feature in the ultraviolet interstellar linear polarization for two lines of sight, confirming the original detection toward HD 197770 and adding HD 147933-4. These are the only two polarization features found in some 30 lines of sight now observed. Both features are centered close to 2175 Angstrom, the position of the ubiquitous ultraviolet extinction bump. Two possibilities are considered for the source of the polarization feature: changes in the mass distribution of the aligned silicate grains responsible for the continuum polarization, and alignment of the small graphite grains responsible for the extinction bump. While the former approach meets with some success for the HD 197770 feature, it is not possible to produce a feature as clearly peaked as in HD 147933-4. The central wavenumbers and widths of both polarization features correspond closely to those of the corresponding extinction bump. Taken together, it seems the graphite grains are a more likely source of the polarization features. Both polarization features have amplitudes that are very small compared to the amount of excess extinction present in the 2175 Angstrom bump, implying poor polarization efficiency. Many other lines of sight have been observed with sufficient signal-to-noise ratios such that features should have been clearly detected if the same polarization efficiency applied, and so real variations in the alignment or shape of the grains responsible seem to occur from one line of sight to another. The weak alignment might be caused by unusually low concentrations of paramagnetic impurities in rapidly spinning small grains.