Ultraviolet interstellar linear polarization. II. The wavelength dependence
We present new ultraviolet (UV) polarimetry of the well-studied interstellar line of sight toward HD 204827 obtained with the Faint Object Spectrograph on Hubble Space Telescope. HD 204827 is of great interest because the dust along this line of sight has extremely low values of both λmax and Rv. Its far-UV extinction is very large, reflecting its small Rv value. In addition, we reexamine the entire sample of 14 interstellar lines of sight for which there are now UV polarization data. We find that the previously suggested relationship between λmax and the wavelength dependence of the polarization in the UV is strongly supported by the data for this larger sample including HD 204827. Seven stars with λmax ≥ 0.54 agree well with an extrapolation of the Serkowski relation into the UV while seven stars with λ≤ ≤ 0.53 μm show polarization in excess of the Serkowski extrapolation (super-Serkowski). However, the division of the observed lines of sight into Serkowski and super-Serkowski categories is artificial. In fact, the amount of polarization in the UV is correlated with a single parameter, λmax. This may indicate that there is a mean interstellar polarization law analogous to the mean interstellar extinction law of Cardelli, Clayton, & Mathis which is based on Rv. The data are consistent with a linear relationship between λ-1max and p(UV)/pmax but more data are needed to define the functional form. We suggest that the Serkowski and super-Serkowski designations be replaced by high and low λmax which are more physically descriptive . At the same time, we note that all seven super-Serkowski (low λmax) stars lie in a relatively small region of the sky between lII = 90°-150° and b = -5°-15°. These stars all lie in or behind a spur of the local Orion spiral arm. Similarly, most of the Serkowski (high λmax) stars lie in or near the Scorpio-Centaurus OB Association. So lines of sight covering larger areas of the sky are needed to test the universality of the λmax/UV polarization relationship. The recent discovery of warm dust near HD 197770 suggests the possibility that a mechanism other than the traditional alignment to the Galactic magnetic field may be invoked to explain its 2175 Å polarization bump.