High precision spectropolarimetry of stars and planets – III. The wavelength dependence of interstellar polarization

Abstract

The interstellar polarization curve has been measured for three stars at a resolution of 50 Å. Low amplitude structure is present in each case. For Zeta Ophiuchi, the star studied in greatest detail (3100–7600 Å), the structure includes an oscillation in the degree of polarization at λ ≳ λmax and a feature about 200 Å wide near 4100 Å. 55 Cygni shows a feature of similar width and amplitude (about 3 per cent of the continuum polarization) but at 4400 Å. The rotation of position angle across the spectrum seen in all three stars may be interpreted in terms of two or more dust clouds along the line-of-sight. These clouds must differ in grain size and grain size distribution as well as in the direction of grain alignment. We argue that the structure in Zeta Ophiuchi beyond λmax may be due to magnetite impurities in silicate grains.