Polarization-wavelength Profile of the Interstellar 4430 Å Absorption Band

Abstract

KNOWLEDGE of the scattering and polarizing properties of interstellar grains has now reached the stage where very specific models are being proposed for the composition and size distribution of these grains. It is evident that high resolution studies of the wavelength dependence of interstellar extinction and polarization will provide important tests of the proposed grain models. Although much work has been done on detailed studies of interstellar extinction, the detailed wavelength dependence of interstellar polarization at a higher resolution needs more study. Another related problem concerns the possibility that the diffuse interstellar bands, the most intense of which occurs at 4430 Å, may be polarized differently from the neighbouring continuum: the amount and nature of this difference, if any, may give a clue to their origin. Spectrophotometry of these bands is always difficult because of the width and shallowness of the features, and polarization studies are correspondingly more difficult. By the use of a Wollaston prism at the Cassegrain focus of the 36-inch telescope of the Royal Observatory, Edinburgh, we have been able to derive a polarization profile across the 4430 band. This report describes the method and presents the preliminary results.