INTERSTELLAR CIRCULAR-POLARIZATION AND THE DIELECTRIC NATURE OF DUST GRAINS

The relationship between interstellar linear and circular polarization is reexamined. Results of Mie theory calculations show that the observed coincidence of the crossover wavelength of circular polarization and the peak wavelength of linear polarization is independent of the optical constants assumed for the interstellar grains and is therefore not a critical constraint on the grain properties. This result agrees with the general argument derived in several studies from the Kramers-Kronig relations. It is shown that the apparent upper limit on the visual absorptivity of grains derived from earlier Mie theory calculations, which has been regarded as one of the principal constraints on grain properties, resulted from a violation of the Kramers-Kronig relations in the assumed optical constants of the particles. The properties of dielectric core-mantle grains in the diffuse interstellar medium are discussed. The optical constants derived from available laboratory data indicate that the mantle material should be a dielectric with a low-energy gap (∼1 eV) and relatively high absorptivity at visible wavelength (the imaginary part of the index of refraction at least as high as 0.15 at 5500 A). The model which incorporates the silicate core-organic refractory mantle grains, as well as smaller particles required to account for the ultraviolet extinction, is shown to satisfy all available constraints on the properties of grains in the visual.