Wavelength dependence of polarization. IX. Interstellar particles

Comparison of the wavelength dependence of interstellar polarization with that observed in reflection nebulae appears to rule out metallic and purely graphitic grains for the general interstellar medium. The observations on reflection nebulae are best explained with composite grains that have an absorptive nucleus of diameter 0.05 p and an icy shell of diameter 0.3 p. The visual albedo of the grains is near 0.5 and the asymmetry factor 0.6. Because of this asymmetry, reflection nebulae generally are not observable if the star is in front of the nebula. NGC 7023 has 4X 10-is grains/cm2 and the visual optical depth within the nebula is only 0.2.