Dust Grains in Reflection Nebulae

Abstract

Abstract.Observations and Mie theory calculations of colors and polarization in several reflection nebulae are compared.A new study of light scattering in reflection nebulae was begun in 1968. Details of the observations and of the theoretical treatment are given elsewhere (Zellner, 1970(Zellner, , 1973. Briefly, observations of color and polarization were made in a few selected regions of NGC 2068, NGC 7023, and IC 5067 with the two-channel digital polarimeter described by Coyne and Gehrels (1967). Most of the data were taken at the 154-cm Catalina reflector of the Lunar and Planetary Laboratory. About 40 h of integration time were invested per region, in six filters ranging in effective wavenumber from 1.21 to 3.03 per micron. The color data were generally in good agreement with previous results, but polarizations in NGC 2068 and NGC 7023 were found to be systematically larger than those reported by Elvius and Hall (1966).The observations were compared with single-scattering Mie calculations for a wide variety of complex refractive indices m* = rri -im". Two types of broad size distribu tions were used, namely the Oort-van de Hulst distribution as modified by Greenberg (OHG distribution) (Greenberg, 1966), n(a) = n 0 exp -5[a/a 0 ] 3 , and the exponential distribution, n(a) = n 0 exp -5\a/a 0 ]. The factor of 5 is introduced in order to make the scale radius a 0 roughly comparable with the equivalent single radius a in the absence of a size distribution. Wavelength-independent refractive indices were assumed, so that a 0 could be left as a free parameter to be determined by the agreement between theory and observation. In most theoretical studies of reflection nebulae, the dust cloud has been assumed to be homogeneous and bounded by a simple geometrical figure such as a sphere or a plane-parallel slab. As illustrated in Figure 1, my models are free of geometrical constraints; in each separate nebular region, the extreme scat tering angles 6 t and 6 2 are independently adjusted to best match the observations. The computed polarization is very sensitive both to the scattering geometry and to the scale radius. The color data, being more perturbed by internal and foreground extinc tion, are only indicative.Reflection nebula polarizations usually decrease linearly with wavenumber from the infrared into the ultraviolet. This behavior is characteristic of forward-scattering dielectric grains, but can be produced by particles with a large m" only under contrived conditions of internal attenuation. Thus pure graphite or metallic grains are imme diately ruled out. It has been stated (Hanner, 1971) that silicate grains (m' = 1.5 to 1.7) would give negative polarization at short wavelengths, contrary to all observations. Figure 2, however, shows this statement to be a halftruth. With an appropriate shift Greenberg and Van de Hulst (eds.)