The Chemical Composition and Distribution of Interstellar Grains

Abstract

The chemical composition of interstellar grains is derived here on the basis of (1) the cosmic abundance of the elements; (2) the wavelength dependence of extinction and polarization; (3) the average total extinction; (4) the ratio of polarization to extinction; (5) the predominantly dielectric character of grains in the visible spectral region; and (6) infrared spectral characteristics of grains. It is indicated that the major portion of the grains, by mass, consist of core-mantle particles in the 0.1 μm size range, whose cores consist largely of silicates and whose mantles are a solid mixture of O, C, and N with H in a heterogeneous combination of simple and complex molecules with frozen free radicals. A minor constituent of the solid particles exist in the form of very small uncoated particles generally less than 10 ^-6 cm in size whose precise composition is not certain. Inferences of the core-mantle model with respect to spatial distribution are consistent with the proposition that growth of the mantles occurs in the galactic shock region predicted by the density-wave theory. Estimates of the total visual extinction toward the galactic center and the consequent estimates of the total amount of far infrared radiation are shown to depend critically on the grain model. Variations of the ratio of far ultraviolet to visual extinction are correlated with the conditions for growth of mantles on the bare small particles which are generally prevented from accreting mantles primarily because of their extreme temperature fluctuations produced by the ultraviolet photons in the radiation field.