IRAS Cirrus Observations and the Nature of Dust

Abstract

Two theories regarding the nature of interstellar dust are considered. One, “MRN”, has bare silicate and carbon particles. The other, by Greenberg and his associates, has silicates with refractory mantles providing the extinction in the visible region of the spectrum. Some relevant observations are reviewed. The most important of these is the fact that linear polarization reaches a maximum at the same wavelength at which the circular polarization goes through zero; this fact implies that the polarizing grains, which apparently are silicates, are nonabsorbing. Since the albedo of real dust is well below unity, there must be another component besides the silicates which provides the absorption. The nature of the absorbing component, and of others, is discussed. The IRAS cirrus 60/100 µm color is that predicted by MRN. The 12/100 µm color is indicating the emission of the 11.4 µm band which is seen in many objects, and which has plausibly been attributed to polycyclic aromatic hydrocarbons. It seems to vary in strength from one region to another. The 20 µm band may be influenced by silicate emission in the 18 µm band.