OPTICAL-PROPERTIES OF OXIDE DUST GRAINS

In this paper, we present the optical properties of a series of well-c haracterized metal oxides (FexMg1-xO, x=1.0, 0.9, 0.8, 0.7, 0.5, 0.4) in the wavelength range from 200 nm to 500 mu m. A number of oxygen-ri ch stars show circumstellar dust features between 10 and 14 mu m and a round 19 mu m which cannot be explained by ordinary cosmic silicates b ut may be modelled by a mixture of aluminium oxide and iron-magnesium oxides. Calculated absorption spectra for small spheres show one vibra tional band in the range from 19.9 mu m (FeO) to 17.7 mu m (Fe0.4Mg0.6 O) in the Rayleigh limit. The absolute peak values of the oxide absorp tion bands are more than a factor of two larger than the 18 mu m band of amorphous silicates, indicating that a small admixture of oxides ca n change the 18 mu m/10 mu m band ratio dramatically. The near-infrare d opacity of the iron-rich oxides is of comparable magnitude to that o f Fe3O4. Therefore, small amounts of iron-rich oxides in addition to s ilicates raise the absorbance in this spectral range in a very efficie nt way.