OPTICAL-PROPERTIES OF OXIDE DUST GRAINS

In this paper, we present the optical properties of a series of well-characterized 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-rich stars show circumstellar dust features between 10 and 14 mu m and around 19 mu m which cannot be explained by ordinary cosmic silicates but may be modelled by a mixture of aluminium oxide and iron-magnesium oxides. Calculated absorption spectra for small spheres show one vibrational band in the range from 19.9 mu m (FeO) to 17.7 mu m (Fe0.4Mg0.6O) in the Rayleigh limit. The absolute peak values of the oxide absorption 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 can change the 18 mu m/10 mu m band ratio dramatically. The near-infrared opacity of the iron-rich oxides is of comparable magnitude to that of Fe3O4. Therefore, small amounts of iron-rich oxides in addition to silicates raise the absorbance in this spectral range in a very efficient way.