Absorption properties of synthetic Cr-doped spinels in the UV, visible and infrared range and their astronomical implications

Summary Among the oxides, spinels are relatively abundant constituents of stardust, as has been inferred from studies of presolar grains in meteorites. Up to now, only pure Mg–Al-spinels have been considered as a possible stardust component. However, cosmically abundant transition metals such as iron or chromium may well be incorporated in spinels in the process of their formation in stellar atmospheres. We have produced synthetic Cr-doped spinels in order to study their UV, visible and IR spectra. Mass absorption coefficients (MACs) have been derived from transmission spectroscopy over a large wavelength range. For a Cr content of 5%, a maximum MAC in the UV (close to 200 nm) of 1000 cm²/g was found. For a Cr content of 10%, the maximum UV-MAC of spinel exceeds the maximum IR-MAC of 2000 cm²/g. The MIR bands of Cr-doped spinels are shifted to longer wavelengths with increasing Cr content, namely by ∼0.1 μm per 5% Cr in the range covered by our measurements. We conclude that a Cr content of spinel amounting to a few percents (<10%) is compatible with astronomical observations of spinel-bearing dusty environments, while a larger chromium content of spinels is not consistent with the presently available astronomical data.