Near-infrared absorption properties of oxygen-rich stardust analogs. The influence of coloring metal ions

Context. Several astrophysically relevant solid oxides and silicates have extremely small opacities in the visual and near-infrared in their pure forms. Datasets for the opacities and for the imaginary part k of their complex indices of refraction are hardly available in these wavelength ranges.
Aims: We aimed at determining k for spinel, rutile, anatase, and olivine, especially in the near-infrared region. Our measurements were made with impurity-containing, natural, and synthetic stardust analogs.
Methods: Two experimental methods were used: preparing small sections of natural minerals and synthesizing melt droplets under the electric arc furnace. In both cases, the aborption properties of the samples were measured by transmission spectroscopy.
Results: For spinel (MgAl₂O₄), anatase, rutile (both TiO₂), and olivine ((Mg,Fe)₂SiO₄), the optical constants have been extended to the visual and near-infrared. We highlight that the individual values of k(λ) and the absorption cross section Q_abs(λ) depend strongly on the content in transition metals like iron. Based on our measurements, we infer that k values below 10^-5 are very rare in natural minerals including stardust grains, if they occur at all.
Conclusions: Data for k and Q_abs(λ) are important for various physical properties of stardust grains such as temperature and radiation pressure. With increasing Q_abs(λ) due to impurities, the equilibrium temperature of small grains in circumstellar shells increases as well. We discuss why and to what extent this is the case.