Near-infrared absorption properties of oxygen-rich stardust analogs

The influence of coloring metal ions

¹ Astrophysikalisches Institut, Schillergässchen 2-3, 07745 Jena, Germany
e-mail: sz@astro.uni-jena.de; mutschke@astro.uni-jena.de
² Institut für Astronomie, Türkenschanzstraße 17, 1180 Wien, Austria
e-mail: thomas.posch@univie.ac.at; hannes.richter@univie.ac.at
³ Institut für Optik und Quantenelektronik, Max-Wien-Platz 1, 07743 Jena, Germany
e-mail: ortrud.wehrhan@uni-jena.de

Received: 16 June 2010
Accepted: 15 October 2010

Abstract

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.