(Sub-)millimeter absorption of silicates

The continuum dust opacity at far-infrared and millimeter wavelengths is an astronomically very important quantity, for instance for determining the amount of cold dust in interstellar clouds or in cold disks. Observations have found indications for a strong T-dependence of the spectral opacity slope for galactic dust [1] which might be an effect of low-energetic absorption processes, such as defect-induced phonon absorption, relaxation absorption, phonon-difference processes. Although these absorption mechanisms have been discovered decades ago, data for cosmic-dust candidate materials such as amorphous and crystalline silicates are scarce.

We studied the far-infrared and sub-millimeter dust opacity of silicates within the DFG priority programme SPP 1573 "Physics of the Interstellar Medium" in a collaboration with Frank Lewen from I. Phys. Institut der Universität Köln (Cologne). In Cologne, a millimeter-wave spectrometer for wavelengths up to 4 mm, has been developed [2]. From the results of this project, we derived complex refractive indices of Mg/Fe silicate glasses over a wide wavelength range (see Fig. 1). In addition, we measured the far-infrared absorption coefficient of olivine and enstatite crystals [3]. The latter study was additionally supported by DFG within the Research Unit FOR2285 “Debris disks in Planetary Systems”.