Far-infrared dust opacity

 

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. Since from laboratory work there are only few data available in this wavelength range, this quantity and its wavelength and temperature dependence are not yet very well known. However, observations have found indications for a strong T-dependence of the spectral opacity slope for galactic dust [1].

The interstellar-dust opacity at these wavelengths should be determined by the structurally amorphous silicate dust, with likely additional contributions from conducting constituents, such as iron and carbon. The low-energetic absorption processes, which are responsible for the high far-infrared absorption/emission coefficient of silicate glasses and its T-dependence, have been partly investigated by solid-state physicists in the 70ies. Astrophysicists have tackled this problem for more relevant silicates in the end of the 90ies (see references in [2]). An attempt to extend these studies has been undertaken in 2003-2005 in our laboratory in a collaboration with colleagues from Toulouse [2]. They have also recently obtained new results on sol-gel produced silicates [3]. Much less data are available for crystalline silicates, carbonaceous materials, and even for water ice, which is an abundant constituent of dust in dark clouds and in the outer regions of circumstellar disks.

We study the far-infrared and sub-millimeter dust opacity within two DFG-funded research projects. One of them is focusing on interstellar dust and is part of the DFG priority programme SPP 1573 "Physics of the Interstellar Medium". This project is a collaboration with Frank Lewen from I. Phys. Institut der Universität Köln (Cologne). Since 2012 we work on the T-dependent absorption of our lab-produced Fe-containing silicate glasses (see Fig. 1), but also of aggregate particles composed of more than one material. An example of FTIR- (Jena) and Microwave- (Cologne) measured absorption coefficients (at room-temperature) is shown in Fig.2. In a following project phase, the absorption coefficients of amorphous carbon materials will be in the focus of the research.

The second project, which has started recently, is part of the DFG Research Unit (FOR 2285) "Debris disks in planetary systems" and is dedicated to dust ejected from minor bodies (mainly by collisions) in planetary systems. Here, we collaborate with theorists, mineralogists, and other laboratory astrophysicists to derive information on such systems from  observations of dust (thermal emission, scattering of starlight). Data on dust optics, collision mechanics, mineralogy, and irradiation physics is needed for that, apart from a complex theory. To measure more effectively dust opacities at low temperatures and long wavelengths (where the cold dust located far from the star emits), our lab has been equipped with a Time-domain THz spectrometer by DFG. Currently, we set up this instrument for low-temperature measurements.

[1] Desert, F.-X.; Macias-Perez, J. F.; Mayet, F.; et al., 2008, "Submillimetre point sources from the Archeops experiment: very cold clumps in the Galactic plane", Astron. Astrophys., 481, 411

[2] Boudet, N.; Mutschke, H.; Nayral, C.; Jäger, C.; Bernard, J.-P.; Henning, T.; Meny, C., 2005, "Temperature Dependence of the Submillimeter Absorption Coefficient of Amorphous Silicate Grains", Astrophys. J., 633, 272

[3] Coupeaud, A.; Demyk, K.; Meny, C.; et al., 2011, "Low-temperature FIR and submillimetre mass absorption coefficient of interstellar silicate dust analogues", Astron. Astrophys., 535, A124

Project Team: Pierre Mohr, Harald Mutschke, Frank Lewen (Cologne), Gabriele Born

Collaboration: Cornelia Jäger, Falko Langenhorst, Stefan Nolte (Jena), Jürgen Blum (Braunschweig), Karine Demyk (Toulouse), Sari Vilaplana (Alcoy), Sebastian Wolf (Kiel), Alexander Krivov, Torsten Löhn (Jena)

 

schmelze   sio2 abs p1

Fig. 1 Synthesis of silicate glasses by melting/quenching

Fig. 2 Measured FIR and microwave absorption coefficient of spherical SiO2 particles (d=1.5µm) at room temperature. The masses given in the legend indicate the amount of sample used in the measurements (pressed to pellets of 1.3 cm diameter).

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