Low temperature MIR to submillimeter mass absorption coefficient of interstellar dust analogues: I. Mg-rich glassy silicates

Abstract

Context. The submillimeter spectral domain has been extensively explored by the Herschel and Planck satellites and is now reachable from the ground with ALMA. A wealth of data, revealing cold dust thermal emission, is available for astronomical environments ranging from interstellar clouds, cold clumps, circumstellar envelops, and protoplanetary disks. The interpretation of these observations relies on the understanding and modeling of cold dust emission and on the knowledge of the dust optical properties. Aims. The aim of this work is to provide astronomers with a set of spectroscopic data of realistic interstellar dust analogues that can be used to interpret the observations. It pursues the experimental effort aimed at characterizing the spectroscopic properties of interstellar dust analogues at low temperature in the mid-infrared (MIR) to millimeter spectral domain. Compared to previous studies, it extends the range of studied dust analogues in terms of composition and of structure of the material. Methods. Glassy silicates of mean composition (1 - x)MgO - xSiO2 with x = 0:35 (close to forsterite, Mg2SiO4), 0.50 (close to enstatite, MgSiO3) and 0.40 (close to Mg1.5SiO3.5 or MgSiO3:Mg2SiO4 = 50:50) were synthesized. The mass absorption coefficient (MAC) of the samples was measured in the spectral domain 30-1000 μm for grain temperature in the range 300-10 K and at room temperature in the 5-40 μm domain. Results. We find that the MAC of all samples varies with the grains temperature and that its spectral shape cannot be approximated by a single power law in λ-β. In the FIR/submm, and above 30 K, the MAC value at a given wavelength increases with the temperature as thermally activated absorption processes appear. The studied materials exhibit dggerent and complex behaviors at long wavelengths (λ ≥ 200 to 700 μm depending on the samples). These behaviors are attributed to the amorphous nature of dust and to the amount and nature of the defects within this amorphous structure. We do not observe MAC variations in the 10-30 K range. Above 20 μm, the measured MAC are much higher than the MAC calculated from interstellar silicate dust models indicating that the analogues measured in this study are more emissive than the silicates in cosmic dust models. Conclusions. The underestimated value of the MAC deduced from cosmic dust models in the FIR/submm has important astrophysical implications because masses are overestimated by the models. Moreover, constraints on elemental abundance of heavy elements in cosmic dust models are relaxed.