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تسجيل مستخدم جديدLow temperature MIR to submillimeter mass absorption coefficient of interstellar dust analogues I: Mg-rich glassy silicates
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A wealth of data from the Herschel and Planck satellites and now from ALMA, 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. 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. Glassy silicates of mean composition (1-x)MgO - xSiO2 with x = 0.35, 0.40 and 0.50 were synthesized. The mass absorption coefficient (MAC) of the samples was measured in the spectral domain 30 - 1000 $mu$m for grain temperature in the range 300 K - 10 K and at room temperature in the 5 - 40 $mu$m domain. We find that the MAC of all samples varies with the grains temperature. In the FIR/submm, and above 30K, the MAC value at a given wavelength increases with the temperature as thermally activated absorption processes appear. The studied materials exhibit different and complex behaviors at long wavelengths (lambda $geq$ 200 to 700 $mu$m depending on the samples) and the MAC cannot be approximated by a single power law in ${lambda}^{-beta}$. These behaviors are attributed to the amorphous nature of dust and to the amount and nature of the defects within this amorphous structure. Above 20 $mu$m, 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. This 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
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To model the cold dust emission observed in the diffuse interstellar medium, in dense molecular clouds or in cold clumps that could eventually form new stars, it is mandatory to know the physical and spectroscopic properties of this dust and to under
stand its emission. This work is a continuation of previous studies aiming at providing astronomers with spectroscopic data of realistic cosmic dust analogues for the interpretation of observations. Ferromagnesium amorphous silicate dust analogues were produced with a mean composition close to $mathrm{Mg_{1-x}Fe_{x}SiO_3}$ with x = 0.1, 0.2, 0.3, 0.4. Part of each sample was annealed at 500$^{circ}$C for two hours in a reducing atmosphere to modify the oxidation state of iron. We have measured the mass absorption coefficient (MAC) of these ferromagnesium amorphous silicate dust analogues in the spectral domain 30 - 1000 $mu$m for grain temperature in the range 10 - 300 K and at room temperature in the 5 - 40 $mu$m range. The MAC of ferromagnesium samples behaves in the same way as the MAC of pure Mg-rich amorphous silicate samples. In the 30 - 300 K range, the MAC increases with increasing grain temperature whereas in the range 10 - 30 K, we do not see any change of the MAC. The MAC cannot be described by a single power law in ${lambda}^{-beta}$. The MAC of all the samples is much higher than the MAC calculated by dust models. The complex behavior of the MAC of amorphous silicates with wavelength and temperature is observed whatever the exact silicate composition (Mg vs. Fe amount). It is a universal characteristic of amorphous materials, and therefore of amorphous cosmic silicates, that should be taken into account in astronomical modeling. The enhanced MAC of the measured samples compared to the MAC calculated for cosmic dust model implies that dust masses are overestimated by the models.
Cold dust grains emission in the FIR/submm is usually expressed as a modified black body law in which the dust mass absorption coefficient (MAC), is described with a temperature- and wavelength-independent emissivity spectral index, beta. However, nu
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