Low-temperature FIR and submillimetre mass absorption coefficient of interstellar silicate dust analogues

Abstract

Context. Cold dust grains are responsible for the far-infrared and submillimetre (FIR/submm) emission observed by Herschel and Planck. Their thermal emission is usually expressed as a modified black body law in which the FIR/submm dust opacity, or mass absorption coefficient (MAC), is described by the MAC at a given wavelength κ λ 0 and the temperature-and wavelength-independent emissivity spectral index β. However, numerous data from previous space and balloon-borne missions and recently from Herschel and Planck show that the dust emission is not well understood, as revealed for example by the observed anti-correlation of β with the grain temperature. Aims. The aim of this work is to measure the optical properties of interstellar dust analogues at low temperatures to give astronomers the necessary data for interpreting FIR/submm observations such as those from the Herschel and Planck satellites. Methods. We synthesised, via sol-gel methods, analogues of interstellar amorphous silicate grains, rich in Mg and Ca, and having stoichiometry of olivine and pyroxene. The samples are characterised by various techniques to determine their composition, size, amorphisation degree. All the amorphous samples are annealed at 1100• C to study the crystallised materials for comparison. We measured the MAC of all the samples in the 2-25 μm range at room temperature and in the 100-1000/1500 μm range for grain temperatures varying from 300 to 10 K. Results. The experimental results show that, for all the amorphous samples, the grain MAC decreases when the grain temperature decreases and that the local spectral index, β, defined as the slope of the MAC curve, is anti-correlated with the grain temperature. These variations, which are not observed in the crystallised samples, are related to the amorphous nature of the samples. In addition, the spectral shape of the MAC is complex and cannot be described by a single spectral index over the 100-1500 μm range. At short wavelengths (λ ≤ 500/700 μm), β is in the range 1.6-2.1 for all grain temperature and grain composition. However, at longer wavelengths (λ ≥ 500/700 μm), β ≤ 2 for samples with a pyroxene stoichiometry and β ≥ 2 for samples with an olivine stoichiometry. Conclusions. The dust properties in the FIR/submm domain and at low temperature are more complicated than expected. The simplifying asymptotic expression based on a single temperature-and wavelength-independent spectral index used by astronomers is not appropriate to describe the dust MAC, hence the dust emission, and may induce significant errors on the derived parameters, such as the dust mass and the dust physical and chemical properties. Instead, dust emission models should use the dust MAC as a function of wavelength and temperature.

Search for grain growth toward the center of L1544

“…However, the measured changes are not significant (within 20%) at mm-wavelengths, when the temperature varies between 6 and 10 K, the temperature range relevant for the central regions of prestellar cores. Boudet et al (2005) and Coupeaud et al (2011) found similar results for amorphous material, as they observed an increase in the spectral index (decrease in the opacity) while decreasing the temperature (for T > 10 K). However, when they let the material crystallize, no temperature dependence was detected.…”

“…Laboratory measurements of the opacity and the spectral index at millimeter wavelengths found a dependence of the mass absorption with temperature for different grain compositions (Agladze et al 1996;Mennella et al 1998;Boudet et al 2005;Coupeaud et al 2011;Demyk et al 2013). Agladze et al (1996) found two different behaviors depending on the temperature range: for very low temperatures (1.2 -20 K) the millimeter opacity decreases with increasing temperature, while for temperatures between 20 and 30 K, it increases or is constant with temperature.…”

Planckearly results. XXIII. The first all-sky survey of Galactic cold clumps

“…15; Désert et al 2008), although the Archeops relation is clearly compatible with our results given the uncertainties. Although a β(T ) relation similar to that observed is expected for amorphous grains with disordered structure (Meny et al 2007;Boudet et al 2005) and for silicates, as shown more recently in laboratory experiments (Coupeaud et al 2011), a robust physical interpretation cannot be inferred from our results because the clump sample is not a well-defined and homogeneous class of objects. Nevertheless, these studies suggest caution when interpreting T c -β relations for samples containing star-forming clouds.…”

Variation in the dust emissivity index across M 33 withHerschelandSpitzer(HerM 33es)

“…The difference in dust spectra between 10 K and 20 K is very difficult to identify in laboratory experiments (e.g. Coupeaud et al 2011). It is currently unclear whether laboratory measurements support a physical β − T anti-correlation for dust in the 10−20 K range.…”