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 mass absorption coefficient at a given wavelength , { \kappa } _ { \lambda _ { 0 } } , and the temperature- and wavelength-independent emissivity spectral index , \beta .
However , numerous data from previous space and balloon-born 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 \beta with the grain temperature . The aim of this work is to measure the optical properties of interstellar dust analogues at low temperatures to give astronomers the necessary data to interpret FIR/submm observations such as those from the Herschel and Planck satellites . We synthesised , via sol-gel methods , analogues of interstellar amorphous silicate grains , rich in Mg and Ca , and having stiochiometry of olivine and pyroxene .
The samples are characterised by various techniques to determine their composition , size , amorphization degree .
All the amorphous samples are annealed at 1100 ^ { \circ } C to study the crystallised materials for comparison .
We measured the MAC of all the samples , in the 2-25 \mu m range at room temperature and in the 100-1000/1500 \mu m range for grain temperatures varying from 300 to 10 K . 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 , \beta , 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 : it can not be described by a single spectral index over the 100-1500 \mu m range .
At short wavelengths ( \lambda \leq 500/700 \mu m ) , \beta is in the range 1.6 - 2.1 for all grain temperature and grain composition .
However , at longer wavelengths ( \lambda \geq 500/700 \mu m ) , \beta \leq 2 for samples with a pyroxene stoichiometry and \beta \geq 2 for samples with an olivine stoichiometry . 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 and 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 .