Low temperature FIR and submm mass absorption coefficient of
  interstellar silicate dust analogues
  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, numerous data from space and balloon-born missions and
recently from Herschel and Planck show that dust emission is not well
understood, as revealed by the observed anti-correlation of beta with the grain
temperature. In order to give astronomers the necessary data to interpret
FIR/submm observations, we synthesised analogues of interstellar amorphous and
crystalline silicate grains, rich in Mg and Ca, and having stiochiometry of
olivine and pyroxene and measured their MAC, in the 100-1000/1500 \mum range
for grain temperatures varying from 300 to 10 K. We find that 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: at short wavelengths (lambda < 500/700
\mum), beta is in the range 1.6 - 2.1 for all grain temperature and grain
composition whereas at longer wavelengths (lambda > 500/700 \mum), beta < 2 for
samples with a pyroxene stoichiometry and beta > 2 for samples with an olivine
stoichiometry. Hence, 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 thus 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.
