Low-temperature MIR to submillimeter mass absorption coefficient of
  interstellar dust analogues II: Mg and Fe-rich amorphous silicates
  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 understand 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.
