Low temperature optical properties of interstellar and circumstellar icy
  silicate grain analogues in the mid-infrared spectral region
  Two different silicate/water ice mixtures representing laboratory analogues
of interstellar and circumstellar icy grains were produced in the laboratory.
For the first time, optical constants, the real and imaginary parts of the
complex refractive index, of such silicate/water ice mixtures were
experimentally determined in the mid-infrared spectral region at low
temperatures. In addition, optical constants of pure water ice and pure
silicates were derived in the laboratory. Two sets of constants were compared,
namely, measured constants calculated from the transmission spectra of
silicate/water ice samples and effective constants calculated from the optical
constants of pure silicates and pure water ice samples using different mixing
rules (effective medium approaches). Differences between measured and effective
constants show that a mixing (averaging) of optical constants of water ice and
silicates for the determination of the optical properties of silicate/ice
mixtures can lead to incorrect results. Also, it is shown that a part of water
ice molecules is trapped in/on silicate grains and does not desorb up to 200 K.
Our unique data are just in time with respect to the new challenging space
mission, James Webb Space Telescope, which will be able to bring novel detailed
information on interstellar and circumstellar grains, and suitable laboratory
data are extremely important for the decoding of astronomical spectra.
