Low temperature optical properties of interstellar and circumstellar icy silicate grain analogues in the mid-infrared spectral region

TL;DR

This study experimentally determined the optical constants of interstellar and circumstellar icy silicate grain analogues at low temperatures in the mid-infrared, providing essential data for interpreting astronomical spectra, especially relevant for the James Webb Space Telescope.

Contribution

First experimental determination of optical constants of silicate/water ice mixtures at low temperatures in the mid-infrared, highlighting limitations of mixing rules for optical properties.

Findings

Measured optical constants differ from effective medium calculations.

Water ice molecules are trapped in silicate grains up to 200 K.

Laboratory data support interpretation of space telescope observations.

Abstract

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.