Dust/ice mixing in cold regions and solid-state water in the diffuse interstellar medium

Abstract

Whether ice in cold cosmic environments is physically separated from the silicate dust or mixed with individual silicate moieties is not known. However, different grain models give very different compositions and temperatures of grains. The aim of the present study is to compare the mid-infrared spectra of laboratory silicate grain/water ice mixtures with astronomical observations to evaluate the presence of dust/ice mixtures in interstellar and circumstellar environments. The laboratory data can explain the observations, assuming reasonable mass-averaged temperatures for the protostellar envelopes and protoplanetary disks, demonstrating that a substantial fraction of water ice may be mixed with silicate grains. On the basis of the combination of laboratory data and infrared observations, we provide evidence of the presence of solid-state water in the diffuse interstellar medium. Our results have implications for future laboratory studies investigating cosmic dust grain analogues and for future observations trying to identify the structure, composition and temperature of grains in different astrophysical environments. Laboratory spectra of dust/ice aggregates are comparable to those observed in astrophysical environments, questioning the traditional onion-like layered ice model. According to observational spectra, such icy mixtures could harbour water ice in the diffuse interstellar medium, as well as in low-temperature circumstellar environments.