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 a comparison of the mid-IR spectra of laboratory silicate-grains/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. Based on 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 trying to investigate cosmic dust grain analogues and for future observations trying to identify the structure, composition, and temperature of grains in different astrophysical environments.2 Cosmic dust grains with carbonaceous and siliceous composition 1,2 represent the most pristine starting material for planetary systems, influence the thermodynamic properties of the medium by absorption and emission of stellar light, and provide a surface for key astrochemical reactions. Dust grains in cold dense astrophysical environments, such as dense interstellar clouds, protostellar envelopes and planet-forming disks beyond the snow line, are typically considered to be mixtures of dust particles with molecular ices. Water is the main constituent of these ices accounting for more than 60% of the ice in most lines of sight 3 . Ices are believed either to cover the surface of a dust core and/or to be physically mixed with dust. While the first case, ice-on-dust, has been intensively studied in the laboratory in recent decades, the second case, ice-mixed-with-dust, presents practically uncharted territory.The ice-on-dust case is typically modelled by ice mixtures deposited onto standard laboratory substrates, such as gold, copper or KBr, which are not characteristic of cosmic dust grains. In addition to these studies, there are studies on physical-chemical processes, such as formation, desorption, and diffusion of molecules, on the surface of cosmic dust analogues: amorphous carbon grains, atomic carbon foils, graphite, amorphous silica, and amorphous and crystalline silicate grains. The reader can find examples of and references to these experimental studies in recent papers [4][5][6][7][8][9] .Concerning ice-mixed-with-dust, this is a new direction of research started very recently by us [10][11][12] . One of the conclusions of our previous study of the optical constants of dust/ice mixtures 11 was that differences between measured constants and constants calculated using effective medium approaches show that a mathematical mixing (averaging) of the optical constants of water ice and silicates f...

Physics and chemistry on the surface of cosmic dust grains: a laboratory view

“…Analysis of cometary dust particles and laboratory analogues of cosmic dust grains; and evolutionary models of dust grains lead to this conclusion (see [280] and references therein).…”

“…Experimental studies of dust/ice mixtures are a new direction of research pioneered by one of the authors of this review [49,50,280,281]. Some results, on desorption of H2O and CO mixed with carbon and silicate grains and on the photochemistry of carbon grains/H2O ice mixtures, have been already discussed in Sections 6.3 and 7.2.…”

“…Recently, the mid-IR spectra of laboratory water ice samples and silicate grains/water ice mixtures were compared with astronomical observations to evaluate the presence of dust/ice mixtures in interstellar environments and circumstellar environments of young stars [280].…”

Fitting infrared ice spectra with genetic modelling algorithms

“…Despite the challenges, new laboratory data combining ice and dust could provide a better understanding of the link between ice and dust in star-forming regions. To this end, Potapov et al (2018Potapov et al ( , 2021 measured spectrum and optical constants of silicate (MgSiO 3 ) and H 2 O ice, and showed that this experimental data can partially fit the 3 µm of some astronomical objects.…”

Fitting infrared ice spectra with genetic modelling algorithms. Presenting the ENIIGMA fitting tool

“…Despite the challenges, new laboratory data combining ice and dust could provide a better understanding of the link between ice and dust in star-forming regions. With this goal, Potapov et al (2018Potapov et al ( , 2021 have measured spectrum and optical constants of silicate (MgSiO 3 ) and H 2 O ice, and showed that this experimental data can partially fit the 3 µm of some astronomical objects.…”