Freeze-out of the gas-phase elements onto cold grains in dense interstellar and circumstellar media builds up ice mantles consisting of molecules that are mostly formed in situ (H2O, NH3, CO2, CO, CH3OH, and more). This review summarizes the detected infrared spectroscopic ice features and compares the abundances across Galactic, extragalactic, and Solar System environments. A tremendous amount of information is contained in the ice band profiles. Laboratory experiments play a critical role in the analysis of the observations. Strong evidence is found for distinct ice formation stages, separated by CO freeze-out at high densities. The ice bands have proven to be excellent probes of the thermal history of their environment. The evidence for the long-held idea that processing of ices by energetic photons and cosmic rays produces complex molecules is weak. Recent state-of-the-art observations show promise for much progress in this area with planned infrared facilities.

Observations of the Icy Universe

Freeze-out of the gas phase elements onto cold grains in dense interstellar and circumstellar media builds up ice mantles consisting of molecules that are mostly formed in situ (H2O, NH3, CO2, CO, CH3OH, and more). This review summarizes the detected infrared spectroscopic ice features and compares the abundances across Galactic, extragalactic, and solar system environments. A tremendous amount of information is contained in the ice band profiles. Laboratory experiments play a critical role in the analysis of the observations. Strong evidence is found for distinct ice formation stages, separated by CO freeze out at high densities. The ice bands have proven to be excellent probes of the thermal history of their environment. The evidence for the long-held idea that processing of ices by energetic photons and cosmic rays produces complex molecules is weak. Recent state of the art observations show promise for much progress in this area with planned infrared facilities.

Many dense interstellar clouds are observable in emission in the near-IR, commonly referred to as "Cloudshine", and in the mid-IR, the so-called "Coreshine". These C-shine observations have usually been explained with grain growth but no model has yet been able to self-consistently explain the dust spectral energy distribution from the near-IR to the submm. We want to demonstrate the ability of our new core/mantle evolutionary dust model THEMIS (The Heterogeneous dust Evolution Model at the IaS), which has been shown to be valid in the far-IR and submm, to reproduce the C-shine observations. Our starting point is a physically motivated core/mantle dust model. It consists of 3 dust populations: small aromatic-rich carbon grains; bigger core/mantle grains with mantles of aromatic-rich carbon and cores either made of amorphous aliphatic-rich carbon or amorphous silicate. We assume an evolutionary path where these grains, when entering denser regions, may first form a second aliphatic-rich carbon mantle (coagulation of small grains, accretion of carbon from the gas phase), second coagulate together to form large aggregates, and third accrete gas phase molecules coating them with an ice mantle. To compute the corresponding dust emission and scattering, we use a 3D Monte-Carlo radiative transfer code. We show that our global evolutionary dust modelling approach THEMIS allows us to reproduce C-shine observations towards dense starless clouds. Dust scattering and emission is most sensitive to the cloud central density and to the steepness of the cloud density profile. Varying these two parameters leads to changes, which are stronger in the near-IR, in both the C-shine intensity and profile. With a combination of aliphatic-rich mantle formation and low-level coagulation into aggregates, we can self-consistently explain the observed C-shine and far-IR/submm emission towards dense starless clouds.

/pdf/mantle-formation-coagulation-and-the-origin-of-cloud-core-1ey4leo5cp.pdf

Mantle formation, coagulation and the origin of cloud/core-shine: II. Comparison with observations

Context. The project Galactic Cold Cores has carried out Herschel photometric observations of interstellar clouds where the Planck satellite survey has located cold and compact clumps. The sources represent different stages of cloud evolution from starless clumps to protostellar cores and are located in different Galactic environments. 
Aims. We examine this sample of 116 Herschel fields to estimate the submillimetre dust opacity and to search for variations that might be attributed to the evolutionary stage of the sources or to environmental factors, including the location within the Galaxy. 
Methods. The submillimetre dust opacity was derived from Herschel data, and near-infrared observations of the reddening of background stars are converted into near-infrared optical depth. We investigated the systematic errors affecting these parameters and used modelling to correct for the expected biases. The ratio of 250 μm and J band opacities is correlated with the Galactic location and the star formation activity. We searched for local variations in the ratio τ(250 μm)/τ(J) using the correlation plots and opacity ratio maps. 
Results. We find a median ratio of τ(250 μm) /τ(J) = (1.6 ± 0.2) × 10^(-3), which is more than three times the mean value reported for the diffuse medium. Assuming an opacity spectral index β = 1.8 instead of β = 2.0, the value would be lower by ~ 30%. No significant systematic variation is detected with Galactocentric distance or with Galactic height. Examination of the τ(250 μm) /τ(J) maps reveals six fields with clear indications of a local increase of submillimetre opacity of up to τ(250 μm) /τ(J) ~ 4 × 10^(-3) towards the densest clumps. These are all nearby fields with spatially resolved clumps of high column density. 
Conclusions. We interpret the increase in the far-infrared opacity as a sign of grain growth in the densest and coldest regions of interstellar clouds.

/pdf/galactic-cold-cores-v-dust-opacity-4n0nk8tkpf.pdf

Galactic cold cores - V. Dust opacity

Context. Using observations to deduce dust properties, grain-size distribution, and physical conditions in molecular clouds is a highly degenerate problem.Aims. The coreshine phenomenon, a scattering process at 3.6 and 4.5 μm that dominates absorption, has revealed its ability to explore the densest parts of clouds. We use this effect to constrain the dust parameters. The goal is to investigate to what extent grain growth (at constant dust mass) inside molecular clouds is able to explain the coreshine observations. We aim to find dust models that can explain a sample of Spitzer coreshine data. We also examine the consistency with near-infrared data we obtained for a few clouds.Methods. We selected four regions with a very high occurrence of coreshine cases: Taurus-Perseus, Cepheus, Chameleon, and L183/L134. We built a grid of dust models and investigated the key parameters to reproduce the general trend of surface brightnesses and intensity ratios of both coreshine and near-infrared observations with the help of a 3D Monte Carlo radiative transfer code. The grid parameters allowed us to investigate the effect of coagulation upon spherical grains up to 5 μm in size derived from the DustEm diffuse interstellar medium grains. Fluffiness (porosity or fractal degree), ices, and a handful of classical grain-size distributions were also tested. We used the near- and mostly mid-infrared intensity ratios as strong discriminants between dust models.Results. The determination of the background-field intensity at each wavelength is a key issue. In particular, an especially strong background field explains why we do not see coreshine in the Galactic plane at 3.6 and 4.5 μm. For starless cores, where detected, the observed 4.5 μm/3.6 μm coreshine intensity ratio is always lower than ~0.5, which is also what we find in the models for the Taurus-Perseus and L183 directions. Embedded sources can lead to higher fluxes (up to four times higher than the strongest starless core fluxes) and higher coreshine ratios (from 0.5 to 1.1 in our selected sample). Normal interstellar radiation-field conditions are sufficient to find suitable grain models at all wavelengths for starless cores. The standard interstellar grains are not able to reproduce observations and, because of the multiwavelength approach, only a few grain types meet the criteria set by the data. Porosity does not affect the flux ratios, while the fractal dimension helps to explain coreshine ratios, but does not seem able to reproduce near-infrared observations without a mix of other grain types.Conclusions. Combined near- and mid-infrared wavelengths confirm the potential of revealing the nature and size distribution of dust grains. Careful assessment of the environmental parameters (interstellar and background fields, embedded or nearby reddened sources) is required to validate this new diagnostic.

/pdf/dust-properties-inside-molecular-clouds-from-coreshine-3xl5et45cu.pdf

Dust properties inside molecular clouds from coreshine modeling and observations

Context: Observations of scattered light in the 1-5 $\mu$m range have revealed dust grains in molecular cores with sizes larger than commonly inferred for the diffuse interstellar medium. It is currently unclear whether these grains are grown within the molecular cores or are an ubiquitous component of the interstellar medium. Aims: We investigate whether the large grains necessary for efficient scattering at 1-5 mum are associated with the abundance of water-ice within molecular clouds and cores. Methods: We combined water-ice abundance measurements for sight lines through the Lupus IV molecular cloud complex with measurements of the scattered light at 3.6 mum for the same sight lines. Results: We find that there is a similar threshold for the cores in emission in scattered light at 3.6 mum (tau_9.7=0.15pm0.05, A_K=0.4pm0.2 as water-ice (tau_9.7=0.11pm0.01, A_K=0.19pm0.04) and that the scattering efficiency increases as the relative water-ice abundance increases. The ice layer increases the average grain size, which again strongly increases the albedo. Conclusions: The higher scattering efficiency is partly due to layering of ice on the dust grains. Although the layer can be relatively thin it can enhance the scattering substantially.

/pdf/a-common-column-density-threshold-for-scattering-at-3-6-mum-3jj905ex9c.pdf

A common column density threshold for scattering at 3.6 mum and water-ice in molecular clouds

Recently discovered scattered light at 3-5 $\mu$m from low-mass cores (so-called "coreshine") reveals the presence of grains around 1 $\mu$m, which is larger than the grains found in the low-density interstellar medium. But only about half of the 100+ cores investigated so far show the effect. This prompts further studies on the origin of this detection rate. From the 3D continuum radiative transfer equation, we derive the expected scattered light intensity from a core placed in an arbitrary direction seen from Earth. We use the approximation of single scattering, consider extinction up to 2nd-order Taylor approximation, and neglect spatial gradients in the dust size distribution. The impact of the directional characteristics of the scattering on the detection of scattered light from cores is calculated for a given grain size distribution, and local effects like additional radiation field components are discussed. The surface brightness profiles of a core with a 1D density profile are calculated for various Galactic locations, and the results are compared to the approximate detection limits. We find that for optically thin radiation and a constant size distribution, a simple limit for detecting scattered light from a low-mass core can be derived that holds for grains with sizes smaller than 0.5 $\mu$m. The extinction by the core prohibits detection in bright parts of the Galactic plane, especially near the Galactic center. For scattered light received from low-mass cores with grain sizes beyond 0.5 $\mu$m, the directional characteristics of the scattering favors the detection of scattered light above and below the Galactic center, and to some extent near the Galactic anti-center. We identify the local incident radiation field as the major unknown causing deviations from this simple scheme.

/pdf/detecting-scattered-light-from-low-mass-molecular-cores-at-3-1hbswlfgce.pdf

Detecting scattered light from low-mass molecular cores at 3.6 $\mu$m - Impact of global effects on the observation of coreshine

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A common column density threshold for scattering at 3.6 mum and water-ice in molecular clouds

Detecting scattered light from low-mass molecular cores at 3.6 $\mu$m - Impact of global effects on the observation of coreshine