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.Comment: To appear in Annual Review of Astronomy and Astrophysics, volume 53, 2015. Updated 08/May/2015: corrected numbers in elemental budget section, updated references and typo

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To study the physical and chemical evolution of ices in solar-mass systems, a spectral survey is conducted of a sample of 41 low-luminosity YSOs (L $ 0:1Y10 L ) using 3Y38 m Spitzer and ground-based spectra. The sample is complemented with previously published Spitzer spectra of background stars and with ISO spectra of well-studied massive YSOs (L $ 10 5 L ). The long-known 6.0 and 6.85 m bands are detected toward all sources, with the Class 0Y type YSOs showing the deepest bands ever observed. The 6.0 m band is often deeper than expected from the bending mode of pure solid H 2 O. The additional 5Y7 m absorption consists of five independent components, which, by comparison to laboratory studies, must be from at least eight different carriers. Much of this absorption is due to simple species likely formed by grain surface chemistry, at abundances of 1%Y30% for CH 3 OH, 3%Y8% for NH 3 , 1%Y5% for HCOOH, $6% for H 2 CO, and $0.3% for HCOO À relative to solid H 2 O. The 6.85 m band has one or two carriers, of which one may be less volatile than H 2 O. Its carrier(s) formed early in the molecular cloud evolution and do not survive in the diffuse ISM. If an NH þ 4 -containing salt is the carrier, its abundance relative to solid H 2 O is $7%, demonstrating the efficiency of low-temperature acid-base chemistry or cosmic-rayYinduced reactions. Possible origins are discussed for enigmatic, very broad absorption between 5 and 8 m. Finally, the same ices are observed toward massive and low-mass YSOs, indicating that processing by internal UV radiation fields is a minor factor in their early chemical evolution.

This paper presents Spitzer-IRS λ/∆λ ∼ 600 spectroscopy of the CO 2 15.2 µm bending mode toward a sample of 50 embedded low-mass stars in nearby star-forming clouds, taken mostly from the "Cores to Disks (c2d)" Legacy program. The average abundance of solid CO 2 relative to water in low-mass protostellar envelopes is 0.32 ± 0.02, significantly higher than that found in quiescent molecular clouds and in massive star forming regions. It is found that a decomposition of all the observed CO 2 bending mode profiles requires a minimum of five unique components. In general, roughly 2/3 of the CO 2 ice is found in a water-rich environment, while most of the remaining 1/3 is found in a CO environment with strongly varying relative concentrations of CO 2 to CO along each line of sight. Ground-based observations of solid CO toward a large subset of the c2d sample are used to further constrain the CO 2 :CO component and suggest a model in which low-density clouds form the CO 2 :H 2 O component and higher density clouds form the CO 2 :CO ice during and after the freeze-out of gas-phase CO. The abundance of the CO 2 :CO component is consistent with cosmic ray processing of the CO-rich part of the ice mantles, although a more quiescent formation mechanism is not ruled out. It is suggested that the subsequent evolution of the CO 2 and CO profiles toward low-mass protostars, in particular the appearance of the splitting of the CO 2 bending mode due to pure, crystalline CO 2 , is first caused by distillation of the CO 2 :CO component through evaporation of CO due to thermal processing to ∼ 20 − 30 K in the inner regions of infalling envelopes. The formation of pure CO 2 via segregation from the H 2 O rich mantle may contribute to the band splitting at higher levels of thermal processing ( 50 K), but is harder to reconcile with the physical structure of protostellar envelopes around low-luminosity objects.

This paper presents, for the Ðrst time, a complete 2.4È25 km spectrum of the dust-embedded young stellar object W33A. The spectrum was obtained with the Short Wavelength Spectrometer of the Infrared Space Observatory at a mean resolving power of D750. The spectrum displays deep ice and silicate 1 Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries : France, Germany, the Netherlands, and the United Kingdom) and with the participation of ISAS and NASA.

Spectra of interstellarice absorption features at a resolving power of j/*j B 1500È2000 are pre-CO 2 sented for 14 lines of sight. The observations were made with the Short-Wavelength Spectrometer (SWS) of the Infrared Space Observatory (ISO). Spectral coverage includes the primary stretching mode of CO 2 near 4.27 km in all sources ; the bending mode near 15.2 km is also detected in 12 of them. The selected sources include massive protostars (Elias 29 [in o Oph], GL 490, GL 2136, GL 2591, GL 4176, NGC 7538 IRS 1, NGC 7538 IRS 9, S140, W3 IRS 5, and W33 A), sources associated with the Galactic Center (Sgr A*, GCS 3 I, and GCS 4), and a background star behind a quiescent dark cloud in Taurus (Elias 16) ; they thus probe a diverse range of environments. Column densities of interstellar ice relative to CO 2 ice fall in the range 10%È23% : this ratio displays remarkably little variation for such a physically H 2 O diverse sample. Comparison of the observed proÐles with laboratory data for ice mixtures CO 2 -bearing indicates that generally exists in at least two phases, one polar dominant) and one nonpolar CO 2 (H 2 O dominant). The observed proÐles may also be reproduced when the nonpolar components are (CO 2 CO 2 replaced with thermally annealed ices. Formation and evolutionary scenarios for and implications CO 2 for grain mantle chemistry are discussed. Our results support the conclusion that thermal annealing, rather than energetic processing due to UV photons or cosmic rays, dominates the evolution of CO 2 -ices.

Abstract. Medium resolution (λ/∆λ = 5000−10 000) VLT-ISAAC M-band spectra are presented of 39 young stellar objects in nearby low-mass star forming clouds showing the 4.67 µm stretching vibration mode of solid CO. By taking advantage of the unprecedentedly large sample, high S/N ratio and high spectral resolution, similarities in the ice profiles from source to source are identified. It is found that excellent fits to all the spectra can be obtained using a phenomenological decomposition of the CO stretching vibration profile at 4.67 µm into 3 components, centered on 2143.7 cm −1 , 2139.9 cm −1 and 2136.5 cm −1 with fixed widths of 3.0, 3.5 and 10.6 cm −1 , respectively. All observed interstellar CO profiles can thus be uniquely described by a model depending on only 3 linear fit parameters, indicating that a maximum of 3 specific molecular environments of solid CO exist under astrophysical conditions. A simple physical model of the CO ice is presented, which shows that the 2139.9 cm −1 component is indistinguishable from pure CO ice. It is concluded, that in the majority of the observed lines of sight, 60−90% of the CO is in a nearly pure form. In the same model the 2143.7 cm −1 component can possibly be explained by the longitudinal optical (LO) component of the vibrational transition in pure crystalline CO ice which appears when the background source is linearly polarised. The model therefore predicts the polarisation fraction at 4.67 µm, which can be confirmed by imaging polarimetry. The 2152 cm −1 feature characteristic of CO on or in an unprocessed water matrix is not detected toward any source and stringent upper limits are given. When this is taken into account, the 2136.5 cm −1 component is not consistent with the available water-rich laboratory mixtures and we suggest that the carrier is not yet fully understood. A shallow absorption band centered between 2165 cm −1 and 2180 cm −1 is detected towards 30 sources. For low-mass stars, this band is correlated with the CO component at 2136.5 cm −1 , suggesting the presence of a carrier different from XCN at 2175 cm −1 . Furthermore the absorption band from solid 13 CO at 2092 cm −1 is detected towards IRS 51 in the ρ Ophiuchi cloud complex and an isotopic ratio of 12 CO/ 13 CO = 68 ± 10 is derived. It is shown that all the observed solid 12 CO profiles, along with the solid 13 CO profile, are consistent with grains with an irregularly shaped CO ice mantle simulated by a Continuous Distribution of Ellipsoids (CDE), but inconsistent with the commonly used models of spherical grains in the Rayleigh limit.

Ices regulate much of the chemistry during star formation and account for up to 80% of the available oxygen and carbon. In this paper, we use the Spitzer c2d ice survey, complimented with data sets on ices in cloud cores and high-mass protostars, to 1 Hubble Fellow 2 Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, the Netherlands 3 Max Planck Institut für Extraterrestrische Physik (MPE), Giessenbachstr.1, 85748 Garching, Germany -2 -determine standard ice abundances and to present a coherent picture of the evolution of ices during low-and high-mass star formation. The median ice composition H 2 O:CO:CO 2 :CH 3 OH:NH 3 :CH 4 :XCN is 100:29:29:3:5:5:0.3 and 100:13:13:4:5:2:0.6 toward low-and high-mass protostars, respectively, and 100:31:38:4:-:-:-in cloud cores. In the low-mass sample, the ice abundances with respect to H 2 O of CH 4 , NH 3 , and the component of CO 2 mixed with H 2 O typically vary by <25%, indicative of co-formation with H 2 O. In contrast, some CO and CO 2 ice components, XCN and CH 3 OH vary by factors 2-10 between the lower and upper quartile. The XCN band correlates with CO, consistent with its OCN − identification. The origin(s) of the different levels of ice abundance variations are constrained by comparing ice inventories toward different types of protostars and background stars, through ice mapping, analysis of cloud-to-cloud variations, and ice (anti-)correlations. Based on the analysis, the first ice formation phase is driven by hydrogenation of atoms, which results in a H 2 O-dominated ice. At later prestellar times, CO freezes out and variations in CO freeze-out levels and the subsequent CO-based chemistry can explain most of the observed ice abundance variations. The last important ice evolution stage is thermal and UV processing around protostars, resulting in CO desorption, ice segregation and formation of complex organic molecules. The distribution of cometary ice abundances are consistent with with the idea that most cometary ices have a protostellar origin.