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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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.

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Observations of interstellar linear polarization in the spectral range 0.35È2.2 km are presented for several stars reddened by dust in the Taurus region. Combined with a previously published study by Whittet et al., these results represent the most comprehensive data set available on the spectral dependence of interstellar polarization in this nearby dark cloud (a total of 27 sight lines). Extinction data for these and other reddened stars in Taurus are assembled for the same spectral range, combining

We discuss the composition of dust and ice along the line of sight to the Galactic center (GC) based on analysis of mid-infrared spectra (2.4È13 km) from the Short Wavelength Spectrometer on the Infrared Space Observatory (ISO). We have analyzed dust absorption features arising in the molecular cloud material and the di †use interstellar medium along the lines of sight toward Sgr A* and the Quintuplet sources, GCS 3 and GCS 4. It is evident from the depth of the 3.0 km and the 4.27 km ice H 2 O C O 2 features that there is more molecular cloud material along the line of sight toward Sgr A* than toward GCS 3 and GCS 4. In fact, Sgr A* has a rich infrared ice spectrum with evidence for the presence of solid and possibly HCOOH. Hydrocarbon dust in the di †use interstellar medium along the CH 4 , NH 3 , line of sight to the GC is characterized by absorption features centered at 3.4, 6.85, and 7.25 km. Ground-based studies have identiÐed the 3.4 km feature with aliphatic hydrocarbons, and ISO has given us the Ðrst meaningful observations of the corresponding modes at longer wavelengths. The integrated strengths of these three features suggest that hydrogenated amorphous carbon is their carrier. We attribute an absorption feature centered at 3.28 km in the GCS 3 spectrum to the CwH stretch in aromatic hydrocarbons. This feature is not detected, and its CwC stretch counterpart appears to be weaker, in the Sgr A* spectrum. A key question now is whether or not aromatics are a widespread component of the di †use interstellar medium, analogous to aliphatic hydrocarbons.