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

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.

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

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We present the first Spitzer Infrared Spectrograph observations of the 15.2 mm bending mode of CO 2 ice toward field stars behind a quiescent dark cloud. CO 2 ice is detected toward two field stars (Elias 16 and Elias 3) and a single protostar (HL Tau) with an abundance of ∼15%-20% relative to water ice. CO 2 ice is not detected toward the source with lowest extinction in our sample, Tamura 17 (A V p 3.9 mag). A comparison of the Elias 16 spectrum with laboratory data demonstrates that the majority of CO 2 ice is embedded in a polar, H 2 O-rich ice component, with ∼15% of CO 2 residing in an apolar, H 2 O-poor mantle. This is the first detection of apolar CO 2 toward a field star. We find that the CO 2 extinction threshold is A V p 4 ‫1ע‬ mag, comparable to the threshold for water ice, but significantly less than the threshold for CO ice, the likely precursor of CO 2 . Our results confirm that CO 2 ice forms in tandem with H 2 O ice along quiescent lines of sight. This argues for CO 2 ice formation by means of a mechanism similar to that responsible for H 2 O ice formation, viz., simple catalytic reactions on grain surfaces.

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