Observations of the Icy Universe

Abstract

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

Formation of Glycerol through Hydrogenation of CO Ice under Prestellar Core Conditions

“…Therefore, the CO hydrogenation composes the core of the reaction scheme presented in Figure 1. CH 3 OH ice has been observed in dense clouds (Pontoppidan et al 2003;Bottinelli et al 2010;Boogert et al 2015), and its abundance is in line with the predictions of theoretical and modeling work (Tielens & Hagen 1982;Shalabiea & Greenberg 1994;Geppert et al 2005;Cuppen et al 2009Cuppen et al , 2011Vasyunin & Herbst 2013a). Recent experimental and theoretical studies demonstrated that twocarbon-bearing COMs, i.e., glycolaldehyde and ethylene glycol, are also formed at this stage (Woods et al 2012;Fedoseev et al 2015;Butscher et al 2015;Chuang et al 2015).…”

Spectroscopic constraints on CH3OH formation: CO mixed with CH3OH ices towards young stellar objects

“…Observational studies show that H 2 O freezes out at much lower extinctions (Whittet et al 2001;Chiar et al 2011;Boogert et al 2013;Whittet et al 2013) than CO, which needs a higher density to freeze out (Chiar et al 1994). Methanol forms at even higher extinction as shown by ice observations (Boogert et al 2011;Chiar et al 2011;Whittet et al 2011), which is consistent with the picture that a relatively high density is needed to efficiently convert CO into CH 3 OH Boogert, Gerakines & Whittet 2015).…”

A study of interstellar aldehydes and enols as tracers of a cosmic ray-driven nonequilibrium synthesis of complex organic molecules

“…To conclude, our study identified two keto-enol tautomer pairs, acetaldehyde-vinyl alcohol along with propanal-(E)/(Z)-1-propenol, synthesized upon interaction of ionizing radiation with ices of carbon monoxide-methane and carbon monoxide-ethane ices representing interstellar analogue ices toward high-mass starforming regions (10,17). The aldehydes along with their enol tautomers are formed via a vigorous cosmic-ray-driven chemistry inside the ices at ultralow temperatures before their sublimation into star-forming regions, but not through traditional radicalradical recombination in the warm-up phase of the irradiated ices as postulated for the last decades.…”

“…These apolar interstellar model ices composed of carbon monoxide with methane or ethane under anhydrous conditions were chosen to explore the proof of concept that enols along with their keto tautomers can be formed via interaction with ionizing radiation. Accounting for data from the Spitzer space telescope, ices containing CO at levels of up to 50% were observed toward high-mass star-forming regions (17). Apolar mixtures with methane (CH 4 ) at levels of up to 11% were also confirmed (17).…”

“…Accounting for data from the Spitzer space telescope, ices containing CO at levels of up to 50% were observed toward high-mass star-forming regions (17). Apolar mixtures with methane (CH 4 ) at levels of up to 11% were also confirmed (17). It should be noted that ethane (C 2 H 6 ) has not yet been detected on interstellar grains, but laboratory experiments verify that ethane can be formed easily by subjecting methanebearing ices to ionizing radiation.…”