Interstellar ice

Abstract: Of the over 150 different molecular species detected in the interstellar and circumstellar media, approximately 50 contain 6 or more atoms. These molecules, labeled complex by astronomers if not by chemists, all contain the element carbon and so can be called organic. In the interstellar medium, complex molecules are detected in the denser sources only. Although, with one exception, complex molecules have only been detected in the gas phase, there is strong evidence that they can be formed in ice mantles on interstellar grains. The nature of the gaseous complex species depends dramatically on the source where they are found: in cold, dense regions they tend to be unsaturated (hydrogen-poor) and exotic, whereas in young stellar objects, they tend to be quite saturated (hydrogen-rich) and terrestrial in nature. Based on both their spectra and chemistry, complex molecules are excellent probes of the physical conditions and history of the sources where they reside. Because they are detected in young stellar objects, complex molecules are expected to be common ingredients for new planetary systems. In this review, we discuss both the observation and chemistry of complex molecules in assorted interstellar regions in the Milky Way.

Abstract: 1. The galactic ecosystem 2. Cooling processes 3. Heating processes 4. Chemical processes 5. Interstellar dust 6. Interstellar polycyclic aromatic hydrocarbon molecules 7. HII regions 8. The phases of the ISM 9. Photodissociation regions 10. Molecular clouds 11. Interstellar shocks 12. Dynamics of the interstellar medium 13. The lifecycle of interstellar dust.

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.

Abstract: We present 2.5-30 mum spectra from the Short-Wavelength Spectrometer of the Infrared Space Observatory for a total of 23 sources. The sources include embedded young stellar objects spanning a wide range of mass and luminosity, together with field stars sampling quiescent dark clouds and the diffuse interstellar medium. Expanding on results of previous studies, we use these spectra to investigate ice composition as a function of environment. The spectra reveal an extremely rich set of absorption features attributed to simple molecules in the ices. We discuss the observed properties of these absorption features and review their assignments. Among the species securely identified are H2O, CO, CO2, CH3OH, and CH4. Likely identified species include OCS, H2CO, and HCOOH. There is also evidence for NH3 and OCN- ice features, but these identifications are more controversial. Features that continue to defy identification include the 3.3-3.7 mum "ice band wing'' and the bulk of the 6.8 mum feature. In addition, we find evidence for excess absorption at 6.0 mum that cannot be attributed to H2O ice. We examine the degree of intercorrelation of the 6.8 mum, 4.62 mum ("XCN'') and 6.0 mum (excess) features. Our results are consistent with the interpretation of the 6.8 and 4.62 mum features as due to NH4+ and OCN- ions, respectively, though alternative explanations cannot currently be ruled out. We find that the optical depth correlations are dependent on the profile of the 6.8 mum feature but not on the mass of the YSO nor the ice temperature along the line of sight. We discuss the implications for our current understanding of ice processing. We briefly discuss the composition, origin, and evolution of interstellar ices.