Interstellar Ice: The Infrared Space Observatory Legacy

We present 2.5-30 μm 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 μm ``ice band wing'' and the bulk of the 6.8 μm feature. In addition, we find evidence for excess absorption at 6.0 μm that cannot be attributed to H<sub>2</sub>O ice. We examine the degree of intercorrelation of the 6.8 μm, 4.62 μm (``XCN'') and 6.0 μm (excess) features. Our results are consistent with the interpretation of the 6.8 and 4.62 μm features as due to NH⁺₄ 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 μm 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.