2011MNRAS.417.2809C

With this paper we provide a solution to a disagreement between astronomical- and laboratory-based CO-ice spectroscopic data. In observations towards icy sources, the CO-ice stretching band comprises a prominent and broad `red component' around 2136.5/cm. This feature is generally attributed to solid CO mixed in a hydrogen-bonded environment like H₂O, but, as far as we are aware, laboratory spectra have not been able to fully reproduce this feature. Water-containing CO ice cannot reproduce the observed band position and bandwidth without simultaneously producing a shoulder at 2152/cm (4.647 µm). This band, believed to originate from the interaction of dangling-OH bonds with CO, is not observed in astronomical spectra. Fraser et al. suggested that the 2152/cm feature is suppressed in astronomical ices by blocking of dangling-OH bonds by other species such as CO₂ and CH₃OH. In the present paper, we test this hypothesis by a systematic spectroscopic study of different H₂O:CO:CO₂mixtures. It is shown that even though the 2152/cm band is suppressed in low-temperature spectra, the width and peak position of the red component cannot be reproduced at the same time. An ice mixture containing only CO and CH₃OH, however, does reproduce the spectra at low temperatures, both in terms of peak position and width of the red component, and the 2152/cm band does not appear. This indicates that CO may reside in water-poor (rather than water-rich) ice in space. The astrophysical implications are discussed.