Infrared spectroscopy of solid CO: the   Ophiuchi molecular cloud*

Infrared spectra centered on the solid CO feature at 4.76 μm are presented for seven embedded objects in the ρ Oph molecular cloud. Synthetic spectra based on laboratory data for various ice mixtures containing CO are compared with the observations, in an attempt to constrain the abundances of both CO and other grain mantle constituents. The profiles indicate that the CO is embedded in only two types of matrix: pure (or nearly pure) CO, and H2O. A large abundance of CO2 mixed with CO in the mantles is ruled out. There are at least two components to the absorption in most cases, attributed to two or more different grain environments. Column densities of solid CO in these regions are presented, along with upper limits on the depth of the CO feature for a further eight lines of sight in the cloud. The features reflect varying physical and chemical conditions within the ρ Oph cloud. The composite features are fundamentally different within current observational limits from those seen in the Taurus dark cloud.