2006A&A...453L..47P

Ices in dense star-forming cores contain the bulk of volatile molecules apart from H₂ and thus represent a large fraction of dark cloud chemistry budget. Mm observations of gas provide indirect evidence for significant freeze-out of CO in the densest cores. To directly constrain the freeze-out profile of CO, the formation route of CO₂ and the carrier of the 6.8µm band, the spatial distribution of the CO/CO₂ ice system and the 6.8µm band carrier are measured in a nearby dense core. VLT-ISAAC, ISOCAM-CVF and Spitzer-IRS archival mid-infrared (3-20µm) spectroscopy of young stellar objects is used to construct a map of the abundances of CO and CO₂ ices in the Oph-F star-forming core, probing core radii from 2x10³ to 14x10³AU or densities from 5x10⁴ to 5x10⁵cm^–3 with a resolution of ∼3000AU. The line-of-sight averaged abundances relative to water ice of both CO and CO₂ ices increase monotonously with decreasing distance to the core center. The map traces the shape of the CO abundance profile between freeze-out ratios of 5-60%. and shows that the CO₂ ice abundance increases by a factor of 2 as the CO freezes out. It is suggested that this indicates a formation route of CO₂ on a CO ice surface to produce a CO₂ component dilute in CO ice, in addition to a fraction of the CO₂ formed at lower densities along with the water ice mantle. It is predicted that the CO₂ bending mode band profile should reflect a high CO:CO₂ number ratio in the densest parts of dark clouds. In contrast to CO and CO₂, the abundance of the carrier of the 6.8µm band remains relatively constant throughout the core. A simple freeze-out model of the CO abundance profile is used to estimate the binding energy of CO on a CO ice surface to 814±30K.