Spatial mapping of ices in the Ophiuchus-F core -: A direct measurement of CO depletion and the formation of CO2

Aims. Ices in dense star-forming cores contain the bulk of volatile molecules apart from H-2 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 6.8 mu m band, the spatial distribution of the CO/CO2 ice system and the 6.8 mu m band carrier are measured in a nearby dense core. Methods. VLT-ISAAC, ISOCAM-CVF and Spitzer-IRS archival mid-infrared (3-20 mu m) 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 2 x 103 to 14 x 10(3) AU or densities from 5 x 10(4) to 5 x 10(5) cm(-3) with a resolution of similar to 3000 AU. Results. 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 6.8 mu m 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 +/- 30 K.