EDP Sciences
Free access
Issue
A&A
Volume 453, Number 3, July III 2006
Page(s) L47 - L50
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:20065569


A&A 453, L47-L50 (2006)
DOI: 10.1051/0004-6361:20065569

Letter

Spatial mapping of ices in the Ophiuchus-F core

A direct measurement of CO depletion and the formation of CO $\mathsf{_2}$
K. M. Pontoppidan1, 2

1  California Institute of Technology, Division for Geological and Planetary Sciences, MS 150-21, Pasadena, CA 91125, USA
    e-mail: pontoppi@gps.caltech.edu
2  Leiden Observatory, PO Box 9513, 2300 RA, Leiden, The Netherlands

(Received 9 May 2006 / Accepted 22 May 2006)

Abstract
Aims.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 6.8 $\mu$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 $2\times 10^3$ to $14\times 10^3\,$AU or densities from $5\times 10^4$ to $5\times 10^5\,\rm cm^{-3}$ with a resolution of $\sim 3000\,$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 $\rm 814\pm 30 \,\rm K$.


Key words: astrochemistry -- molecular processes -- ISM: molecules -- infrared: ISM



© ESO 2006

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