Spatial mapping of ices in the Ophiuchus-F core

Free access article

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

A direct measurement of CO depletion and the formation of CO $\mathsf{_2}$

K. M. Pontoppidan^{1, 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

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