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Astron. Astrophys. 342, 257-270 (1999)


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Depletion of CO in a cold dense cloud core of IC 5146

C. Kramer 1, J. Alves 2, C.J. Lada 2, E.A. Lada 3, A. Sievers 4, H. Ungerechts 4 and C.M. Walmsley 5

1 I. Physikalisches Institut, Universität zu Köln, Zülpicher Strasse 77, D-50937 Köln, Germany
2 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
3 Astronomy Department, University of Florida, Gainesville, FL 32608, USA
4 Instituto de Radioastronomía Milimétrica (IRAM), Avenida Divina Pastora 7, Núcleo Central, E-18012 Granada, Spain
5 Osservatorio di Arcetri, Largo E. Fermi 5, I-50125 Firenze, Italy

Received 2 June 1998 / Accepted 20 October 1998

Abstract

We have used the IRAM 30m telescope to map the [FORMULA] (1[FORMULA]0) and (2[FORMULA]1) transitions in a region of [FORMULA] pc2 ([FORMULA]) in the nearby (460 pc distance) molecular cloud IC 5146 at resolutions of down to 0.025 pc. Additionally, we have observed the corresponding [FORMULA] transitions towards 24 positions in the central region of the cloud as a check on optical depth effects. On the basis of these observations, we conclude that the [FORMULA] emission observed by us is optically thin and that the observed [FORMULA] (2[FORMULA]1)/(1[FORMULA]0) ratio is consistent with gas at 10 K and density in the range [FORMULA] to [FORMULA] cm-3. We have estimated [FORMULA] column densities over the region mapped, using an LVG program to correct for excitation effects, and have compared them, at the same spatial resolution of [FORMULA], with the dust extinction derived from the NIR images of Lada et al. (1998). From this, we find a roughly constant, though with considerable dispersion, ratio of [FORMULA] column density to visual extinction ([FORMULA]) in directions with [FORMULA] less than [FORMULA] magnitudes. The ratio is compatible with previous determinations of the [[FORMULA]]/[H2] abundance ratio, e.g. [FORMULA] as obtained by Frerking et al. (1982). At extinctions higher than [FORMULA] magnitudes, however, there is a systematic fall-off in the ratio N([FORMULA])/[FORMULA] which we attribute to depletion of [FORMULA] onto dust ice mantles within the dense, [FORMULA] cm-3, and cold, [FORMULA] K, [FORMULA] K, core interior. This fall-off is also seen, though with less statistical significance, in our observed dependence of N([FORMULA])/[FORMULA] with [FORMULA]. We thus think it unlikely that optical depth effects are influencing our conclusions. We have additionally made pointed [FORMULA] (1[FORMULA]0) and (2[FORMULA]1) observations towards 94 background stars with extinction greater than 10 magnitudes from the study of Lada et al. (1994). These in general confirm our mapping results.

Key words: ISM: molecules – ISM: kinematics and dynamics – ISM: individual objects: IC 5146 – ISM: dust, extinction – ISM: clouds – ISM: abundances

Send offprint requests to: C. Kramer

Correspondence to: kramer@ph1.uni-koeln.de

© European Southern Observatory (ESO) 1999

Online publication: December 22, 1998

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