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Issue A&A
Volume 383, Number 1, February III 2002
Page(s) L9 - L13
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:20020001



A&A 383, L9-L13 (2002)
DOI: 10.1051/0004-6361:20020001

Letter

HD 112 $\mathsf{\mu}$m in absorption and extreme CO depletion in a cold molecular cloud

E. Caux1, C. Ceccarelli2, 3, L. Pagani4, S. Maret1, A. Castets2 and J. R. Pardo5

1  CESR CNRS-UPS, BP 4346, 31028 Toulouse Cedex 04, France
2  Observatoire de Bordeaux, BP 89, 33270 Floirac, France
3  Laboratoire d'Astrophysique, Observatoire de Grenoble, BP 53, 38041 Grenoble Cedex 09, France
4  LRM-DEMIRM, 77 avenue Denfert Rochereau, 75014 Paris, France
5  Consejo Superior de Investigaciones Cientificas (CSIC), Madrid

(Received 13 November 2001 / Accepted 21 December 2001)

Abstract
We present ISO-LWS observations at high spectral resolution ( $R \sim 10^{4}$) towards a cold molecular cloud in the line of sight of W49. The HD ground state transition at 112 $\mu$m is detected, showing the first observation of this line in absorption outside the solar system. The 112 $\mu$m absorption depth gives a straightforward measure of the lower limit of the cloud's HD column density, $N{\rm (HD)} > 8 \times 10^{18}$ cm -2. We also mapped the same line of sight in the 12CO (2-1) and (3-2), and 13CO (2-1) transitions at the CSO, and in the C 18O and C 17O (1-0) and (2-1) transitions at the IRAM 30-m. From these observations we derive an upper limit to the CO column density, $N{\rm (CO)} < 7 \times 10^{17}$ cm -2. Assuming a standard CO abundance ( $1 \times 10^{-4}$) would imply a [D]/[H] abundance two orders of magnitude larger than the average [D]/[H] value observed in the solar neighborhood. The alternative explanation that we defend here is that CO is highly depleted (by a factor 100) in this cloud. This is the first measurement of such a depletion factor in a relatively massive cold molecular cloud (~10 $^{3}~M_\odot$).


Key words: ISM: abundances -- ISM: lines and bands -- ISM: molecules -- ISM: clouds -- ISM: individual objects: W49 -- infrared: ISM -- radio lines: ISM

Offprint request: E. Caux, caux@cesr.fr

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© ESO 2002


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