Volume 383, Number 1, FebruaryIII 2002
|Page(s)||L9 - L13|
|Published online||15 February 2002|
Letter to the Editor
HD 112 m in absorption and extreme CO depletion in a cold molecular cloud*
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
Corresponding author: E. Caux, email@example.com
Accepted: 21 December 2001
We present ISO-LWS observations at high spectral resolution () towards a cold molecular cloud in the line of sight of W49. The HD ground state transition at 112 μm is detected, showing the first observation of this line in absorption outside the solar system. The 112 μm absorption depth gives a straightforward measure of the lower limit of the cloud's HD column density, 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 C18O and C17O (1–0) and (2–1) transitions at the IRAM 30-m. From these observations we derive an upper limit to the CO column density, cm-2. Assuming a standard CO abundance () 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).
Key words: ISM: abundances / ISM: lines and bands / ISM: molecules / ISM: clouds / ISM: individual objects: W49 / infrared: ISM / radio lines: ISM
© ESO, 2002
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