Letter to the Editor

HD 112 m in absorption and extreme CO depletion in a cold molecular cloud^*

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

Corresponding author: E. Caux, caux@cesr.fr

Received: 13 November 2001
Accepted: 21 December 2001

Abstract

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).