2002A&A...383L...9C

We present ISO-LWS observations at high spectral resolution (R∼10⁴) towards a cold molecular cloud in the line of sight of W49. The HD ground state transition at 112µm is detected, showing the first observation of this line in absorption outside the solar system. The 112µm absorption depth gives a straightforward measure of the lower limit of the cloud's HD column density, N(HD) > 8x10¹⁸cm^–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, N(CO)<7x10¹⁷cm^–2. Assuming a standard CO abundance (1x10^–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 (∼10³M_☉).