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

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 μ m is detected, showing the first observation of this line in absorption outside the solar system. The 112 μ 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 12 CO (2–1) and (3–2), and 13 CO (2–1) transitions at the CSO, and in the C 18 O and C 17 O (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)} -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$^{3}~M_\odot$).