HD 112 μm in absorption and extreme CO depletion in a cold molecular cloud

We present ISO-LWS observations at high spectral resolution (Rsimilar to10(4)) towards a cold molecular cloud in the line of sight of W49. The HD ground state transition at 112 mum is detected, showing the first observation of this line in absorption outside the solar system. The 112 mum absorption depth gives a straightforward measure of the lower limit of the cloud's HD column density, N(HD) > 8x10(18) cm(-2). We also mapped the same line of sight in the (CO)-C-12 (2-1) and (3-2), and 13 CO (2-1) transitions at the CSO, and in the (CO)-O-18 and (CO)-O-17 (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) < 7x10(17) cm(-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 (similar to 10(3) M-circle dot).