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

Abstract

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 112 mu m is detected, showing the first observation of this line in absorption outside the solar system. The 112 mu m absorption depth gives a straightforward measure of the lower limit of the cloud's HD column density, N(HD) > 8 x 10¹⁸ 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, N(CO) < 7 x 10¹⁷ cm^-2. Assuming a standard CO abundance (1 x 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 ( ~ 10³ M_sun). Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, The Netherlands and the UK) with the participation of ISAS and NASA.