Large [O]/[CO] Ratios in Cold Molecular Clouds Towards W49N

We present ISO-LWS high spectral resolution (R 10 4 ) observations of the (OI) 63 m and 145m lines towards the high mass star formation region W49N. At 145m the ob- served spectrum shows an emission component only, while at 63 m it shows both emission and absorption components. We performed an analysis of the observed spectra using measured profiles of the LWS Fabry-Pspectral responses. We also present 12 CO and 13 CO J=1 ! 0a nd J=2 ! 1 ob- servations of the same line of sight and detected seven molecular clouds. We interpret the observed (OI) 63 m line absorption as due to the oxygen associated with both atomic hydrogen clouds and molecular clouds present on the line of sight. We found that HI clouds are responsible only for a fraction of this absorption and that at least 15 times more O than CO has to be present in the molecular clouds to reproduce the observed 63 m absorption. This implies that the gaseous oxygen is almost totally in atomic form and that carbon is deficient by more than a factor six in these molecular clouds.