1999A&A...347L...1C

We present spectroscopic ISO-LWS observations of the [OI] (63µm and 145µm), the [CII] (158µm) and the H₂O (179µm) lines towards the molecular cloud L1689N. From the observed ratio of the two [OI] lines, we deduce a mean gas temperature of (26±0.5)K, an H₂ density ≥3x10⁴cm^–3 and an [OI] column density ≥5x10¹⁹cm^–2. Combining these observations with previous CO observations, we obtain [OI]/[CO]∼50. This ratio implies that up to 98% of oxygen abundance is in atomic form in the gas phase. Furthermore, assuming all the gaseous carbon is locked into the CO, carbon has to be depleted by more than a factor 24. Finally, the upper limit derived for the H₂O (179µm) line (3x10^–13erg/s/cm²) implies that the water abundance in this region is less than 6x10^–7 with respect to H nuclei.