Large atomic oxygen abundance towards the molecular cloud L1689N

We present spectroscopic ISO-LWS observations of the [OI] (63 mu m and 145 mu m), the [CII] (158 mu m) and the H2O (179 mu m) lines towards the molecu lar cloud L1689N. From the observed ratio of the two [OI] lines, we deduce a mean gas temperature of (26 +/- 0.5) K, an H-2 density greater than or eq ual to 3 x 10(4) cm(-3) and an [OI] column density greater than or equal to 5 x 10(19) cm(-2). Combining these observations with previous CO observati ons, we obtain [OI]/[OD] similar to 50. This ratio implies that up to 98% o f oxygen abundance is in atomic form in the gas phase. Furthermore, assumin g all the gaseous carbon is locked into the CO, carbon has to be depleted b y more than a factor 24. Finally, the upper limit derived for the H2O (179 mu m) line (3 x 10(-13) erg s(-1) cm(-2)) implies that the water abundance in this region is less than 6 x 10(-7) with respect to H nuclei.