Large atomic oxygen abundance towards the molecular cloud L1689N

Abstract

We present spectroscopic ISO-LWS observations of the [OI] (63 mu m and 145 mu m), the [CII] (158 mu m) and the H_2O (179 mu m) 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 >= 3 x 10(4) cm(-3) and an [OI] column density >= 5 x 10(19) cm(-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 H_2O (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. 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 United Kingdom) with the participation of ISAS and NASA.