Caux, E.; Ceccarelli, C.; Castets, A.; Vastel, C.; Liseau, R.; Molinari, S.; Nisini, B.; Saraceno, P. and White, G. J.
Large atomic oxygen abundance towards the molecular cloud L1689N.
Astronomy and Astrophysics, 347 L1-L4.
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We present spectroscopic ISO-LWS observations of the [OI] (63 μm and 145 μm), the [CII] (158 μm) and the H2O (179 μm) 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 H2O (179 μm) 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.
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