ASTROPHYSICS Searching for 16 O 18

After our first tentative detection of the molecular oxygen 16O18O isotopomer obtained during the winter ’92/93 (Pagani et al. 1993), we attempted to confirm our results by making a deep search in other sources and in other positions in L134N. We have not been able to confirm the original detection in any other place in L134N but we obtain good upper limits towards NGC 2264(IRS2), TMC2, OMC3 and L134N (4’, -1’) and (5.2’, -1) which give an O2/CO ratio upper limit near 0.1 — the lowest yet obtained toward galactic sources— instead of 0.3–0.4 as predicted by gas phase chemical equilibriummodels. As our sample consists of eight cold or lukewarm dark clouds, the low abundance of O2 with respect to CO seems to be a general characteristic of dark clouds.