DIRECT SEARCH FOR O-2 IN A SINGLE MOLECULAR CLOUD

We report a new sensitive but still negative search for molecular oxyg en in interstellar space. We observed a dark molecular cloud at a reds hift z=0.685 that allows us to search for the 368 and 424 GHz oxygen l ines, without being absorbed by atmospheric molecular oxygen. The abso rbing cloud is seen in projection onto a background continuum source. Although we detect an optically thick (CO)-O-18 line in absorption in this dark cloud, we fail to detect the 424GHz O-2 line in absorption. We confirm that the O-2 abundance in the gas phase must be lower than 1.4 10(-2) that of CO (at 1 sigma), a result that has previously been obtained only on average over an entire galaxy. The large spatial reso lution provided by absorption measurements allows us to draw this conc lusion locally, on a typical dark cloud scale. The best limit at 1 sig ma towards a single galactic dark cloud was up to now O-2/CO less than or equal to 0.07, and we improve this result by a factor 5.