SIMBAD references

We report a new sensitive but still negative search for molecular oxygen in interstellar space. We observed a dark molecular cloud at a redshift z=0.685 that allows us to search for the 368 and 424GHz oxygen lines, without being absorbed by atmospheric molecular oxygen. The absorbing cloud is seen in projection onto a background continuum source. Although we detect an optically thick C¹⁸O line in absorption in this dark cloud, we fail to detect the 424GHz O2 line in absorption. We confirm that the 02 abundance in the gas phase must be lower than 1.4x10^–2 that of CO (at 1σ), a result that has previously been obtained only on average over an entire galaxy. The large spatial resolution provided by absorption measurements allows us to draw this conclusion locally, on a typical dark cloud scale. The best limit at 1σ towards a single galactic dark cloud was up to now O2/CO≤0.07, and we improve this result by a factor 5.