2016ApJ...830..102W

The general lack of molecular oxygen in molecular clouds is an outstanding problem in astrochemistry. Extensive searches with the Submillimeter Astronomical Satellite, Odin, and Herschel have only produced two detections; upper limits to the O₂ abundance in the remaining sources observed are about 1000 times lower than predicted by chemical models. Previous atomic oxygen observations and inferences from observations of other molecules indicated that high abundances of O atoms might be present in dense cores exhibiting large amounts of CO depletion. Theoretical arguments concerning the oxygen gas-grain interaction in cold dense cores suggested that, if O atoms could survive in the gas after most of the rest of the heavy molecular material has frozen out onto dust, then O₂ could be formed efficiently in the gas. Using Herschel HIFI, we searched a small sample of four depletion cores-L1544, L694-2, L429, and Oph D-for emission in the low excitation O₂ N_J = 3₃-1₂ line at 487.249 GHz. Molecular oxygen was not detected and we derive upper limits to its abundance in the range of N(O₂)/N(H₂) ~ (0.6\mbox–1.6) × 10^–7. We discuss the absence of O₂ in the light of recent laboratory and observational studies.