A Search for O_2 in CO-depleted Molecular Cloud Cores with Herschel
  The general lack of molecular oxygen in molecular clouds is an outstanding
problem in astrochemistry. Extensive searches with SWAS, Odin and Herschel have
only produced two detections; upper limits to the O_2 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 on to dust, then O_2 could be formed
efficiently in the gas. Using Herschel HIFI we searched a small sample of four
depletion cores - L1544, L694-2, L429, Oph D - for emission in the low
excitation O_2 N_J=3_3-1_2 line at 487.249 GHz. Molecular oxygen was not
detected and we derive upper limits to its abundance in the range N(O_2)/N(H_2)
= (0.6 - 1.6)x10^{-7}. We discuss the absence of O_2 in the light of recent
laboratory and observational studies.
