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Context: Dark cloud chemical models usually predict large amounts of O2,often above observational limits. Aims: We investigate the reason for thisdiscrepancy from a theoretical point of view, inspired by the studies ofJenkins and Whittet on oxygen depletion. Methods: We use the gas-grain codeNautilus with an up-to-date gas-phase network to study the sensitivity of themolecular oxygen abundance to the oxygen elemental abundance. We use the ratecoefficient for the reaction O + OH at 10 K recommended by the KIDA (KIneticDatabase for Astrochemistry) experts. Results: The updates of rate coefficientsand branching ratios of the reactions of our gas-phase chemical network,especially N + CN and H3+ + O, have changed the model sensitivity to the oxygenelemental abundance. In addition, the gas-phase abundances calculated with ourgas-grain model are less sensitive to the elemental C/O ratio than thosecomputed with a pure gas-phase model. The grain surface chemistry plays therole of a buffer absorbing most of the extra carbon. Finally, to reproduce thelow abundance of molecular oxygen observed in dark clouds at all times, we needan oxygen elemental abundance smaller than 1.6E-4. Conclusion: The chemistry ofmolecular oxygen in dense clouds is quite sensitive to model parameters thatare not necessarily well constrained. That O2 abundance may be sensitive tonitrogen chemistry is an indication of the complexity of interstellarchemistry.

Oxygen depletion in dense molecular clouds: a clue to a low O2 abundance?