Abstract

We have developed the first gas-grain chemical model for oxygen fractionation (also including sulphur fractionation) in dense molecular clouds, demonstrating that gas-phase chemistry generates variable oxygen fractionation levels, with a particularly strong effect for NO, SO, O-2, and SO2. This large effect is due to the efficiency of the neutral O-18 + NO, O-18 + SO, and O-18 + O-2 exchange reactions. The modelling results were compared to new and existing observed isotopic ratios in a selection of cold cores. The good agreement between model and observations requires that the gas-phase abundance of neutral oxygen atoms is large in the observed regions. The (SO)-O-16/(SO)-O-18 ratio is predicted to vary substantially over time, showing that it can be used as a sensitive chemical proxy for matter evolution in dense molecular clouds.