Oxygen fractionation in dense molecular clouds

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, O2, and SO2. This large effect is due to the efficiency of the neutral 18O + NO, 18O + SO, and 18O + O2 exchange reactions. The modeling 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 S16O/S18O 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.

22 pages, 5 figures

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• Molecule dependent oxygen isotopic ratios in the coma of comet 67P/Churyumov-Gerasimenko
• A sensitive 3 mm line survey of L483: a broad view of the chemical composition of a core around a Class 0 object
• Detection of HC18O+ in a protoplanetary disk: exploring oxygen isotope fractionation of CO
• Chemical compositions of five Planck cold clumps
• Theoretical studies of carbon isotopic fractionation in reactions of C with C${}_2$: dynamics, kinetics, and isotopologue equilibria