Oxygen fractionation in dense molecular clouds

22 pags., 5 figs.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, and SO. This large effect is due to the efficiency of the neutral O + NO, O + SO, and O + O 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/SO 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.This work was supported by the program “Physique et Chimie du Milieu Interstellaire” (PCMI) funded by CNRS and CNES. VW researches are funded by the ERC Starting Grant (3DICE, grant agreement 336474). Computer time was provided by the Pôle Modélisation HPC facilities of the Institut des Sciences Moléculaires UMR 5255 CNRS − Université de Bordeaux, co-funded by the Nouvelle Aquitaine region. M.A., N.M., J.C. and J.R.C. thank the Spanish MICIU for funding support under grants AYA2016-75066-C2-1-P and AYA2017-85111-P, and the European Research Council for support under grant ERC2013-Syg-610256-NANOCOSMOS. M.A. also acknowledges funding support from the Ramón y Cajal programme of Spanish MICIU (RyC-2014-16277). IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain)