Oxygen fractionation in dense molecular clouds

TL;DR

This paper introduces a novel gas-grain chemical model for oxygen fractionation in dense molecular clouds, revealing significant variability driven by exchange reactions and aligning well with observations, thus providing a new tool for studying cloud evolution.

Contribution

It is the first model to incorporate oxygen and sulfur fractionation in dense clouds, highlighting the role of exchange reactions and matching observed isotopic ratios.

Findings

Gas-phase chemistry causes variable oxygen fractionation levels.

The S16O/S18O ratio varies significantly over time, serving as a chemical proxy.

Model predictions align well with observed isotopic ratios in cold cores.

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.