# 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.

## Key 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.

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Source: https://tomesphere.com/paper/1902.08840