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Oxygen fractionation in dense molecular clouds.

Jean-Christophe Loison1, Valentine Wakelam2, Pierre Gratier2

  • 1Institut des Sciences Moléculaires (ISM), CNRS, Univ. Bordeaux, 351 cours de la Libération, 33400, Talence, France.

Monthly Notices of the Royal Astronomical Society
|August 21, 2019
PubMed
Summary

This study introduces a new chemical model for oxygen and sulfur fractionation in dense molecular clouds. The model shows gas-phase chemistry significantly impacts isotopic ratios, particularly for molecules like sulfur monoxide (SO), and can serve as a proxy for cloud evolution.

Keywords:
ISM: abundancesISM: cloudsPhysical Data and Processes: astrochemistry

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Area of Science:

  • Astrochemistry
  • Molecular Cloud Chemistry
  • Isotopic Fractionation

Background:

  • Dense molecular clouds are crucial sites for star and planet formation.
  • Understanding chemical processes, including isotopic fractionation, is key to interpreting observations of these regions.

Purpose of the Study:

  • To develop the first gas-grain chemical model for oxygen and sulfur fractionation in dense molecular clouds.
  • To investigate the role of gas-phase chemistry in generating variable isotopic ratios.
  • To assess the potential of isotopic ratios as tracers of cloud evolution.

Main Methods:

  • Development of a novel gas-grain chemical model.
  • Inclusion of key oxygen and sulfur fractionation pathways.
  • Comparison of model predictions with observational data of isotopic ratios in cold cores.

Main Results:

  • Gas-phase chemistry significantly influences oxygen fractionation, with notable effects on NO, SO, O2, and SO2.
  • Efficient neutral 18O exchange reactions drive the observed fractionation.
  • Model results show good agreement with observations when assuming a large gas-phase abundance of neutral oxygen atoms.

Conclusions:

  • The developed model successfully reproduces observed isotopic ratios in dense molecular clouds.
  • Oxygen fractionation is highly variable and driven by specific gas-phase reactions.
  • The sulfur-16O/sulfur-18O (S16O/S18O) ratio can serve as a sensitive chemical proxy for tracking the evolution of matter in dense molecular clouds.