Volume 584, December 2015
|Number of page(s)||11|
|Section||Atomic, molecular, and nuclear data|
|Published online||27 November 2015|
Hydrogen isotope exchanges between water and methanol in interstellar ices
Univ. Grenoble Alpes, IPAG,
2 CNRS, IPAG, 38000 Grenoble, France
3 Aix-Marseille Université, PIIM UMR-CNRS 7345, 13397 Marseille, France
Received: 8 May 2015
Accepted: 22 September 2015
The deuterium fractionation of gas-phase molecules in hot cores is believed to reflect the composition of interstellar ices. The deuteration of methanol is a major puzzle, however, because the isotopologue ratio [CH2DOH]/[CH3OD], which is predicted to be equal to 3 by standard grain chemistry models, is much larger (~20) in low-mass hot corinos and significantly lower (~1) in high-mass hot cores. This dichotomy in methanol deuteration between low-mass and massive protostars is currently not understood. In this study, we report a simplified rate equation model of the deuterium chemistry occurring in the icy mantles of interstellar grains. We apply this model to the chemistry of hot corinos and hot cores, with IRAS 16293-2422 and the Orion KL Compact Ridge as prototypes, respectively. The chemistry is based on a statistical initial deuteration at low temperature followed by a warm-up phase during which thermal hydrogen/deuterium (H/D) exchanges occur between water and methanol. The exchange kinetics is incorporated using laboratory data. The [CH2DOH]/[CH3OD] ratio is found to scale inversely with the D/H ratio of water, owing to the H/D exchange equilibrium between the hydroxyl (-OH) functional groups of methanol and water. Our model is able to reproduce the observed [CH2DOH]/[CH3OD] ratios provided that the primitive fractionation of water ice [HDO]/[H2O] is ~2% in IRAS 16293-2422 and ~0.6% in Orion KL. We conclude that the molecular D/H ratios measured in hot cores may not be representative of the original mantles because molecules with exchangeable deuterium atoms can equilibrate with water ice during the warm-up phase.
Key words: astrochemistry / molecular processes / solid state: volatile / ISM: molecules
© ESO, 2015
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