Hydrogen isotope exchanges between water and methanol in interstellar ices

Abstract

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.

11 pages, 6 figures, accepted in Astronomy and Astrophysics

References in corpus (7)

• First detection of triply-deuterated methanol
• Multilayer formation and evaporation of deuterated ices in prestellar and protostellar cores
• Deuterated methanol in the pre-stellar core L1544
• The deuterium fractionation of water on solar-system scales in deeply-embedded low-mass protostars
• Warm water deuterium fractionation in IRAS 16293-2422 - The high-resolution ALMA and SMA view
• Thermal H/D exchange in polar ice - deuteron scrambling in space
• The D/H Ratio of Water Ice at Low Temperatures

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