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Observations of star forming environments revealed that the abundances ofsome deuterated interstellar molecules are markedly larger than the cosmic D/Hratio of 10-5. Possible reasons for this pointed to grain surface chemistry.How- ever, organic molecules and water, which are both ice constituents, do notenjoy the same deuteration. For example, deuterated formaldehyde is veryabundant in comets and star forming regions, while deuterated water rarely is.In this article, we explain this selective deuteration by following theformation of ices (using the rate equation method) in translucent clouds, aswell as their evolu- tion as the cloud collapses to form a star. Ices startwith the deposition of gas phase CO and O onto dust grains. While reaction ofoxygen with atoms (H or D) or molecules (H2) yields H2O (HDO), CO only reactswith atoms (H and D) to form H2CO (HDCO, D2CO). As a result, the deuteration offormaldehyde is sensitive to the gas D/H ratio as the cloud undergoesgravitational collapse, while the deuteration of water strongly depends on thedust temperature at the time of ice formation. These results reproduce well thedeuterium fractionation of formaldehyde observed in comets and star formingregions and can explain the wide spread of deuterium fractionation of waterobserved in these environments.

Interstellar ices as witnesses of star formation: selective deuteration of water and organic molecules unveiled