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Recent interferometer observations have found that the D2O/HDO abundanceratio is higher than that of HDO/H2O by about one order of magnitude in thevicinity of low-mass protostar NGC 1333-IRAS 2A, where water ice hassublimated. Previous laboratory and theoretical studies show that the D2O/HDOice ratio should be lower than the HDO/H2O ice ratio, if HDO and D2O ices areformed simultaneously with H2O ice. In this work, we propose that the observedfeature, D2O/HDO > HDO/H2O, is a natural consequence of chemical evolution inthe early cold stages of low-mass star formation: 1) majority of oxygen islocked up in water ice and other molecules in molecular clouds, where waterdeuteration is not efficient, and 2) water ice formation continues with muchreduced efficiency in cold prestellar/protostellar cores, where deuterationprocesses are highly enhanced due to the drop of the ortho-para ratio of H2,the weaker UV radiation field, etc. Using a simple analytical model and gas-iceastrochemical simulations tracing the evolution from the formation of molecularclouds to protostellar cores, we show that the proposed scenario canquantitatively explain the observed HDO/H2O and D2O/HDO ratios. We also findthat the majority of HDO and D2O ices are likely formed in coldprestellar/protostellar cores rather than in molecular clouds, where themajority of H2O ice is formed. This work demonstrates the power of thecombination of the HDO/H2O and D2O/HDO ratios as a tool to reveal the pasthistory of water ice formation in the early cold stages of star formation andwhen the enrichment of deuterium in the bulk of water occurred. Furtherobservations are needed to explore if the relation, D2O/HDO > HDO/H2O, iscommon in low-mass protostellar sources.

Reconstructing the history of water ice formation from HDO/H2O and D2O/HDO ratios in protostellar cores