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Context. Millimetric observations have measured high degrees of moleculardeuteration in several species seen around low-mass protostars. The HerschelSpace Telescope, launched in 2009, is now providing new measures of thedeuterium fractionation of water, the main constituent of interstellar ices.Aims. We aim at theoretically studying the formation and the deuteration ofwater, which is believed to be formed on interstellar grain surfaces inmolecular clouds. Methods. We used our gas-grain astrochemical model GRAINOBLE,which considers the multilayer formation of interstellar ices. We variedseveral input parameters to study their impact on water deuteration. Weincluded the treatment of ortho- and para-states of key species, including H2,which affects the deuterium fractionation of all molecules. The model alsoincludes relevant laboratory and theoretical works on the water formation anddeuteration on grain surfaces. In particular, we computed the transmissionprobabilities of surface reactions using the Eckart model, and we consideredice photodissociation following molecular dynamics simulations. Results. Theuse of a multilayer approach allowed us to study the influence of variousparameters on the abundance and the deuteration of water. Deuteration of wateris found to be very sensitive to the ortho-to-para ratio of H2 and to the totaldensity, but it also depends on the gas/grain temperatures and the visualextinction of the cloud. Since the deuteration is very sensitive to thephysical conditions, the comparison with sub-millimetric observation towardsthe low-mass protostar IRAS 16293 allows us to suggest that water ice is formedtogether with CO2 in molecular clouds with limited density, whilst formaldehydeand methanol are mainly formed in a later phase, where the condensation becomesdenser and colder.

Water ice deuteration: a tracer of the chemical history of protostars