  Understanding gas-grain chemistry of deuterium in star-forming objects may
help to explain their history and present state. We aim to clarify how
processes in ices affect the deuterium fractionation. In this regard, we
investigate a Solar-mass protostellar envelope using an astrochemical
rate-equation model that considers bulk-ice chem- istry. The results show a
general agreement with the molecular D/H abundance ratios observed in low-mass
protostars. The simultaneous processes of ice accumulation and rapid synthesis
of HD on grain surfaces in the prestellar core hampers the deuteration of icy
species. The observed very high D/H ratios exceeding 10 per cent, i.e., super-
deuteration, are reproduced for formaldehyde and dimethyl ether, but not for
other species in the protostellar envelope phase. Chemical transformations in
bulk ice lower D/H ratios of icy species and do not help explaining the
super-deuteration. In the protostellar phase, the D2O/HDO abundance ratio was
calculated to be higher than the HDO/H2O ratio owing to gas-phase chemistry.
Species that undergo evaporation from ices have high molecular D/H ratio and a
high gas-phase abundance.
