Chemical fractionation of deuterium in the protosolar nebula

Understanding the 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 chemistry. 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 pre-stellar core hamper 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 a high molecular D/H ratio and a high gas-phase abundance.