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IRAS 16293-2422 is a well studied low-mass protostar characterized by astrong level of deuterium fractionation. In the line of sight of theprotostellar envelope, an additional absorption layer, rich in singly anddoubly deuterated water has been discovered by a detailed multiline analysis ofHDO. To model the chemistry in this source, the gas-grain chemical codeNautilus has been used with an extended deuterium network. For the protostellarenvelope, we solve the chemical reaction network in infalling fluid parcels ina protostellar core model. For the foreground cloud, we explored severalphysical conditions (density, cosmic ionization rate, C/O ratio). The mainresults of the paper are that gas-phase abundances of H2O, HDO and D2O observedin the inner regions of IRAS16293-2422 are lower than those predicted by a 1Ddynamical/chemical (hot corino) model in which the ices are fully evaporated.The abundance in the outer part of the envelope present chaotic profiles due toadsorption/evaporation competition, very different from the constant abundanceassumed for the analysis of the observations. We also found that the largeabundances of gas-phase H2O, HDO and D2O observed in the absorption layer aremore likely explained by exothermic surface reactions rather thanphotodesorption processes.

Chemical modeling of water deuteration in IRAS16293-2422