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A detailed radiative transfer analysis of the observed continuum andmolecular line emission toward the deeply embedded young stellar object IRAS16293-2422 is performed. The continuum modelling is used to constrain thetemperature and density distributions in the envelope, enabling quantitativeestimates of various molecular abundances. The molecular excitation analysisreveals that the emission from some molecular species is well reproducedassuming a constant fractional abundance throughout the envelope. Theabundances and isotope ratios are generally close to typical values found incold molecular clouds in these cases, and there is a high degree of deuteriumfractionation. There are, however, a number of notable exceptions. Linescovering a wide range of excitation conditions indicate for some molecules,e.g., H2CO, CH3OH, SO, SO2 and OCS, a drastic increase in their abundances inthe warm and dense inner region of the circumstellar envelope. The location atwhich this increase occurs is consistent with the radius at which ices areexpected to thermally evaporate off the grains. In all, there is strongevidence for the presence of a `hot core' close to the protostar, whosephysical properties are similar to those detected towards most high massprotostars except for a scaling factor. However, the small scale of the hot gasand the infalling nature of the envelope lead to very different chemical timescales between low mass and high mass hot cores, such that only very rapidlyproduced second-generation complex molecules can be formed in IRAS 16293-2422.Alternatively, the ices may be liberated due to grain-grain collisions inturbulent shear zones where the outflow interacts with the envelope.Comment: 23 pages, 9 figures, accepted by A&

Does IRAS 16293–2422 have a hot core? Chemical inventory and abundance changes in its protostellar environment