  A detailed radiative transfer analysis of the observed continuum and
molecular line emission toward the deeply embedded young stellar object IRAS
16293-2422 is performed. The continuum modelling is used to constrain the
temperature and density distributions in the envelope, enabling quantitative
estimates of various molecular abundances. The molecular excitation analysis
reveals that the emission from some molecular species is well reproduced
assuming a constant fractional abundance throughout the envelope. The
abundances and isotope ratios are generally close to typical values found in
cold molecular clouds in these cases, and there is a high degree of deuterium
fractionation. There are, however, a number of notable exceptions. Lines
covering a wide range of excitation conditions indicate for some molecules,
e.g., H2CO, CH3OH, SO, SO2 and OCS, a drastic increase in their abundances in
the warm and dense inner region of the circumstellar envelope. The location at
which this increase occurs is consistent with the radius at which ices are
expected to thermally evaporate off the grains. In all, there is strong
evidence for the presence of a `hot core' close to the protostar, whose
physical properties are similar to those detected towards most high mass
protostars except for a scaling factor. However, the small scale of the hot gas
and the infalling nature of the envelope lead to very different chemical time
scales between low mass and high mass hot cores, such that only very rapidly
produced second-generation complex molecules can be formed in IRAS 16293-2422.
Alternatively, the ices may be liberated due to grain-grain collisions in
turbulent shear zones where the outflow interacts with the envelope.
