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.
Comment: 23 pages, 9 figures, accepted by A&A
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