Sulfur chemistry: 1D modeling in massive dense cores
  The main sulfur-bearing molecules OCS, H2S, SO, SO2, and CS have been
observed in four high mass dense cores (W43-MM1, IRAS 18264, IRAS 05358, and
IRAS 18162). Our goal is to put some constraints on the relative evolutionary
stage of these sources by comparing these observations with time-dependent
chemical modeling. We used the chemical model Nahoon, which computes the
gas-phase chemistry and gas-grain interactions of depletion and evaporation.
Mixing of the different chemical compositions shells in a 1D structure through
protostellar envelope has been included since observed lines suggest nonthermal
supersonic broadening. Observed radial profiles of the temperature and density
are used to compute the chemistry as a function of time. With our model, we
underproduce CS by several orders of magnitude compared to the other S-bearing
molecules, which seems to contradict observations, although some uncertainties
in the CS abundance observed at high temperature remain. The OCS/SO2, SO/SO2,
and H2S/SO2 abundance ratios could in theory be used to trace the age of these
massive protostars since they show a strong dependence with time, but the
sources are too close in age compared to the accuracy of chemical models and
observations. Our comparison between observations and modeling may, however,
indicate that W43-MM1 could be chemically younger than the three other sources.
Turbulent diffusivity through the protostellar envelopes has to be less
efficient than 2e14 cm2s-1. Otherwise, it would have smoothed out the abundance
profiles, and this would have been observed. The sulfur chemistry depends
strongly on the 1D physical conditions. In our case, no conclusion can be given
on the relative age of IRAS 18264, IRAS 18162 and IRAS 05358 except that they
are very close. W43-MM1 seems younger than the other sources.
