Polysulfanes on interstellar grains as a possible reservoir of
  interstellar sulphur
  The form of depleted sulphur in dense clouds is still unknown. Until now,
only two molecules, OCS and SO2, have been detected in interstellar ices but
cannot account for the elemental abundance of sulphur observed in diffuse
medium. Chemical models suggest that solid H2S is the main form of sulphur in
denser sources but observational constraints exist that infirm this hypothesis.
We have used the Nautilus gas-grain code in which new chemical reactions have
been added, based on recent experiments of H2S ice irradiation with UV photons
and high energy protons. In particular, we included the new species Sn, H2Sn
and C2S. We found that at the low temperature observed in dense clouds, i.e. 10
K, these new molecules are not efficiently produced and our modifications of
the network do not change the previous pre- dictions. At slightly higher
temperature, 20 K in less dense clouds or in the proximity of protostars, H2S
abundance on the surfaces is strongly decreased in favor of the polysulfanes
H2S3. Such a result can also be obtained if the diffusion barriers on the
grains are less im- portant. In the context of the life cycle of interstellar
clouds and the mixing between diffuse and denser parts of the clouds, the
depletion of sulphur in the form of polysulfanes or other sulphur polymers, may
have occurred in regions where the temperature is slightly higher than the cold
inner parts of the clouds.
