Polysulfanes on interstellar grains as a possible reservoir of interstellar sulphur

Abstract

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.