2012MNRAS.426..354D

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 the diffuse medium. Chemical models suggest that solid H₂ S is the main form of sulphur in denser sources but observational constraints exist that greatly weaken this hypothesis. We have used the Nautilus gas–grain code in which new chemical reactions have been added, based on recent experiments of H₂ S ice irradiation with UV photons and high energy protons. In particular, we included the new species S_n, H₂S_n and C₂S. We found that at the low temperature observed in dense clouds, i.e. 10K, these new molecules are not efficiently produced and our modifications of the network do not change the previous predictions. At slightly higher temperature, 20K in less dense clouds or in the proximity of protostars, H₂ S abundance on the surfaces is strongly decreased in favour of the polysulphanes H₂S₃. Such a result can also be obtained if the diffusion barriers on the grains are less important. 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 polysulphanes or other sulphur polymers may have occurred in regions where the temperature is slightly higher than the cold inner parts of the clouds.