Sulfur ice chemistry: Photolysis and radiolysis of OCS and H_2S in interstellar ice analogs

This work aims to provide new insights into the long-standing problem of sulfur depletion in dense interstellar clouds through laboratory simulations of the chemistry induced by ultraviolet (UV) photons and cosmic rays in ice mantles containing OCS and H_2S. Analogs of interstellar ices containing OCS and H_2S were irradiated under vacuum with 5 keV electrons and UV photons from a D_2 lamp. A special emphasis was paid to ternary ice mixtures of the S-bearing molecules with H_2O and CO_2. Changes in the ice composition upon irradiation were monitored with infrared (IR) spectroscopy. Additional information was derived from thermal programmed desorption (TPD) measurements In particular, OCS and H_2S were swiftly destroyed, both by electrons and by VUV photons; whereas H_2S_2, SO_2, and CS_2, as well as compounds related to sulfuric acid (HSO_3^-, HSO_4^-, SO_4^2-, and H_2SO_4), were identified in the IR spectra. The proportion of the latter species grew with increasing oxygen content in the mixture. Polysulfanes, H_2S_ with n=3,4 were also found in the TPD measurements. Part of the initial S atoms remain undetected after irradiation. The presence of H_2S was significant in all the irradiated mixtures, but the formation of OCS was not favored Sulfur in interstellar ices is probably distributed among many species including H_2S, SO_2, CS_2, and OCS, as well as compounds related to sulfuric acid and sulfur polymers, such as polysulfanes and sulfur allotropes. A relatively high proportion of H_2S and a significant amount of compounds related to H_2SO_4 should be expected. The detection of many of these species is problematic due to their low IR activity or to interferences from other ice components. We find that OCS and H_2S are destroyed in a time much shorter than the lifetime of dense clouds. Efficient formation mechanisms must be presumed to counter this fast destruction.