The fate of S-bearing species after ion irradiation of interstellar icy grain mantles

Context. Chemical models predict the presence of S-bearing molecules such as hydrogen sulfide (H2S) in interstellar icy grain mantles in dense molecular clouds. Up to now only two S-bearing molecules, namely sulfur dioxide (SO2) and carbonyl sulfide (OCS), have been detected in the solid phase towards young stellar objects (YSOs), while upper limits for solid H2S have been reported towards the same lines of sight. The estimated abundance of S-bearing molecules in icy grain mantles is not able to account for the cosmic S abundance. Aims. In this paper we studied the effects of ion irradiation on different icy targets formed by carbon monoxide (CO) and SO2 or H2S as mixtures and, for the first time, as layers. Methods. We carried out several irradiation experiments on ices containing SO2 or H2S mixed or layered with CO. The samples were irradiated with 200 keV protons in a high-vacuum chamber ($P < 10^{-7}$ mbar) at a temperature of 16–20 K. IR spectra of the samples were recorded after various steps of irradiation and after warm-up. Results. We have found that the column density of H2S and SO2, as well as CO, decreases after irradiation, and the formation of new molecular species is observed. In the case of CO:SO2 samples, OCS, sulfur trioxide (SO3), ozone (O3), and carbon dioxide (CO2) are the most abundant species formed. In the case of CO:H2S samples the most abundant species formed are OCS, SO2, carbon disulfide (CS2), hydrogen persulfide (H2S2), and CO2. The profile of the OCS band formed after irradiation of the CO:H2S mixture compares well with the profile of the OCS band detected towards the high mass YSO W33