M. Garozzo

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 H2 Sa nd 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 )a re 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 W33A. Conclusions. Our results show that on a time scale comparable to the molecular cloud lifetime, the column density of H2S is strongly reduced and we suggest that this could explain the failure of its detection in the solid phase in the lines of sight of YSOs. We suggest that the solid OCS and SO2 detected in dense molecular clouds are formed after ion irradiation of icy grain mantles.

Ion irradiation of astrophysical ices

Ices, silicates and carbonaceous materials have been detected in several astrophysical environments such as interstellar molecular clouds, comets, and planetary surfaces. These solids are continuously exposed to ion irradiation and UV photolysis. Our knowledge on the properties of solids and molecules and on the modification induced by fast ions (keV-MeV) and UV photons is mainly based on laboratory experiments and on the comparison of experimental results with observations. Here we will give a few examples of the role of laboratory experiments to our understanding of the physical and chemical properties of ices in space.

Space weathering of asteroidal surfaces - Influence on the UV-Vis spectra

Context. The surfaces of airless bodies in the Solar System are continuously altered by the bombardment of micrometeoroids and irradiation by solar wind, flares,and cosmic particles.Major effects of thisprocess ‐ space weathering ‐ are darkening and “reddening” of the spectra of surface materials, as well as a “degrading” of absorption features. Aims. We studied the changes induced by energetic ion irradiation in the ultraviolet-visual-near-infrared (UV-Vis-NIR) (0.2‐0.98 μm) reflectance spectra of targets selected to mimic the surfaces of airless bodies in the inner Solar System. Our chosen targets are olivine pellets, pure or covered by an organic polymer (polystyrene), which is transparent before irradiation. Polystyrene is used as a template for organic matter of low volatility that can be present on asteroidal surfaces. Moreover we measured the changes induced by ion irradiation in the absorption coefficient of the polymer. The purpose was to have a tool to better compare laboratory with observed spectra and distinguish between planetary objects with pure silicate surfaces and those whose surface is covered by organic matter exposed to cosmic ion bombardment. Methods. The samples were irradiated in vacuum, at room temperature, with 200keV protons or 200‐400 keV argon ions. Before, during, and after irradiation diffuse reflectance spectra were acquired. Polystyrene films were also deposited on quartz substrates and irradiated while transmittance spectra were recorded. Results. We measured the variations of the absorption coefficient of polystyrene as a function of ion fluence. We showed that after ion irradiation the diffuse reflectance spectra of the samples covered by organics exhibit a much more significant variation than those of pure silicates. The spectra of targets made of olivine plus polystyrene can be fitted by using the measured absorption coefficient of polystyrene. Conclusions. The results obtained for pure olivine extend to the UV the spectral range of previous experiments. The data concerning the absorption coefficient of polystyrene are available on our web site (http://web.ct.astro.it/weblab/dbindex.html )a nd can be used to compare laboratory with astronomical spectra. This will possibly allow us to obtain information about organic matter on the surface of a given object, as well as on the relevance of the exposure to cosmic ions (space weathering).