2017A&A...608A..12S

Context. Carbon dioxide (CO₂) is one of the most abundant species detected in icy grain mantles in star forming regions. Laboratory experiments have shown that CO₂ molecules are efficiently formed in the solid state under interstellar conditions. Specifically, solid CO₂ can be formed through energetic (e.g. UV photolysis, electron and ion bombardment) and non-energetic mechanisms (atom-addition reactions).
Aims. Here we investigate the role of low-energy cosmic-ray bombardment in the formation of solid CO₂ in quiescent dense molecular clouds.
Methods. We performed laboratory experiments to study the formation of CO₂ after ion irradiation with 200 keV H⁺ of astrophysical relevant ice mixtures. Laboratory spectra are used to fit the profile of the CO₂ bending mode band observed at about 15.2µm (660cm^–1) by the Spitzer Space Telescope in the line of sight to background sources.
Results. From a qualitative point of view, good fits to observations are obtained by considering either three or four laboratory components. From a quantitative point of view, a better result is obtained with four components, i.e. when a spectrum of CO₂ formed after ion irradiation of CH₃OH ice is added to the fitting procedure.
Conclusions. Our results support the hypothesis that energetic processing of icy grain mantles is an efficient formation mechanism of CO₂ ice also in quiescent dark cloud regions, and indirectly suggest the presence of CH₃OH in icy grain mantles in interstellar cold regions.