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 15.2 μm (660 cm-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.