2013A&A...554A..34I

Solid interstellar CO₂ is an abundant component of ice dust mantles. Its ubiquity towards quiescent molecular clouds, as well as protostellar envelopes, has recently been confirmed by the IRS (InfraRed Spectrograph) aboard the Spitzer Space Telescope. Although it has been shown that CO₂ cannot be efficiently formed in the gas phase, the CO₂ surface formation pathway is still unclear. To date several CO₂ surface formation mechanisms induced by energetic (e.g., UV photolysis and cosmic ray irradiation) and non-energetic (e.g., cold atom addition) input have been proposed. Our aim is to investigate the contribution of cosmic ray irradiation to the formation of CO₂ in different regions of the interstellar medium (ISM). To achieve this goal we compared quantitatively laboratory data with the CO₂ bending mode band profile observed towards several young stellar objects (YSOs) and a field star by the Spitzer Space Telescope. All the experiments presented here were performed at the Laboratory for Experimental Astrophysics in Catania (Italy). The interstellar relevant samples were all irradiated with fast ions (30-200keV) and subsequently annealed in a stainless steel high vacuum chamber (P<10^–7mbar). Chemical and structural modifications of the ice samples were monitored by means of infrared spectroscopy. Laboratory spectra were then used to fit some thirty observational spectra. A qualitative analysis shows that a good fit can be obtained with a minimum of two components. The choice of the laboratory components is based on the chemical-physical condition of each source. A quantitative analysis of the sources with known visual extinction (A_V) and methanol abundances highlights that the solid carbon dioxide can be efficiently and abundantly formed after ion irradiation of interstellar ices in all the selected YSOs in a time compatible with cloud lifetimes (3x10⁷ years). Only in the case of field stars can the expected CO₂ column density formed upon energetic input not explain the observed abundances. This result, to be confirmed along the line of sight to different quiescent clouds, gives an indirect indication that CO₂ can also be formed in an early cloud stage through surface reactions induced by non-energetic mechanisms. In a later stage, when ices are exposed to higher UV and cosmic ray doses, the CO₂ total abundance is strongly affected by energetic formation mechanisms. Our results indicate that energetic processing of icy grain mantles significantly contribute to the formation of solid phase interstellar CO₂.