Nitrogen oxides and carbon chain oxides formed after ion irradiation of CO:N2 ice mixtures

Context. High CO depletion as well as depletion of N-bearing species is observed in dense pre-stellar cores. It is generally accepted that depleted species freeze out onto dust grains to form icy mantles and that these ices suffer energetic processing due to cosmic ion irradiation and ion-induced UV photons. Aims. The aim of this work is to study the chemical and structural effects induced by ion irradiation on different CO:N-2 mixtures at low temperature (16 K) to simulate the effects of cosmic ion irradiation of icy mantles. Methods. Different CO:N-2 mixtures and pure CO and pure N-2 were irradiated with 200 keV H+ at 16 K. Infrared transmittance spectra of the samples were obtained in situ before and after irradiation. The samples were warmed up and spectra were taken at different temperatures. The residues left over on the substrate at room temperature were analysed ex situ by micro Raman spectroscopy. Results. Several new absorption features are present in the infrared spectra after irradiation, indicating that new species are formed. The most abundant are nitrogen oxides (such as NO, NO2 and N2O), carbon chain oxides (such as C2O, C3O and C3O2), carbon chains (such as C-3 and C-6), O-3 and N-3. A refractory residue is also formed after ion irradiation and is clearly detected by Raman spectroscopy. Conclusions. We suggest that carbon chains and nitrogen oxides observed in the gas phase towards star-forming regions are formed in the solid phase after cosmic ion irradiation of icy grain mantles and are released into the gas phase after desorption of grain mantles. We expect that the Atacama LargeMillimeter/submillimeter Array (ALMA), thanks to its high sensitivity and resolution, will increase the number of nitrogen oxides and carbon chain oxides detected towards star-forming regions.