Characterization of H2O:N2 ice under bombardment by cosmic rays – I. Reaction rates and chemical equilibrium

ABSTRACT In space, nitrogen-rich ice is constantly exposed to ionizing radiation, which triggers chemical reactions and desorption processes allowing a chemical enhancement of interstellar medium (ISM). Here, we present the first part of a series of studies on the effect of cosmic ray bombardment (40 MeV Ni$^{11+}$ ions) on H$_2$O:N$_2$ (1:5) ice at 15 K, employing the PROCODA code as the modelling tool including 28 chemical species and 930 chemical coupled equations (also including desorption). This first part focuses on the reaction rates and chemical equilibrium stage due to radiation processing. Among the results, we characterize the molecular abundances at chemical equilibrium, including experimentally observed and non-observed species (predicted) suggesting some candidates as a target for astronomical observation. The best-fitting models provided the effective rate coefficients, which can be employed in astrochemical models to understand the chemistry of cold space environments. The findings also help to clarify the chemical processes of N-bearing species in the ISM and frozen surfaces of the Solar system, including the moon of giant planets, outer solar system objects, and ices in the interstellar and protostellar medium.