2015MNRAS.448.2438H

Cyanide species are the most common metal-containing molecules in the circumstellar gas. The exact determination of their abundances requires accurate molecular data. For this purpose, collisional excitation rate coefficients are essential in order to simulate the excitation processes and hence to model the emission lines. In this paper, we use the aluminium cyanide (AlCN), aluminium isocyanide (AlNC), magnesium cyanide (MgCN) and magnesium isocyanide (MgNC) collisional data recently computed to simulate the excitation of these molecules in the circumstellar gas. We perform radiative transfer calculations for typical physical conditions encountered in the circumstellar gas. We obtain the brightness and excitation temperatures of selected lines frequently observed towards the circumstellar envelopes. In circumstellar gas, we have found that local thermodynamic equilibrium conditions are not fulfilled for these species and that radiative transfer calculations are needed in order to accurately determine their abundances. The calculations also show that the estimations of the cyanides/isocyanides abundance ratios deduced from line intensities ratios lead to large differences compared to exact radiative transfer calculations. Finally, the results confirm that AlCN and MgCN are significantly less abundant than AlNC and MgNC, respectively. This shows again the evidence of selective cyanide chemistry.