Cyanides/isocyanides abundances in the interstellar medium – IV. Temperature dependence of SiCN/SiNC rate coefficients and astrophysical applications
Résumé
Accurate determination of collisional rate coefficients is an essential step in the estimation of the SiCN and SiNC abundances in the interstellar and circumstellar media. In this paper, we carry out calculations of rate coefficients for the rotational (de-)excitation of SiCN and SiNC molecules in collision with He. The calculations are based on new two-dimensional potential energy surfaces obtained from highly correlated ab initio calculations. Coupled-States quantum approximation was used in the scattering calculations to obtain collisional (de-)excitation cross-sections of SiCN and SiNC by He. The spin–orbit coupling and Λ-doublet splitting of SiCN and SiNC levels were taken into account explicitly. Rate coefficients for transitions among the first 92 rotational levels of SiCN and SiNC were calculated for temperatures ranging from 5 to 100 K. Moderate differences exist between the rate coefficients of both isomers. Subsequently, the new collisional data are used to simulate the excitation of SiCN and SiNC in the circumstellar gas. We obtain the brightness and excitation temperatures of selected lines frequently observed towards the circumstellar envelopes and we find that local thermodynamic equilibrium conditions are not fulfilled for these species. Radiative transfer calculations are then needed in order to accurately determine their abundances. Our results also show that previous estimations of the cyanides/isocyanides abundance ratios were incorrect and the present calculations show that SiCN, the most stable isomer, is more abundant than SiNC. This shows again the evidence of selective cyanide chemistry.
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