Non-LTE modelling of the HC 2 NC and HNC 3 abundance in astrophysical environments

The isomers of HC 3 N, namely HC 2 NC and HNC 3 , are widely observed in the interstellar medium and in circumstellar envelopes. Their abundance has been determined under the assumption of local thermodynamic equilibrium (LTE) conditions or non-LTE radiative transfer models, but in considering the collisional excitation of HC 3 N as the same for all isomers. Chemical models for the prototypical cold cores, TMC-1 and L1544, reproduced the abundance of HC 3 N fairly well, but they tend to overestimate the abundances of HC 2 NC and HNC 3 with respect to the observations. It is therefore worth revisiting the interpretation of the observational spectra of these isomers using a rigorous non-LTE modelling. The abundance of HC 2 NC and HNC 3 were then determined using non-LTE radiative transfer calculations based on the proper rate coefficients for the first time in this work. Modelling the brightness temperature of HC 2 NC and HNC 3 when using their proper collision rate coefficients shows that models based on LTE or non-LTE with approximate collision data may lead to deviations of up to a factor of ~1.5. Reinterpreting the observational spectra led us to significant differences relative to the observed abundances previously determined. Our findings suggest quite similar abundance ratios for the TMC-1 and L1544 cold cores as well as the L483 protostar. This work will encourage further modelling with more robust non-LTE radiative transfer calculations and future studies to revisit the chemistry of HC 3 N and its isomers in cold molecular clouds.