Guidelines for non-LTE modelling of cyanopolyynes

Cyanopolyynes are abundant and widespread in cold molecular clouds. Their abundances are typically determined under the assumption of local thermodynamic equilibrium (LTE) or through non-LTE analysis based on approximate collisional rate coefficients. Here, we present the first quantum mechanical scattering data for HC5N and HC7N in collisions with helium. We used both close-coupling (CC) and coupled-states (CS) methods to compute excitation cross-sections. We then derived rate coefficients for the first energy levels of HC5N and HC7N. A comparative analysis between the CC and CS results reveals an agreement better than a factor of two for high-magnitude rate coefficients. However, we found that the derivation of approximate HC5N and HC7N rate coefficients from those of HCN and HC3N using a size proportionality relationship, as has been previously done in the literature, leads to an overestimation of the collisional data by up to two orders of magnitude. To assess the sensitivity of cyanopolyyne line intensities to collisional rate coefficients, we compared LTE predictions with various non-LTE models using different rate coefficients. Under the typical physical conditions of cold molecular clouds, the LTE assumption and radiative transfer calculations based on approximate collisional rate coefficients strongly overestimate high-frequency line intensities, while the non-LTE models based on the HC3N collisional rate coefficients as a proxy for HC5N and HC7N remain accurate within 10%. We therefore recommend revisiting the abundance estimates for HC5N and HC7N using the new rate coefficients. For HC9N and HC11N, adopting HC7N collisional data in line modelling is expected to be highly accurate.