Guidelines for non-LTE modelling of cyanopolyynes

¹ Univ Rennes, CNRS, IPR (Institut de Physique de Rennes) – UMR 6251, 35000 Rennes, France
² Nantes Université, CNRS, CEISAM, UMR 6230, 44000 Nantes, France

^★ Corresponding author; cheikhtidiane.bop@ucad.edu.sn

Received: 7 January 2025
Accepted: 10 February 2025

Abstract

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.