2023MNRAS.518.3533B

Tricarbon and pentacarbon monoxides have been detected towards the Taurus Molecular Cloud (TMC-1) with relatively important abundances. Understanding the chemical formation of these molecules requires interpreting their observational spectra by mean of non-local thermodynamical equilibrium modelling. For this purpose, we report rate coefficients of C₃O and C₅O induced by collision with He for temperatures up to 100 K. These data are obtained by calculating inelastic cross sections for the 31 low-lying rotational levels of C₃O and C₅O using the close-coupling approach. The comparison of the new rate coefficients with those of HC₃N and HC₅N, previously used to interpret the observational spectra of C₃O and C₅O, reveals differences of up to an order of magnitude. The effect of the new collisional rate coefficients in radiative transfer calculations is checked by computing the excitation temperatures for some transitions and simulating the C₃O and C₅O column densities observed towards TMC-1. Our findings suggest that the use of HC_nN as template for C_nO may lead to local thermodynamic equilibrium conditions for gas densities as low as ∼10³ cm^–3. Regarding the interpretation of the observational spectra, using radiative transfer modelling based on the actual C_nO collisional rate coefficients instead of rotational diagram analysis leads to underestimate the column densities reported in the literature by up to 25 per cent and accordingly the C₃O/ C₅O abundance ratio by up to 50 per cent. We expect that the new rate coefficients and the radiative transfer calculations presented in this work will encourage further modellings of the C_nO abundance and accordingly constrain the chemistry.