Fine-structure excitation of C₂O by He: Rate coefficients and astrophysical modeling

Context. C 2 O molecules are very good probes of oxygen chemistry in interstellar molecular clouds. The accurate derivation of their abundance requires non-local thermodynamic equilibrium (LTE) modeling of their emission spectra. Aims. This study aims to provide highly accurate fine-structure resolved excitation rate coefficients of C 2 O induced by collisions with He, enabling the improvement of the modeling of C 2 O emission spectra in (cold) molecular clouds. Methods. A new potential energy surface for the C 2 O–He system was calculated using the spin-restricted coupled-cluster method together with a complete atomic basis set extrapolation. Quantum scattering calculations were performed using the exact close-coupling approach, explicitly accounting for the fine structure of C 2 O. Excitation calculations using a radiative transfer model were conducted in order to interpret observations of C 2 O in TMC-1. Results. Rate coefficients for transitions up to the rotational state N = 20 and temperatures up to 70 K were obtained. The analysis of the excitation calculations revealed non-LTE effects of C 2 O emission lines at typical densities of TMC-1 ( n (H 2 )∼10 4 cm −3 ), reflecting a balance between collisional excitation and radiative relaxation. These effects significantly influence the derived physical conditions. The column density of C 2 O in TMC-1 was estimated to be N C 2 O ≈ 9 · 10 11 cm −2 . This refined value, derived using the newly calculated rate coefficients, highlights the limitations of previous LTE-based estimates and underscores the importance of non-LTE modeling. Conclusions. The new accurate collisional data enable a more confident modeling of C 2 O excitation in interstellar clouds and improve the interpretation of C 2 O emission spectra in molecular clouds, highlighting again the necessity of having accurate molecular data in astrochemical studies.