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

Abstract

Context. C_{2O 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 C2O induced by collisions with He, enabling the improvement of the modeling of C2O emission spectra in (cold) molecular clouds.Methods. A new potential energy surface for the C2O–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 C2O. Excitation calculations using a radiative transfer model were conducted in order to interpret observations of C2O 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 C2O emission lines at typical densities of TMC-1 (n(H2)∼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 C2O in TMC-1 was estimated to be NC2O ≈ 9 · 1011 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 C2O excitation in interstellar clouds and improve the interpretation of C2O emission spectra in molecular clouds, highlighting again the necessity of having accurate molecular data in astrochemical studies.}}