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

¹ Departamento de Química Física, University of Salamanca, Plaza Caídos s/n, E-37008 Salamanca, Spain
² Univ. Rennes, CNRS, IPR (Institut de Physique de Rennes) – UMR 6251, 35000 Rennes, France
³ Physics Department, Khalifa University, Abu-Dhabi, United Arab Emirates
⁴ Nantes Université, CNRS, CEISAM, UMR 6230, F-44000 Nantes, France

^⋆ Corresponding authors: anzheves@usal.es, francois.lique@univ-rennes.fr

Received: 18 April 2025
Accepted: 16 May 2025

Abstract

Context. C₂O 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)∼10⁴ 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 N_C2O ≈ 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.