2019A&A...628A..72U

Context. C₂O and C₃O belong to the carbon chain oxides family. Both molecules have been detected in the gas phase towards several star-forming regions, and to explain the observed abundances, ion-molecule gas-phase reactions have been invoked. On the other hand, laboratory experiments have shown that carbon chain oxides are formed after energetic processing of CO-rich solid mixtures. Therefore, it has been proposed that they are formed in the solid phase in dense molecular clouds after cosmic ion irradiation of CO-rich icy grain mantles and released in the gas phase after their desorption.
Aims. In this work, we contribute to the understanding of the role of both gas-phase reactions and energetic processing in the formation of simple carbon chain oxides that have been searched for in various low-mass star-forming regions.
Methods. We present observations obtained with the Noto-32m and IRAM-30 m telescopes towards star-forming regions. We compare these with the results of a gas-phase model that simulates C₂O and C₃O formation and destruction, and laboratory experiments in which both molecules are produced after energetic processing (with 200 keV protons) of icy grain mantle analogues. Results. New detections of both molecules towards L1544, L1498, and Elias 18 are reported. The adopted gas phase model is not able to reproduce the observed C₂O/C₃O ratios, while laboratory experiments show that the ion bombardment of CO-rich mixtures produces C₂O/C₃O ratios that agree with the observed values.Conclusions. Based on the results obtained here, we conclude that the synthesis of both species is due to the energetic processing of CO-rich icy grain mantles. Their subsequent desorption because of non-thermal processes allows the detection in the gas-phase of young star-forming regions. In more evolved objects, the non-detection of both C₂O and C₃O is due to their fast destruction in the warm gas.