Abstract
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.
Highlights
Despite the harsh conditions, chemistry thrives in the interstellar medium (ISM), in particular in 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 of icy grain mantle analogues
We show that (i) C3O is detected towards TMC-1, L1544, L1498, and Elias 18 and that C2O is detected towards TMC-1, L1544, and Elias 18 while neither C2O nor C3O is detected at 3σ level towards the other sources investigated; (ii) the adopted gas-phase model is not able to reproduce the C2O/C3O ratio observed towards the star-forming regions where both species have been detected; and (iii) the experimental results support the hypothesis that both C2O and C3O are formed in the solid phase after cosmic-ion bombardment of icy grain mantles and are injected into the gas-phase after mantle desorption
Summary
Chemistry thrives in the interstellar medium (ISM), in particular in star-forming regions. Various processes are invoked to explain the formation of molecules observed in the gas phase in star-forming regions: (1) reactions taking place on the grain surfaces during the cold prestellar period of grain mantle formation When processes (1) and (4) occur, molecules formed in the solid phase are observed in the gas phase after desorption of the icy grain mantles. All these processes are very likely at work, it is not clear whether or not one or more of them is dominant, and if so, when. Distinguishing what process operates in what conditions is crucial if we want to understand how the molecular complexity builds up in the ISM in general, and, in regions where planetary systems like the solar one will
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