2021A&A...650A..85C

Context. The simultaneous detection of organic molecules of the form C₂H_nO, such as ketene (CH₂CO), acetaldehyde (CH₃CHO), and ethanol (CH₃CH₂OH), toward early star-forming regions offers hints of a shared chemical history. Several reaction routes have been proposed and experimentally verified under various interstellar conditions to explain the formation pathways involved. Most noticeably, the non-energetic processing of C₂H₂ ice with OH-radicals and H-atoms was shown to provide formation routes to ketene, acetaldehyde, ethanol, and vinyl alcohol (CH₂CHOH) along the H₂O formation sequence on grain surfaces in translucent clouds.
Aims. In this work, the non-energetic formation scheme is extended with laboratory measurements focusing on the energetic counterpart, induced by cosmic rays penetrating the H₂O-rich ice mantle. The focus here is on the H⁺ radiolysis of interstellar C₂H₂:H₂O ice analogs at 17 K.
Methods. Ultra-high vacuum experiments were performed to investigate the 200 keV H⁺ radiolysis chemistry of predeposited C₂H₂:H₂O ices, both as mixed and layered geometries. Fourier-transform infrared spectroscopy was used to monitor in situ newly formed species as a function of the accumulated energy dose (or H⁺ fluence). The infrared spectral assignments are further confirmed in isotope labeling experiments using H₂¹⁸O.
Results. The energetic processing of C₂H₂:H₂O ice not only results in the formation of (semi-) saturated hydrocarbons (C₂H₄ and C₂H₆) and polyynes as well as cumulenes (C₄H₂ and C₄H₄), but it also efficiently forms O-bearing COMs, including vinyl alcohol, ketene, acetaldehyde, and ethanol, for which the reaction cross-section and product composition are derived. A clear composition transition of the product, from H-poor to H-rich species, is observed as a function of the accumulated energy dose. Furthermore, the astronomical relevance of the resulting reaction network is discussed.