Abstract
Context. The simultaneous detection of organic molecules of the form C2HnO, 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.
Highlights
Interstellar complex organic molecules (COMs), which are referred to as organic compounds consisting of more than six atoms, are present in various star-forming stages, stretching from interstellar clouds to comets in our Solar System (Herbst & van Dishoeck 2009; Biver et al 2014; Altwegg et al 2017; Herbst 2017)
In spectrum (b) of Fig. 1, the peak intensity of signals due to C2H2 and H2O significantly decrease after H+ radiolysis
Astrochemical implication and conclusions This laboratory study validates a solid reaction network for forming interstellar COMs described by the formula C2HnO, such as vinyl alcohol, acetaldehyde, ketene, and ethanol, through
Summary
Interstellar complex organic molecules (COMs), which are referred to as organic compounds consisting of more than six atoms, are present in various star-forming stages, stretching from interstellar clouds to comets in our Solar System (Herbst & van Dishoeck 2009; Biver et al 2014; Altwegg et al 2017; Herbst 2017). The simultaneous detection of these COMs, characterized by their degree of hydrogenation, implies that they are likely to share similar formation mechanisms Such a chemical link is found in another group of O-bearing COMs expressed by the formula C2HnO (n = 2, 4, and 6), namely, ketene (CH2CO), acetaldehyde (CH3CHO), and ethanol (CH3CH2OH). These species have been detected toward several protostellar sources, such as NGC 7129 FIRS 2, SVS13-A, IRAS 16923-2422, and the L1157-B1 shock region (Bisschop et al 2007; Fuente et al 2014; Lefloch et al 2017; Bianchi et al 2018; Jørgensen et al 2020)
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