A&A 635, A199 (2020) Astronomy https://doi.org/10.1051/0004-6361/201937302 & © K.-J. Chuang et al. 2020 Astrophysics Formation of complex molecules in translucent clouds: acetaldehyde, vinyl alcohol, ketene, and ethanol via “nonenergetic” processing of C2H2 ice K.-J. Chuang1,2, G. Fedoseev1,3, D. Qasim1, S. Ioppolo4, C. Jäger2, Th. Henning5, M. E. Palumbo3, E. F. van Dishoeck6, and H. Linnartz1 1 Laboratory for Astrophysics, Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands 2 Laboratory Astrophysics Group of the Max Planck Institute for Astronomy at the Friedrich Schiller University Jena, Institute of Solid State Physics, Helmholtzweg 3, 07743 Jena, Germany e-mail: [email protected] 3 INAF – Osservatorio Astrofisico di Catania, Via Santa Sofia 78, 95123 Catania, Italy 4 School of Electronic Engineering and Computer Science, Queen Mary University of London, Mile End Road, London E1 4NS, UK 5 Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany 6 Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands Received 11 December 2019 / Accepted 2 February 2020 ABSTRACT Context. Complex organic molecules (COMs) have been identified toward high- and low-mass protostars as well as molecular clouds, suggesting that these interstellar species originate from the early stage(s) of starformation. The reaction pathways resulting in COMs described by the formula C2HnO, such as acetaldehyde (CH3CHO), vinyl alcohol (CH2CHOH), ketene (CH2CO), and ethanol (CH3CH2OH), are still under debate. Several of these species have been detected in both translucent and dense clouds, where chemical processes are dominated by (ground-state) atom and radical surface reactions. Therefore, efficient formation pathways are needed to account for their appearance well before the so-called catastrophic CO freeze-out stage starts. Aims. In this work, we investigate the laboratory possible solid-state reactions that involve simple hydrocarbons and OH-radicals along 3 3 with H O ice under translucent cloud conditions (1 A 5 and n 10 cm− ). We focus on the interactions of C H with H-atoms 2 ≤ V ≤ H ∼ 2 2 and OH-radicals, which are produced along the H2O formation sequence on grain surfaces at 10 K. Methods. Ultra-high vacuum experiments were performed to study the surface chemistry observed during C2H2 + O2 + H code- position, where O2 was used for the in situ generation of OH-radicals. These C2H2 experiments were extended by a set of similar experiments involving acetaldehyde (CH3CHO) – an abundant product of C2H2 + O2 + H codeposition. Reflection absorption infrared spectroscopy was applied to in situ monitor the initial and newly formed species. After that, a temperature-programmed desorption experiment combined with a quadrupole mass spectrometer was used as a complementary analytical tool. The IR and QMS spectral 18 assignments were further confirmed in isotope labeling experiments using O2. Results. The investigated 10 K surface chemistry of C2H2 with H-atoms and OH-radicals not only results in semi and fully satu- rated hydrocarbons, such as ethylene (C2H4) and ethane (C2H6), but it also leads to the formation of COMs, such as vinyl alcohol, acetaldehyde, ketene, ethanol, and possibly acetic acid. It is concluded that OH-radical addition reactions to C2H2, acting as a molecu- lar backbone, followed by isomerization (i.e., keto-enol tautomerization) via an intermolecular pathway and successive hydrogenation provides so far an experimentally unreported solid-state route for the formation of these species without the need of energetic input. The kinetics of acetaldehyde reacting with impacting H-atoms leading to ketene and ethanol is found to have a preference for the saturated product. The astronomical relevance of the reaction network introduced here is discussed. Key words. astrochemistry – methods: laboratory: solid state – infrared: ISM – ISM: atoms – ISM: molecules – molecular processes 1. Introduction Recent laboratory studies focusing on grain surface chemistry have shown that CO hydrogenation at temperatures as low as Astronomical infrared observations in star-forming regions 13 K efficiently leads to the formation of H2CO and CH3OH have revealed ice features of a number of abundant species, (∼Watanabe et al. 2003; Fuchs et al. 2009). Moreover, it offers a such as H2O, CO2, CO, CH3OH, NH3, and CH4 (see review nonenergetic (i.e., in the absence of processing by high energetic Boogert et al. 2015 and references therein). The origin of these particles) pathway toward O-bearing species containing multiple interstellar ices has been intensively studied, both observation- carbon atoms (chemical formula: C2HnO2 and C3HmO3), such ally and in the laboratory, showing that most interstellar ices as sugar-like molecules (e.g., glycolaldehyde and glyceralde- comprise of two separate ice layers, namely a H2O-rich and hyde) and sugar alcohols (e.g., ethylene glycol and glycerol) a CO-rich layer on top that follow two sequential accretion (Fedoseev et al. 2015, 2017; Butscher et al. 2015, 2017; Chuang stages (Tielens et al. 1991; Gibb et al. 2004; Pontoppidan 2006; et al. 2016). These biologically relevant species are referred to Öberg et al. 2011; Mathews et al. 2013; Boogert et al. 2015). as interstellar complex organic molecules (COMs; see review A199, page 1 of 16 Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Open Access funding provided by Max Planck Society. A&A 635, A199 (2020) paper: Herbst & van Dishoeck 2009; Herbst 2017). Additionally, identified in interstellar ice mantles through their C-H deforma- methanol, a fully hydrogen-saturated derivative of CO, is also tion transitions at 7.41 and 7.25 µm, respectively (Schutte regarded as a starting point in the formation of COMs upon et al. 1999; Öberg∼ et al. 2011).∼ Moreover, acetaldehyde is one “energetic” processing via the interaction of UV-photons, of the most commonly detected COMs in massive hot cores and cosmic particles, and X-rays (Gerakines et al. 1996; Moore et al. giant molecular clouds, such as Orion KL, Sgr B2N, and TMC- 10 9 1996; Garrod et al. 2006; Öberg et al. 2009; Modica & Palumbo 1, with a relatively high fractional abundance of χH2 10− –10− 2010; Chen et al. 2013; Boamah et al. 2014; Maity et al. 2015; (Matthews et al. 1985; Turner 1991; Ikeda et≈ al. 2001; Henderson & Gudipati 2015; Paardekooper et al. 2016; Chuang Charnley 2004). The isomers of acetaldehyde, (anti)syn-vinyl et al. 2017; Rothard et al. 2017). These solid-state laboratory alcohol (CH2CHOH), and ethylene oxide (c-C2H4O) have higher 1 studies of energetic and nonenergetic scenarios all suggest an icy enthalpies by (62.7)72.4 and 96 kJ mol− , respectively, than origin of COMs in star-forming regions. Systematic experimen- acetaldehyde. Both isomers have been identified in Sgr B2N by tal studies also show that the corresponding relative yields of Turner & Apponi(2001). Ethylene oxide has been reported in these complex species depend on the involved chemical triggers, the PILS survey1 of the low-mass protostar IRAS 16293-2422 the initial ice composition and substrate material underneath the by Lykke et al.(2017), but a recent observational search for vinyl ices (Chuang et al. 2017; Ciaravella et al. 2018). alcohol toward multiple solar-type protostars only resulted in Furthermore, these newly formed species may participate in upper limits (Melosso et al. 2019). Besides for high-mass proto- gas-phase reactions, once they or their fragments are released stars, the detection of acetaldehyde, usually together with ketene, from the ice (Balucani et al. 2015; Bertin et al. 2016; Taquet has been reported toward solar-type dense molecular clouds, et al. 2016). Several mechanisms have been studied in the recent such as B1-b, L1689B, and B5 (Bisschop et al. 2008; Öberg et al. past to understand the transfer from solid state material into 2010; Cernicharo et al. 2012; Bacmann et al. 2012; Taquet et al. the gas phase. Thermal desorption is well characterized for 2017) and detections even have been reported for translucent many different ices (Brown & Bolina 2007; Burke & Brown clouds, such as CB 17, CB 24, and CB 228, with a surprisingly 8 2010), but in dense dark molecular clouds, thermal desorption is high χH2 10− at low temperatures (Turner et al. 1999). These ≈ highly unlikely due to the very low temperatures. Therefore, sev- observations hint at a cold formation pathway of acetaldehyde eral nonthermal desorption mechanisms (e.g., photodesorption, and chemical derivatives (e.g., isomers and (de)hydrogenated chemical desorption, and ion sputtering) have been investigated products), during the very early stage of molecular clouds that to explain the net transfer from the solid state into the gas is governed by H2O ice formation, namely, before CO accretion phase (Garrod et al. 2007; Andersson & van Dishoeck 2008; starts. The goal of the present study is to investigate this pathway van Hemert et al. 2015; Bertin et al. 2016; Cruz-Diaz et al. under controlled laboratory conditions. 2016; Minissale et al. 2016; Chuang et al. 2018a; Oba et al. The formation of acetaldehyde and its isomers has been 2018; Dartois et al. 2019). The resulting numbers, do not provide proposed to take place through either gas-phase or solid-state conclusive efficiencies so far. chemistry in star-forming regions. However, in the gas-phase COM gas phase signatures have been unambiguously routes, the model derived formation yields of C2H4O are usually detected in various astronomical environments from massive (1–2 orders of magnitude) lower than the observed abundances sources in the Galactic Center to comets in our solar system (Herbst & Leung 1989; Lee et al. 1996). Moreover, the proposed (Herbst & van Dishoeck 2009; Biver et al. 2014; Altwegg et al.