Formation of complex molecules in translucent clouds: acetaldehyde, vinyl alcohol, ketene, and ethanol via “nonenergetic” processing of C2H2ice

K.-J Chuang,E F Van Dishoeck,D Qasim,C Jäger,M E Palumbo,H Linnartz,S Ioppolo,G Fedoseev,Th Henning

doi:10.1051/0004-6361/201937302

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 with H2O ice under translucent cloud conditions (1 ≤AV≤ 5 andnH~ 103cm−3). We focus on the interactions of C2H2with H-atoms 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 codeposition, where O2was used for the in situ generation of OH-radicals. These C2H2experiments 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 assignments were further confirmed in isotope labeling experiments using18O2.Results.The investigated 10 K surface chemistry of C2H2with H-atoms and OH-radicals not only results in semi and fully saturated 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 molecular 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.

Highlights

Astronomical infrared observations in star-forming regions have revealed ice features of a number of abundant species, such as H2O, CO2, CO, CH3OH, NH3, and CH4
Ultra-high vacuum experiments were performed to study the surface chemistry observed during C2H2 + O2 + H codeposition, where O2 was used for the in situ generation of OH-radicals
Complex organic molecules (COMs) composition The formation yields or relative intensity of products have been derived by two independent methods as previously described in Sect. 2; via (1) RAIR data followed by the modified Beer-Lambert law using known or estimated absorption band strengths or via (2) TPD-QMS data followed by a series of calibrations taking into account any species’ fragmentation fraction, ionization cross-section, and detection sensitivity

Summary

Introduction

Astronomical infrared observations in star-forming regions have revealed ice features of a number of abundant species, such as H2O, CO2, CO, CH3OH, NH3, and CH4 (see review Boogert et al 2015 and references therein). Recent laboratory studies focusing on grain surface chemistry have shown that CO hydrogenation at temperatures as low as ∼13 K efficiently leads to the formation of H2CO and CH3OH (Watanabe et al 2003; Fuchs et al 2009) It offers a nonenergetic (i.e., in the absence of processing by high energetic particles) pathway toward O-bearing species containing multiple carbon atoms (chemical formula: C2HnO2 and C3HmO3), such as sugar-like molecules (e.g., glycolaldehyde and glyceraldehyde) and sugar alcohols (e.g., ethylene glycol and glycerol) (Fedoseev et al 2015, 2017; Butscher et al 2015, 2017; Chuang et al 2016).

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