2017ApJ...841...96B

The structural isomers ethanol (CH₃CH₂OH) and dimethyl ether (CH₃OCH₃) were detected in several low-, intermediate-, and high-mass star-forming regions, including Sgr B2, Orion, and W33A, with the relative abundance ratios of ethanol/dimethyl ether varying from about 0.03 to 3.4. Until now, no experimental data regarding the formation mechanisms and branching ratios of these two species in laboratory simulation experiments could be provided. Here, we exploit tunable photoionization reflectron time-of-flight mass spectrometry (PI-ReTOF-MS) to detect and analyze the production of complex organic molecules (COMs) resulting from the exposure of water/methane (H₂O/CH₄) ices to energetic electrons. The main goal is to understand the formation mechanisms in star-forming regions of two C₂H₆O isomers: ethanol (CH₃CH₂OH) and dimethyl ether (CH₃OCH₃). The results show that the experimental branching ratios favor the synthesis of ethanol versus dimethyl ether (31 ± 11:1). This finding diverges from the abundances observed toward most star-forming regions, suggesting that production routes on interstellar grains to form dimethyl ether might be missing; alternatively, ethanol can be overproduced in the present simulation experiments, such as via radical-radical recombination pathways involving ethyl and hydroxyl radicals. Finally, the PI-ReTOF-MS data suggest the formation of methylacetylene (C₃H₄), ketene (CH₂CO), propene (C₃H₆), vinyl alcohol (CH₂CHOH), acetaldehyde (CH₃CHO), and methyl hydroperoxide (CH₃OOH), in addition to ethane (C₂H₆), methanol (CH₃OH), and CO₂ detected from infrared spectroscopy. The yield of all the confirmed species is also determined.