First detection of ethylene oxide and acetaldehyde in hot core G358.93$-$0.03 MM1: Tracing prebiotic oxygen chemistry

Abstract

Ethylene oxide (c-C$_{2}$H$_{4}$O) and its isomer, acetaldehyde (CH$_{3}$CHO), are important complex organic molecules owing to their potential role in the formation of amino acids (R-CH(NH$_{2}$)-COOH) in ISM. The detection of c-C$_{2}$H$_{4}$O in hot molecular cores suggests that the possible existence of larger ring-shaped molecules containing more than three carbon atoms, such as furan (c-C$_{4}$H$_{4}$O), which shares structural similarities with ribose (C$_{5}$H$_{10}$O5), the sugar component of DNA. In this study, we report the first detection of the rotational emission lines of c-C$_{2}$H$_{4}$O and CH$_{3}$CHO towards the hot molecular core G358.93$-$0.03 MM1, based on observations from the Atacama Large Millimeter/Submillimeter Array (ALMA) in band 7. The fractional abundances of c-C$_{2}$H$_{4}$O and CH$_{3}$CHO relative to H$_{2}$ are $(2.1\pm0.2)\times10^{-9}$ and $(7.1\pm0.9)\times10^{-9}$, respectively. The column density ratio between CH$_{3}$CHO and c-C$_{2}$H$_{4}$O is $3.4\pm0.7$. A Pearson correlation heat map reveals strong positive correlations ($r$ $>$ 0.5) between the abundances and excitation temperatures of c-C$_{2}$H$_{4}$O and CH$_{3}$CHO, suggesting a possible chemical connection between those two molecules. To investigate this further, we conducted a two-phase warm-up chemical model using the gas-grain chemical code UCLCHEM. A comparison between our derived abundances and the predictions from our chemical model and existence model demonstrates good agreement within factors of 0.73 and 0.74, respectively. We propose that c-C$_{2}$H$_{4}$O may form in G358.93$-$0.03 MM1 via the grain surface reaction between C$_{2}$H$_{4}$ and O, but CH$_{3}$CHO may be produced through the surface reaction between CH$_{3}$ and HCO.