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1. Modelling the insertion of O(1D) into methane on the surface of interstellar ice mantles

   https://doi.org/10.1093/mnras/stab2619  

   Carder, Joshua T; Ochs, Wyatt; Herbst, Eric (October 2021, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT The detection of many complex organic molecules (COMs) in interstellar space has sparked the study of their origins. While the formation of COMs detected in hot cores is attributed to photochemistry on warming grain surfaces followed by recombination of radicals and desorption, the formation routes in colder regions are still a debated issue with a number of theories such as cosmic ray bombardment on interstellar ice mantles or non-diffusive surface chemistry. Here, we present another method with reactions involving metastable atomic oxygen in the O(1D) state, which is initially produced by photodissociation of oxygen-containing species in interstellar ices. As a first example, we study the reactions of metastable oxygen atoms and methane in ices to form both formaldehyde and methanol. The reaction is studied incorporating two different surface processes: diffusive and non-diffusive chemistry. The formation of methanol and formaldehyde via metastable oxygen atoms is compared with well-known formation routes of both to understand the O(1D) contributions at different temperatures. 

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2. On modelling cosmic ray sputtering of interstellar grain ices

   https://doi.org/10.1093/mnras/stac2507  

   Paulive, Alec; Carder, Joshua T; Herbst, Eric (September 2022, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT In the interstellar medium (ISM), the formation of complex organic molecules (COMs) is largely facilitated by surface reactions. However, in cold dark clouds, thermal desorption of COMs is inefficient because of the lack of thermal energy to overcome binding energies to the grain surface. Non-thermal desorption methods are therefore important explanations for the gas-phase detection of many COMs that are primarily formed on grains. Here, we present a new non-thermal desorption process: cosmic ray sputtering of grain ice surfaces based on water, carbon dioxide, and a simple mixed ice. Our model applies estimated rates of sputtering to the three-phase rate equation model nautilus-1.1, where this inclusion results in enhanced gas-phase abundances for molecules produced by grain reactions such as methanol (CH3OH) and methyl formate (HCOOCH3). Notably, species with efficient gas-phase destruction pathways exhibit less of an increase in models with sputtering compared to other molecules. These model results suggest that sputtering is an efficient, non-specific method of non-thermal desorption that should be considered as an important factor in future chemical models. 

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3. Detection of complex organic molecules in young starless core L1521E

   https://doi.org/10.1093/mnras/stab1151  

   Scibelli, Samantha; Shirley, Yancy; Vasyunin, Anton; Launhardt, Ralf (May 2021, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Determining the level of chemical complexity within dense starless and gravitationally bound pre-stellar cores is crucial for constructing chemical models, which subsequently constrain the initial chemical conditions of star formation. We have searched for complex organic molecules (COMs) in the young starless core L1521E, and report the first clear detection of dimethyl ether (CH3OCH3), methyl formate (HCOOCH3), and vinyl cyanide (CH2CHCN). Eight transitions of acetaldehyde (CH3CHO) were also detected, five of which (A states) were used to determine an excitation temperature to then calculate column densities for the other oxygen-bearing COMs. If source size was not taken into account (i.e. if filling fraction was assumed to be one), column density was underestimated, and thus we stress the need for higher resolution mapping data. We calculated L1521E COM abundances and compared them to other stages of low-mass star formation, also finding similarities to other starless/pre-stellar cores, suggesting related chemical evolution. The scenario that assumes formation of COMs in gas-phase reactions between precursors formed on grains and then ejected to the cold gas via reactive desorption was tested and was unable to reproduce observed COM abundances, with the exception of CH3CHO. These results suggest that COMs observed in cold gas are formed not by gas-phase reactions alone, but also through surface reactions on interstellar grains. Our observations present a new, unique challenge for existing theoretical astrochemical models. 

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4. The role of radiolysis in the modelling of C2H4O2 isomers and dimethyl ether in cold dark clouds

   https://doi.org/10.1093/mnras/staa3458  

   Paulive, Alec; Shingledecker, Christopher N; Herbst, Eric (December 2020, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT Complex organic molecules (COMs) have been detected in a variety of interstellar sources. The abundances of these COMs in warming sources can be explained by syntheses linked to increasing temperatures and densities, allowing quasi-thermal chemical reactions to occur rapidly enough to produce observable amounts of COMs, both in the gas phase, and upon dust grain ice mantles. The COMs produced on grains then become gaseous as the temperature increases sufficiently to allow their thermal desorption. The recent observation of gaseous COMs in cold sources has not been fully explained by these gas-phase and dust grain production routes. Radiolysis chemistry is a possible non-thermal method of producing COMs in cold dark clouds. This new method greatly increases the modelled abundance of selected COMs upon the ice surface and within the ice mantle due to excitation and ionization events from cosmic ray bombardment. We examine the effect of radiolysis on three C2H4O2 isomers – methyl formate (HCOOCH3), glycolaldehyde (HCOCH2OH), and acetic acid (CH3COOH) – and a chemically similar molecule, dimethyl ether (CH3OCH3), in cold dark clouds. We then compare our modelled gaseous abundances with observed abundances in TMC-1, L1689B, and B1-b. 

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5. Chemical modeling of aminoketene, ethanolamine, and glycine production in interstellar ices

   https://doi.org/10.1051/0004-6361/202554598  

   Willis, Sydney A; Krasnokutski, Serge A; Morin, Nathaniel J; Garrod, Robin T (November 2025, Astronomy & Astrophysics)

   Context. Icy interstellar dust grains are a source of complex organic molecule (COM) production, although the formation mechanisms of these molecules are debated. Laboratory experiments show that atomic carbon deposited onto interstellar ice analogs can readily react with solid-phase ammonia to form the CHNH₂radical, a possible precursor to COMs, including aminoketene (NH₂CHCO). Aims. We used astrochemical kinetics models to explore the role of the reaction of atomic C with ammonia as well as the subsequent reaction with CO in the formation of aminoketene and other COMs, including ethanolamine (NH₂CH₂CH₂OH) and glycine (NH₂CH₂COOH). Methods. We applied the three-phase chemical model MAGICKAL to hot molecular core conditions from the cold-collapse through to the hot-core stage. The chemical network was extended to include NH₂CHCO and a range of associated gas-phase, grain-surface, and bulk-ice products and reactions. We also implemented a model approximating conditions in a shocked cloud, including sputtering of the ice mantles. Results. Aminoketene is formed on grains at low temperatures (∼ 10 K) with a peak solid-phase abundance of ∼ 2 × 10⁻¹⁰n_H. Its formation is driven by nondiffusive reactions, in particular the Eley-Rideal reaction of C with surface NH₃, followed by immediate reaction with CO. Surface hydrogenation of aminoketene produces ethanolamine with a significant abundance of ∼ 8 × 10⁻⁸n_H. In the gas-phase, although ethanolamine reaches a modest abundance peak immediately following its desorption from grains under hot-core conditions, it is destroyed more rapidly due to its high proton affinity. Molecular survival is much higher in the shocked regions, where these species seem most likely to be detected. Glycine abundances are modestly enhanced by the new chemistry. Conclusions. Aminoketene is produced efficiently on simulated interstellar grain surfaces, acting subsequently as an important precursor to more complex organics, including ethanolamine and glycine. Ion-molecule gas-phase destruction of amine-bearing COMs is less efficient in (weakly) shocked lower-density regions, in contrast to hot cores, enhancing their abundances and lifetimes. 

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