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The observation and synthesis of organic molecules in interstellar space is one of the most exciting and rapidly growing topics in astrochemistry. Spectroscopic observations especially with millimeter and submillimeter waves have resulted in the detection of more than 250 molecules in the interstellar clouds from which stars and planets are ultimately formed. In this review, we focus on the diverse suggestions made to explain the formation of Complex Organic Molecules (COMs) in the low-temperature interstellar medium. The dominant mechanisms at such low temperatures are still a matter of dispute, with both gas-phase and granular processes, occurring on and in ice mantles, thought to play a role. Granular mechanisms include both diffusive and nondiffusive processes. A granular explanation is strengthened by experiments at 10 K that indicate that the synthesis of large molecules on granular ice mantles under space-like conditions is exceedingly efficient, with and without external radiation. In addition, the bombardment of carbon-containing ice mantles in the laboratory by cosmic rays, which are mainly high-energy protons, can lead to organic species even at low temperatures. For processes on dust grains to be competitive at low temperatures, however, non-thermal desorption mechanisms must be invoked to explain why the organic molecules are detected in the gas phase. Although much remains to be learned, a better understanding of low-temperature organic syntheses in space will add both to our understanding of unusual chemical processes and the role of molecules in stellar evolution. 

Abstract Chemical models and experiments indicate that interstellar dust grains and their ice mantles play an important role in the production of complex organic molecules (COMs). To date, the most complex solid-phase molecule detected with certainty in the interstellar medium is methanol, but the James Webb Space Telescope (JWST) may be able to identify still larger organic species. In this study, we use a coupled chemodynamical model to predict new candidate species for JWST detection toward the young star-forming core Cha-MMS1, combining the gas–grain chemical kinetic code MAGICKAL with a 1D radiative hydrodynamics simulation using Athena++ . With this model, the relative abundances of the main ice constituents with respect to water toward the core center match well with typical observational values, providing a firm basis to explore the ice chemistry. Six oxygen-bearing COMs (ethanol, dimethyl ether, acetaldehyde, methyl formate, methoxy methanol, and acetic acid), as well as formic acid, show abundances as high as, or exceeding, 0.01% with respect to water ice. Based on the modeled ice composition, the infrared spectrum is synthesized to diagnose the detectability of the new ice species. The contribution of COMs to IR absorption bands is minor compared to the main ice constituents, and the identification of COM ice toward the core center of Cha-MMS1 with the JWST NIRCAM/Wide Field Slitless Spectroscopy (2.4–5.0 μ m) may be unlikely. However, MIRI observations (5–28 μ m) toward COM-rich environments where solid-phase COM abundances exceed 1% with respect to the column density of water ice might reveal the distinctive ice features of COMs. 

A new, more comprehensive model of gas–grain chemistry in hot molecular cores is presented, in which nondiffusive reaction processes on dust-grain surfaces and in ice mantles are implemented alongside traditional diffusive surface/bulk-ice chemistry. We build on our nondiffusive treatments used for chemistry in cold sources, adopting a standard collapse/warm-up physical model for hot cores. A number of other new chemical model inputs and treatments are also explored in depth, culminating in a final model that demonstrates excellent agreement with gas-phase observational abundances for many molecules, including some (e.g., methoxymethanol) that could not be reproduced by conventional diffusive mechanisms. The observed ratios of structural isomers methyl formate, glycolaldehyde, and acetic acid are well reproduced by the models. The main temperature regimes in which various complex organic molecules (COMs) are formed are identified. Nondiffusive chemistry advances the production of many COMs to much earlier times and lower temperatures than in previous model implementations. Those species may form either as by-products of simple-ice production, or via early photochemistry within the ices while external UV photons can still penetrate. Cosmic ray-induced photochemistry is less important than in past models, although it affects some species strongly over long timescales. Another production regime occurs during the high-temperature desorption of solid water, whereby radicals trapped in the ice are released onto the grain/ice surface, where they rapidly react. Several recently proposed gas-phase COM-production mechanisms are also introduced, but they rarely dominate. New surface/ice reactions involving CH and CH₂are found to contribute substantially to the formation of certain COMs.