Formation Of Complex Organic Molecules In Hot Molecular Cores Through Nondiffusive Grain-surface And ice-mantle Chemistry

By Keith Cowing
October 19, 2021
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Formation Of Complex Organic Molecules In Hot Molecular Cores Through Nondiffusive Grain-surface And ice-mantle Chemistry
Layer-by-layer ice composition produced during stage 1 (collapse) of the final model setup. All abundances are given as a fraction of the total ice composition (by number), and are local to each layer (rather than cumulative over the total ice thickness formed at a given time). Abundances shown for any particular layer or time in the model (top axis) are those recorded at the moment that that material is incorporated into the bulk ice. Material in the bulk continues to be processed after its incorporation.

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.

Observed ratios of structural isomers methyl formate, glycolaldehyde and acetic acid are well reproduced by the models. The main temperature regimes are identified in which various complex organic molecules (COMs) are formed. 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 CH2 are found to contribute substantially to the formation of certain COMs.

Robin T. Garrod, Mihwa Jin, Kayla A. Matis, Dylan Jones, Eric R. Willis, Eric Herbst

Comments: 92 pages, 21 figures, 21 tables. Accepted for publication in Astronomy & Astrophysics Supplements
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2110.09743 [astro-ph.GA] (or arXiv:2110.09743v1 [astro-ph.GA] for this version)
Submission history
From: Robin Garrod
[v1] Tue, 19 Oct 2021 05:23:59 UTC (994 KB)
Astrobiology, Astrochemistry,

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻