UV-photoprocessing Of Acetic Acid (CH3COOH)-bearing Interstellar Ice Analogs

By Keith Cowing
Status Report
December 6, 2023
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UV-photoprocessing Of Acetic Acid (CH3COOH)-bearing Interstellar Ice Analogs
Tentative detection of oxalic acid (HOOC-COOH) during warm-up of an UV-irradiated CH3COOH:H2O ice sample (Exp. 11). Left panel: QMS signal for 𝑚 𝑧 = 44, 45 and 46. These fragments are representative of the oxalic acid, while 𝑚 𝑧 = 21 is not expected to correspond to any molecule and is used as a reference. Centre panel: mass spectrum of oxalic acid taken from NIST database. Right panel: scheme to form oxalic acid via radical-radical reaction from photon and thermal processing of acetic acid-containing ices. The formation temperature of oxalic acid could not be determined, since oxalic acid was only detected from QMS data in the gas phase during thermal desorption. — astro-ph.IM

Acetic acid (CH3COOH) was detected in the gas toward interstellar clouds, hot cores, protostars and comets. Its formation in ice mantles was proposed and acetic acid awaits detection in the infrared spectra of the ice as most other COMs except methanol.

The thermal annealing and UV-irradiation of acetic acid in the ice was simulated experimentally in this work under astrophysically relevant conditions. The experiments were performed under ultra-high vacuum conditions. An ice layer was formed by vapor deposition onto a cold substrate, and was warmed up or exposed to UV photons.

The ice was monitored by infrared spectroscopy while the molecules desorbing to the gas phase were measured using a quadrupole mass spectrometer. The transformation of the CH3COOH monomers to cyclic dimers occurs at 120 K and the crystal form composed of chain polymers was observed above 160 K during warm-up of the ice. Ice sublimation proceeds at 189 K in our experiments. Upon UV-irradiation simpler species and radicals are formed, which lead to a residue made of complex molecules after warm-up to room temperature. The possible formation of oxalic acid needs to be confirmed. The photodestruction of acetic acid molecules is reduced when mixed with water in the ice.

This work may serve to search for the acetic acid photoproducts in lines of sight where this species is detected. A comparison of the reported laboratory infrared spectra with current JWST observations allows to detect or set upper imits on the CH3COOH abundances in interstellar and circumstellar ice mantles.

C. del Burgo Olivares, H. Carrascosa, B. Escribano, G. M. Muñoz Caro, R. Martín-Doménech

Comments: Accepted to MNRAS
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM); Earth and Planetary Astrophysics (astro-ph.EP); Astrophysics of Galaxies (astro-ph.GA); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2312.01845 [astro-ph.IM] (or arXiv:2312.01845v1 [astro-ph.IM] for this version)
Submission history
From: Carlos Del Burgo Olivares
[v1] Mon, 4 Dec 2023 12:26:18 UTC (2,135 KB)
Astrobiology, Astrochemistry

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