Hydrogenation Of Accreting C-atoms And CO Molecules -- Simulating Ketene And Acetaldehyde Formation Under Dark And Translucent Cloud Conditions

In the left panel the IR spectra are shown obtained after the simultaneous co-deposition of H2O molecules with C-atoms and (a) CO molecules and H atoms; (b) CO molecules and D atoms; (c) a mixture of CO with C18O molecules with H-atoms at a substrate temperature 10 K. All co-depositions are performed for a period of 1920 seconds. Spectra (d) and (e) are obtained after warm up of the ice presented in spectrum (c) to 55 K and 165 K, respectively. The H2O:C:CO:(C18O):H(D) ratio is estimated to be 16:1:1.5:(3):4(3) with the used Catom flux equal to 5x1011 cm-2. The right panel shows a zoom-in of the 2200-2050 cm-1 region where the main absorption features of regular and isotopic labelled CO and H2CCO molecules are situated (note that the vertical axis is not identical to that used in the left panel). Spectra are offset for clarity. Blue and red colours are used to indicate D and 18O labelled experiments and assignments, respectively. Tentative assignments are presented in brackets.

Simple and complex organic molecules (COMs) are observed along different phases of star and planet formation and have been successfully identified in prestellar environments such as dark and translucent clouds.

Yet the picture of organic molecule formation at those earliest stages of star formation is not complete and an important reason is the lack of specific laboratory experiments that simulate carbon atom addition reactions on icy surfaces of interstellar grains. Here we present experiments in which CO molecules as well as C- and H-atoms are co-deposited with H2O molecules on a 10 K surface mimicking the ongoing formation of an "H2O-rich" ice mantle.

To simulate the effect of impacting C-atoms and resulting surface reactions with ice components, a specialized C-atom beam source is used, implemented on SURFRESIDE3, an UHV cryogenic setup. Formation of ketene (CH2CO) in the solid state is observed "in situ" by means of reflection absorption IR spectroscopy. C18O and D isotope labelled experiments are performed to further validate the formation of ketene. Data analysis supports that CH2CO is formed through C-atom addition to a CO-molecule, followed by successive hydrogenation transferring the formed :CCO into ketene.

Efficient formation of ketene is in line with the absence of an activation barrier in C+CO reaction reported in the literature. We also discuss and provide experimental evidence for the formation of acetaldehyde (CH3CHO) and possible formation of ethanol (CH3CH2OH), two COM derivatives of CH2CO hydrogenation. The underlying reaction network is presented and the astrochemical implications of the derived pathways are discussed.

Gleb Fedoseev, Danna Qasim, Ko-Ju Chuang, Sergio Ioppolo, Thanja Lamberts, Ewine F. van Dishoeck, Harold Linnartz

Comments: Accepted by ApJ
Subjects: Astrophysics of Galaxies (astro-ph.GA); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2111.08548 [astro-ph.GA] (or arXiv:2111.08548v1 [astro-ph.GA] for this version)
Submission history
From: Gleb Fedoseev
[v1] Tue, 16 Nov 2021 15:22:31 UTC (2,541 KB)
Astrobiology,, Astrochemistry,

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