Astrochemistry

Production of Linear Alkanes via the Solid-state Hydrogenation of Interstellar Polyynes

By Keith Cowing
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astro-ph.EP
January 12, 2025
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Production of Linear Alkanes via the Solid-state Hydrogenation of Interstellar Polyynes
Schematic representation of the overall suggested mechanism. Various polyynes observed in dark clouds (e.g., TMC-1) are presented in the left part of the reaction scheme. The accretion of polyynes on the surface of the grains is followed by hydrogenation of their unsaturated CC triple bonds, resulting in the formation of their semi-saturated and fully saturated counterparts. Besides CH2CHCCCHO, all of the firstgeneration products obtained by such hydrogenation have also been observed in the gas phase in TMC-1 or other dark clouds (Brown 1981; Irvine & Hjalmarson 1984; Agúndez et al. 2021; Cernicharo et al. 2001, 2020, 2021a,b, 2024; Lee et al. 2021; McGuire et al. 2017). In the right part of the reaction scheme, we present the fully saturated aliphatic organic compounds. Most of these molecules have been observed in the comet 67P/Churyumov-Gerasimenko. The heavy molecules whose identifications are still lacking are marked with asterisks. It should be noted that UVphotolysis and cosmic ray processing of fully saturated aliphatic compounds result in their efficient dehydrogenation. The high saturation degree of the detected aliphatic compounds hints at their partial formation through surface hydrogenation mechanisms (Altwegg et al. 2017; Schuhmann et al. 2019; Hänni et al. 2021, 2022, 2023). The image of comet 67P/Churyumov-Gerasimenko is taken by ESA Rosetta on September 2, 2014. — astro-ph.EP

Highly unsaturated carbon chains, including polyynes, have been detected in many astronomical regions and planetary systems.

With the success of the QUIJOTE survey of the TMC-1, the community has seen a “boom” in the number of detected carbon chains. On the other hand, the Rosetta mission revealed the release of fully saturated hydrocarbons, C3H8, C4H10, C5H12, and (under specific conditions) C6H14 with C7H16, from the comet 67P/Churyumov-Gerasimenko.

The detection of the latter two is attributed to dust-rich events. Similarly, the analysis of samples returned from asteroid Ryugu by Hayabusa2 mission indicates the presence of long saturated aliphatic chains in Ryugu’s organic matter. The surface chemistry of unsaturated carbon chains under conditions resembling those of molecular clouds can provide a possible link among these independent observations.

However, laboratory-based investigations to validate such a chemistry is still lacking. In the present study, we aim to experimentally verify the formation of fully saturated hydrocarbons by the surface hydrogenation of C2nH2 (n>1) polyynes under ultra-high vacuum conditions at 10 K.

We undertook a two-step experimental technique. First, a thin layer of C2H2 ice was irradiated by UV-photons (≥ 121 nm) to achieve a partial conversion of C2H2 into larger polyynes: C4H2 and C6H2. Afterwards, the obtained photoprocessed ice was exposed to H atoms to verify the formation of various saturated hydrocarbons.

In addition to C2H6, which was investigated previously, the formation of larger alkanes, including C4H10 and (tentatively) C6H14, is confirmed by our study. A qualitative analysis of the obtained kinetic data indicates that hydrogenation of HCCH and HCCCCH triple bonds proceeds at comparable rates, given a surface temperature of 10 K.}

G. Fedoseev, X. Li, G. A. Baratta, M. E. Palumbo, K.-J. Chuang

Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Astrophysics of Galaxies (astro-ph.GA); Solar and Stellar Astrophysics (astro-ph.SR); Chemical Physics (physics.chem-ph)
Cite as: arXiv:2501.03031 [astro-ph.EP] (or arXiv:2501.03031v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2501.03031
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Submission history
From: Ko-Ju Chuang
[v1] Mon, 6 Jan 2025 14:18:26 UTC (3,017 KB)
https://arxiv.org/abs/2501.03031
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