Issue |
A&A
Volume 696, April 2025
|
|
---|---|---|
Article Number | A180 | |
Number of page(s) | 16 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361/202453102 | |
Published online | 18 April 2025 |
Carbon accretion and desorption by interstellar polycyclic aromatic hydrocarbons
1
Sorbonne Université, UPMC Université Paris 6 and CNRS, UMR 7095, Institut d’Astrophysique de Paris,
Paris,
France
2
Institut für Organische Chemie, Universität Tübingen,
Auf der Morgenstelle 18,
72076
Tübingen,
Germany
★ Corresponding authors; omont@iap.fr; holger.bettinger@uni-tuebingen.de
Received:
21
November
2024
Accepted:
12
February
2025
Two key questions of the chemistry of polycyclic aromatic hydrocarbons (PAHs) in the interstellar medium (ISM) are addressed: (i) the way carbon is returned from PAHs to the interstellar gas after the very efficient accretion of C+ ions onto PAHs, and (ii) the PAH contribution to the high abundance of small carbon molecules observed in UV-irradiated regions. They are addressed based on the structure and stability of the various isomers of the complexes formed by PAHs and their cations with atomic carbon. Carbon complexes with coronene are studied by B3LYP/6-311+G** calculations, in order to determine the behaviour of C+ and C complexes with larger pericondensed interstellar PAHs, which are thought to be dominant in the ISM. The most stable forms of the [C-coronene]+ cation include 7C and 4C rings, C+ insertion into a CH bond, and a 5C ring with a short exocyclic cumulene chain, and similarly for neutral [C-coronene]. The subsequent evolution of similar complexes with pericondensed PAHs, in diffuse clouds, is discussed under the action of interstellar UV photons and H atoms as a function of the PAH size. Despite the complexity of this processing, it seems probable that, for small PAHs, these complexes efficiently lose a C2H2 molecule from repeated photodissociations. However, this conclusion needs to be confirmed by the identification of reaction paths and the computation of activation energies. The case of the evolution of larger [C-PAH] complexes is less clear. The processing may explain the observed balance between C+ and PAHs, at least in the diffuse ISM. The formation of C2H2 from PAH catalysis is a key input for the chemistry of small carbon molecules in diffuse clouds. C+ accretion might frequently form stable PAHs that contain a peripheral pentagonal ring and form a significant fraction of interstellar PAHs.
Key words: ISM: clouds / dust, extinction / ISM: molecules / photon-dominated region (PDR)
© The Authors 2025
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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