Polycyclic Aromatic Hydrocarbon Molecules and the 2175 Angstrom Interstellar Extinction Bump

By Keith Cowing
Status Report
August 9, 2023
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Polycyclic Aromatic Hydrocarbon Molecules and the 2175 Angstrom Interstellar Extinction Bump
Target molecules for TD-DFT computations of their electronic transitions. — astro-ph.SR

The exact nature of the 2175 Angstrom extinction bump, the strongest spectroscopic absorption feature superimposed on the interstellar extinction curve, remains unknown ever since its discovery in 1965. Popular candidate carriers for the extinction bump include nano-sized graphitic grains and polycyclic aromatic hydrocarbon (PAH) molecules.

To quantitatively evaluate PAHs as a possible carrier, we perform quantum chemical computations for the electronic transitions of 30 compact, pericondensed PAH molecules and their cations as well as anions with a wide range of sizes from 16 to 96 C atoms and a mean size of 43 C atoms. It is found that a mixture of such PAHs, which individually exhibit sharp absorption features, show a smooth and broad absorption band that resembles the 2175 Angstrom interstellar extinction bump.

Arising from Ο€βˆ—β†Ο€ transitions, the width and intensity of the absorption bump for otherwise randomly-selected and uniformly-weighted PAH mixtures, do not vary much with PAH sizes and charge states, whereas the position shifts to longer wavelengths as PAH size increases.

While the computed bump position, with the computational uncertainty taken into account, appears to agree with that of the interstellar extinction bump, the computed width is considerably broader than the interstellar bump which remains unaccounted for as far as the PAH mixtures considered here are concerned. It appears that, to account for the observed bump width, one has to resort to PAH species of specific sizes and structures.

Qi Lin, X.J. Yang, Aigen Li

Comments: 13 pages, 8 figures, accepted for publication by MNRAS
Subjects: Astrophysics of Galaxies (astro-ph.GA); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2308.04084 [astro-ph.GA] (or arXiv:2308.04084v1 [astro-ph.GA] for this version)
Submission history
From: Xuejuan Yang
[v1] Tue, 8 Aug 2023 06:47:50 UTC (2,169 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) πŸ––πŸ»