Size distribution of polycyclic aromatic hydrocarbons in space: an old new light on the 11.2/3.3 μm intensity ratio

Alexander K. Lemmens, Cameron J. Mackie, Alessandra Candian, Timothy M.J. Lee, Alexander G.G.M. Tielens, Anouk M. Rijs, Wybren Jan Buma*

*Corresponding author for this work

Research output: Contribution to JournalArticleAcademicpeer-review

Abstract

The intensity ratio of the 11.2/3.3 μm emission bands is considered to be a reliable tracer of the size distribution of polycyclic aromatic hydrocarbons (PAHs) in the interstellar medium (ISM). This paper describes the validation of the calculated intrinsic infrared (IR) spectra of PAHs that underlie the interpretation of the observed ratio. The comparison of harmonic calculations from the NASA Ames PAH IR spectroscopic database to gas-phase experimental absorption IR spectra reveals a consistent underestimation of the 11.2/3.3 μm intensity ratio by 34%. IR spectra based on higher level anharmonic calculations, on the other hand, are in very good agreement with the experiments. While there are indications that the 11.2/3.3 μm ratio increases systematically for PAHs in the relevant size range when using a larger basis set, it is unfortunately not yet possible to reliably calculate anharmonic spectra for large PAHs. Based on these considerations, we have adjusted the intrinsic ratio of these modes and incorporated this in an interstellar PAH emission model. This corrected model implies that typical PAH sizes in reflection nebulae such as NGC 7023 - previously inferred to be in the range of 50 to 70 carbon atoms per PAH are actually in the range of 40 to 55 carbon atoms. The higher limit of this range is close to the size of the C60 fullerene (also detected in reflection nebulae), which would be in line with the hypothesis that, under appropriate conditions, large PAHs are converted into the more stable fullerenes in the ISM.

Original languageEnglish
Pages (from-to)380-390
Number of pages11
JournalFaraday Discussions
Volume245
Early online date2 Feb 2023
DOIs
Publication statusPublished - 1 Sept 2023

Bibliographical note

Publisher Copyright:
© 2023 The Royal Society of Chemistry.

Funding

We dedicate this article to Tim Lee who passed away November 3, 2022. His contributions have been key to advancing quantum chemistry in general and astrochemistry in particular. He will be sorely missed. We would like to thank Dr Christof Iserlohe for his computational support. This work is part of the Dutch Astrochemistry Network (DAN) II (project no. 648.000.029).

FundersFunder number
Dutch Astrochemistry Network648.000.029

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