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

doi:10.1039/D2FD00180B

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

Alexander K. Lemmens,

^ab Cameron J. Mackie,^cd Alessandra Candian,

^ae Timothy M. J. Lee,

^f Alexander G. G. M. Tielens,

^g Anouk M. Rijs

^h and Wybren Jan Buma

*^ab

Author affiliations

* Corresponding authors

^a Van 't Hoff Institute for Molecular Sciences, University of Amsterdam, 1098 XH Amsterdam, The Netherlands
E-mail: w.j.buma@uva.nl

^b Radboud University, Institute for Molecules and Materials, FELIX Laboratory, 6525 ED Nijmegen, The Netherlands

^c Kenneth S. Pitzer Center for Theoretical Chemistry, Department of Chemistry, University of California, Berkeley, California 94720, USA

^d Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

^e Anton Pannekoek Institute for Astronomy, University of Amsterdam, 1098 XH Amsterdam, The Netherlands

^f NASA Ames Research Center, Moffett Field, California 94035-1000, USA

^g Leiden Observatory, Leiden University, 2333 CA Leiden, The Netherlands

^h Division of BioAnalytical Chemistry, AIMMS Amsterdam Institute of Molecular and Life Sciences, Faculty of Science, Vrije Universiteit Amsterdam, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands

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.

This article is part of the themed collection: Astrochemistry at high resolution

Faraday Discussions

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

Abstract

Associated articles

Supplementary files

Article information

Download Citation

Permissions

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

Social activity

Search articles by author

Spotlight

Advertisements