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

The intensity ratio of the 11.2/3.3 mu m 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 mu m 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 mu m 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 C-60 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.

Automated summary

This study validates the reliability of polycyclic aromatic hydrocarbons (PAHs) size distribution in the interstellar medium (ISM) using the calculated intrinsic infrared (IR) spectra. By comparing calculations from the NASA Ames PAH IR spectroscopic database to gas-phase experimental absorption IR spectra, it was found that the 11.2/3.3 mu m intensity ratio is consistently underestimated by 34%. However, higher level anharmonic calculations show good agreement with the experiments. Based on these findings, the intrinsic ratio of these modes was adjusted and incorporated into an interstellar PAH emission model, suggesting that typical PAH sizes in reflection nebulae are actually in the range of 40 to 55 carbon atoms.