On the origin of the 11.3 micron unidentified infrared emission feature

TL;DR

This study challenges the PAH hypothesis for the 11.3 micron UIE feature, suggesting that oxygen and magnesium containing molecules or aromatic/aliphatic structures may better explain the astronomical observations.

Contribution

It provides quantum mechanical analysis of PAHs and proposes alternative molecular carriers for the 11.3 micron emission feature.

Findings

Pure PAH molecules cannot fit astronomical spectra.

Oxygen and magnesium containing molecules improve spectral fits.

Aromatic/aliphatic structures show consistent vibrational modes.

Abstract

The 11.3 $\mu$m emission feature is a prominent member of the family of unidentified infrared emission (UIE) bands and is frequently attributed to out-of-plane bending modes of polycyclic aromatic hydrocarbon (PAH) molecules. We have performed quantum mechanical calculations of 60 neutral PAH molecules and found that it is difficult to reconcile the observed astronomical feature with any or a mix of these PAH molecules. We have further analyzed the fitting of spectra of several astronomical objects by the NASA PAH database program and found that reasonable fittings to the observed spectra are only possible by including significant contributions from oxygen and/or magnesium containing molecules in the mix. A mixed of pure PAH molecules, even including units of different sizes, geometry and charged states, is unable to fit the astronomical spectra. Preliminary theoretical results on the vibrational spectra of simple molecules with mixed aromatic/aliphatic structures show that these structures have consistent bundles of vibrational modes and could be viable carriers of the UIE bands.