Interpreting the subtle spectral variations of the 11.2 and 12.7 {\mu}m   polycyclic aromatic hydrocarbon bands
  We report new properties of the 11 and 12.7 {\mu}m emission complexes of polycyclic aromatic hydrocarbons (PAHs) by applying a Gaussian-based decomposition technique. Using high-resolution \textit{Spitzer} Space Telescope data, we study in detail the spectral and spatial characteristics of the 11 and 12.7 {\mu}m emission bands in maps of reflection nebulae NGC 7023 and NGC 2023 (North and South) and the star-forming region M17. Profile variations are observed in both the 11 and 12.7 {\mu}m emission bands. We identify a neutral contribution to the traditional 11.0 {\mu}m PAH band and a cationic contribution to the traditional 11.2 {\mu}m band, the latter of which affects the PAH class of the 11.2 {\mu}m emission in our sample. The peak variations of the 12.7 {\mu}m complex are explained by the competition between two underlying blended components. The spatial distributions of these components link them to cations and neutrals. We conclude that the 12.7 {\mu}m emission originates in both neutral and cationic PAHs, lending support to the use of the 12.7/11.2 intensity ratio as a charge proxy. 
M. J. Shannon, D. J. Stock, E. Peeters
astro-ph.GA