The Profiles of the 3-12 Micron Polycyclic Aromatic Hydrocarbon Features

We present spectra of the 3.3 mu m and 11.2 mu m polycyclic aromatic hydrocarbon (PAH) features of a large number of stellar sources, planetary nebulae, reflection nebulae, H II regions, and galaxies, obtained with Infrared Space Observatory Short Wavelength Spectrometer. Clear variations are present in the profiles of these features. Most of the sources show a symmetric 3.3 mu m feature peaking at [approx]3.290 mu m, while only very few show an asymmetric 3.3 mu m feature peaking at a slightly longer wavelength. The profiles of the 11.2 mu m feature are distinctly asymmetric. The majority of the sources has a 11.2 mu m feature peaking between 11.20 and 11.24 mu m, with a very steep blue rise and a low tail-to-top ratio. A few sources show a 11.2 mu m feature with a peak position of [approx]11.25 mu m, a less steep blue rise, and a high tail-to-top ratio. The sources are classified independently on the basis of the 3.3 and 11.2 mu m feature profiles and peak positions. Correlations between these classes and those based on the 6-9 mu m features (Peeters et al.) are found. In particular, sources with the most common profiles in the 6-9 mu m region also show the most common 3.3 and 11.2 mu m feature profiles. However, the uncommon profiles do not correlate with each other. Also, these classifications depend on the type of object. In general, H II regions, nonisolated Herbig AeBe stars and young stellar objects show the same profiles for all 3-12 mu m features. Many planetary nebulae and post-asymptotic giant branch stars show uncommon feature profiles. The three galaxies in our sample show the same profiles as the H II regions for all but the 11.2 mu m feature, being similar to that of evolved stars. The observed pronounced contrast in the spectral variations for the CH modes (3.3 and 11.2 mu m bands) versus the CC modes (6.2, 7.7, and 8.6 mu m bands) is striking: the peak wavelengths of the features attributed to CC modes vary by [approx]15-80 cm super(-1), while for the CH modes the variations are [approx]4-6.5 cm super(-1). We summarize existing laboratory data and theoretical calculations of the modes emitting in the 3-12 mu m region of PAH molecules and complexes. In contrast to the 6.2 and 7.7 mu m components, which are attributed to PAH cations, the 3.3 mu m feature appears to originate in neutral and/or negatively charged PAHs. We attribute the variations in peak position and profile of these IR emission features to the composition of the PAH family.