2002A&A...390.1089P

IR spectroscopy provides a valuable tool for the characterisation and identification of interstellar molecular species. Here, we present 6-9µm spectra of a sample of reflection nebulae, HII regions, YSOs, evolved stars and galaxies that show strong unidentified infrared bands, obtained with the SWS spectrograph on board ISO. The IR emission features in this wavelength region show pronounced variations. 1) The 6.2µm feature shifts from 6.22 to 6.3µm and clearly shows profile variations. 2) The 7.7µm complex is comprised of at least two subpeaks peaking at 7.6 and one longwards of 7.7µm. In some cases the main peak can apparently shift up to 8µm. Two sources do not exhibit a 7.7µm complex but instead show a broad emission feature at 8.22µm. 3) The 8.6µm feature has a symmetric profile in all sources and some sources exhibit this band at slightly longer wavelengths. For the 6.2, 7.7 and 8.6µm features, the sources have been classified independently based on their profile and peak position. The classes derived for these features are directly linked with each other. Sources with a 6.2µm feature peaking at ∼6.22µm exhibit a 7.7µm complex dominated by the 7.6µm component. In contrast, sources with a 6.2µm profile peaking longwards of 6.24µm show a 7.7µm complex with a dominant peak longwards of 7.7µm and a 8.6µm feature shifted toward the red. Furthermore, the observed 6-9µm spectrum depends on the type of object. All ISM-like sources and a few PNe and Post-AGB stars belong to the first group while isolated Herbig AeBe stars, a few Post-AGB stars and most PNe belong to the second group. We summarise existing laboratory data and theoretical quantum chemical calculations of the modes emitting in this wavelength region of PAH molecules. We discuss the variations in peak position and profile in view of the exact nature of the carrier. We attribute the observed 6.2µm profile and peak position to the combined effect of a PAH family and anharmonicity with pure PAHs representing the 6.3µm component and substituted/complexed PAHs representing the 6.2µm component. The 7.6µm component is well reproduced by both pure and substituted/complexed PAHs but the 7.8µm component remains an enigma. In addition, the exact identification of the 8.22µm feature remains unknown. The observed variations in the characteristics of the IR emission bands are linked to the local physical conditions. Possible formation and evolution processes that may influence the interstellar PAH class are highlighted.