Abstract. IR spectroscopy provides a valuable tool for the characterisation and identification of interstellar molecular species.Here, we present 6-9 µm 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 6.2 µm feature shifts from 6.22 to 6.3 µm and clearly shows profile variations.2) The 7.7 µm complex is comprised of at least two subpeaks peaking at 7.6 and one longwards of 7.7 µm. In some cases the main peak can apparently shift up to 8 µm. Two sources do not exhibit a 7.7 µm complex but instead show a broad emission feature at 8.22 µm.3) The 8.6 µm 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 8.6 µm 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 6.2 µm feature peaking at ∼6.22 µm exhibit a 7.7 µm complex dominated by the 7.6 µm component. In contrast, sources with a 6.2 µm profile peaking longwards of 6.24 µm show a 7.7 µm complex with a dominant peak longwards of 7.7 µm and a 8.6 µm feature shifted toward the red. Furthermore, the observed 6-9 µm 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 6.2 µm profile and peak position to the combined effect of a PAH family and anharmonicity with pure PAHs representing the 6.3 µm component and substituted/complexed PAHs representing the 6.2 µm component. The 7.6 µm component is well reproduced by both pure and substituted/complexed PAHs but the 7.8 µm component remains an enigma. In addition, the exact identification of the 8.22 µm 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.

In recent decades, a number of molecules and diverse dust features have been identified by astronomical observations in various environments. Most of the dust that determines the physical and chemical characteristics of the interstellar medium is formed in the outflows of asymptotic giant branch stars and is further processed when these objects become planetary nebulae. We studied the environment of Tc 1, a peculiar planetary nebula whose infrared spectrum shows emission from cold and neutral C60 and C70. The two molecules amount to a few percent of the available cosmic carbon in this region. This finding indicates that if the conditions are right, fullerenes can and do form efficiently in space.

Abstract. We present 10−15 µm spectra of a sample of H ii regions, YSOs and evolved stars that show strong unidentified infrared emission features, obtained with the ISO/SWS spectrograph on-board ISO. These spectra reveal a plethora of emission features with bands at 11.0, 11.2, 12.0, 12.7, 13.5 and 14.2 µm. These features are observed to vary considerably in relative strength to each-other from source to source. In particular, the 10-15 µm spectra of the evolved stars are dominated by the 11.2 µm band while for H ii regions the 12.7 is typically as strong as the 11.2 µm band. Analysing the ISO data we find a good correlation between the 11.2 µm band and the 3.3 µm band, and between the 12.7 µm and the 6.2 µm band. There is also a correlation between the ratio of the UIR bands to the total dust emission and the 12.7 over 11.2 µm ratio. Bands in the 10-15 µm spectral region are due to CH out−of−plane (OOP) bending modes of polycyclic aromatic hydrocarbons (PAHs). We summarise existing laboratory data and theoretical quantum chemical calculations of these modes for neutral and cationic PAHs. Due to mode coupling, the exact peak position of these bands depends on the number of adjacent CH groups and hence the observed interstellar 10−15 µm spectra can be used to determine the molecular structure of the interstellar PAHs emitting in the different regions. We conclude that evolved stars predominantly inject compact 100−200 C-atom PAHs into the ISM where they are subsequently processed, resulting in more open and uneven PAH structures.

Infrared (IR) emission features at 3. 3, 6.2, 7.7, 8.6 and 11.3 µm are generally attributed to IR fluorescence from (mainly) FUV pumped large Polycyclic Aromatic Hydrocarbon (PAH) molecules. As such, these features trace the FUV stellar flux and are thus a measure of star formation. We examined the IR spectral characteristics of Galactic massive star forming regions and of normal and starburst galaxies, as well as AGNs and ULIRGs. The goal of this study is to analyze if PAH features are a good qualitative and/or quantitative tracer of star formation and hence the application of PAH emission as a diagnostic tool in order to identify the dominant processes contributing to the infrared emission from Seyfert's and ULIRGs. We develop a new MIR/FIR diagnostic diagram based upon our Galactic sample and compare it to the diagnostic tools of Genzel et al. (1998) and(Laurent et al. 2000), with these diagnostic tools also applied to our Galactic sample. This MIR/FIR diagnostic is derived from the FIR normalized 6.2 µm PAH flux and the FIR normalized 6.2 µm continuum flux. Within this diagram, the Galactic sources form a sequence spanning a range of 3 orders of magnitude in these ratios, ranging from embedded compact H II regions to exposed Photo Dissociation Regions (PDRs) and the (diffuse) ISM. However, the variation in the 6.2 µm PAH feature-to-continuum ratio is relative small. Comparison -2 -of our extragalactic sample with our Galactic sources revealed an excellent resemblance of normal and starburst galaxies to exposed PDRs. While Seyfert-2's coincide with the starburst trend, Seyfert-1's are displaced by at least a factor 10 in 6.2 µm continuum flux, in accordance with general orientation dependent unification schemes for AGNs. ULIRGs show a diverse spectral appearance. Some show a typical AGN hot dust continuum. More, however, are either starburstlike or show signs of strong dust obscuration in the nucleus. One characteristic of the ULIRGs also seems to be the presence of more prominent FIR emission than either starburst galaxies or AGNs. We discuss the observed variation in the Galactic sample in view of the evolutionary state and the PAH/dust abundance and discuss the use of PAHs as quantitative tracers of star formation activity. Based on these investigations we find that PAHs may be better suited as a tracer of B stars, which dominate the Galactic stellar energy budget, than as a tracer of massive star formation (O stars).