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.

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.

We present spectra of the 3.3 m and 11.2 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 m feature peaking at $3.290 m, while only very few show an asymmetric 3.3 m feature peaking at a slightly longer wavelength. The profiles of the 11.2 m feature are distinctly asymmetric. The majority of the sources has a 11.2 m feature peaking between 11.20 and 11.24 m, with a very steep blue rise and a low tail-to-top ratio. A few sources show a 11.2 m feature with a peak position of $11.25 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 m feature profiles and peak positions. Correlations between these classes and those based on the 6-9 m features (Peeters et al.) are found. In particular, sources with the most common profiles in the 6-9 m region also show the most common 3.3 and 11.2 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 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 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 m bands) versus the CC modes (6.2, 7.7, and 8.6 m bands) is striking: the peak wavelengths of the features attributed to CC modes vary by $15-80 cm À1 , while for the CH modes the variations are $4-6.5 cm À1 . We summarize existing laboratory data and theoretical calculations of the modes emitting in the 3-12 m region of PAH molecules and complexes. In contrast to the 6.2 and 7.7 m components, which are attributed to PAH cations, the 3.3 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. The variations in FWHM of the 3.3 m feature remains an enigma, while those of the 11.2 m can be explained by anharmonicity and molecular structure. The possible origin of the observed contrast in profile variations between the CH modes and the CC modes is highlighted.

Abstract.We present an analysis of ISO-SWS observations of the Herbig Ae/Be stars HD 97048 and Elias 1. Besides the well-known family of IR emission bands at 3.3, 6.2, "7.7", 8.6 and 11.2 µm these objects show strong, peculiar emission features at 3.43 and 3.53 µm. The latter two features show pronounced substructure which is very similar in the two sources. Comparison of the spectra of HD 97048 and Elias 1 with laboratory spectra of H-terminated diamond surfaces show excellent and very convincing agreement in peak position and spectral detail (Guillois et al. 1999). The position of the 3.53 µm band indicates a temperature of ∼1000 K. An analysis of the radiative energy budget makes us conclude that the diamond carrier of the 3.53 µm feature has typical sizes of 1-10 nm for HD 97048. A fit of the 3.53 µm feature with a theoretical, calculated profile indicates that the emitting diamonds in HD 97048 see a FUV flux of 5.The derived diamond mass, 1.5 × 10 −10 M , is only a tiny fraction of the total circumstellar dust mass and corresponds to only about 1 parts per billion relative to hydrogen. We discuss the origin of the diamond around these Herbig Ae/Be stars and conclude that most likely they are formed in situ. The implications for the nanodiamonds discovered in meteorites are also discussed.

Context. Infrared (IR) spectra provide a prime tool to study the characteristics of polycyclic aromatic hydrocarbon (PAH) molecules in regions of star formation. Herbig Ae/Be stars are a class of young pre-main sequence stellar objects of intermediate mass. They are known to have varying amounts of natal cloud material still present in their direct vicinity. Aims. We characterise the IR emission bands, due to fluorescence by PAH molecules, in the spectra of Herbig Ae/Be stars and link observed variations to spatial aspects of the mid-IR emission. Methods. We analysed two PAH dominated spectra from a sample of 15 Herbig Ae/Be stars observed with the Spitzer Space Telescope. Results. We derived profiles of the major PAH bands by subtracting appropriate continua. The shape and the measured band characteristics show pronounced variations between the two Spitzer spectra investigated. Those variations parallel those found between three infrared space observatory (ISO) spectra of other, well-studied, Herbig Ae/Be stars. The derived profiles are compared to those from a broad sample of sources, including reflection nebulae, planetary nebulae, H ii regions, young stellar objects, evolved stars and galaxies. The Spitzer and ISO spectra exhibit characteristics commonly interpreted respectively as interstellar matter-like (ISM), non-ISM-like, or a combination of the two. Conclusions. We argue that the PAH emission detected from the sources exhibiting a combination of ISM-like and non-ISM-like characteristics indicates the presence of two dissimilar, spatially separated, PAH families. As the shape of the individual PAH band profiles reflects the composition of the PAH molecules involved, this demonstrates that PAHs in subsequent, evolutionary linked stages of star formation are different from those in the general ISM, implying active chemistry. None of the detected PAH emission can be associated with the (unresolved) disk and is thus associated with the circumstellar (natal) cloud. This implies that chemical changes may already occur in the (collapsing?) natal cloud and not necessarily in the disk.