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Astron. Astrophys. 357, 1013-1019 (2000)

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1. Introduction

Ground-based, airborne, and space-based observations have shown that the mid-infrared spectra of many objects with associated dust and gas are dominated by the well-known emission features at 3.3, 6.2, 7.7, 8.6, and 11.2 [FORMULA] (Gillett et al. 1973; Cohen et al. 1986; Geballe et al. 1985; Roche et al. 1989; Roelfsema et al. 1996; Beintema et al. 1996). These bands are now generally attributed to vibrational relaxation of UV-pumped Polycyclic Aromatic Hydrocarbon molecules (PAHs) containing [FORMULA]50 carbon atoms (Allamandola et al. 1989, hereafter ATB; Léger & Puget 1984, 1989; Tielens et al. 1999). The PAH molecules do not only dominate the IR spectral characteristics of the ISM, they also play an important role in the life cycle of the interstellar medium. The photoelectric effect on PAHs and PAH clusters dominates the heating of diffuse interstellar gas and therefore, PAHs are an essential ingredient of the phase structure of the ISM. Because of their large abundance, PAHs are also important for the charge balance inside dense interstellar clouds and hence they regulate important processes such as ambipolar diffusion (Lepp & Dalgarno 1988). Finally, interstellar PAHs and PAH clusters provide a large surface area for reactions, leading to the formation of H2 and other species (Allen & Robinson 1975). The origin and evolution of the interstellar medium is therefore intimately interwoven with the origin and evolution of interstellar PAHs.

The ISO mission 1 was ideally suited to study these IR emission features spectroscopically in a large variety of objects. We have initiated a project to study the origin and evolution of PAH molecules in compact H II regions, young stellar objects - in particular Herbig AeBe stars - and stars in the later stages of their evolution (post-AGB objects and planetary nebulae (PNe)) using 2.5-45 µm spectra obtained with the Short Wavelength Spectrometer (SWS) on board of ISO. In all there are about 75 sources in the combined samples. First results of these studies have been presented in Roelfsema et al. (1996), Molster et al. (1996), Tielens et al. (1999), Cox et al. (1999), Van Kerckhoven et al. (1999), and Peeters et al. (1999).

PAH molecules will show emission longward 15 µm associated with the bending modes of the carbon-skeleton (ATB; Zhang et al. 1996; Moutou et al. 1996). For small molecules, these modes are more molecule specific than the modes giving rise to the shorter wavelength features and hence the 15-20 µm region holds great promise from a molecular identification point of view. Here we discuss the infrared characteristics in the 15-20 µm region of a subset of the sources in the samples. In Sect. 2, we present the Short Wavelength Spectrometer (SWS) observations of the sources in our sample. The characteristics of the newly discovered 15-20 µm plateau are discussed in Sect. 3. The infrared spectroscopy of PAH molecules in this wavelength region are reviewed in Sect. 4. Finally, in Sect. 5, the implications of this plateau for the interstellar PAH size distribution are considered. Our results are summarized in Sect. 6.

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© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000