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Paper: The Unidentified Infrared Features After ISO
Volume: 309, Astrophysics of Dust
Page: 141
Authors: Peeters, E.; Allamandola, L.J.; Hudgins, D.M.; Hony, S.; Tielens, A.G.G.M.
Abstract: The Infrared Space Observatory (ISO) has provided the first complete mid-IR spectra for a wide range of objects. Almost all of these spectra are dominated by the well-known infrared emission features at 3.3, 6.2, 7.7, and 11.2 μm, the so-called Unidentified Infra-Red (UIR) features. Besides the major features, there is an array of minor features and broad plateaux stretching from 3 to 20 μm which reveal subtle details of conditions in the emission zones and properties of the carriers. Generally attributed to the vibrational relaxation of UV-pumped Polycyclic Aromatic Hydrocarbon molecules (PAHs) containing some 50—100 C-atoms, these UIR spectra are a treasure trove of information. The ISO spectra have, for the first time, allowed a systematic analysis of the spectral characteristics of the UIR features in a wide variety of environments. The peak positions, profiles, and relative strengths of the major features vary from source to source and spatially within sources. Variations in the 6.2 and 7.7 μm features are particularly noteworthy; the former peaks between 6.25—6.3 μm in many PNe, but falls at 6.2 μm in RNe and H ii regions. Similarly, the broad 7.7 μm feature consists of major components at 7.6 and "7.8" μmwhose relative strengths vary. The 7.6 μm component dominates in H ii regions and reflection nebulae, while the "7.8" μm feature takes over in most PNe. These specific profiles are not unique to certain object types but can occur within each individual source. While the 3.3 and 11.2 μm also show variations in peak position and profile, these are much less pronounced. In addition, the 3.3 μm feature intensity correlates quite well with that of the 11.2 μm feature, a correlation which does not extend to the CH modes between 12 and 14 μm. Also, variations occurs in the relative strengths of the CC modes in the 6—9 μm range relative to the CH modes 3.3 and 11.2 μm. Such behavior requires that the UIR features are carried by a family of related compounds whose detailed physical and/or chemical characteristics vary in response to local physical conditions. Here, we review ISO and recent ground-based observations and assess some of their implications.
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