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Paper: Models of the Unidentifed Infrared Emission Features
Volume: 309, Astrophysics of Dust
Page: 731
Authors: Bakes, E.L.O.; Bauschlicher, C., Jr.; Tielens, A.
Abstract: Strong infrared (IR) emission features between 3.3 and 12.7 microns are ubiquitous and are the most luminous mid-IR spectral features originating from the ambient interstellar medium (ISM). First discovered in 1973 by Gillett, Forrest and Merrill, they are valuable diagnostics of its chemical composition, thermal excitation and evolution. IR emission features at 3.3, 6.2, 7.7, 8.6, 11.2 microns and 12.7 microns are observed in many astronomical sources at many stages of evolution. Debate has raged about these mysterious features, which are generally attributed to polycyclic aromatic hydrocarbon (PAH) molecules, although exact identification of the carriers with specific molecules has remained elusive, leading to their being termed the "unidentified infrared (UIR) emission features". PAH molecules play a critical role in the thermodynamics and chemistry of star forming regions. Using the models described in this review, we are able to better describe the size, charge, structure, origins and chemical evolution of the carriers of the "UIR" bands as carbonaceous material is processed through early to late phases of the evolution of stars and their environs in terms of a limited and well-defined set of chemical processes. Our models are based on a quantum chemical and laboratory database of the spectral characteristics of neutral, negative and positively charged PAHs (up to size 96 carbon atoms and charge +3). They simultaneously account for realistic IR characteristics of specific PAH species, photoelectric charging and electron recombination, ultraviolet (UV) photon excitation (single and multiple) followed by IR relaxation and PAH photochemistry, driven by the stellar radiation field and the (ion-molecule) chemistry with the gas. Using these models, we can interpret interstellar data from ISO, SOFIA and SIRTF and Cassini-orbiter data from Titan, Saturn's largest satellite, where features resembling the astrophysical "UIR" emission have been observed by ISO. The Cassini mission promises to elucidate the nature of the "UIR" emitters, which are effectively located in our own "back yard", available for collection, unlike their interstellar counterparts.
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