IRAS low-resolution spectra were extracted for 11,224 IRAS sources. These spectra were classiÐed into astrophysical classes, based on the presence of emission and absorption features and on the shape of the continuum. Counterparts of these IRAS sources in existing optical and infrared catalogs are identiÐed, and their optical spectral types are listed if they are known. The correlations between the photospheric/ optical and circumstellar/infrared classiÐcation are discussed.
Unidentified infrared emission bands at wavelengths of 3-20 micrometres are widely observed in a range of environments in our Galaxy and in others. Some features have been identified as the stretching and bending modes of aromatic compounds, and are commonly attributed to polycyclic aromatic hydrocarbon molecules. The central argument supporting this attribution is that single-photon excitation of the molecule can account for the unidentified infrared emission features observed in 'cirrus' clouds in the diffuse interstellar medium. Of the more than 160 molecules identified in the circumstellar and interstellar environments, however, not one is a polycyclic aromatic hydrocarbon molecule. The detections of discrete and broad aliphatic spectral features suggest that the carrier of the unidentified infrared emission features cannot be a pure aromatic compound. Here we report an analysis of archival spectroscopic observations and demonstrate that the data are most consistent with the carriers being amorphous organic solids with a mixed aromatic-aliphatic structure. This structure is similar to that of the organic materials found in meteorites, as would be expected if the Solar System had inherited these organic materials from interstellar sources.
This authoritative volume provides a comprehensive review of the origin and evolution of planetary nebulae. It covers all the stages of their evolution, carefully synthesizes observations from across the spectrum, and clearly explains all the key physical processes at work. Particular emphasis is placed on observations from space, using the Hubble Space Telescope, the Infrared Space Observatory, and the ROSAT satellite. This book presents a thoroughly modern understanding of planetary nebulae, integrating developments in stellar physics with the dynamics of nebular evolution. It also describes exciting possibilities such as the use of planetary nebulae in determining the cosmic distance scale, the distribution of dark matter and the chemical evolution of galaxies. This book provides graduate students with an accessible introduction to planetary nebulae, and researchers with an authoritative reference. It can also be used as an advanced text on the physics of the interstellar medium.
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