http://arxiv.org/abs/1608.06704
The so-called unidentified infrared emission (UIE) features at 3.3, 6.2, 7.7, 8.6, and 11.3 micrometer are ubiquitously seen in a wide variety of astrophysical regions. The UIE features are characteristic of the stretching and bending vibrations of aromatic hydrocarbon materials, e.g., polycyclic aromatic hydrocarbon (PAH) molecules. The 3.3 micrometer aromatic C–H stretching feature is often accompanied by a weaker feature at 3.4 micrometer. The latter is often thought to result from the C–H stretch of aliphatic groups attached to the aromatic systems. The ratio of the observed intensity of the 3.3 micrometer aromatic C–H feature to that of the 3.4 micrometer aliphatic C–H feature allows one to estimate the aliphatic fraction of the UIE carriers, provided that the intrinsic oscillator strengths of the 3.3 micrometer aromatic C–H stretch (A3.3) and the 3.4 micrometer aliphatic C–H stretch (A3.4) are known. While previous studies on the aliphatic fraction of the UIE carriers were mostly based on the A3.4/A3.3 ratios derived from the mono-methyl derivatives of small PAH molecules, in this work we employ density functional theory to compute the infrared vibrational spectra of several PAH molecules with a wide range of sidegroups including ethyl, propyl, butyl, and several unsaturated alkyl chains, as well as all the isomers of dimethyl-substituted pyrene. We find that, except PAHs with unsaturated alkyl chains, the corresponding A3.4/A3.3 ratios are close to that of mono-methyl PAHs. This confirms the predominantly-aromatic nature of the UIE carriers previously inferred from the A3.4/A3.3 ratio derived from mono-methyl PAHs.
X. Yang, A. Li, R. Glaser, et. al.
Thu, 25 Aug 16
12/43
Comments: 19 pages, 5 figures, 2 tables; accepted for publication in The Astrophysical Journal
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