The so-called "unidentified" infrared emission (UIE) features at 3.3, 6.2, 7.7, 8.6, and 11.3 μm 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 μm aromatic C-H stretching feature is often accompanied by a weaker feature at 3.4 μm. The latter is generally 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 μm aromatic C-H feature to that of the 3.4 μm 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 μm aromatic C-H stretch (A3.3) and the 3.4 μm 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 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 for 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 et al., "The C-H Stretching Features at 3.2-3.5 μm of Polycyclic Aromatic Hydrocarbons with Aliphatic Sidegroups," Astrophysical Journal, vol. 825, no. 1, Institute of Physics Publishing, Jun 2016.
The definitive version is available at https://doi.org/10.3847/0004-637X/825/1/22
Keywords and Phrases
Dust; Extinction; ISM: lines and bands; ISM: molecules
International Standard Serial Number (ISSN)
Article - Journal
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01 Jun 2016