2013ApJ...776..110Y

The unidentified infrared emission (UIE) features at 3.3, 6.2, 7.7, 8.6, and 11.3 µm, commonly attributed to polycyclic aromatic hydrocarbon (PAH) molecules, have recently been ascribed to mixed aromatic/aliphatic organic nanoparticles. More recently, an upper limit of <9% was placed on the aliphatic fraction (i.e., the fraction of carbon atoms in aliphatic form) of the UIE carriers based on the observed intensities of the 3.4 µm and 3.3 µm emission features by attributing them to aliphatic and aromatic C-H stretching modes, respectively, and assuming A_3.4/A_3.3~ 0.68 derived from a small set of aliphatic and aromatic compounds, where A_3.4 and A_3.3are, respectively, the band strengths of the 3.4 µm aliphatic and 3.3 µm aromatic C-H bonds. To improve the estimate of the aliphatic fraction of the UIE carriers, here we analyze 35 UIE sources exhibiting both the 3.3 µm and 3.4 µm C-H features and determine I_3.4/I_3.3, the ratio of the power emitted from the 3.4 µm feature to that from the 3.3 µm feature. We derive the median ratio to be < I_3.4/I_3.3 > ~ 0.12. We employ density functional theory to compute A_3.4/A_3.3 for a range of methyl-substituted PAHs. The resulting A_3.4/A_3.3ratio well exceeds ∼1.4, with an average ratio of A_3.4/A_3.3~ 1.76. By attributing the 3.4 µm feature exclusively to aliphatic C-H stretch (i.e., neglecting anharmonicity and superhydrogenation), we derive the fraction of C atoms in aliphatic form from I_3.4/I_3.3~ 0.12 and A_3.4/A _3.3~ 1.76 to be ∼2%. We therefore conclude that the UIE emitters are predominantly aromatic.