A Tale of Two Sightlines: Comparison of Hydrocarbon Dust Absorption Bands toward Cygnus OB2-12 and the Galactic Center

Infrared spectra of hydrocarbon dust absorption bands toward the bright hypergiant Cygnus OB2-12 are compared to published spectra of the Quintuplet Cluster, a sightline to the Galactic center. The Cyg OB2-12 data include a new ground-based 2.86−3.70 μ m spectrum and a previously published, but here further analyzed, spectrum of the 5.50–7.34 μ m region. Higher-spectral-resolution data for the Cyg OB2-12 sightline in the 3 μ m region allows a detailed comparison of the 3.4 μ m aliphatic bands to those observed toward the Quintuplet. Despite differences in interstellar environments along each sightline, strong similarities are observed in the central wavelengths and relative strengths for bands at ∼3.3, 3.4, 5.85, 6.2, and 6.85 μ m. Analysis of these bands, produced by aromatic, aliphatic, olefinic, hydrogenated, and oxygenated components, shows that carbonaceous dust is a significant component of the diffuse interstellar medium (ISM), second in abundance only to silicates, and is primarily aromatic in nature. The grains producing these bands likely consist of large aromatic carbon cores with thin aliphatic mantles composed of hydrogenated amorphous carbon. Laboratory analog spectra reproduce the observed aliphatic absorption bands well, supporting the presence of such mantles. We present evidence that the carriers of both the 3.4 μ m aliphatic and the 3.3 μ m aromatic bands reside exclusively in the diffuse ISM, and that the 3.3 μ m bands observed in the diffuse ISM differ from the 3.25 μ m band seen in dense clouds, implying chemically distinct carriers.