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

TL;DR

This study compares hydrocarbon dust absorption features along two different interstellar sightlines, revealing that aromatic carbon dust is prevalent and likely consists of large aromatic cores with aliphatic mantles, primarily in the diffuse ISM.

Contribution

It provides a detailed comparison of hydrocarbon dust absorption bands toward Cygnus OB2-12 and the Galactic Center, highlighting the aromatic nature and composition of interstellar dust.

Findings

Strong similarities in absorption bands despite environmental differences

Carbonaceous dust is second in abundance after silicates in the ISM

Carriers of the aromatic and aliphatic bands are confined to the diffuse ISM

Abstract

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 microns spectrum and a previously published, but here further analyzed, spectrum of the 5.50-7.34 microns region. Higher spectral resolution data for the Cyg OB2-12 sightline in the 3 micron region allows a detailed comparison of the 3.4 micron 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 microns. 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, 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 (HAC). 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 micron aliphatic and the 3.3 micron aromatic bands reside exclusively in the diffuse ISM, and that the 3.3 micron bands observed in the diffuse ISM differ from the 3.25 micron band seen in dense clouds, implying chemically distinct carriers.