Detection of PAH Absorption and Determination of the Mid-infrared Diffuse Interstellar Extinction Curve from the Sight Line toward Cyg OB2-12

The sight line toward the luminous blue hypergiant Cyg OB2-12 is widely used to study interstellar dust on account of its large extinction (A_V ≃ 10 mag) and the fact that this extinction appears to be dominated by dust typical of the diffuse interstellar medium. We present a new analysis of archival Infrared Space Observatory Short Wavelength Spectrometer and Spitzer IRS observations of Cyg OB2-12 using a model of the emission from the star and its stellar wind to determine the total extinction A_λ from 2.4 to 37 μm. In addition to the prominent 9.7 and 18 μm silicate features, we robustly detect absorption features associated with polycyclic aromatic hydrocarbons, including the first identification of the 7.7 μm feature in absorption. The 3.3 μm aromatic feature is found to be much broader in absorption than is typically seen in emission. The 3.4 and 6.85 μm aliphatic hydrocarbon features are observed with relative strengths that are consistent with observations of these features on sight lines toward the Galactic center. We identify and characterize more than 60 spectral lines in this wavelength range, which may be useful in constraining models of the star and its stellar wind. Based on this analysis, we present an extinction curve ${A}_{\lambda }/{A}_{2.2\mu {\rm{m}}}$ that extrapolates smoothly to determinations of the mean Galactic extinction curve at shorter wavelengths and to dust opacities inferred from emission at longer wavelengths, providing a new constraint on models of interstellar dust in the mid-infrared.