Detection of PAH Absorption and Determination of the Mid-Infrared Diffuse Interstellar Extinction Curve from the Sightline Toward Cyg OB2-12.

The sightline toward the luminous blue hypergiant Cyg OB2-12 is widely used in studying interstellar dust on account of its large extinction ($A_V \simeq 10$ 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 ISO-SWS 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 $A_\lambda$ from 2.4--37 $\mu$m. In addition to the prominent 9.7 and 18 $\mu$m silicate features, we robustly detect absorption features associated with polycyclic aromatic hydrocarbons (PAHs), including the first identification of the 7.7 $\mu$m feature in absorption. The 3.3 $\mu$m aromatic feature is found to be much broader in absorption than is typically seen in emission. The 3.4 and 6.85 $\mu$m aliphatic hydrocarbon features are observed with relative strengths consistent with observation of these features on sightlines toward the Galactic Center. We identify and characterize more than sixty 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 $A_\lambda/A_{2.2 \mu m}$ 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.