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The sightline toward the luminous blue hypergiant Cyg OB2-12 is widely usedin studying interstellar dust on account of its large extinction ($A_V \simeq10$ mag) and the fact that this extinction appears to be dominated by dusttypical of the diffuse interstellar medium. We present a new analysis ofarchival ISO-SWS and Spitzer IRS observations of Cyg OB2-12 using a model ofthe emission from the star and its stellar wind to determine the totalextinction $A_\lambda$ from 2.4--37 $\mu$m. In addition to the prominent 9.7and 18 $\mu$m silicate features, we robustly detect absorption featuresassociated with polycyclic aromatic hydrocarbons (PAHs), including the firstidentification of the 7.7 $\mu$m feature in absorption. The 3.3 $\mu$m aromaticfeature is found to be much broader in absorption than is typically seen inemission. The 3.4 and 6.85 $\mu$m aliphatic hydrocarbon features are observedwith relative strengths consistent with observation of these features onsightlines toward the Galactic Center. We identify and characterize more thansixty spectral lines in this wavelength range, which may be useful inconstraining 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 extrapolatessmoothly to determinations of the mean Galactic extinction curve at shorterwavelengths and to dust opacities inferred from emission at longer wavelengths,providing a new constraint on models of interstellar dust in the mid-infrared.

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