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.
