Analysis of the 3.2–3.3 μm Interstellar Absorption Feature on Three Milky Way Sightlines

We report new analyses of spectra of the 3.2–3.3 μm absorption feature observed in the diffuse interstellar medium toward three Milky Way sources: 2MASS J17470898 ‑ 2829561 (2M1747) and the Quintuplet Cluster, both located in the Galactic center, and Cygnus OB2-12. The 3.2–3.3 μm interval coincides with the CH-stretching region for compact polycyclic aromatic hydrocarbons (PAHs). We focus on the 2M1747 spectrum. Its published optical depth spectrum contains residual telluric transmission features, which arise from the 0.06 difference in mean airmasses between the observations of the source and its telluric standard star. We corrected the published spectrum by adding the airmass residual optical depth spectrum. The corrected spectrum is well fit by a superposition of four Gaussians. The absorption spectra of the other two sources were also fit by four Gaussians, with similar central wavelengths, widths, and relative peak opacities. We associate the three longer wavelength Gaussians covering the 3.23–3.31 μm interval with compact PAHs in positive, neutral, and negative charge states. We identify the shortest-wavelength Gaussian, near 3.21 μm, with irregularly shaped PAHs. Constraints imposed by spectral smoothness on the corrected 2M1747 spectrum, augmented by a PAH cluster formation model for post-asymptotic giant branch stars, suggests that >99% of the PAHs in the diffuse interstellar medium reside in small clusters. This study supports the PAH hypothesis, and it suggests that a family of primarily compact PAHs with a C₆₆H₂₀ (circumvalene) parent is consistent with the observed mid-infrared and ultraviolet interstellar absorption spectrum.