Searching for the Shortest-wavelength Aromatic Infrared Bands: No Evidence for the Predicted 1.05 μ m Polycyclic Aromatic Hydrocarbon Feature

Abstract Polycyclic aromatic hydrocarbons (PAHs) are responsible for a variety of near- and mid-infrared spectral features in Galactic and extragalactic sources. A feature at 1.05 μ m arising from electronic transitions in PAH cations is predicted by laboratory experiments but has never been observationally confirmed. We conduct a dedicated search for this feature in absorption on a highly extinguished sight line toward BD+40 4223, a blue supergiant in Cyg OB2, using the TripleSpec spectrograph at Palomar Observatory. We place a 5 σ upper limit on the feature strength of Δ τ 1.05 / A V < 5.6 × 10 −3 , ruling out theoretical estimates with >10 σ significance. We constrain the effective temperature of BD+40 4223 to be log 10 T eff = 4.41 ± 0.03 and infer that it is veiled by 6.39 ± 0.05 mag of visual extinction, consistent with but more constraining than previous determinations. As dust on the sight line toward BD+40 4223 appears typical of the diffuse interstellar medium, this nondetection challenges existing models of PAH material properties and/or charge distribution.