Searching for the Shortest-wavelength Aromatic Infrared Bands: No Evidence for the Predicted 1.05 $\mu$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 $\mu$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 $5\sigma$ upper limit on the feature strength of $\Delta {\tau }_{1.05}/A_V<5.6\times 10^{-3}$, ruling out theoretical estimates with $>10\sigma$ significance. We constrain the effective temperature of BD+40 4223 to be ${\log }_{10}\left(T_{\mathrm{eff}}\right)=4.41\pm 0.03$ and infer that it is veiled by $6.39\pm 0.05$ magnitudes 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 non-detection challenges existing models of PAH material properties and/or charge distribution.

12 pages, 5 figures, 1 table. Accepted for publication in ApJ