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

TL;DR

This study searched for a predicted 1.05 μm PAH feature in interstellar space but found no evidence, challenging existing models of PAH properties and charge states.

Contribution

It provides the first observational upper limit on the 1.05 μm PAH feature, constraining theoretical models and refining the understanding of PAH characteristics in the diffuse interstellar medium.

Findings

Placed a 5σ upper limit on the 1.05 μm feature strength.

Ruled out theoretical estimates with >10σ significance.

Constrained the physical parameters of the observed sight line.

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 $\Delta\tau_{1.05}/A_{V} < 5.6 \times10^{-3}$, ruling out theoretical estimates with $> 10\sigma$ significance. We constrain the effective temperature of BD+40 4223 to be $\log_{10}\left(T_{\rm eff}\right)=4.41\pm0.03$ and infer that it is veiled by $6.39\pm0.05$ 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.