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

TL;DR

This study tests the long-predicted 1.05 μm PAH absorption feature arising from PAH cations by targeting the heavily extinguished sight line toward BD+40 4223 with near-IR spectroscopy. Using an extinguished blackbody continuum plus a Drude-profile PAH component and extracting the continuum via line subtraction, the authors perform an MCMC fit to derive Teff, A_V, and a strict upper limit on Δτ_{1.05}/A_V. They find no detection of the 1.05 μm feature, establishing a 5σ upper limit of Δτ_{1.05}/A_V < 5.6 × 10^{-3}, which is inconsistent with the predicted strength by modern PAH models by more than an order of magnitude. The results imply revisions to PAH charge distributions or species on diffuse interstellar sight lines and underscore the need for broader observational and laboratory efforts. The work also identifies other spectral features and validates that the BD+40 4223 sight line is representative of the diffuse ISM, reinforcing the broader implication that 1.05 μm PAH absorption may be rarer or weaker than anticipated.

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 $5σ$ upper limit on the feature strength of $Δτ_{1.05}/A_{V} < 5.6 \times10^{-3}$, ruling out theoretical estimates with $> 10σ$ 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.

Figures (6)

• Figure 1: Absorption and emission lines in the TripleSpec observations that are fit and subsequently subtracted from the observed spectrum following the procedure in Section \ref{['subsec:linemodel_subtraction']}. In addition to numerous hydrogen recombination lines---mostly in the Brackett series---these also include various helium lines and an iron line. Unidentified lines are unlabelled but are still modeled and subtracted.
• Figure 2: TripleSpec observations, shown in black, for BD+40 4223. Regions of the high atmospheric contamination have been masked. Visually identified emission and absorption lines have been subtracted following the procedure in Section \ref{['subsec:linemodel_subtraction']}. The best fit model spectrum---fit only to the TripleSpec observation---is shown as the orange line. The Gaia XP spectrum is shown in purple 2016AA...595A...1G2023AA...674A...1G2023AA...674A...2D2023AA...674A...3M. We also plot measured 2MASS near-infrared fluxes in $J$, $H$, and $K_{\rm S}$ band as the blue, green, and red crosses, respectively 2006AJ....131.1163S. The locations of the 770 nm, 850 nm, and 1.05 $\mu$m features are indicated.
• Figure 3: BD+40 4223 Best-fit and Literature Values
• Figure 4: The posterior distributions from the MCMC sampling using emcee for BD+40 4223. The best fit model (listed in Table \ref{['tab:bestfitparams']}) is shown as as the dashed line and the grey band.
• Figure 5: TripleSpec observations, shown in black with $1\sigma$ uncertainties shown in gray. The best-fit model spectrum, fit to the TripleSpec observations, is shown as the orange line. The blue curve indicates a model sharing the same extinction and stellar parameters as our best fit model spectrum, but with the depth of the 1.05 $\mu$m feature is set to the theoretical model from 2023ApJ...948...55H. The residuals are shown in the lower panel.