Role of Polycyclic Aromatic Hydrocarbons with Edge Defects in Explaining Astronomical Infrared Emission Observations

Abstract

A systematic study was performed on the spectral properties of polycyclic aromatic hydrocarbons (PAHs) with edge defects using harmonic density functional theory calculations. Their potential astronomical relevance was assessed through direct comparison with NIRSpec and MIRI-MRS spectra of the atomic photodissociation region of the Orion Bar from the JWST Early Release Science PDRs4All program. It is found that the astronomical 6.2 μm PAH emission band, including its blue side, is well reproduced by PAHs with edge defects, when taking into account the effects of polarization in the computations, and without a need for PAHs that contain nitrogen. Small neutral PAHs with edge defects explain the blue wing of the 3.3 μm band. A low number of edge defects is required to reproduce the 8.6 and 11.2 μm band profiles, while the 11.0 + 11.2/12.7 μm band intensity ratio is a measure for the number of edge defects. A blind database fit to the Orion Bar spectrum reproduces the 6–15 μm region with an error of 9.9% and shows a clear delineation of charge, with the 6–10 μm PAH bands being carried by PAH cations and the 10–15 μm region by predominantly neutral PAHs. The contribution of anions is negligible. Armchair PAHs fit the 12.7 μm band, simultaneously producing a very weak broad emission feature centered at 3.225 μm. Zigzag PAHs fit the 11.2 μm band. It is concluded that PAHs with a low number of edge defects, in addition to armchair and zigzag PAHs, all contribute to the observed interstellar infrared emission.