Polycyclic aromatic hydrocarbon emission model in photodissociation regions – II. Application to the polycyclic aromatic hydrocarbon and fullerene emission in NGC 7023

Abstract

ABSTRACT We present a charge distribution-based emission model that calculates the infrared spectrum of fullerenes (C60). Analysis of the modelled spectrum of C60 in various charge states shows that the relative intensity of the features in the 5–10 μm versus 15–20 μm can be used to probe the C60 charge state in interstellar spectra. We further used our model to simulate emission from polycyclic aromatic hydrocarbons (PAHs) and C60 at five positions in the cavity of reflection nebula NGC 7023. Specifically, we modelled the 6.2/11.2 band ratio for circumcoronene and circumcircumcoronene and the 7.0/19.0 band ratio for C60 as a function of the ionization parameter γ. A comparison of the model results with the observed band ratios shows that the γ values in the cavity do not vary significantly, suggesting that the emission in the cavity does not originate from locations at the projected distances. Furthermore, we find that the C60-derived γ values are lower than the PAH-derived values by an order of magnitude. We discuss likely scenarios for this discrepancy. In one scenario, we attribute the differences in the derived γ values to the uncertainties in the electron recombination rates of PAHs and C60. In the other scenario, we suggest that PAHs and C60 are not co-spatial resulting in different γ values from their respective models. We highlight that experiments to determine necessary rates will be required in validating either one of the scenarios.