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

We present a charge distribution-based emission model that calculates the infrared spectrum of fullerenes (C-60). Analysis of the modelled spectrum of C-60 in various charge states shows that the relative intensity of the features in the 5-10 mu m versus 15-20 mu m can be used to probe the C-60 charge state in interstellar spectra. We further used our model to simulate emission from polycyclic aromatic hydrocarbons (PAHs) and C-60 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 C-60 as a function of the ionization parameter gamma. A comparison of the model results with the observed band ratios shows that the gamma 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 C-60-derived gamma 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 gamma values to the uncertainties in the electron recombination rates of PAHs and C-60. In the other scenario, we suggest that PAHs and C-60 are not co-spatial resulting in different gamma values from their respective models. We highlight that experiments to determine necessary rates will be required in validating either one of the scenarios.