2023MNRAS.522.3227S

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.