Application of the emission model to PAHs and C$_{60}$ II.Polycyclic Aromatic Hydrocarbon emission model in photodissociation regions

Abstract

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.