Revisiting polycyclic aromatic hydrocarbon emission in Photodissociation regions

The mid-infrared (IR) spectrum of almost all objects in the Universe is dominated by a set of strong emission features characteristic of a class of large organic molecules made of carbon and hydrogen known as polycyclic aromatic hydrocarbons (PAHs). These molecules account for ~15% of the cosmic carbon and ~20% of the total IR power of the Milky Way and star-forming galaxies. They are strong absorbers of ultraviolet (UV) photons and release the absorbed energy through vibrational transitions that result in strong IR emission features. PAHs play a critical role in the evolution of the interstellar medium (ISM) as they drive much of the ISM’s heating and ionization balance. As a result, detailed knowledge of the molecular astrophysics of PAHs, including a thorough understanding of their molecular properties and their interactions with the environment in which they reside, is crucial to understand the evolution of the ISM. Although decades of experimental, theoretical, and observational work have helped gain important insights into the behaviour of PAHs in the ISM, our understanding is far from complete. In this thesis, we investigate the astrophysical behaviour of PAHs from both an observational and theoretical standpoint. Our observational study focuses on identifying the key parameters that drive the PAH behaviour in two well-known Galactic reflection nebulae, NGC 2023 and NGC 7023, using a Principal Component Analysis. We find that the amount of PAH emission, which represents the PAH abundance and excitation, and the PAH charge state are the only two parameters that drive their behaviour in both environments. In our theoretical study, we develop a model that determines the charge distribution of PAHs and uses it to compute the PAH emission spectrum in astrophysical environments. The relative strengths of the PAH emission features predicted by our model in the Orion Bar, NGC 2023, NGC 7023, the Horsehead nebula, and the diffuse ISM compare well to those obtained from observations. Furthermore, the results of our model highlight the necessity of experimentally determined electron-recombination rates of PAHs and the molecular characteristics of PAH anions, both of which are crucial in understanding PAH behaviour but for which the data is scarce to date