Doctor of Philosophy
Planetary Science and Exploration
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.
Summary for Lay Audience
Molecules are powerful messengers in the Universe, providing us with critical information about physical and chemical processes occurring there. One such important class of molecules are polycyclic aromatic hydrocarbons (PAHs). These molecules, which are made up of carbon and hydrogen, pervade the Universe. They regulate the temperature and the ionization balance of the interstellar gas, which ultimately impacts major astronomical processes, including star formation, planet formation, and galaxy evolution. The specifics of PAH-driven small-scale physical processes are determined by their molecular properties, such as charge, size, and molecular structure. These molecular properties are not well constrained in astronomical environments. It is crucial to bridge this gap in our understanding of astronomical PAH molecules to better comprehend the large astrophysical processes.
The infrared (IR) emission features of PAHs that we observe encode the precise molecular properties of PAHs in a given environment. Much work has been done in the past to deduce these underlying properties of PAHs but we are still a long way from uncovering the whole picture. This thesis contributes to solving this puzzle by investigating PAH molecules from an observational and theoretical perspective. For the observational study, we employ an advanced statistical technique called Principal Component Analysis to analyze the IR emission features of PAHs in two different environments and find that in terms of the molecular properties, the charge state is the major driving factor of the observed PAH emission. In our theoretical study, we develop a model based on the results of the observational study to predict the spectral signatures of these molecules in the Universe. This model serves as a test-bed for our understanding of the astrophysics of PAHs required for the small scale physical processes driven by PAHs, which ultimately drive the large-scale processes occurring in the Universe.
Sidhu, Ameek K., "Revisiting polycyclic aromatic hydrocarbon emission in Photodissociation regions" (2021). Electronic Thesis and Dissertation Repository. 8149.
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