Investigating the Spectroscopic Diversity of Luminous Peculiar Type Ia Supernovae

Despite their importance as cosmological probes, the nature of Type Ia supernovae (SNe Ia) remains elusive. While many of these objects are classified as “normal” owing to their relatively homogeneous photometric and spectroscopic properties, growing observational samples have revealed a diverse population of peculiar events spanning a wide range of properties, leading to the identification of several spectroscopic subclasses. This diversity challenges the notion that they arise from a single progenitor–explosion channel. Understanding the physical origin of this diversity is particularly important for the most luminous events, due to their importance for cosmology. This thesis investigates the spectroscopic diversity of luminous peculiar SNe Ia through abundance tomography, a technique that reconstructs the chemical structure of the ejecta from modeling a time-series of spectra. Particular emphasis is placed on objects exhibiting properties that bridge multiple spectroscopic subclasses, in particular, the transitional SN 1999aa, the peculiar 91T-like event iPTF16abc, and the 03fg-like SN 2012dn. SN 1999aa and iPTF16abc reveal an ejecta structure shared with SN 1991T, characterised by the confinement of intermediate-mass elements to a narrow velocity shell. This suggests that the transition from normal to 91T-like SNe Ia cannot be explained by a simple luminosity sequence, but instead requires a distinct outer ejecta structure produced by a sharp decline in burning efficiency. While the progression from SN 1999aa to SN 1991T can largely be understood as a luminosity sequence within this common abundance structure, the lower outer iron abundance and deeply carbon pockets inferred for iPTF16abc despite its luminosity being comparable to that of SN 1991T suggest that variations in burning and ejecta structure contribute to the diversity even within the 91T-like subclass. The inferred kinetic energies and central concentrations of stable neutron-rich Fe-group elements support Chandrasekhar-mass progenitors with a common inner ejecta structure, iii while variations in the outer layers, suggest an explosion scenario that modifies only the outer layers. SN 2012dn contains a central oxygen-rich region and an ejecta mass exceeding the Chandrasekhar limit, strongly favouring a double-degenerate merger origin. The inferred 56Ni mass is insufficient to account for its luminosity, suggesting an additional contribution from interaction with a carbon-rich circumstellar medium. More broadly, these results suggest that mergers may provide a common framework for the 03fg-like subclass, with their diversity reflecting variations in both 56Ni production and the strength of ejecta– circumstellar-medium interaction. The exceptionally low iron abundance inferred in the outer ejecta further points to a sub-solar metallicity progenitor, supporting the picture that 03fg-like events originate in metal-poor environments. Taken together, these results suggest that luminous peculiar SNe Ia do not form a single continuous sequence governed by a smoothly varying parameter such as luminosity, but instead arise from variations in progenitor and explosion mechanisms that produce overlapping observational properties across nominally distinct subclasses.