Superluminous supernovae: from individual extremes to population trends

This mass loss can naturally be explained by pulsational pair-instability (PPI) ejections. The PPI scenario also accounts for the observed spectral signatures for interaction and the pre-explosion light curve plateau through<br>shell–shell collisions. This work suggests that early-time forbidden lines may serve as a diagnostic for identifying future PPI candidates.<br><br>This thesis also presents the largest uniformly processed sample of SLSN spectra to date, nearly 1000 spectra for 234 events.

Principal component analysis finds no statistical evidence for distinct subclasses within the population, but is able to identify outlier events with evidence for energy input from interaction. Measurements of line velocities and velocity gradients indicate line-forming regions close to the photosphere and ejecta in homologous expansion. These findings are consistent with central engine models.

This homogeneous dataset will facilitate future machine learning applications to SLSNe.<br><br>Finally, I explore the ultraviolet (UV) properties of SLSNe through the unusual case of SN 2023taz, whose red UV colours and deep Mg II absorption point to greater intrinsic diversity in the UV than previously understood. As upcoming facilities push SLSN discoveries to higher redshift where rest-frame UV may be the only accessible wavelength range, understanding this diversity at low redshift will be critical.

Coordinated UV–optical campaigns will be essential for developing new classification schemes that can be applied to the next decade of transient surveys.<br>