Observations of Circumstellar Interaction in Diverse CCSNe

In this thesis, I investigate the observational properties of two supernovae displaying strong signatures of interaction with circumstellar material, and how they contribute to furthering our understanding of their respective subtypes. The first part of this thesis concerns the photometric and spectroscopic analysis of SN 2018zd. I coordinated the follow-up campaign for this object on behalf of the NUTS collaboration, acquiring hours of observations in the UV through NIR bands. I determine that this transient event is the result of the Fe core collapse of a 8-10M¿ red supergiant. The high ionisation lines observed in the early epochs of spectra are the observational signature of delayed shock breakout through circumstellar material detached from the progenitor envelope. Finally, I compare SN 2018zd to the sample of objects known as LLEV’s, luminous low expansion velocity transients; these objects have an enhanced plateau magnitude relative to their expansion velocities due to the extra photon source provided by early time interaction with circumstellar material. This enhanced plateau luminosity affects the inclusion of these objects in samples used to derive correlations from “normal” Type II SN characteristics. One such relationship we investigate is the Standard Candle Method, which can be used to estimate distances to Type IIP SNe where spectra are lacking, and we show that SN 2018zd breaks this correlation. I propose that a caveat should be attached to the Standard Candle Method, and other similar photometric distance estimate methods for Type IIP SNe, that in the absence of early spectral observations to rule out interaction with circumstellar material, strict cuts should be applied to the light curves of these transients before they are included in such samples. The second half of this thesis concerns the analysis of an archival dataset for SN 2015G, comprising of one of the highest cadence and lengthy follow-up campaigns of a Type Ibn SNe to date. SN 2015G was one of the closest observed Type Ibn’s at a distance of ~20.9 Mpc, allowing for prolonged observations of the tail phase, up to approximately six and a half months after discovery. We observed undulations in the light curve, and determine that these are the result of prolonged interaction with circumstellar material ejected shortly prior to the SN explosion. This excludes production of the circumstellar material via smooth progenitor winds, and we conclude that the material was most likely stripped by a binary companion. The results presented in this thesis span nearly the full range of possible circumstellar interaction scenarios, from the very brief, early interaction of SN 2018zd, to the interaction observed in SN 2015G which endured for the entire follow-up campaign. Both studies further our understanding of how interaction affects the evolution of transients; they bolster the broad diversity possible even within well established subtypes of SNe, and act as an all important reminder that our understanding of these subtypes is still limited by the cadences, duration, and wavelength coverage of our observational follow-up capabilities.