UC Davis

Although massive stars have a profound influence on the Universe at every scale, the answers to key questions about their mass loss and explodability are unknown. This is because massive stars are rare and the final phases of their evolution and explosions, as supernovae, occur on very short time scales. Over the last 5 years, wide-field surveys have greatly increased the number of supernovae discovered, opening a new window onto massive star evolution. In this thesis I will present the extensive studies of ASASSN-15oz and SN~2018ivc, two hydrogen rich supernovae with linearly declining light curves. In both of these supernovae we find evidence of circumstellar interaction, through radio detections and light curve modeling for ASASSN-15oz and the X-ray detection and uncharacteristic light curve of SN~2018ivc. These observations show that just prior to explosion, red supergiants may undergo eruptive mass loss events typically associated with more massive progenitors. Additionally, through the modeling of pre-explosion observations, the supernova light curve, and nebular spectra, we find that zero-age main sequence mass of the progenitors of these supernovae were less than 17 solar masses. Finally, we analyze progenitor masses from the nebular spectra of 24 supernovae found in the literature. Consistent with progenitor masses determined from pre-explosion observations, we again do not find any high mass red supergiant progenitors from this independent technique, implying that a theoretical explanation may be needed to explain the lack of high mass progenitors.