Study of Core-Collapse Supernovae and their Progenitors

Abstract

Supernovae are the cosmic fireworks that mark the death of stars, wherein heavy elements are forged and dispersed, leading to the galactic enrichment of the Universe. In core-collapse supernovae, the collapse of the inert iron core in massive stars paves the way for the catastrophic explosion. This gives rise to a range of observational signatures due to the diverse nature of the pre-explosion star and its environment. The decoding of the progenitor’s properties and its immediate environment therefore calls for a comprehensive study of these events. The underlying quest of most current studies in this field is to map each of these explosions to a progenitor system in order to construct a complete picture of the theory of stellar evolution. Hydrogen-rich core-collapse supernovae originate from those progenitors that have managed to retain most of its hydrogen envelope prior to the explosion. In this thesis, six hydrogen-rich core-collapse supernovae have been characterized: 2014cx, 2014cy, 2015an, 2015ba, 2015cz, 2016B for probing their progenitor properties and environment. Photometric and spectroscopic monitoring of these events was carried out with Indian and international telescopes. Analysis and modelling of the data has been performed to constrain progenitor properties and explosion parameters. Both light curve and spectral modelling indicate that circumstellar interaction is important in most hydrogen-rich core-collapse supernovae. The classification conundrum of the hydrogenrich supernovae whether or not to be categorised into two subclasses: the plateau supernovae (IIP) and the linearly declining supernovae (IIL), has also been addressed in this thesis. The distinction between the two subclasses has been the subject of much debate in recent years, with some authors arguing for a separate division of classes and others arguing in favour of continuity. Our study supports the latter, as the supernovae studied in this thesis exhibit a transitional nature that is similar to the photometric and spectroscopic properties of both Type IIP and IIL. A deeper understanding of the characteristics of Type II SNe is also important in honing their utility as cosmological probes. In the thesis, we have used the expanding photosphere method, to estimate the distances to four host galaxies. Unlike some other standard candle methods, this approach is independent of the extragalactic distance ladder, and therefore does not invoke external calibration. The dilemma surrounding the low progenitor mass of type II supernovae from direct imaging, popularly known as the ‘Red Supergiant Problem’ has also been explored in this thesis. In the case of SN 2015ba, the progenitor mass is estimated to be approximately 24-26 M , which is higher than the upper limit of 18 M proposed from pre-explosion images. However, the nebular spectra of SN 2015ba exhibited insignificant levels of oxygen, which would otherwise be expected from a massive progenitor. This might be suggestive of the non-monotonical link between O-core masses and the Zero-Age Main-Sequence mass of pre-supernova stars and/or uncertainties in the mixing scenario in the supernova ejecta.