Unravelling the nature of hydrogen-poor thermonuclear supernovae

Abstract

Thermonuclear supernovae take many forms, as demonstrated by the increasing evidence of diversity among type Ia supernovae, particularly at early times, and subclasses of peculiar objects. This thesis presents new observations and modelling tools that explore the diversity of thermonuclear supernovae. We present colour light curves, and optical and near-infrared spectra for SN 2015H, a type Iax supernova, beginning a few days post-explosion. We compare these observations to synthetic light curves and spectra predicted from multi-dimensional explosion models invoking weak deflagrations of Chandrasekhar-mass carbon-oxygen white dwarfs, and find reasonable agreement with a model producing only ∼0.07 M of 56Ni. Our results demonstrate that the observational signatures of this explosion scenario are consistent with type Iax supernovae across a range of luminosities. Our analysis of SN 2015H shows that the strength of the deflagration, and hence amount of 56Ni produced during the explosion, can explain some of the diversity observed among type Iax supernovae. In a subsequent study, we present observations of PS1-12bwh that demonstrate this is not the sole factor affecting the appearance of type Iax supernovae. While the light curve and post-maximum spectra of PS1-12bwh are virtually identical to the prototypical SN 2005hk, the premaximum spectrum of the former does not resemble spectra of the latter at a comparable epoch. We perform spectral modelling and find that the unique pre-maximum appearance of PS1-12bwh is consistent with a lower density in the outer ejecta compared to SN 2005hk. Both objects showed similar light curve peaks and shapes, therefore our analysis indicates that there are additional factors responsible for the diversity of type Iax supernovae apart from the amount of 56Ni produced; hence, type Iax supernovae are not a one-parameter family. The early light curves of type Ia supernovae also display diversity in their colours and shapes. We therefore present a new Monte Carlo radiative transfer code designed for modelling light curves of radioactively driven transients. We perform a parameter study, focusing on the effects of the 56Ni distribution and density profile within the supernova ejecta. Models with 56Ni extending throughout the entire ejecta show brighter and bluer light curves at early times. The density profile also plays a significant role in shaping the early light curve; however, this has been neglected in previous studies. Our models show that comparisons with full colour light curves are necessary to constrain the ejecta properties of type Ia supernovae, such as 56Ni distribution and density profile.

Date of Award	Jul 2019
Original language	English
Awarding Institution	Queen's University Belfast
Supervisor	Stephen Smartt (Supervisor) & Stuart Sim (Supervisor)