Investigating the origins of thermonuclear supernovae through multi-dimensional radiative transfer modelling

Abstract

Despite extensive study the explosion mechanisms that produce type Ia supernovae (SNe Ia) are still poorly understood. Modern transient surveys have demonstrated SNe Ia to be a diverse class, consisting of multiple sub-classes, which likely result from different explosion scenarios. The largest and most diverse peculiar sub-class of SNe Ia are type Iax supernovae (SNe Iax). Previous studies have shown pure deflagrations of Chandrasekhar mass carbon oxygen white dwarfs (CO WDs) are the most promising scenario to explain SNe Iax. In this scenario a thermonuclear runaway is ignited near the centre of a Chandrasekhar mass CO WD with the thermonuclear burning front always propagating below the sound speed of the WD (as a deflagration). Previous simulations of deflagration models predict synthetic observables in reasonable agreement with observed SNe Iax but are unable to match the full diversity of the SNe Iax sub-class and show some systematic differences. In this work we primarily focus on time-dependent Monte Carlo radiative transfer simulations of Chandrasekhar mass CO WD pure deflagration models and their comparisons with SNe Iax to investigate if the differences can be resolved. We first present 3D synthetic observables, calculated using an approximate NLTE treatment in our radiative transfer simulations (ARTIS-CLASSIC), for an extensive parameter study of single spark pure deflagration models. These models produce good agreement with bright and intermediate luminosity SNe Iax although some systematic differences persist. Additionally, such models struggle to reproduce the faintest events. We demonstrate that including the contribution from a luminous remnant, predicted by pure deflagration models and suggested to have been observed for SNe Iax, leads to improved agreement with observations. We also present a simulation of a deflagration model up to 30 days after explosion using an improved NLTE treatment of the plasma in the radiative transfer (ARTIS-NLTE). This represents the first such simulation of a pure deflagration model. Significant NLTE effects are found in the synthetic light curves and spectra which lead to an overall improved agreement with observed SNe Iax. We finally investigate one alternative explosion scenario for Chandrasekhar mass CO WDs known as the Gravitationally Confined Detonation (GCD) scenario. In this scenario the deflagration is followed by a detonation. We present 3D synthetic observables calculated using ARTIS-CLASSIC for a parameter study of the GCD scenario and compare with observed SNe Ia. We find that the GCD scenario remains most promising for the over-luminous 91T-like SNe Ia sub-class but does not provide a good match to any of the other SNe Ia sub-classes. Overall the work presented in this thesis strengthens the case of pure deflagration models as the explanation for SNe Iax however differences still remain. Future work should focus on investigating the nature of the luminous remnant in more detail as well as simulating the radiative transfer for both standard pure deflagration models and those with the remnant contribution included utilising a full NLTE treatment of the plasma conditions.

Date of Award	Dec 2023
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Science & Technology Facilities Council
Supervisor	Stuart Sim (Supervisor) & Stephen Smartt (Supervisor)