Advanced MHD Simulations of White Dwarf Mergers and Thermonuclear Transients

Abstract

White Dwarfs (WDs) -- particularly those in binary systems -- represent a vital research topic in modern astronomy. WD binaries are believed to play a key role in numerous areas, including supernova explosions, galactic chemical evolution, the formation of highly magnetic compact stars, and the gravitational wave background. Advanced magnetohydrodynamic (MHD) codes, coupled with increasing computational resources, now allow astronomers to numerically model WD interactions with improved resolution, extended timescales, and more sophisticated physical processes. Higher dimensionality, more realistic chemical compositions, and larger nuclear reaction networks all contribute to reconciling models with observations. In this thesis, I simulate a variety of WD interactions using new physics-rich simulation frameworks. I employ the moving mesh MHD code AREPO to conduct hydrodynamical simulations and adopt realistic chemical profiles for the WD structures. The first part of the thesis develops a pipeline for generating chemically and thermally self-consistent WD structures using the WDEC code. These structures are used to simulate the merger of a carbon-oxygen WD with a low-mass helium WD, resulting in an edge-lit detonation that produces asymmetric ejecta dominated by Ni56, Si28, and S32. Variation in inspiral rates strongly affects the outcome: slower inspirals produce surviving high-velocity helium WD companions consistent with observed hypervelocity WDs, while faster inspirals lead to a disruption of the companion via extreme tidal forces. These results highlight the importance of physically accurate pre-merger structures and angular-momentum evolution in merger modelling. Subsequent simulations focus on mergers between an oxygen-neon (ONe) and a helium WD using detailed ONe profiles generated with LPCODE. A thermonuclear runaway is triggered at the base of the helium layer, producing about 0.103 solar masses, of ejecta with compositions characteristic of faint, rapidly evolving transients similar to a supernova of type ".Ia". Comparison with simplified constant-composition models shows that neglecting realistic abundance profiles can yield unphysical double-detonation pathways. The outcomes of these models strengthen the link between helium accreting ONe WD systems and observed sub-luminous thermonuclear transients. The final part of the thesis examines how chemical distillation of Ne22 during WD crystallisation alters explosion nucleosynthesis and observables. Artificial detonations of Ne22-enriched core and shell models produce enhanced yields of neutron-rich isotopes such as Ni58, Ni59, and Co55. Spectral differences are only modest at early times but become potentially distinctive in the nebular-phase, where forbidden lines of iron-group elements emerge. Distillation also increases the B-band decline rate without greatly reducing peak luminosity, offering a possible explanation for bright, rapidly declining SNe Ia. These results demonstrate that the evolution of internal composition can leave detectable imprints on explosion products and light curves. Collectively, this work provides new insights into the conditions leading to WD detonations and the diversity of the resulting transients. It underscores the importance of using physically consistent initial models, realistic chemical stratification, and coupled hydrodynamic-nucleosynthetic approaches in simulating thermonuclear events. Show more