Shear instabilities between accretion disks and nova objects
Classical novae are thermonuclear explosions on the surface of white dwarfs caused by accretion from a companion star. Most studies focus on modelling the explosive phase (thermonuclear runaway), while this thesis, instead, focuses on the pre-explosion stage, investigating how shear instabilities and accretion-driven dynamics affect the boundary layer between the accretion disk and the stellar surface. The main goal is to assess how shear-driven mixing modifies the thermodynamic and structural conditions of this region and how this affects the onset of thermonuclear runaway, with additional discussion of possible observational consequences. The introductory chapter reviews classical novae, emphasising the key role of the boundary layer. Chapters 1–3 introduce a simplified local model aimed at identifying the conditions under which shear instabilities, in particular the critical layer instability, arise and drive vertical mixing. The effects of this mixing on the envelope’s composition and thermodynamic state are investigated using a novel approach that couples multidimensional hydrodynamic simulations with one-dimensional thermonuclear evolution, allowing a self-consistent study of pre-ignition mixing and its impact on runaway ignition. In Chapters 4–5, the model is extended to include additional physical processes. The boundary layer is obtained self-consistently from dynamical simulations of an accretion disk around a white dwarf, including realistic viscosity and radiative transport. These results show that viscous dissipation and radiation play a major role in shaping both the boundary layer and larger-scale structures such as outflows. The final chapter summarises the work, highlighting the effectiveness of a progressive transition from minimal local models to global simulations as a framework for future extensions, including magnetic fields.
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