Propagation of the First Flames in Type Ia Supernovae

We consider the competition of the different physical processes that can affect the evolution of a flame bubble in a Type Ia supernovae: burning, turbulence and buoyancy. Even in the vigorously turbulent conditions of a convecting white dwarf, thermonuclear burning that begins at a point near the center (within 100 km) of the star is dominated by the spherical laminar expansion of the flame until the burning region reaches kilometers in size. Consequently, flames that ignite in the inner ~20 km promptly burn through the center, and flame bubbles anywhere must grow quite large (indeed, resolvable by large-scale simulations of the global system) for significant motion or deformation occur. As a result, any hot spot that successfully ignites into a flame can burn a significant amount of white dwarf material. This potentially increases the stochastic nature of the explosion compared to a scenario in which a simmering progenitor can have small early hot spots float harmlessly away. Furthermore, the size at which the laminar flame speed dominates other relevant velocities sets a characteristic scale for fragmentation of larger flame structures, as nothing, by definition, can easily break the burning region into smaller volumes. This enables the development of semianalytic descriptions of the earliest phase of the propagation of burning in a Type Ia supernovae, which we present here. Our analysis is supported by fully resolved numerical simulations of flame bubbles.