On the Thermonuclear Runaway in Type Ia Supernovae: How to Run Away?

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Abstract
Type Ia supernovae (SNe Ia) are thought to be thermonuclear explosions of massive white dwarfs (WDs). We present the first study of multidimensional effects during the final hours prior to the thermonuclear runaway that leads to the explosion. The calculations utilize an implicit, two-dimensional hydrodynamic code. Mixing and the ignition process are studied in detail. We find that the initial chemical structure of the WD is changed, but the material is not fully homogenized. In particular, the exploding WD sustains a central region with a low C/O ratio. This implies that the explosive nuclear burning will begin in a partially carbon-depleted environment. The thermonuclear runaway happens in a well-defined region close to the center. It is induced by compressional heat when matter is brought inward by convective flows. We find no evidence for multiple spot or strong off-center ignition. Convective velocities in the WD are on the order of 100 km s-1, which is well above the effective burning speeds in SNe Ia previously expected right after the runaway. In our calculations, the ignition occurs near the center. Then, for ≈ 0.5-1 s, the speed of the burning front will neither be determined by the laminar speed nor the Rayleigh-Taylor instabilities but by convective flows produced prior to the runaway. The consequences are discussed for our understanding of the detailed physics of the flame propagation, the deflagration to detonation transition, and the nucleosynthesis in the central layers. Our results strongly suggest the preconditioning of the progenitor as a key factor for our understanding of the diversity in SNe Ia.