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

Type Ia Supernovae are thought to be thermonuclear explosions of massivewhite dwarfs (WD). We present the first study of multi-dimensional effectsduring the final hours prior to the thermonuclear runaway which leads to theexplosion. The calculations utilize an implicit, 2-D hydro code.Mixing and theignition process are studied in detail. We find that the initial chemicalstructure of the WD is changed but the material is not fully homogenized. Theexploding WD sustains a central region with a low C/O ratio. This implies thatthe explosive nuclear burning will begin in a partially C-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 inwards byconvective flows. We find no evidence for multiple spot or strong off-centerignition. Convective velocities are of the order of 100 km/sec which is wellabove the effective burning speeds in SNe Ia previously expected right afterthe runaway. For about 0.5 to 1 sec, the speed of the burning front willneither be determined by the laminar speed nor the Rayleigh-Taylorinstabilities but by convective flows produced prior to the runaway. Theconsequences are discussed for our under- standing of the detailed physics ofthe flame propagation, the deflagration detonation transition, and thenucleosynthesis in the central layers. Our results strongly suggest thepre-conditioning of the progenitor as a key-factor for our understanding of thediversity in SNeIa.Comment: 16 pages + 10 figures, ApJ accepted. For high resolution figures send E-mail to pah@hej1.as.utexas.ed