TURBULENT NUCLEAR FLAMES IN TYPE IA SUPERNOVAE

A numerical scheme is presented that allows us to compute numerically the coupling between large-scale motions and unresolved turbulent scal es for thermonuclear burning fronts in carbon-oxygen white dwarfs. In contrast to most previous calculations, the parameters of the turbulen ce model are determined by physical considerations and from results of laboratory combustion experiments, with the exception of a closure re lation that remains largely undetermined but seems to be of great impo rtance. As a first test case the subgrid model is coupled to a code so lving the Euler equation directly by means of the piecewise parabolic method (PPM), and the propagation of a nuclear burning front in a Chan drasekhar mass white dwarf is simulated in two dimensions. We find tha t energy from the well-resolved Rayleigh-Taylor scale is indeed going into turbulent motions, thereby accelerating the flame speed significa ntly beyond its laminar value. However, with a conservative choice of the parameters of the subgrid model, the turbulent velocity never exce eds a few percent of the sound speed. Consequently, the model has very little in common with observed Type Ia supernovae. Possible ways out of this dilemma are briefly discussed.