Thermonuclear explosions of Chandrasekhar-mass C+O white dwarfs

First results of simulations are presented which compute the dynamical evol ution of a Chandrasekhar-mass white dwarf, consisting of equal amounts of c arbon and oxygen, from the onset of violent thermonuclear burning, by means of a new two-dimensional numerical code. Since in the interior of such a m assive white dwarf nuclear burning progresses on microscopic scales as a sh arp discontinuity, a so-called flamelet, which cannot be resolved by any nu merical scheme, and since on macroscopic scales the burning front propagate s due to turbulence, we make an attempt to model both effects explicitly in the framework of a finite-volume hydrodynamics code. Turbulence is include d by a sub-grid model, following the spirit of large eddy simulations, and the well-localized burning front is treated by means of a level set, which allows us to compute the geometrical structure of the front more accurately than with previous methods. The only free parameters of our simulations ar e the location and the amount of nuclear fuel that is ignited as an initial perturbation. We find that models in which explosive carbon burning is ign ited at the center remain bound by the time the front reaches low densities , where we stopped the computations because our description of combustion i s no longer applicable. In contrast, off-center ignition models give rise t o explosions which, however, are still too weak for typical Type Ia superno vae. Possible reasons for this rather disappointing result are discussed.