  The explosion of a carbon-oxygen white dwarf as a Type Ia supernova is known
to be sensitive to the manner in which the burning is ignited. Studies of the
pre-supernova evolution suggest asymmetric, off-center ignition, and here we
explore its consequences in two- and three-dimensional simulations. Compared
with centrally ignited models, one-sided ignitions initially burn less and
release less energy. For the distributions of ignition points studied, ignition
within two hemispheres typically leads to the unbinding of the white dwarf,
while ignition within a small fraction of one hemisphere does not. We also
examine the spreading of the blast over the surface of the white dwarf that
occurs as the first plumes of burning erupt from the star. In particular, our
studies test whether the collision of strong compressional waves can trigger a
detonation on the far side of the star as has been suggested by Plewa et al.
(2004). The maximum temperature reached in these collisions is sensitive to how
much burning and expansion has already gone on, and to the dimensionality of
the calculation. Though detonations are sometimes observed in 2D models, none
ever happens in the corresponding 3D calculations. Collisions between the
expansion fronts of multiple bubbles also seem, in the usual case, unable to
ignite a detonation. "Gravitationally confined detonation" is therefore not a
robust mechanism for the explosion. Detonation may still be possible in these
models however, either following a pulsation or by spontaneous detonation if
the turbulent energy is high enough.
