  We present a new mechanism for Type Ia supernova explosions in massive white
dwarfs. The proposed scenario follows from relaxing the assumption of symmetry
in the model and involves a detonation created in an unconfined environment.
The explosion begins with an essentially central ignition of stellar material
initiating a deflagration. This deflagration results in the formation of a
buoyantly-driven bubble of hot material that reaches the stellar surface at
supersonic speeds. The bubble breakout forms a strong pressure wave that
laterally accelerates fuel-rich outer stellar layers. This material, confined
by gravity to the white dwarf, races along the stellar surface and is focused
at the location opposite to the point of the bubble breakout. These streams of
nuclear fuel carry enough mass and energy to trigger a detonation just above
the stellar surface. The flow conditions at that moment support a detonation
that will incinerate the white dwarf and result in an energetic explosion. The
stellar expansion following the deflagration redistributes stellar mass in a
way that ensures production of intermediate mass and iron group elements
consistent with observations. The ejecta will have a strongly layered structure
with a mild amount of asymmetry following from the early deflagration phase.
This asymmetry, combined with the amount of stellar expansion determined by
details of the evolution (principally the energetics of deflagration, timing of
detonation, and structure of the progenitor), can be expected to create a
family of mildly diverse Type Ia supernova explosions.
