  Mergers of carbon-oxygen (CO) white dwarfs (WDs) are considered as one of the
potential progenitors of type Ia supernovae (SNe Ia). Recent hydrodynamical
simulations showed that the less massive (secondary) WD violently accretes onto
the more massive (primary) one, carbon detonation occurs, the detonation wave
propagates through the primary, and the primary finally explodes as a
sub-Chandrasekhar mass SN Ia. Such an explosion mechanism is called the violent
merger scenario. Based on the smoothed particle hydrodynamics (SPH) simulations
of merging CO WDs, we derived more stringent critical mass ratio (qcr) leading
to the violent merger scenario than the previous results. We conclude that this
difference mainly comes from the differences in the initial condition,
synchronously spinning of WDs or not. Using our new results, we estimated the
brightness distribution of SNe Ia in the violent merger scenario and compared
it with previous studies. We found that our new qcr does not significantly
affect the brightness distribution. We present the direct outcome immediately
following CO WD mergers for various primary masses and mass ratios. We also
discussed the final fate of the central system of the bipolar planetary nebula
Henize 2-428, which was recently suggested to be a double CO WD system whose
total mass exceeds the Chandrasekhar-limiting mass, merging within the Hubble
time. Even considering the uncertainties in the proposed binary parameters, we
concluded that the final fate of this system is almost certainly a
sub-Chandrasekhar mass SN Ia in the violent merger scenario.
