  Mergers of two carbon-oxygen (CO) white dwarfs (WDs) have been considered as
progenitors of Type Ia supernovae (SNe Ia). Based on smoothed particle
hydrodynamics (SPH) simulations, previous studies claimed that mergers of CO
WDs lead to an SN Ia explosion either in the dynamical merger phase or
stationary rotating merger remnant phase. However, the mass range of CO WDs
that lead to an SN Ia has not been clearly identified yet. In the present work,
we perform systematic SPH merger simulations for the WD masses ranging from
$0.5~M_{\odot}$ to $1.1~M_{\odot}$ with higher resolutions than the previous
systematic surveys and examine whether or not carbon burning occurs dynamically
or quiescently in each phase. We further study the possibility of SN Ia
explosion and estimate the mass range of CO WDs that lead to an SN Ia. We found
that when the both WDs are massive, i.e., in the mass range of $0.9~M_{\odot}
{\le} M_{1,2} {\le} 1.1~M_{\odot}$, they can explode as an SN Ia in the merger
phase. On the other hand, when the more massive WD is in the range of
$0.7~M_{\odot} {\le} M_{1} {\le} 0.9~M_{\odot}$ and the total mass exceeds
$1.38~M_{\odot}$, they can finally explode in the stationary rotating merger
remnant phase. We estimate the contribution of CO WD mergers to the entire SN
Ia rate in our galaxy to be of ${\lt} 9\%$. So, it might be difficult to
explain all galactic SNe Ia by CO WD mergers.
