  We systematically explore the evolution of the merger of two carbon-oxygen
(CO) white dwarfs. The dynamical evolution of a 0.9 Msun + 0.6 Msun CO white
dwarf merger is followed by a three-dimensional SPH simulation. We use an
elaborate prescription in which artificial viscosity is essentially absent,
unless a shock is detected, and a much larger number of SPH particles than
earlier calculations. Based on this simulation, we suggest that the central
region of the merger remnant can, once it has reached quasi-static equilibrium,
be approximated as a differentially rotating CO star, which consists of a
slowly rotating cold core and a rapidly rotating hot envelope surrounded by a
centrifugally supported disc. We construct a model of the CO remnant that
mimics the results of the SPH simulation using a one-dimensional hydrodynamic
stellar evolution code and then follow its secular evolution. The stellar
evolution models indicate that the growth of the cold core is controlled by
neutrino cooling at the interface between the core and the hot envelope, and
that carbon ignition in the envelope can be avoided despite high effective
accretion rates. This result suggests that the assumption of forced accretion
of cold matter that was adopted in previous studies of the evolution of double
CO white dwarf merger remnants may not be appropriate. Our results imply that
at least some products of double CO white dwarfs merger may be considered good
candidates for the progenitors of Type Ia supernovae. In this case, the
characteristic time delay between the initial dynamical merger and the eventual
explosion would be ~10^5 yr. (Abridged).
