Remnant evolution after a carbon-oxygen white dwarf merger

We systematically explore the evolution of the merger of two carbon-oxygen (CO) white dwarfs. The dynamical evolution of a 0.9 M circle dot + 0.6 M circle dot CO white dwarf merger is followed by a 3D smoothed particle hydrodynamics (SPH) simulation. The calculation uses a state-of the-art equation of state that is coupled to an efficient nuclear reaction network that accurately approximates all stages from helium burning up to nuclear statistical equilibrium. 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 1D hydrodynamic stellar evolution code and then follow its secular evolution, where we include the effects of rotation on the stellar structure and the transport of angular momentum. The influence of the Keplerian disc is implicitly treated by considering mass accretion from the disc on to the hot envelope. 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. Specifically we find that off-centre carbon ignition, which would eventually lead to the collapse of the remnant to a neutron star, can be avoided if the following conditions are satisfied. (1) When the merger remnant reaches quasi-static equilibrium, the local maximum temperature at the interface between the core and the envelope must be lower than the critical limit for carbon ignition. (2) Angular momentum loss from the central merger remnant should not occur on a time-scale shorter than the local neutrino cooling time-scale at the interface. (3) The mass accretion rate from the centrifugally supported disc must be sufficiently low (M less than or similar to 5 x 10(-6) to 10(-5) M circle dot yr(-1)). Our results imply that at least some products of double CO white dwarfs merger may be considered good candidates for the progenitors of Type la supernovae. In this case, the characteristic time delay between the initial \dynamical merger and the eventual explosion would be similar to 10(5) yr.