Evolving to Type Ia supernovae with long delay times

Recent investigations on the delay time of Type Ia supernovae have set useful constraints on the progenitors of Type Ia supernovae. Here we have calculated the evolution of close binaries consisting of a white dwarf and a main-sequence or subgiant companion. We assume that, once Roche lobe overflow occurs, a small fraction of the lost mass from the system forms a circumbinary disk, which extracts the orbital angular momentum from the system through tidal torques. Our calculations indicate that the existence of a circumbinary disk can enhance the mass transfer rate and cause secular orbital shrinkage. The white dwarf can grow in mass efficiently to trigger Type Ia supernovae even with relatively low mass (less than or similar to 2 M-circle dot) donor stars. Thus, this scenario suggests a new possible evolutionary channel to those Type Ia supernovae with long delay times similar to 1-3 Gyr.