Evolving to Type Ia Supernovae with Long Delay Times

Recent investigations on the delay time of type Ia supernovae have set usefulconstraints on the progenitors of type Ia supernovae. Here we have calculatedthe evolution of close binaries consisting of a white dwarf and a main-sequenceor subgiant companion. We assume that, once Roche lobe overflow occurs a smallfraction of the lost mass from the system forms a circumbinary disk, whichextracts the orbital angular momentum from the system through tidal torques.Our calculations indicate that the existence of circumbinary disk can enhancethe mass transfer rate and cause secular orbital shrinkage. The white dwarf cangrow in mass efficiently to trigger type Ia supernovae even with relativelylow-mass ($\la 2 M_{\odot}$) donor stars. Thus this scenario suggest a newpossible evolutionary channel to those type Ia supernovae with long delay time$\sim 1-3$ Gyr.Comment: accepted for publication in ApJ Letter