Thermal Timescale Mass Transfer and the Evolution of White Dwarf Binaries

The evolution of binaries consisting of evolved main-sequence stars (1 < M d / M ☉ < 3.5) with white dwarf companions (0.7 < M wd / M ☉ < 1.2) is investigated through the thermal mass-transfer phase. Taking into account the stabilizing effect of a strong, optically thick wind from the accreting white dwarf surface, we have explored the formation of several evolutionary groups of systems for progenitors with initial orbital periods of 1 and 2 days. The numerical results show that CO white dwarfs can accrete sufficient mass to evolve to a Type Ia supernova, and ONeMg white dwarfs can be built up to undergo accretion-induced collapse for donors more massive than about 2 M ☉ . For donors less massive than ~2 M ☉ , the system can evolve to form an He and CO or ONeMg white dwarf pair. In addition, sufficient helium can be accumulated (~0.1 M ☉ ) in systems characterized by 1.6 ≲ M d / M ☉ ≲ 1.9 and 0.8 ≲ M wd / M ☉ ≲ 1 such that sub-Chandrasekhar-mass models for Type Ia supernovae, involving off-center helium ignition, are possible for progenitor systems evolving via the Case A mass-transfer phase. For systems characterized by mass ratios ≳3, the system likely merges as a result of the occurrence of a delayed dynamical mass-transfer instability. We develop a semianalytical model to delineate these phases that can be easily incorporated in population synthesis studies of these systems.