Cataclysmic variable evolution with circumbinary disks

The results of a systematic investigation of the evolution of cataclysmic variable binary systems with angular momentum losses associated with a circumbinary disk are presented. A grid of models characterized by initial donor masses ranging from 0.15 to 1.2 M_⊙ and white dwarf masses ranging from 0.3 to 1.4 M_⊙ has been constructed. In contrast to previous studies, the detailed evolution of the binary and that of a geometrically thin, viscous Keplerian circumbinary disk are both fully taken into account in the computations. The numerical results show that the circumbinary disk can promote the mass transfer between the binary components in the system at sufficient rates even for low fractional mass input rates into the circumbinary disk, δ ∼ 10^-4. The existence of only a few systems in the period gap requires that the majority of systems with initial orbital periods greater than about 3 hr undergo a period bounce. Such evolutionary behavior implies that δ is a function of the evolutionary state of the donor, in the sense that δ must be higher for more evolved systems to bounce above an orbital period of 3 hr. For the orbital periods ≲ 2 hr where the majority of dwarf novae exist, small δ are indicated, suggesting that δ is a function of the mean mass transfer rate in the system. In this picture there is no tendency for systems to converge toward a common period as they approach the period minimum, which is consistent with observations indicating that there is not a peak in the period distribution. The implications of these low fractional mass input rates into circumbinary disks on their detectability via their continuum radiation are also discussed.