Theoretical Orbital Period Distributions of Cataclysmic Variables above the Period Gap: Effects of Circumbinary Disks

Abstract

Population synthesis tools are used to investigate the population of nonmagnetic cataclysmic variables with unevolved main-sequence-like donors at orbital periods greater than 2.75 hr. In addition to the angular momentum losses associated with gravitational radiation, magnetic braking, and mass loss from the system, we also include the effects of circumbinary disks on the evolution. For a fractional mass input rate into the disk, corresponding to 3×10-4 of the mass transfer rate, the model systems exhibit a bounce at orbital periods greater than 2.75 hr. The simulations reveal that (1) some systems can exist as dwarf novae throughout their lifetime, (2) dwarf novae can evolve into novalike systems, and (3) novalike systems can evolve back into dwarf novae during their postbounce evolution to longer orbital periods. Among these subclasses, novalike cataclysmic variables would be the best candidates to search for circumbinary disks at wavelengths >~10 μm. The theoretical orbital period distribution is in reasonable accord with the combined population of dwarf novae and novalike systems above the period gap, suggesting the possibility that systems with unevolved donors need not detach and evolve below the period gap as in the disrupted magnetic braking model. The resulting population furthermore reveals the possible presence of systems with small mass ratios and a preference of O/Ne/Mg white dwarfs in dwarf nova systems in comparison to novalike systems. The novalike population furthermore shows a lack of systems with high-mass white dwarfs. The importance of observational bias in accounting for the differing populations is examined, and it is shown that an understanding of these effects is necessary in order to confront the theoretical distributions with the observed ones in a meaningful manner.