Cataclysmic variables with evolved secondaries and the progenitors of AM CVn stars

We present the results of a systematic study of cataclysmic variables (CVs) and related systems, combining detailed binary-population synthesis (BPS) models with a grid of 120 binary evolution sequences calculated with a Henyey-type stellar evolution code. In these sequences, we used three masses for the white dwarf (0.6, 0.8 and 1.0 M-circle dot) and seven masses for the donor star in the range of 0.6-1.4 M-circle dot . The shortest orbital periods were chosen to have initially unevolved secondaries, and the longest orbital period for each secondary mass was taken to be just longer than the bifurcation period (16-22 h), beyond which systems evolve towards long orbital periods. These calculations show that systems that start with evolved secondaries near the end or just after their main-sequence phase become ultracompact systems with periods as short as similar to7 min. These systems are excellent candidates for AM Canum Venaticorum (AM CVn) stars. Using a standard BPS code, we show how the properties of CVs at the beginning of mass transfer depend on the efficiency for common-envelope (CE) ejection and the efficiency of magnetic braking. In our standard model, where CE ejection is efficient, some 10 per cent of all CVs have initially evolved secondaries (with a central hydrogen abundance X-c<0.4) and ultimately become ultracompact systems (implying a Galactic birth rate for AM CVn-like stars of similar to10(-3) yr(-1)). While these systems do not experience a period gap between 2 and 3 h, their presence in the gap does not destroy its distinct appearance. Almost all CVs with orbital periods longer than similar to5 h are found to have initially evolved or relatively massive secondaries. Based on a preliminary analysis, we find that their distribution of effective temperatures appears to be in reasonably good agreement with the distribution of spectral types obtained by Beuermann et al.