Modelling Population II Cataclysmic Variables

We study the long–term evolution of cataclysmic variables (CVs) as a function of the companion star metallicity. It is shown that CVs with secondaries of low metallicity (Z=10−4) which belong to population II (PopII CVs) pass through a detached phase with a smaller period width, a shorter minimum period, and display a slightly higher mass transfer rate, resulting in shorter evolutionary timescales compared to CVs with a solar chemical composition of the secondary. A population synthesis model for PopII CVs with a star formation period lasting for 109yr after the onset of Galaxy formation shows that most PopII CVs have already evolved beyond the minimum period. As these systems are faint, they are difficult to observe. Extracting a magnitude–limited sample from the computed intrinsic models and taking into account the sample’s incompleteness towards fainter apparent magnitudes we derive a period distribution both for PopII CVs and the more common population I CVs. From a differential comparison of the models we calculate the probability that a CV at a given vector distance r from the Sun is member of the population II. Based on our results, we find that a reasonable observable sample of PopII CVs can only be found for z >∼ 2000pc.