The age of cataclysmic variables

Abstract

Applying standard models for the formation and the evolution of cataclysmic variables (CVs) we determine the age structure of a model population of Galactic CVs. By convolving the obtained CV age distribution with the observed age-space velocity relation of single stars we derive the theoretically expected distribution of γ-velocities and the dispersion of γ as a function of orbital period. This dispersion turns out to be essentially independent of selection effects. Therefore the observational determination of γ for a sufficient number of CVs with known orbital period would probe the intrinsic, largely invisible CV population. Our model calculation predicts that most systems above the period gap are younger than 1.5 Gyr, whereas systems below the gap have a typical age of 3-4 Gyr. This age difference is a direct result of disrupted magnetic braking and leads to a significant difference between the velocity dispersion of CVs below and above the period gap; however, observations do not at present support the existence of this significant difference, although the data are sufficiently sparse and uncertain that they do not contradict it either.