A Volume Limited Sample of Cataclysmic Variables from $\mathit{Gaia}$ DR2: Space Density and Population Properties

We present the first volume-limited sample of cataclysmic variables (CVs), selected using the accurate parallaxes provided by the second data release (DR2) of the ESA $\mathit{Gaia}$ space mission. The sample is composed of 42 CVs within $150\,$pc, including two new systems discovered using the $\mathit{Gaia}$ data, and is $(77 \pm 10)\,$per cent complete. We use this sample to study the intrinsic properties of the Galactic CV population. In particular, the CV space density we derive, $\rho=(4.8^{+0.6}_{-0.8}) \times10^{-6}\,\mathrm{pc}^{-3}$, is lower than predicted by most binary population synthesis studies. We also find a low fraction of period bounce CVs, seven per cent, and an average white dwarf mass of $\langle M_\mathrm{WD} \rangle = (0.83 \pm 0.17)\,\mathrm{M}_\odot$. Both findings confirm previous results, ruling out the presence of observational biases affecting these measurements, as has been suggested in the past. The observed fraction of period bounce CVs falls well below theoretical predictions, by at least a factor of five, and remains one of the open problems in the current understanding of CV evolution. Conversely, the average white dwarf mass supports the presence of additional mechanisms of angular momentum loss that have been accounted for in the latest evolutionary models. The fraction of magnetic CVs in the $150\,$pc sample is remarkably high at $36\,$per cent. This is in striking contrast with the absence of magnetic white dwarfs in the detached population of CV progenitors, and underlines that the evolution of magnetic systems has to be included in the next generation of population models.