Kinematics and Space Densities for the Local Cataclysmic Variable Population

Abstract

I investigate the kinematics and space densities of the local cataclysmic variable (CV) population. CVs are close binary star systems composed of a white dwarf that accretes matter from a less compact companion star in Roche lobe overflow. Kinematics provide useful insight into the distances (and hence absolute magnitudes) of stellar populations. My kinematic sample consists of 359 independently measured proper motions, and 306 systemic radial velocities compiled from the literature. The proper motions are the first measurements for some CVs, and are improvements for most of the others. The catalog of CV radial velocities is the most complete to date. I develop a maximum likelihood method to infer mean absolute magnitudes based on a Bayesian kinematic distance estimator. Given an expected underlying velocity distribution, I calculate the probability density function for the absolute magnitude, given a measured proper motion and apparent magnitude. I verify the accuracy of the framework using kinematic data for local main sequence stars. The estimates for CVs are found to be reasonably consistent with absolute magnitudes derived from trigonometric parallaxes. Finally, I use the mean absolute magnitudes for each CV subtype to calculate lower limits on their space densities. While the total derived space density is still an order of magnitude smaller than that predicted by some CV evolution theories, it is consistent with other recent studies.