The cataclysmic variable orbital period gap: More evident than ever

Abstract

Context. Recently, large and homogeneous samples of cataclysmic variables identified by the Sloan Digital Sky Survey (SDSS) were published. In these samples, the famous orbital period gap, which is a dearth of systems in the orbital period range ∼2 − 3 h and the defining feature of most evolutionary models for cataclysmic variables, has been claimed not to be clearly present. If true, this finding would completely change our picture of cataclysmic variable evolution. Aims. In this Letter we focus on potential differences with respect to the orbital period gap between cataclysmic variables in which the magnetic field of the white dwarf is strong enough to connect with that of the donor star, so-called polars, and non-polar cataclysmic variables as the white dwarf magnetic field in polars has been predicted to reduce the strength of angular momentum loss through magnetic braking. Methods. We separated the SDSS I–IV sample of cataclysmic variables into polars and non-polar systems and performed statistical tests to evaluate whether the period distributions are bimodal as predicted by the standard model for cataclysmic variable evolution or not. We also compared the SDSS I–IV period distribution of non-polars to that of other samples of cataclysmic variables. Results. We confirm the existence of a period gap in the SDSS I–IV sample of non-polar cataclysmic variables with > 98% confidence. The boundaries of the orbital period gap are 147 and 191 min, with the lower boundary being different to previously published values (129 min). The orbital period distribution of polars from SDSS I–IV is clearly different and does not show a similar period gap. Conclusions. The SDSS samples as well as previous samples of cataclysmic variables are consistent with the standard theory of cataclysmic variable evolution. Magnetic braking does indeed seem get disrupted around the fully convective boundary, which causes a detached phase during cataclysmic variable evolution. In polars, the white dwarf magnetic field reduces the strength of magnetic braking and consequently the orbital period distribution of polars does not display an equally profound and extended period gap as non-polars. It remains unclear why the breaking rates derived from the rotation of single stars in open clusters favour prescriptions that are unable to explain the orbital period distribution of cataclysmic variables.