The cataclysmic variable orbital period gap: More evident than ever

Abstract

Recently, large and homogeneous samples of cataclysmic variables (CVs) 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 hr and the defining feature of most evolutionary models for CVs, has been claimed not to be clearly present. If true, this finding would completely change our picture of CV evolution. In this Letter we focus on potential differences with respect to the orbital period gap between CVs 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 CVs as the white dwarf magnetic field in polars has been predicted to reduce the strength of angular momentum loss through magnetic braking. We separated the SDSS I-IV sample of CVs 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 CV evolution or not. We confirm the existence of a period gap in the SDSS I-IV sample of non-polar CVs with >98 per cent confidence. The boundaries of the orbital period gap are 147 and 191 minutes, 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. The SDSS samples as well as previous samples of CVs are consistent with the standard theory of CV evolution. Magnetic braking does indeed seem get disrupted around the fully convective boundary, which causes a detached phase during CV 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.

arXiv commentAccepted for publication in Astronomy & Astrophysics Letters

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