The cataclysmic variable orbital period gap: More evident than ever

Abstract

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 $ and the defining feature of most evolutionary models for has been claimed not to be clearly present. If true, this finding would completely change our picture of evolution. In this Letter we focus on potential differences with respect to the orbital period gap between 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 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 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 evolution or not. We also compared the SDSS\,I-IV period distribution of non-polars to that of other samples of cvs. We confirm the existence of a period gap in the SDSS\,I-IV sample of non-polar 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 are consistent with the standard theory of evolution. Magnetic braking does indeed seem get disrupted around the fully convective boundary, which causes a detached phase during 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 cvs.

Evolutionary Period Changes for 52 Cataclysmic Variables, and the Failure for the Most-fundamental Prediction of the Magnetic Braking Model

“…K2011 reports the gap is 2.15-3.18 hr for CVs with superhumps. Further, Schreiber et al (2024) measured the gap to be from 2.45 to 3.18 hr for nonmagnetic systems. These systems are from the large Sloan Sky Survey volume-limited sample, and so are dominated by the lowest accretion rate systems.…”