The cataclysmic variable orbital period gap: More evident than ever
arxiv(2024)
摘要
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.
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