Cataclysmic variable

A cataclysmic variable is a close binary in which a Roche-lobe-filling low-mass star transfers matter to a white dwarf. Angular-momentum loss drives long-term transfer, while unstable burning of accumulated hydrogen produces classical novae.

A cataclysmic variable whose persistent luminous disk resembles a dwarf nova in outburst. A sufficiently high transfer rate can keep its accretion disk on the hot stable branch of the hydrogen-ionization disk instability, avoiding ordinary dwarf-nova cycles.

A cataclysmic variable with recurrent disk outbursts associated with the hydrogen-ionization disk instability. Enhanced accretion powers the bright phase; the event does not require thermonuclear runaway on the white dwarf.

A cataclysmic variable whose white dwarf magnetic field does not dominate the large-scale accretion flow. It normally supports an accretion disk fed through Roche-lobe overflow. The term does not rule out a magnetized donor star or magnetic braking of a binary star.

The bright region where the stream from Roche-lobe overflow strikes an accretion disk. Dissipation of relative kinetic energy produces a localized source of light whose visibility changes around the orbital period.

Ordinary hydrogen-rich cataclysmic variables occur on both sides of a cataclysmic-variable period gap at approximately two to three hours, with a cataclysmic-variable period minimum near 82 minutes. Selection effects change the observed proportions. Knigge, Baraffe and Patterson's donor-based evolutionary study summarizes these empirical landmarks.

For a Roche lobe-filling donor with local radius response $R_2\propto M_2^{\zeta }$, the Roche-lobe-filling period-density relation gives $P\propto M_2^{(3\zeta -1)/2}$. As mass is lost, the period falls for $\zeta >1/3$ and rises for $\zeta <1/3$, with a minimum at the transition. The reversal is called a period bounce. Slow evolution near the turning point produces an accumulation in the period distribution: for a steady flow of systems along one evolutionary branch, $N(P)\propto 1/|\dot{P}|$.

The reversal from decreasing to increasing orbital period during continuing donor mass loss. The Roche-lobe-filling period-density relation gives $d\log P/d\log M_d=(3\zeta -1)/2$, where $\zeta$ is the effective stellar radius response exponent along the mass-losing sequence. A crossing from $\zeta >1/3$ to $\zeta <1/3$ gives a local cataclysmic-variable period minimum.

The cataclysmic-variable period gap is the observed deficit of accreting, ordinary hydrogen-rich cataclysmic variables with periods around two to three hours. The disrupted magnetic braking model interprets it as a detached phase after the donor contracts inside its Roche lobe.

A magnetized wind removes spin angular momentum from a cool donor star. Tidal synchronization replenishes that spin from the orbit, so the wind indirectly removes orbital angular momentum and drives the binary toward shorter separation.

In this model, strong magnetic braking of a binary star inflates the mass-losing donor above the cataclysmic-variable period gap. A reduction in the torque near the fully convective star transition allows thermal contraction and detachment. Gravitational-wave emission from a binary system continues to shrink the orbit until contact returns below the gap. The model requires a braking decrease and a previously inflated donor, not disappearance of stellar magnetism. Knigge's review describes this evolutionary picture and observational tests.

In isotropic re-emission, transferred mass is expelled from the vicinity of the accretor and carries the accretor's specific orbital angular momentum. The total binary mass and orbital angular momentum then decrease even though the accretor retains no net mass.