The accretion flows and evolution of magnetic cataclysmic variables

Norton, A. J.; Butters, O. W.; Parker, T. L. and Wynn, G. A. (2008). The accretion flows and evolution of magnetic cataclysmic variables. The Astrophysical Journal, 672(1) pp. 524–530.



We have used a model of magnetic accretion to investigate the accretion flows of magnetic cataclysmic variables (mCVs). Numerical simulations demonstrate that four types of flow are possible: disks, streams, rings, and propellers. The fundamental observable determining the accretion flow, for a given mass ratio, is the spin-to-orbital-period ratio of the system. If intermediate polars (IPs) are accreting at their equilibrium spin rates, then for a mass ratio of 0.5, those with Pspin/Porb<~0.1 will be disklike, those with 0.1<~Pspin/Porb<~0.6 will be streamlike, and those with Pspin/Porb~0.6 will be ringlike. The spin-to-orbital-period ratio at which the systems transition between these flow types increases as the mass ratio of the stellar components decreases. For the first time we present evolutionary tracks of mCVs, which make it possible to investigate how their accretion flow changes with time. As systems evolve to shorter orbital periods and smaller mass ratios, in order to maintain spin equilibrium their spin-to-orbital-period ratio will generally increase. As a result, the relative occurrence of ringlike flows will increase, and the occurrence of disklike flows will decrease, at short orbital periods. The growing number of systems observed at high spin-to-orbital-period ratios with orbital periods below 2 hr and the observational evidence for ringlike accretion in EX Hya are fully consistent with this picture.

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