The Accretion Flows and Evolution of Magnetic Cataclysmic Variables

TL;DR

This paper models magnetic accretion in cataclysmic variables, identifying four flow types, and presents evolutionary tracks showing how accretion modes change with system evolution and observed properties.

Contribution

It introduces a model linking accretion flow types to spin-to-orbital period ratios and provides the first evolutionary tracks for magnetic cataclysmic variables.

Findings

Four types of accretion flows identified: discs, streams, rings, propellers.

Flow type depends on spin-to-orbital period ratio and mass ratio.

Evolutionary tracks show increasing ring-like flows at shorter periods.

Abstract

We have used a model of magnetic accretion to investigate the accretion flows of magnetic cataclysmic variables. Numerical simulations demonstrate that four types of flow are possible: discs, 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 IPs are accreting at their equilibrium spin rates, then for a mass ratio of 0.5, those with Pspin/Porb < 0.1 will be disc-like, those with 0.1 < Pspin/Porb < 0.6 will be stream-like, and those with Pspin/Porb ~ 0.6 will be ring-like. 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 allow investigation of 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 ring-like flows will increase, and the occurrence of disc-like flows will decrease, at short orbital periods. The growing number of systems observed at high spin-to-orbital period ratios with orbital periods below 2h, and the observational evidence for ring-like accretion in EX Hya, are fully consistent with this picture.