3D structure of truncated accretion discs in close binaries

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Abstract
We use smoothed particle hydrodynamics to investigate the 3D accretion flow in binary systems where the secondary star transfers matter on to a compact primary star via a truncated accretion disc. Our model neglects radiative cooling and has been evolved up to 1.7 orbital periods. Our method of calculation differs from those of previous investigators in our treatment of artificial viscosity. We use a pseudo-viscosity in order to absorb energy cascades at a subgrid level and attempt to simulate large-scale eddies. The resulting flow is locally turbulent and eddies can be seen on several scales ranging from the smoothing length up to half the thickness of the disc, with turbulence being most apparent in the stream–disc interaction region. The model is probably relevant to the class of magnetic cataclysmic variables known as the intermediate polars, where the central regions of the accretion disc are disrupted by interactions with the magnetosphere of a rotating magnetic white dwarf. The calculations show that, as the accretion rate approaches a steady value, the highdensity ring which first forms at the circularization radius continues to maintain its identity within a more extended disc, with the bulk of the mass and angular momentum being transferred directly from the companion star into the vicinity of the ring. The overall structure of the disc is qualitatively similar to that previously inferred for truncated discs in the intermediate polars, based on the observed spin equilibria of the magnetic white dwarfs in these systems. The disc shows 3D structure with evidence for variations in the height of the disc as a function of radius and rotational azimuth. The disc is thickest in its outer rim near the rotational phase Φ = 0.2 (Φ = 0.5 corresponding to the phase where the white dwarf is between the observer and the companion star), and there are further bulges centred at Φ = 0.5 and 0.8.

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