3D STRUCTURE OF TRUNCATED ACCRETION DISKS IN CLOSE BINARIES

We use smoothed particle hydrodynamics to investigate the 3D accretion flow in binary systems where the secondary star transfers matter on t o a compact primary star via a truncated accretion disc. Our model neg lects radiative cooling and has been evolved up to 1.7 orbital periods . Our method of calculation differs from those of previous investigato rs in our treatment of artificial viscosity. We use a pseudoviscosity in order to absorb energy cascades at a subgrid level and attempt to s imulate large-scale eddies. The resulting flow is locally turbulent an d eddies can be seen on several scales ranging from the smoothing leng th up to half the thickness of the disc, with turbulence being most ap parent in the stream-disc interaction region. The model is probably re levant to the class of magnetic cataclysmic variables known as the int ermediate polars, where the central regions of the accretion disc are disrupted by interactions with the magnetosphere of a rotating magneti c white dwarf. The calculations show that, as the accretion rate appro aches a steady value, the high-density ring which first forms at the c ircularization radius continues to maintain its identity within a more extended disc, with the bulk of the mass and angular momentum being t ransferred directly from the companion star into the vicinity of the r ing. The overall structure of the disc is qualitatively similar to tha t 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 variation s in the height of the disc as a function of radius and rotational azi muth. The disc is thickest in its outer rim near the rotational phase PHI = 0.2 (PHI = 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 PHI = 0.5 and 0.8.