The effect of diffusion of magnetic field on flow structure in close binaries Original Research Article

We use 3D numerical simulations to study MHD flows in close binary system SS Cyg. This star belongs to the class of the semi-detached binaries where the secondary star (donor) fills its Roche lobe and loses mass through the inner Lagrangian point image. The matter accretes to the magnetized primary star – the white dwarf (accretor). To describe the magnetic field we take the value image for the magnetic induction on the surface of the compact primary star. We assume that the dipole moment is inclined to the rotation axis at the angle image and the rotation of the accretor is synchronous with the orbital rotation of the binary system. The developed numerical model takes into account the turbulent diffusion of the magnetic field that is determined by the magnetic reconnections and the electric currents dissipation in turbulent vortexes as well as by the magnetic flux tubes buoyancy. To define the influence of the turbulent dissipation of the magnetic field on the flow structure we used the model of the ideal MHD and that of MHD with non-zero turbulent diffusivity. It was found out that in both models the flow structure has the same qualitative features as in pure HD solution: (i) the magnetized accretion disk forms in the system; (ii) all previously discovered waves still exist in the disk: the shock wave (‘’hot line”) forming due to the interaction between the circumdisk halo and the stream from the inner Lagrangian point image, two arms of the tidal shock, the spiral precessional wave, and the bow shock caused by the motion of the accretor and the disk through the circumbinary envelope. It was also shown that in both models the magnetic field in the disk is mainly toroidal, and the presence of the magnetic field leads to the formation of the magnetosphere and the funnel flow near the magnetic poles of the primary star. Besides, we discovered that taking into consideration the turbulent diffusion of the magnetic field leads to the drastic change of the plasma parameter image (from image in ideal MHD model to image for the model with turbulent dissipation) and as a consequence to the significant changes in the obtained MHD flows. As a result, the turbulent diffusion significantly increases the mass of the accretion disk, decreases its outer radius, height and characteristic density. Moreover, we came to the conclusion that the turbulent diffusion decreases the accretion rate (2.5–3 times) of the system that can drastically change the observational properties of the system.