3D MHD SIMULATIONS OF DISK ACCRETION TO AN INCLINED ROTATOR. I. MAGNETOSPHERIC FLOW AT DIFFERENT Q

Authors: M.M. Romanova, G.V. Ustyugova, A.V. Koldoba, J.V. Wick,  R.V.E. Lovelace, 

We present results of fully three-dimensional magnetohydrodynamic (MHD) simulations of disk accretion to a rotating magnetized star with its dipole moment inclined relative to the rotation axis of the disk which is parallel to the star's rotation axis. The star rotates slowly with an angular rate W* = 0.04 WK* , where WK* is the Keplerian rate at the surface of the star. The disk-star interaction was investigated for a range of inclination angles, Q = 0, 5, 15, 30, 45, 60 and 75. We observed that for Q = 0, the flow is axisymmetric and matter flow is close to that observed with an analogous two-dimensional code. At small inclination angles, Q = 5 and Q = 15 the axisymmetry brakes, and matter flows to the star along two streams. At inclination angles Q = 30, 45, & 60, the streams become more complicated, and often split into four streams to give a multi-thread structure. At an even larger inclination angle, 75, matter accretes to the star in two streams, but their geometry is different from the cases of small Q. The streams usually co-rotate with the star, but may precess about the star for small Q. The disk structure is significantly changed inside the radius rbr, where magnetic braking is significant. The density is lower in this area and the rotation rate is sub-Keplerian. For Q  < 30, a dense ring forms around the magnetosphere as in the axisymmetric case. Inner regions of the disk are  warped. Warping re ects the beginning of matter flow from the disk to the magnetosphere, so that the normal of the warped disk has a tendency to be parallel to the magnetic axis of the dipole. This direction is diffrent from one proposed theoretically. The accretion rate to the star dM/dt increases with Q. The torque Nz (relative to the star's rotation axis) is positive for all inclination angles and leads to spinning-up of the star. This is because the present work considers the case where the star's rotation rate is small compared with WK*. Nz  only weakly depends on the inclination angle Q. The torque about the Y- axis, Ny, acts to decrease the inclination angle Q. This torque decreases for small Q and for Q near 90. The torque Nx is comparable with Ny, and it acts to cause precession of the magnetic axis around the rotational axis. The angular momentum flux to the star is transported from the disk to the star predominantly by the magnetic field; the matter contributes ~ 1% of the total flux. Results of the present study are applicable to classical T Tauri stars, cataclysmic variables and X-ray pulsars. We predict that in addition to the half-period variability associated with \hot spots", there may be shorter scale variability associated with occultation of stellar hot spot emission by magnetospheric streams.

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Last updated on 29.01.07