Magnetically Driven Warping, Precession and Resonances in Accretion Disks

The inner region of the accretion disk onto a rotating magnetized central star (neutron star, white dwarf or T Tauri star) is subjected to magnetic torques which induce warping and precession of the disk. The origin of these torques lies in the interaction between the (induced) surface current on the disk and the horizontal magnetic field (parallel to the disk) produced by the inclined magnetic dipole. Under quite general conditions, there exists a magnetic warping instability in which the magnetic torque drives the disk plane away from the equatorial plane of the star toward a state where the disk normal vector is perpendicular to the spin axis. Viscous stress tends to suppress the warping instability at large radii, but the magnetic torque always dominates as the disk approaches the magnetosphere boundary. The magnetic torque also drives the tilted inner disk into retrograde precession around the stellar spin axis. Moreover, resonant magnetic forcing on the disk can occur which may affect the dynamics of the disk. The magnetically driven warping instability and precession may be related to a number observational puzzles, including: (1) Spin evolution (torque reversal) of accreting X-ray pulsars; (2) Quasi-periodic oscillations in low-mass X-ray binaries; (3) Super-orbital periods in X-ray binaries; (4) Photometric period variations of T Tauri stars.Comment: 39 pages including 1 ps figure; Published version; ApJ, 524, 1030-1047 (1999

Time Evolution of Relativistic Force-Free Fields Connecting a Neutron Star and its Disk

We study the magnetic interaction between a neutron star and its disk by solving the time-dependent relativistic force-free equations. At the initial state, we assume that the dipole magnetic field of the neutron star connects the neutron star and its equatorial disk, which deeply enters into the magnetosphere of the neutron star. Magnetic fields are assumed to be frozen to the star and the disk. The rotation of the neutron star and the disk is imposed as boundary conditions. We apply Harten-Lax-van Leer (HLL) method to simulate the evolution of the star-disk system. We carry out simulations for (1) a disk inside the corotation radius, in which the disk rotates faster than the star, and (2) a disk outside the corotation radius, in which the neutron star rotates faster than the disk. Numerical results indicate that for both models, the magnetic field lines connecting the disk and the star inflate as they are twisted by the differential rotation between the disk and the star. When the twist angle exceeds pi radian, the magnetic field lines expand with speed close to the light speed. This mechanism can be the origin of relativistic outflows observed in binaries containing a neutron star.Comment: 10 pages, 6figures, accepted for publication in PAS

Outflows driven by Giant Protoplanets

We investigate outflows driven by a giant protoplanet using three-dimensional MHD nested grid simulations. We consider a local region around the protoplanet in the protoplanetary disk, and calculate three models: (a) unmagnetized disk model, (b) magnetized disk model having magnetic field azimuthally parallel to the disk, and (c) magnetic field perpendicular to the disk. Outflows with velocities, at least, 10 km/s are driven by the protoplanets in both magnetized disk models, while outflow does not appear in unmagnetized disk model. Tube-like outflows along the azimuthal direction of the protoplanetary disk appear in model with magnetic field being parallel to the disk. In this model, the magnetically dominated regions (i.e., density gap) are clearly contrasted from other regions and spiral waves appear near the protoplanet. On the other hand, in model with magnetic field being perpendicular to the disk, outflows are driven by a protoplanet with cone-like structure just as seen in the outflow driven by a protostar. Magnetic field lines are strongly twisted near the protoplanet and the outflows have well-collimated structures in this model.These outflows can be landmarks for searching exo-protoplanets in their formation stages. Our results indicate that the accretion rate onto the protoplanet tend to have a larger value than that expected from previous hydrodynamical calculations, since a fraction of the angular momentum of circum-planetary disk is removed by outflows, enhanced non-axisymmetric patterns caused by magnetic field, and magnetic braking. Possible implications for observation are also briefly discussed.Comment: 11 pages, 3 figures, Submitted to ApJL, For high resolution figures see http://www2.scphys.kyoto-u.ac.jp/~machidam/jupiter/doc/resubmit_0703.pd