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Session 27 - Variable Stars, Novae, & Supernovae.
Display session, Tuesday, June 10
South Main Hall,

[27.11] Quasi-steady Disks, Magnetically Cataclysmic Disks and Jet Forming Disks

R. Matsumoto, T. Matsuzaki (Chiba U.), T. Tajima (U.Texas), K. Shibata (NAOJ)

We carried out three-dimensional magnetohydrodynamic simulations of differentially rotating disks. First, we present the results of local simulations of a gravitationally stratified, Keplerian disk initially threaded by azimuthal magnetic field. Numerical results indicate that magnetic accretion disks have two states; a gas pressure dominated quasi-steady state and a magnetic pressure dominated cataclysmic state. In weakly magnetized disks, we confirmed the results by Stone et al. (1996) that the disk approaches to a quasi-steady state with \beta=P_gas/P_mag \sim 30 and \alpha_B=-\left \sim 0.01. Once the magnetic pressure is comparable to the gas pressure, however, the Balbus amp; Hawley instability coupled with the Parker instability further amplifies magnetic fields. We found that the disk stays in a low-beta (\beta \le 1) state for time scale longer than the rotation period. Large amplitude sporadic time variations in low-state disks may be due to the magnetic energy release in low-beta disks.

By including the effects of radial boundaries and curvature of azimuthal magnetic field lines, we also carried out global three-dimensional simulations of the whole disk. We assumed a differentially rotating polytropic plasma threaded by azimuthal magnetic field (B_\varphi \propto 1/r). In a cylindrical model neglecting vertical stratification, we found that when the rotation law is nearly Keplerian, non-axisymmetric high-m (azimuthal wave number) modes grow. The essential results of local simulations of high-beta disks are confirmed by global simulations. When the initial magnetic field is parallel to the rotation axis, since the rotating cylinder tends to become low-beta, we included vertical gravity and simulated the evolution of a torus from which plasma can expand along the rotation axis. We show that a bipolar jet is ejected from the torus and that non-axisymmetric, helical structures are created in the jet.

Program listing for Tuesday