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