(1) Superbubbles in the Galactic Disk and Halo:
Magnetohydrodynamic Simulations
The well-known study in the late 1970's by Weaver, Castor,
McCray, Shapiro, and Moore [1977; ApJ, 218, 377]
of stellar-wind-driven interstellar bubbles powered
by mass loss from individual early-type stars
subsequently found new application in the 1980's with the discovery of
interstellar superbubbles, believed to arise from the collective
effect of sequential supernova explosions and/or winds from stellar OB
associations in the Galactic disk.
These superbubbles are believed to be an important mechanism by which
supernova explosion energy
and shock-heated interstellar gas are supplied
to the Galactic halo to drive the Galactic Fountain proposed by Shapiro
and Field [1976; ApJ, 205, 762]. As such, the questions of
whether and how superbubbles can break out of the Galactic disk and vent
their mass and energy into the halo are central to understanding the
large-scale dynamics of the ISM, in general, and the Galactic Fountain,
in particular.
The possible importance of the
Galactic magnetic field in determining the outcome of this phenomenon
led Mineshige, Shibata, and Shapiro
(1993a;
1993b) to study
the evolution of interstellar superbubbles
in the presence of the Galactic magnetic field
numerically, using a two-dimensional magnetohydrodynamics (MHD) code.
They found that in the presence of horizontal magnetic fields of strength
comparable to that in the Galactic disk, B~5 microgauss, the
vertical expansion of the superbubble (the contact surface) can, under some
conditions, be significantly inhibited by the effect of a decelerating
JxB force. At the same time, the outermost effect of the
disturbance actually propagates somewhat faster than in nonmagnetic cases,
as an MHD fast shock or nonlinear wave. The implications of these results
for galactic supershells, the Galactic Fountain, observed activity
in starburst galaxies, and supernova remnants was briefly discussed.