97 research outputs found
Second order perturbations in the radius stabilized Randall-Sundrum two branes model II -- Effect of relaxing strong coupling approximation --
We discuss gravitational perturbations in the Randall-Sundrum two branes
model with radius stabilization. Following the idea by Goldberger and Wise for
the radius stabilization, we introduce a scalar field which has potentials
localized on the branes in addition to a bulk potential. In our previous paper
we discussed gravitational perturbations induced by static, spherically
symmetric and nonrelativistic matter distribution on the branes under the
condition that the values of the scalar field on the respective branes cannot
fluctuate due to its extremely narrow brane potentials. We call this case the
strong coupling limit. Our concern in this paper is to generalize our previous
analysis relaxing the limitation of taking the strong coupling limit. We find
that new corrections in metric perturbations due to relaxing the strong
coupling limit enhance the deviation from the 4D Einstein gravity only in some
exceptional cases. In the case that matter fields reside on the negative
tension brane, the stabilized radion mass becomes very small when the new
correction becomes large.Comment: 12 pages, No figures, typos correcte
Induced Core Formation Time in Subcritical Magnetic Clouds by Large-Scale Trans-Alfv\'enic Flows
We clarify the mechanism of accelerated core formation by large-scale
nonlinear flows in subcritical magnetic clouds by finding a semi-analytical
formula for the core formation time and describing the physical processes that
lead to them. Recent numerical simulations show that nonlinear flows induce
rapid ambipolar diffusion that leads to localized supercritical regions that
can collapse. Here, we employ non-ideal magnetohydrodynamic simulations
including ambipolar diffusion for gravitationally stratified sheets threaded by
vertical magnetic fields. One of the horizontal dimensions is eliminated,
resulting in a simpler two-dimensional simulation that can clarify the basic
process of accelerated core formation. A parameter study of simulations shows
that the core formation time is inversely proportional to the square of the
flow speed when the flow speed is greater than the Alfv\'en speed. We find a
semi-analytical formula that explains this numerical result. The formula also
predicts that the core formation time is about three times shorter than that
with no turbulence, when the turbulent speed is comparable to the Alfv\'en
speed.Comment: 22 pages, 9 figures, accepted for publication in Ap
The Primordial Origin Model of Magnetic Fields in Spiral Galaxies
We propose a primordial-origin model for the composite configurations of
global magnetic fields in spiral galaxies. We show that uniform tilted magnetic
field wound up into a rotating disk galaxy can evolve into composite magnetic
configurations comprising bisymmetric spiral (S=BSS), axisymmetric spiral
(A=ASS), plane-reversed spiral (PR), and/or ring (R) fields in the disk, and
vertical (V) fields in the center. By MHD simulations we show that these
composite galactic fields are indeed created from weak primordial uniform
field, and that the different configurations can co-exist in the same galaxy.
We show that spiral fields trigger the growth of two-armed gaseous arms. The
centrally accumulated vertical fields are twisted and produce jet toward the
halo. We find that the more vertical was the initial uniform field, the
stronger is the formed magnetic field in the galactic disk.Comment: 11 pages, 14 figures, accepted for publication in PAS
The Acceleration Mechanism of Resistive MHD Jets Launched from Accretion Disks
We analyzed the results of non-linear resistive magnetohydrodynamical (MHD)
simulations of jet formation to study the acceleration mechanism of
axisymmetric, resistive MHD jets. The initial state is a constant angular
momentum, polytropic torus threaded by weak uniform vertical magnetic fields.
The time evolution of the torus is simulated by applying the CIP-MOCCT scheme
extended for resistive MHD equations. We carried out simulations up to 50
rotation period at the innermost radius of the disk created by accretion from
the torus. The acceleration forces and the characteristics of resistive jets
were studied by computing forces acting on Lagrangian test particles. Since the
angle between the rotation axis of the disk and magnetic field lines is smaller
in resistive models than in ideal MHD models, magnetocentrifugal acceleration
is smaller. The effective potential along a magnetic field line has maximum
around in resistive models, where is the radius where the
density of the initial torus is maximum. Jets are launched after the disk
material is lifted to this height by pressure gradient force. Even in this
case, the main acceleration force around the slow magnetosonic point is the
magnetocentrifugal force. The power of the resistive MHD jet is comparable to
the mechanical energy liberated in the disk by mass accretion. Joule heating is
not essential for the formation of jets.Comment: 15 pages, 15 figures, 1 table, accepted for publication in Ap
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