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 $z \sim 0.5r_0$ in resistive models, where $r_0$ 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