1,254 research outputs found
Origin of black string instability
It is argued that many nonextremal black branes exhibit a classical
Gregory-Laflamme (GL) instability. Why does the universal instability exist? To
find an answer to this question and explore other possible instabilities, we
study stability of black strings for all possible types of gravitational
perturbation. The perturbations are classified into tensor-, vector-, and
scalar-types, according to their behavior on the spherical section of the
background metric. The vector and scalar perturbations have exceptional
multipole moments, and we have paid particular attention to them. It is shown
that for each type of perturbations there is no normalizable negative
(unstable) modes, apart from the exceptional mode known as s-wave perturbation
which is exactly the GL mode. We discuss the origin of instability and comment
on the implication for the correlated-stability conjecture.Comment: 19 pages (revtex4), 4 figures; references added, minor correction
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
Six-dimensional localized black holes: numerical solutions
To test the strong-gravity regime in Randall-Sundrum braneworlds, we consider
black holes bound to a brane. In a previous paper, we studied numerical
solutions of localized black holes whose horizon radii are smaller than the AdS
curvature radius. In this paper, we improve the numerical method and discuss
properties of the six dimensional (6D) localized black holes whose horizon
radii are larger than the AdS curvature radius. At a horizon temperature
, the thermodynamics of the localized black
hole undergo a transition with its character changing from a 6D Schwarzschild
black hole type to a 6D black string type. The specific heat of the localized
black holes is negative, and the entropy is greater than or nearly equal to
that of the 6D black strings with the same thermodynamic mass. The large
localized black holes show flattened horizon geometries, and the intrinsic
curvature of the horizon four-geometry becomes negative near the brane. Our
results indicate that the recovery mechanism of lower-dimensional Einstein
gravity on the brane works even in the presence of the black holes.Comment: 17 pages, 9 figures, RevTeX4, typos correcte
Three-dimensional MHD Simulations of Jets from Accretion Disks
We report the results of 3-dimensional magnetohydrodynamic (MHD) simulations
of a jet formation by the interaction between an accretion disk and a large
scale magnetic field. The disk is not treated as a boundary condition but is
solved self-consistently. To investigate the stability of MHD jet, the
accretion disk is perturbed with a non-axisymmetric sinusoidal or random
fluctuation of the rotational velocity. The dependences of the jet velocity
, mass outflow rate , and mass accretion rate
on the initial magnetic field strength in both non-axisymmetric cases are
similar to those in the axisymmetric case. That is, ,
and where is the
initial magnetic field strength. The former two relations are consistent with
the Michel's steady solution, , although
the jet and accretion do not reach the steady state. In both perturbation
cases, a non-axisymmetric structure with appears in the jet, where
means the azimuthal wave number. This structure can not be explained by
Kelvin-Helmholtz instability and seems to originate in the accretion disk.
Non-axisymmetric modes in the jet reach almost constant levels after about 1.5
orbital periods of the accretion disk, while all modes in the accretion disk
grow with oscillation. As for the angular momentum transport by Maxwell stress,
the vertical component, , in the wide range of initial magnetic field
strength.Comment: Accepted for publication in ApJ. The pdf file with high resolution
figures can be downloaded at
http://www.kusastro.kyoto-u.ac.jp/~hiromitu/3j050806.pd
Cortical and Subcortical Organization Participating in Tactual Pattern Perception in Man: A PET study
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General Relativistic Simulations of Jet Formation in a Rapidly Rotating Black Hole Magnetosphere
To investigate the formation mechanism of relativistic jets in active
galactic nuclei and micro-quasars, we have developed a new general relativistic
magnetohydrodynamic code in Kerr geometry. Here we report on the first
numerical simulation of jet formation in a rapidly-rotating (a=0.95) Kerr black
hole magnetosphere. We study cases in which the Keplerian accretion disk is
both co-rotating and counter-rotating with respect to the black hole rotation.
In the co-rotating disk case, our results are almost the same as those in
Schwarzschild black hole cases: a gas pressure-driven jet is formed by a shock
in the disk, and a weaker magnetically-driven jet is also generated outside the
gas pressure-driven jet. On the other hand, in the counter-rotating disk case,
a new powerful magnetically-driven jet is formed inside the gas pressure-driven
jet. The newly found magnetically-driven jet in the latter case is accelerated
by a strong magnetic field created by frame dragging in the ergosphere. Through
this process, the magnetic field extracts the energy of the black hole
rotation.Comment: Co-rotating and counter-rotating disks; 8 pages; submitted to ApJ
letter
Effects of substrate differences on water availability for Arctic lichens during the snow-free summers in the High Arctic glacier foreland
AbstractWe used observational and experimental analyses to investigate the photosynthetic activity and water relationships of five lichen species attached to different substrates in a glacier foreland in the High Arctic, Ny-Ă
lesund, Svalbard (79°N) during the snow-free season in 2009 and 2010. After the rains ceased, lichens and their attached substrates quickly dried, whereas photosynthetic activity in the lichens decreased gradually. The in situ photosynthetic activity was estimated based on the relative electron transportation rate (rETR) in four fruticose lichens: Cetrariella delisei, Flavocetraria nivalis, Cladonia arbuscula ssp. mitis, and Cladonia pleurota. The rETR approached zero around noon, although the crustose lichen Ochrolechia frigida grown on biological soil crust (BSC) could acquire water from the BSC and retain its WC to perform positive photosynthesis. The light-rETR relationship curves of the five well-watered lichens were characterized into two types: shade-adapted with photoinhibition for the fruticose lichens, and light-adapted with no photoinhibition for O. frigida. The maximum rETR was expected to occur when they could acquire water from the surrounding air or from substrates during the desiccation period. Our results suggest that different species of Arctic lichens have different water availabilities due to their substrates and/or morphological characteristics, which affect their photosynthetic active periods during the summer
Quadrupole formula for Kaluza-Klein modes in the braneworld
The quadrupole formula in four-dimensional Einstein gravity is a useful tool
to describe gravitational wave radiation. We derive the quadrupole formula for
the Kaluza-Klein (KK) modes in the Randall-Sundrum braneworld model. The
quadrupole formula provides transparent representation of the exterior weak
gravitational field induced by localized sources. We find that a general
isolated dynamical source gives rise to the 1/r^2 correction to the leading 1/r
gravitational field. We apply the formula to an evaluation of the effective
energy carried by the KK modes from the viewpoint of an observer on the brane.
Contrary to the ordinary gravitational waves (zero mode), the flux of the
induced KK modes by the non-spherical part of the quadrupole moment vanishes at
infinity and only the spherical part contributes to the flux. Since the effect
of the KK modes appears in the linear order of the metric perturbations, the
effective energy flux observed on the brane is not always positive, but can
become negative depending on the motion of the localized sources.Comment: 9 pages, no figures, REVTeX 4; version accepted for publication in
CQ
Formation of protostellar jets - effects of magnetic diffusion
We investigate the evolution of a disk wind into a collimated jet under the
influence of magnetic diffusivity, assuming that the turbulent pattern in the
disk will also enter the disk corona and the jet. Using the ZEUS-3D code in the
axisymmetry option we solve the time-dependent resistive MHD equations for a
model setup of a central star surrounded by an accretion disk. We find that the
diffusive jets propagate slower into the ambient medium. Close to the star we
find that a quasi stationary state evolves after several hundred (weak
diffusion) or thousand (strong diffusion) disk rotations. Magnetic diffusivity
affects the protostellar jet structure as follows. The jet poloidal magnetic
field becomes de-collimated. The jet velocity increases with increasing
diffusivity, while the degree of collimation for the hydrodynamic flow remains
more or less the same. We suggest that the mass flux is a proper tracer for the
degree of jet collimation and find indications of a critical value for the
magnetic diffusivity above which the jet collimation is only weak.Comment: 16 pages, 12 figs, accepted by Astron. and Astrop
Small localized black holes in a braneworld: Formulation and numerical method
No realistic black holes localized on a 3-brane in the Randall-Sundrum
infinite braneworld have been found so far. The problem of finding a static
black hole solution is reduced to a boundary value problem. We solve it by
means of a numerical method, and show numerical examples of a localized black
hole whose horizon radius is small compared to the bulk curvature scale. The
sequence of small localized black holes exhibits a smooth transition from a
five-dimensional Schwarzschild black hole, which is a solution in the limit of
small horizon radius. The localized black hole tends to flatten as its horizon
radius increases. However, it becomes difficult to find black hole solutions as
its horizon radius increases.Comment: RevTeX, 13 pages, 6 figures, references corrected, typos corrected;
to appear in Phys.Rev.
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