1,114 research outputs found
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
Nonlinear Hydromagnetic Wave Support of a Stratified Molecular Cloud
We perform numerical simulations of nonlinear MHD waves in a gravitationally
stratified molecular cloud that is bounded by a hot and tenuous external
medium. We study the relation between the strength of the turbulence and
various global properties of a molecular cloud, within a 1.5-dimensional
approximation. Under the influence of a driving source of Alfvenic
disturbances, the cloud is lifted up by the pressure of MHD waves and reaches a
steady-state characterized by oscillations about a new time-averaged
equilibrium state. The nonlinear effect results in the generation of
longitudinal motions and many shock waves; however, the wave kinetic energy
remains predominantly in transverse, rather than longitudinal, motions. There
is an approximate equipartition of energy between the transverse velocity and
fluctuating magnetic field (aspredicted by small-amplitude theory) in the
region of the stratified cloud which contains most of the mass; however, this
relation breaks down in the outer regions, particularly near the cloud surface,
where the motions have a standing-wave character. This means that the
Chandrasekhar-Fermi formula applied to molecular clouds must be significantly
modified in such regions. Models of an ensemble of clouds show that, for
various strengths of the input energy, the velocity dispersion in the cloud
, where is a characteristic size of the
cloud.Furthermore, is always comparable to the mean Alfven velocity of
the cloud, consistent with observational results.Comment: 16 pages, 15 figures, emulateapj, to appear in ApJ, 2003 Oct 1,
higher resolution figures at http://www.astro.uwo.ca/~basu/pub.html or
http://www.astro.uwo.ca/~kudoh/pub.htm
Analytical Studies on the Structure and Emission of the SS433 Jets
We study the structure and emission of the SS 433 jets in the X-ray emitting
region and in the inner and hotter portion inside the X-ray emitting region. In
order to consider the jet structure from the inner to outer regions we develop
the hybrid model combining the conical beam and the model beam whose cross
section grows with the distance more slowly. We find that the jet beams in the
inner and hotter portion are of two-temperature and emit a large amount of high
energy gamma photons. Our analyses suggest the thick absorbing envelope to
exist in the SS 433 system. Based on our results, we discuss the possible
acceleration mechanism for the SS 433 jets.Comment: 15 pages. Accepted for publication by Publ. Astron. Soc. Japa
Gravitational Wave Background from Neutrino-Driven Gamma-Ray Bursts
We discuss the gravitational wave background (GWB) from a cosmological
population of gamma-ray bursts (GRBs). Among various emission mechanisms for
the gravitational waves (GWs), we pay a particular attention to the vast
anisotropic neutrino emissions from the accretion disk around the black hole
formed after the so-called failed supernova explosions. The produced GWs by
such mechanism are known as burst with memory, which could dominate over the
low-frequency regime below \sim 10Hz. To estimate their amplitudes, we derive
general analytic formulae for gravitational waveform from the axisymmetric
jets. Based on the formulae, we first quantify the spectrum of GWs from a
single GRB. Then, summing up its cosmological population, we find that the
resultant value of the density parameter becomes roughly \Omega_{GW} \approx
10^{-20} over the wide-band of the low-frequency region, f\sim 10^{-4}-10^1Hz.
The amplitude of GWB is sufficiently smaller than the primordial GWBs
originated from an inflationary epoch and far below the detection limit.Comment: 6 pages, 4 figures, accepted for publication in MNRA
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
Second order perturbations in the radius stabilized Randall-Sundrum two branes model
The nonlinear gravitational interaction is investigated in the Randall-Sundrum two branes model with the radius stabilization mechanism. As the stabilization model, we assume a single scalar field which has the potential in the bulk and the potential on each brane. We develop the formulation of the second order gravitational perturbations under the assumption of the static and axial-symmetric 5-dimensional metric that is spherically symmetric in the 4-dimensional sense. After deriving the formal solutions for the perturbations, we discuss the gravity on each brane induced by the matter on its own side, taking the limit of the large coupling of the scalar field interaction term on the branes. We show using the Goldberger-Wise stabilization model that the 4-dimensional Einstein gravity is approximately recovered in the second order perturbations
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
3-D General Relativistic MHD Simulations of Generating Jets
We have performed a first fully 3-D GRMHD simulation with Schwarzschild black
hole with a free falling corona. The initial simulation results show that a jet
is created as in previous axisymmetric simulations. However, the time to
generate the jet is longer than in the 2-D simulations. We expect that due to
the additional azimuthal dimension the dynamics of jet formation can be
modified.Comment: 4 pages Proc. Oxford Radio Galaxy Workshop ed. R. Laing & K. Blundell
(San Francisco: PASP) in press (revised
Thermodynamics of four-dimensional black objects in the warped compactification
We reinvestigate the thermodynamics of black objects (holes and strings) in
four-dimensional braneworld models that are originally constructed by Emparan,
Horowitz and Myers based on the anti-de Sitter (AdS) C-metric. After proving
the uniqueness of slicing the AdS C-metric, we derive thermodynamic quantities
of the black objects by means of the Euclidean formulation and find that we
have no necessity of requiring any regularization to calculate their classical
action. We show that there exist the Bekenstein-Hawking law and the
thermodynamic first law. The thermodynamic mass of the localized black hole on
a flat brane is negative, and it differs from the one previously derived. We
discuss the thermodynamic stabilities and show that the BTZ black string is
more stable than the localized black holes in a canonical ensemble, except for
an extreme case. We also find a braneworld analogue of the Hawking-Page
transition between the BTZ black string and thermal AdS branes. The localized
black holes on a de Sitter brane is discussed by considering Nariai instanton,
comparing the study of "black cigar" in the five-dimensional braneworld model.Comment: 15 pages, 4 figures, RevTex4, typos fixed, minor correction
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