9,585 research outputs found
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
X-Ray Flares and Mass Outflows Driven by Magnetic Interaction between a Protostar and its Surrounding Disk
We propose a model of hard X-ray flares in protostars observed by ASCA
satellite. Assuming that the dipole magnetic field of the protostar threads the
protostellar disk, we carried out 2.5-dimensional magnetohydrodynamic (MHD)
simulations of the disk-star interaction. The closed magnetic loops connecting
the central star and the disk are twisted by the rotation of the disk. As the
twist accumulates, magnetic loops expand and finally approach to the open field
configuration. A current sheet is formed inside the expanding loops. In the
presence of resistivity, magnetic reconnection takes place in the current
sheet. Outgoing magnetic island and post flare loops are formed as a result of
the reconnection. The time scale of this `flare' is the order of the rotation
period of the disk. The released magnetic energy partly goes into the thermal
energy and heats up the flaring plasma up to K. The length of the
flaring loop is several times of the radius of the central star, consistent
with observations. The speed of the hot plasmoid ejected by the reconnection is
km s when the footpoint of the loop is at 0.03 AU from 1
M protostar. The hot plasma outflow can explain the speed and mass flow
rate of optical jets. Dense, cold, magnetically accelerated wind ( km s) emanates from the surface of the disk along the partially
open magnetic field lines threading the disk. This dense, cold wind may
correspond to high velocity neutral winds.Comment: 14 pages, uses aasms4.sty,2 PostScript figures, tar'ed and
gzip'ed.Full postscript text, figures (color) and mpeg simulations available
at http://pleiades.c.chiba-u.ac.jp/~hayashi/lanlxxx.html Accepted for
publication in 'ApJ Letters
Outflows and Jets from Collapsing Magnetized Cloud Cores
Star formation is usually accompanied by outflow phenomena. There is strong
evidence that these outflows and jets are launched from the protostellar disk
by magneto-rotational processes. Here, we report on our three dimensional,
adaptive mesh, magneto-hydrodynamic simulations of collapsing, rotating,
magnetized Bonnor-Ebert-Spheres whose properties are taken directly from
observations. In contrast to the pure hydro case where no outflows are seen,
our present simulations show an outflow from the protodisk surface at ~ AU and
a jet at ~ 0.07 AU after a strong toroidal magnetic field build up. The large
scale outflow, which extends up to ~ AU at the end of our simulation, is driven
by toroidal magnetic pressure (spring), whereas the jet is powered by
magneto-centrifugal force (fling). At the final stage of our simulation these
winds are still confined within two respective shock fronts. Furthermore, we
find that the jet-wind and the disk-anchored magnetic field extracts a
considerable amount of angular momentum from the protostellar disk. The initial
spin of our cloud core was chosen high enough to produce a binary system. We
indeed find a close binary system (separation ~ 3 R_sol) which results from the
fragmentation of an earlier formed ring structure. The magnetic field strength
in these protostars reaches ~ 3 kG and becomes about 3 G at 1 AU from the
center in agreement with recent observational results.Comment: revised version, accepted for publication in ApJ, a higher resolution
version of this paper can be downloaded at
http://www.physics.mcmaster.ca/~banerjee/outflows.pd
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
Magnetic reconnection and stochastic plasmoid chains in high-Lundquist-number plasmas
A numerical study of magnetic reconnection in the large-Lundquist-number
(), plasmoid-dominated regime is carried out for up to . The
theoretical model of Uzdensky {\it et al.} [Phys. Rev. Lett. {\bf 105}, 235002
(2010)] is confirmed and partially amended. The normalized reconnection rate is
\normEeff\sim 0.02 independently of for . The plasmoid flux
() and half-width () distribution functions scale as and . The joint distribution of and
shows that plasmoids populate a triangular region ,
where is the reconnecting field. It is argued that this feature is due to
plasmoid coalescence. Macroscopic "monster" plasmoids with % of the
system size are shown to emerge in just a few Alfv\'en times, independently of
, suggesting that large disruptive events are an inevitable feature of
large- reconnection.Comment: 5 pages, 6 figures, submitted for publicatio
Magnetically Driven Jets in the Kerr Metric
We compute a series of three-dimensional general relativistic
magnetohydrodynamic simulations of accretion flows in the Kerr metric to
investigate the properties of the unbound outflows that result. The overall
strength of these outflows increases sharply with increasing black hole
rotation rate, but a number of generic features are found in all cases. The
mass in the outflow is concentrated in a hollow cone whose opening angle is
largely determined by the effective potential for matter orbiting with angular
momentum comparable to that of the innermost stable circular orbit. The
dominant force accelerating the matter outward comes from the pressure of the
accretion disk's corona. The principal element that shapes the outflow is
therefore the centrifugal barrier preventing accreting matter from coming close
to the rotation axis. Inside the centrifugal barrier, the cone contains very
little matter and is dominated by electromagnetic fields that rotate at a rate
tied closely to the rotation of the black hole. These fields carry an
outward-going Poynting flux whose immediate energy source is the rotating
spacetime of the Kerr black hole. When the spin parameter a/M of the black hole
exceeds ~0.9, the energy carried to infinity by these outflows can be
comparable to the nominal radiative efficiency predicted in the Novikov-Thorne
model. Similarly, the expelled angular momentum can be comparable to that
accreted by the black hole. Both the inner electromagnetic part and the outer
matter part can contribute in significant fashion to the energy and angular
momentum of the outflow.Comment: 43 pages 12 figures To Appear in the Astrophysical Journal replaced
figure 3c with correct imag
Possible explanation for star-crushing effect in binary neutron star simulations
A possible explanation is suggested for the controversial star-crushing
effect seen in numerical simulations of inspiraling neutron star binaries by
Wilson, Mathews and Marronetti (WMM). An apparently incorrect definition of
momentum density in the momentum constraint equation used by WMM gives rise to
a post-1-Newtonian error in the approximation scheme. We show by means of an
analytic, post-1-Newtonian calculation that this error causes an increase of
the stars' central densities which is of the order of several percent when the
stars are separated by a few stellar radii, in agreement with what is seen in
the simulations.Comment: 4 pages, 1 figure, uses revetx macros, minor revision
Stripe State in the Lowest Landau Level
The stripe state in the lowest Landau level is studied by the density matrix
renormalization group (DMRG) method. The ground state energy and pair
correlation functions are systematically calculated for various
pseudopotentials in the lowest Landau level. We show that the stripe state in
the lowest Landau level is realized only in a system whose width perpendicular
to the two-dimensional electron layer is smaller than the order of magnetic
length.Comment: 4 pages, 6 figures, to appear in J. Phys. Soc. Jpn. vol.73 No.1
(2004
Various features of quasiequilibrium sequences of binary neutron stars in general relativity
Quasiequilibrium sequences of binary neutron stars are numerically calculated
in the framework of the Isenberg-Wilson-Mathews (IWM) approximation of general
relativity. The results are presented for both rotation states of synchronized
spins and irrotational motion, the latter being considered as the realistic one
for binary neutron stars just prior to the merger. We assume a polytropic
equation of state and compute several evolutionary sequences of binary systems
composed of different-mass stars as well as identical-mass stars with adiabatic
indices gamma=2.5, 2.25, 2, and 1.8. From our results, we propose as a
conjecture that if the turning point of binding energy (and total angular
momentum) locating the innermost stable circular orbit (ISCO) is found in
Newtonian gravity for some value of the adiabatic index gamma_0, that of the
ADM mass (and total angular momentum) should exist in the IWM approximation of
general relativity for the same value of the adiabatic index.Comment: Text improved, some figures changed or deleted, new table, 38 pages,
31 figures, accepted for publication in Phys. Rev.
Real Space Effective Interaction and Phase Transition in the Lowest Landau Level
The transition between the stripe state and the liquid state in a high
magnetic field is studied by the density-matrix renormalization-group (DMRG)
method. Systematic analysis on the ground state of two-dimensional electrons in
the lowest Landau level shows that the transition from the stripe state to the
liquid state at v=3/8 is caused by a reduction of repulsive interaction around
r=3. The same reduction of the interaction also stabilizes the incompressible
liquid states at v=1/3 and 2/5, which shows a similarity between the two liquid
states at v=3/8 and 1/3. It is also shown that the strong short-range
interaction around r=1 in the lowest Landau level makes qualitatively different
stripe correlations compared with that in higher Landau levels.Comment: 5 pages, to appear in J. Phys. Soc. Jpn. Vol.73, No.8 (2004
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