404 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
Can Superflares Occur on Our Sun?
Recent observations of solar type stars with the Kepler satellite by Maehara
et al. have revealed the existence of superflares (with energy of 10^33 - 10^35
erg) on Sun-like stars, which are similar to our Sun in their surface
temperature (5600 K - 6000 K) and slow rotation (rotational period > 10 days).
From the statistical analysis of these superflares, it was found that
superflares with energy 10^34 erg occur once in 800 years and superflares with
10^35 erg occur once in 5000 years on Sun-like stars. In this paper, we examine
whether superflares with energy of 10^33 - 10^35 erg could occur on the present
Sun through the use of simple order-of-magnitude estimates based on current
ideas relating to the mechanisms of the solar dynamo.Comment: Accepted by Publ. Astron. Soc. Japan on Dec. 6, 2012 (to be published
on PASJ vol. 65, No. 3, (2013) June 25
Effects of resonant single-particle states on pairing correlations
Effects of resonant single-particle (s.p.) states on the pairing correlations
are investigated by an exact treatment of the pairing Hamiltonian on the Gamow
shell model basis. We introduce the s.p. states with complex energies into the
Richardson equations. The solution shows the property that the resonant s.p.
states with large widths are less occupied. The importance of many-body
correlations between bound and resonant prticle pairs is shown.Comment: 4 pages, 3 figures, to be published in Phys. Rev.
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
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