12,013 research outputs found
Global axisymmetric stability analysis for a composite system of two gravitationally coupled scale-free discs
In a composite system of gravitationally coupled stellar and gaseous discs,
we perform linear stability analysis for axisymmetric coplanar perturbations
using the two-fluid formalism. The background stellar and gaseous discs are
taken to be scale-free with all physical variables varying as powers of
cylindrical radius with compatible exponents. The unstable modes set in as
neutral modes or stationary perturbation configurations with angular frequency
.Comment: 7 pages using AAS styl
New variable separation approach: application to nonlinear diffusion equations
The concept of the derivative-dependent functional separable solution, as a
generalization to the functional separable solution, is proposed. As an
application, it is used to discuss the generalized nonlinear diffusion
equations based on the generalized conditional symmetry approach. As a
consequence, a complete list of canonical forms for such equations which admit
the derivative-dependent functional separable solutions is obtained and some
exact solutions to the resulting equations are described.Comment: 19 pages, 2 fig
model and Higgs mass in standard model calculated by Gaussian effective potential approach with a new regularization-renormalization method
Basing on new regularization-renormalization method, the
model used in standard model is studied both perturbatively and
nonperturbatively (by Gaussian effective potential). The invariant property of
two mass scales is stressed and the existence of a (Landau) pole is emphasized.
Then after coupling with the SU(2)U(1) gauge fields, the Higgs mass in
standard model (SM) can be calculated as 138GeV. The critical
temperature () for restoration of symmetry of Higgs field, the critical
energy scale (, the maximum energy scale under which the lower
excitation sector of the GEP is valid) and the maximum energy scale
(, at which the symmetry of the Higgs field is restored) in the
standard model are 476 GeV, GeV
and GeVv respectively.Comment: 12 pages, LaTex, no figur
Variational ground states of 2D antiferromagnets in the valence bond basis
We study a variational wave function for the ground state of the
two-dimensional S=1/2 Heisenberg antiferromagnet in the valence bond basis. The
expansion coefficients are products of amplitudes h(x,y) for valence bonds
connecting spins separated by (x,y) lattice spacings. In contrast to previous
studies, in which a functional form for h(x,y) was assumed, we here optimize
all the amplitudes for lattices with up to 32*32 spins. We use two different
schemes for optimizing the amplitudes; a Newton/conjugate-gradient method and a
stochastic method which requires only the signs of the first derivatives of the
energy. The latter method performs significantly better. The energy for large
systems deviates by only approx. 0.06% from its exact value (calculated using
unbiased quantum Monte Carlo simulations). The spin correlations are also well
reproduced, falling approx. 2% below the exact ones at long distances. The
amplitudes h(r) for valence bonds of long length r decay as 1/r^3. We also
discuss some results for small frustrated lattices.Comment: v2: 8 pages, 5 figures, significantly expanded, new optimization
method, improved result
MHD tidal waves on a spinning magnetic compact star
In an X-ray binary system, the companion star feeds the compact neutron star
with plasma materials via accretions. The spinning neutron star is likely
covered with a thin "magnetized ocean" and may support {\it magnetohydrodynamic
(MHD) tidal waves}. While modulating the thermal properties of the ocean, MHD
tidal waves periodically shake the base of the stellar magnetosphere that traps
energetic particles, including radiating relativistic electrons. For a radio
pulsar, MHD tidal waves in the stellar surface layer may modulate radio
emission processes and leave indelible signatures on timescales different from
the spin period. Accretion activities are capable of exciting these waves but
may also obstruct or obscure their detections meanwhile. Under fortuitous
conditions, MHD tidal waves might be detectable and offer valuable means to
probe properties of the underlying neutron star. Similar situations may also
occur for a cataclysmic variable -- an accretion binary system that contains a
rotating magnetic white dwarf. This Letter presents the theory for MHD tidal
waves in the magnetized ocean of a rotating degenerate star and emphasizes
their potential diagnostics in X-ray and radio emissions.Comment: ApJ Letter paper already publishe
Exact Solutions of the Klein-Gordon Equation in the Presence of a Dyon, Magnetic Flux and Scalar Potential in the Specetime of Gravitational Defects
In this paper we analyse the relativistic quantum motion of a charged spin-0
particle in the presence of a dyon, Aharonov-Bohm magnetic field and scalar
potential, in the spacetimes produced by an idealized cosmic string and global
monopole. In order to develop this analysis, we assume that the dyon and the
Aharonov-Bohm magnetic field are superposed to both gravitational defects. Two
distinct configurations for the scalar potential, , are considered:
the potential proportional to the inverse of the radial distance, i.e.,
, and the potential proportional to this distance, i.e.,
. For both cases the center of the potentials coincide with the
dyon's position. In the case of the cosmic string the Aharonov-Bohm magnetic
field is considered along the defect, and for the global monopole this magnetic
field pierces the defect. The energy spectra are computed for both cases and
explicitly shown their dependence on the electrostatic and scalar coupling
constants. Also we analyse scattering states of the Klein-Gordon equations, and
show how the phase shifts depend on the geometry of the spacetime and on the
coupling constants parameter.Comment: To be published in CQG. Minor comments adde
Beam Orientation Optimization for Intensity Modulated Radiation Therapy using Adaptive l1 Minimization
Beam orientation optimization (BOO) is a key component in the process of IMRT
treatment planning. It determines to what degree one can achieve a good
treatment plan quality in the subsequent plan optimization process. In this
paper, we have developed a BOO algorithm via adaptive l_1 minimization.
Specifically, we introduce a sparsity energy function term into our model which
contains weighting factors for each beam angle adaptively adjusted during the
optimization process. Such an energy term favors small number of beam angles.
By optimizing a total energy function containing a dosimetric term and the
sparsity term, we are able to identify the unimportant beam angles and
gradually remove them without largely sacrificing the dosimetric objective. In
one typical prostate case, the convergence property of our algorithm, as well
as the how the beam angles are selected during the optimization process, is
demonstrated. Fluence map optimization (FMO) is then performed based on the
optimized beam angles. The resulted plan quality is presented and found to be
better than that obtained from unoptimized (equiangular) beam orientations. We
have further systematically validated our algorithm in the contexts of 5-9
coplanar beams for 5 prostate cases and 1 head and neck case. For each case,
the final FMO objective function value is used to compare the optimized beam
orientations and the equiangular ones. It is found that, our BOO algorithm can
lead to beam configurations which attain lower FMO objective function values
than corresponding equiangular cases, indicating the effectiveness of our BOO
algorithm.Comment: 19 pages, 2 tables, and 5 figure
Dynamic Evolution Model of Isothermal Voids and Shocks
We explore self-similar hydrodynamic evolution of central voids embedded in
an isothermal gas of spherical symmetry under the self-gravity. More
specifically, we study voids expanding at constant radial speeds in an
isothermal gas and construct all types of possible void solutions without or
with shocks in surrounding envelopes. We examine properties of void boundaries
and outer envelopes. Voids without shocks are all bounded by overdense shells
and either inflows or outflows in the outer envelope may occur. These
solutions, referred to as type void solutions, are further
divided into subtypes and
according to their characteristic behaviours across the sonic critical line
(SCL). Void solutions with shocks in envelopes are referred to as type
voids and can have both dense and quasi-smooth edges.
Asymptotically, outflows, breezes, inflows, accretions and static outer
envelopes may all surround such type voids. Both cases of
constant and varying temperatures across isothermal shock fronts are analyzed;
they are referred to as types and
void shock solutions. We apply the `phase net matching procedure' to construct
various self-similar void solutions. We also present analysis on void
generation mechanisms and describe several astrophysical applications. By
including self-gravity, gas pressure and shocks, our isothermal self-similar
void (ISSV) model is adaptable to various astrophysical systems such as
planetary nebulae, hot bubbles and superbubbles in the interstellar medium as
well as supernova remnants.Comment: 24 pages, 13 figuers, accepted by ApS
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