27,385 research outputs found
Solutions of the Yang-Mills-Higgs equations in 2+1 dimensional anti-de Sitter space-time
The solutions of the Bogomolny equation in anti-de Sitter space-time are
obtained by using Darboux transformations with both constant spectral
parameters and variable "spectral parameters". These solutions give the
Yang-Mills-Higgs fields in anti-de Sitter space-time. Some examples in SU(2)
case are considered and qualitative asymptotic behaviors of the solutions as t
tends to infinity are discussed in detail.Comment: LaTeX, 18 pages, 11 PS figure
Inflation from Geometrical Tachyons
We propose an alternative formulation of tachyon inflation using the
geometrical tachyon arising from the time dependent motion of a BPS -brane
in the background geometry due to parallel 5-branes arranged around a
ring of radius . Due to the fact that the mass of this geometrical tachyon
field is times smaller than the corresponding open-string tachyon
mass, we find that the slow roll conditions for inflation and the number of
e-foldings can be satisfied in a manner that is consistent with an effective
4-dimensional model and with a perturbative string coupling. We also show that
the metric perturbations produced at the end of inflation can be sufficiently
small and do not lead to the inconsistencies that plague the open string
tachyon models. Finally we argue for the existence of a minimum of the
geometrical tachyon potential which could give rise to a traditional reheating
mechanism.Comment: Latex, 20 pages, 4 figures; correction of algebraic errors in section
5 concerning the tachyon potential near its minimum. Conclusions unchange
A Fast Potential and Self-Gravity Solver for Non-Axisymmetric Disks
Disk self-gravity could play an important role in the dynamic evolution of
interaction between disks and embedded protoplanets. We have developed a fast
and accurate solver to calculate the disk potential and disk self-gravity
forces for disk systems on a uniform polar grid. Our method follows closely the
method given by Chan et al. (2006), in which an FFT in the azimuthal direction
is performed and a direct integral approach in the frequency domain in the
radial direction is implemented on a uniform polar grid. This method can be
very effective for disks with vertical structures that depend only on the disk
radius, achieving the same computational efficiency as for zero-thickness
disks. We describe how to parallelize the solver efficiently on distributed
parallel computers. We propose a mode-cutoff procedure to reduce the parallel
communication cost and achieve nearly linear scalability for a large number of
processors. For comparison, we have also developed a particle-based fast
tree-code to calculate the self-gravity of the disk system with vertical
structure. The numerical results show that our direct integral method is at
least two order of magnitudes faster than our optimized tree-code approach.Comment: 8 figures, accepted to ApJ
Type I Migration in Radiatively Efficient Discs
We study Type I migration of a planet in a radiatively efficient disk using
global two dimensional hydrodynamic simulations. The large positive corotation
torque is exerted on a planet by an adiabatic disk at early times when the disk
has the steep negative entropy gradient. The gas on the horseshoe orbit of the
planet is compressed adiabatically during the change of the orbit from the slow
orbit to the fast orbit, increasing its density and exerting the positive
torque on the planet. The planet would migrate outward in the adiabatic disk
before saturation sets in. We further study the effect of energy dissipation by
radiation on Type I migration of the planet. The corotation torque decreases
when the energy dissipates effectively because the density of the gas on the
horseshoe orbit does not increase by the compression compared with the gas of
the adiabatic disk. The total torque is mainly determined by the negative
Lindblad torque and becomes negative. The planet migrates inward toward the
central star in the radiatively efficient disk. The migration velocity is
dependent on the radiative efficiency and greatly reduced if the radiative
cooling works inefficiently.Comment: 12 pages, 10 figures, 1 table, Accepted for publication in MNRA
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