35,791 research outputs found
Molecular dynamics simulations of ballistic annihilation
Using event-driven molecular dynamics we study one- and two-dimensional
ballistic annihilation. We estimate exponents and that describe
the long-time decay of the number of particles () and of
their typical velocity (). To a good accuracy our results
confirm the scaling relation . In the two-dimensional case our
results are in a good agreement with those obtained from the Boltzmann kinetic
theory.Comment: 4 pages; some changes; Physical Review E (in press
Jet Collimation by Small-Scale Magnetic Fields
A popular model for jet collimation is associated with the presence of a
large-scale and predominantly toroidal magnetic field originating from the
central engine (a star, a black hole, or an accretion disk). Besides the
problem of how such a large-scale magnetic field is generated, in this model
the jet suffers from the fatal long-wave mode kink magnetohydrodynamic
instability. In this paper we explore an alternative model: jet collimation by
small-scale magnetic fields. These magnetic fields are assumed to be local,
chaotic, tangled, but are dominated by toroidal components. Just as in the case
of a large-scale toroidal magnetic field, we show that the ``hoop stress'' of
the tangled toroidal magnetic fields exerts an inward force which confines and
collimates the jet. The magnetic ``hoop stress'' is balanced either by the gas
pressure of the jet, or by the centrifugal force if the jet is spinning. Since
the length-scale of the magnetic field is small (< the cross-sectional radius
of the jet << the length of the jet), in this model the jet does not suffer
from the long-wave mode kink instability. Many other problems associated with
the large-scale magnetic field are also eliminated or alleviated for
small-scale magnetic fields. Though it remains an open question how to generate
and maintain the required small-scale magnetic fields in a jet, the scenario of
jet collimation by small-scale magnetic fields is favored by the current study
on disk dynamo which indicates that small-scale magnetic fields are much easier
to generate than large-scale magnetic fields.Comment: 14 pages, no figur
The cosmological behavior of Bekenstein's modified theory of gravity
We study the background cosmology governed by the Tensor-Vector-Scalar theory
of gravity proposed by Bekenstein. We consider a broad family of potentials
that lead to modified gravity and calculate the evolution of the field
variables both numerically and analytically. We find a range of possible
behaviors, from scaling to the late time domination of either the additional
gravitational degrees of freedom or the background fluid.Comment: 10 pages, 8 figures, A few typos corrected in the text and figures.
Version published in PR
Are Magnetic Wind-Driving Disks Inherently Unstable?
There have been claims in the literature that accretion disks in which a
centrifugally driven wind is the dominant mode of angular momentum transport
are inherently unstable. This issue is considered here by applying an
equilibrium-curve analysis to the wind-driving, ambipolar diffusion-dominated,
magnetic disk model of Wardle & Konigl (1993). The equilibrium solution curves
for this class of models typically exhibit two distinct branches. It is argued
that only one of these branches represents unstable equilibria and that a real
disk/wind system likely corresponds to a stable solution.Comment: 5 pages, 2 figures, to be published in ApJ, vol. 617 (2004 Dec 20).
Uses emulateapj.cl
Lorentz-breaking effects in scalar-tensor theories of gravity
In this work, we study the effects of breaking Lorentz symmetry in
scalar-tensor theories of gravity taking torsion into account. We show that a
space-time with torsion interacting with a Maxwell field by means of a
Chern-Simons-like term is able to explain the optical activity in syncrotron
radiation emitted by cosmological distant radio sources. Without specifying the
source of the dilaton-gravity, we study the dilaton-solution. We analyse the
physical implications of this result in the Jordan-Fierz frame. We also analyse
the effects of the Lorentz breaking in the cosmic string formation process. We
obtain the solution corresponding to a cosmic string in the presence of torsion
by keeping track of the effects of the Chern-Simons coupling and calculate the
charge induced on this cosmic string in this framework. We also show that the
resulting charged cosmic string gives us important effects concerning the
background radiation.The optical activity in this case is also worked out and
discussed.Comment: 10 pages, no figures, ReVTex forma
Building analytical three-field cosmological models
A difficult task to deal with is the analytical treatment of models composed
by three real scalar fields, once their equations of motion are in general
coupled and hard to be integrated. In order to overcome this problem we
introduce a methodology to construct three-field models based on the so-called
"extension method". The fundamental idea of the procedure is to combine three
one-field systems in a non-trivial way, to construct an effective three scalar
field model. An interesting scenario where the method can be implemented is
within inflationary models, where the Einstein-Hilbert Lagrangian is coupled
with the scalar field Lagrangian. We exemplify how a new model constructed from
our method can lead to non-trivial behaviors for cosmological parameters.Comment: 11 pages, and 3 figures, updated version published in EPJ
Modelling of epitaxial film growth with a Ehrlich-Schwoebel barrier dependent on the step height
The formation of mounded surfaces in epitaxial growth is attributed to the
presence of barriers against interlayer diffusion in the terrace edges, known
as Ehrlich-Schwoebel (ES) barriers. We investigate a model for epitaxial growth
using a ES barrier explicitly dependent on the step height. Our model has an
intrinsic topological step barrier even in the absence of an explicit ES
barrier. We show that mounded morphologies can be obtained even for a small
barrier while a self-affine growth, consistent with the Villain-Lai-Das Sarma
equation, is observed in absence of an explicit step barrier. The mounded
surfaces are described by a super-roughness dynamical scaling characterized by
locally smooth (faceted) surfaces and a global roughness exponent .
The thin film limit is featured by surfaces with self-assembled
three-dimensional structures having an aspect ratio (height/width) that may
increase or decrease with temperature depending on the strength of step
barrier.Comment: To appear in J. Phys. Cond. Matter; 3 movies as supplementary
materia
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