9,267 research outputs found
Patterning of dielectric nanoparticles using dielectrophoretic forces generated by ferroelectric polydomain films
A theoretical study of a dielectrophoretic force, i.e. the force acting on an
electrically neutral particle in the inhomogeneous electric field, which is
produced by a ferroelectric domain pattern, is presented. It has been shown by
several researchers that artificially prepared domain patterns with given
geometry in ferroelectric single crystals represent an easy and flexible method
for patterning dielectric nanoobjects using dielectrophoretic forces. The
source of the dielectrophoretic force is a strong and highly inhomogeneous
(stray) electric field, which exists in the vicinity of the ferroelectric
domain walls at the surface of the ferroelectric film. We analyzed
dielectrophoretic forces in the model of a ferroelectric film of a given
thickness with a lamellar 180 domain pattern. The analytical formula
for the spatial distribution of the stray field in the ionic liquid above the
top surface of the film is calculated including the effect of free charge
screening. The spatial distribution of the dielectrophoretic force produced by
the domain pattern is presented. The numerical simulations indicate that the
intersection of the ferroelectric domain wall and the surface of the
ferroelectric film represents a trap for dielectric nanoparticles in the case
of so called positive dielectrophoresis. The effects of electrical neutrality
of dielectric nanoparticles, free charge screening due to the ionic nature of
the liquid, domain pattern geometry, and the Brownian motion on the mechanism
of nanoparticle deposition and the stability of the deposited pattern are
discussed.Comment: Accepted in the Journal of Applied Physics, 10 pages, 5 figure
Scattered light images of spiral arms in marginally gravitationally unstable discs with an embedded planet
Scattered light images of transition discs in the near-infrared often show
non-axisymmetric structures in the form of wide-open spiral arms in addition to
their characteristic low-opacity inner gap region. We study self-gravitating
discs and investigate the influence of gravitational instability on the shape
and contrast of spiral arms induced by planet-disc interactions.
Two-dimensional non-isothermal hydrodynamical simulations including viscous
heating and a cooling prescription are combined with three-dimensional dust
continuum radiative transfer models for direct comparison to observations. We
find that the resulting contrast between the spirals and the surrounding disc
in scattered light is by far higher for pressure scale height variations, i.e.
thermal perturbations, than for pure surface density variations. Self-gravity
effects suppress any vortex modes and tend to reduce the opening angle of
planet-induced spirals, making them more tightly wound. If the disc is only
marginally gravitationally stable with a Toomre parameter around unity, an
embedded massive planet (planet-to-star mass ratio of ) can trigger
gravitational instability in the outer disc. The spirals created by this
instability and the density waves launched by the planet can overlap resulting
in large-scale, more open spiral arms in the outer disc. The contrast of these
spirals is well above the detection limit of current telescopes.Comment: Accepted for publication in MNRAS; 13 pages, 8 figure
HESS J1632-478: an energetic relic
HESS J1632-478 is an extended and still unidentified TeV source in the
galactic plane. In order to identify the source of the very high energy
emission and to constrain its spectral energy distribution, we used a deep
observation of the field obtained with XMM-Newton together with data from
Molonglo, Spitzer and Fermi to detect counterparts at other wavelengths. The
flux density emitted by HESS J1632-478 peaks at very high energies and is more
than 20 times weaker at all other wavelengths probed. The source spectrum
features two large prominent bumps with the synchrotron emission peaking in the
ultraviolet and the external inverse Compton emission peaking in the TeV. HESS
J1632-478 is an energetic pulsar wind nebula with an age of the order of 10^4
years. Its bolometric (mostly GeV-TeV) luminosity reaches 10% of the current
pulsar spin down power. The synchrotron nebula has a size of 1 pc and contains
an unresolved point-like X-ray source, probably the pulsar with its wind
termination shock.Comment: A&A accepted, 9 pages, 5 figures, 4 table
Non-thermal high-energy emission from colliding winds of massive stars
Colliding winds of massive star binary systems are considered as potential
sites of non-thermal high-energy photon production. This is motivated merely by
the detection of synchrotron radio emission from the expected colliding wind
location. Here we investigate the properties of high-energy photon production
in colliding winds of long-period WR+OB-systems. We found that in the
dominating leptonic radiation process anisotropy and Klein-Nishina effects may
yield spectral and variability signatures in the gamma-ray domain at or above
the sensitivity of current or upcoming gamma-ray telescopes. Analytical
formulae for the steady-state particle spectra are derived assuming diffusive
particle acceleration out of a pool of thermal wind particles, and taking into
account adiabatic and all relevant radiative losses. For the first time we
include their advection/convection in the wind collision zone, and distinguish
two regions within this extended region: the acceleration region where spatial
diffusion is superior to convective/advective motion, and the convection region
defined by the convection time shorter than the diffusion time scale. The
calculation of the Inverse Compton radiation uses the full Klein-Nishina cross
section, and takes into account the anisotropic nature of the scattering
process. This leads to orbital flux variations by up to several orders of
magnitude which may, however, be blurred by the geometry of the system. The
calculations are applied to the typical WR+OB-systems WR 140 and WR 147 to
yield predictions of their expected spectral and temporal characteristica and
to evaluate chances to detect high-energy emission with the current and
upcoming gamma-ray experiments. (abridged)Comment: 67 pages, 24 figures, submitted to Ap
Demonstrating the feasibility of standardized application program interfaces that will allow mobile/portable terminals to receive services combining UMTS and DVB-T
Crucial to the commercial exploitation of any service combining UMTS and DVB-T is the availability of standardized API’s adapted to the hybrid UMTS and DVB-T network and to the technical limitations of mobile/portable terminals. This paper describes work carried out in the European Commission Framework Program 5 (FP5) project CONFLUENT to demonstrate the feasibility of such Application Program Interfaces (API’s) by enabling the reception of a Multimedia Home Platform (MHP) based application transmitted over DVB-T on five different terminals with parts of the service running on a mobile phone
A current driven instability in parallel, relativistic shocks
Recently, Bell has reanalysed the problem of wave excitation by cosmic rays
propagating in the pre-cursor region of a supernova remnant shock front. He
pointed out a strong, non-resonant, current-driven instability that had been
overlooked in the kinetic treatments, and suggested that it is responsible for
substantial amplification of the ambient magnetic field. Magnetic field
amplification is also an important issue in the problem of the formation and
structure of relativistic shock fronts, particularly in relation to models of
gamma-ray bursts. We have therefore generalised the linear analysis to apply to
this case, assuming a relativistic background plasma and a monoenergetic,
unidirectional incoming proton beam. We find essentially the same non-resonant
instability noticed by Bell, and show that also under GRB conditions, it grows
much faster than the resonant waves. We quantify the extent to which thermal
effects in the background plasma limit the maximum growth rate.Comment: 8 pages, 1 figur
Limits on Supersymmetric Dark Matter From EGRET Observations of the Galactic Center Region
In most supersymmetic models, neutralino dark matter particles are predicted
to accumulate in the Galactic center and annihilate generating, among other
products, gamma rays. The EGRET experiment has made observations in this
region, and is sensitive to gamma rays from 30 MeV to 30 GeV. We have
used an improved point source analysis including an energy dependent point
spread function and an unbinned maximum likelihood technique, which has allowed
us to significantly lower the limits on gamma ray flux from the Galactic
center. We find that the present EGRET data can limit many supersymmetric
models if the density of the Galactic dark matter halo is cuspy or spiked
toward the Galactic center. We also discuss the ability of GLAST to test these
models.Comment: 4 pages, 3 figure
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