917 research outputs found
Gravitational Waves and GRBs from Tidal Disruption of Stars in the Center of Galaxies
Recent measurements of the Chandra satellite have shown that a supermassive
black hole of is located in the Galactic
Center; it seems probable that, from other observations, this fact is common in
the majority of galaxies. On the other hand, GRB explosions are typical
phenomenon linked to the galactic dynamics. In the present paper we discuss the
possibility that GRBs are tidal disruption of stars by supermassive black holes
located in the center of galaxies. This conjecture can be tested by a
gravitational wave detector of the class of AURIGA.Comment: 4 pages, 2 figures talk at Gamma Ray Burst Symposium - Santa Fe, New
Mexico (USA) September 8-12, 2003. There were added four new references and
the entrances of the two figures were written in a more clear wa
Clustering and gelation of hard spheres induced by the Pickering effect
A mixture of hard-sphere particles and model emulsion droplets is studied
with a Brownian dynamics simulation. We find that the addition of nonwetting
emulsion droplets to a suspension of pure hard spheres can lead to both
gas-liquid and fluid-solid phase separations. Furthermore, we find a stable
fluid of hard-sphere clusters. The stability is due to the saturation of the
attraction that occurs when the surface of the droplets is completely covered
with colloidal particles. At larger emulsion droplet densities a percolation
transition is observed. The resulting networks of colloidal particles show
dynamical and mechanical properties typical of a colloidal gel. The results of
the model are in good qualitative agreement with recent experimental findings
[E. Koos and N. Willenbacher, Science 331, 897 (2011)] in a mixture of
colloidal particles and two immiscible fluids.Comment: 5 figures, 5 page
The Response of Test Masses to Gravitational Waves in the Coordinates of a Local Observer
The response of laser interferometers to gravitational waves has been
calculated in a number of different ways, particularly in the
transverse-traceless and the local Lorentz gauges. At first sight, it would
appear that these calculations lead to different results when the separation
between the test masses becomes comparable to the wavelength of the
gravitational wave. In this paper this discrepancy is resolved. We describe the
response of free test masses to plane gravitational waves in the coordinate
frame of a local observer and show that it acquires contributions from three
different effects: the displacement of the test masses, the apparent change in
the photon velocity, and the variation in the clock speed of the local
observer, all of which are induced by the gravitational wave. Only when taken
together do these three effects represent a quantity which is translationally
invariant. This translationally-invariant quantity is identical to the response
function calculated in the transverse-traceless gauge. We thus resolve the
well-known discrepancy between the two coordinates systems, and show that the
results found in the coordinate frame of a local observer are valid for large
separation between the masses.Comment: 25 pages, 3 figures, Latex2
Computer simulations of colloidal transport on a patterned magnetic substrate
We study the transport of paramagnetic colloidal particles on a patterned
magnetic substrate with kinetic Monte Carlo and Brownian dynamics computer
simulations. The planar substrate is decorated with point dipoles in either
parallel or zigzag stripe arrangements and exposed to an additional external
magnetic field that oscillates in time. For the case of parallel stripes we
find that the magnitude and direction of the particle current is controlled by
the tilt angle of the external magnetic field. The effect is reliably obtained
in a wide range of ratios between temperature and magnetic permeability.
Particle transport is achieved only when the period of oscillation of the
external field is greater than a critical value. For the case of zigzag stripes
a current is obtained using an oscillating external field normal to the
substrate. In this case, transport is only possible in the vertex of the
zigzag, giving rise to a narrow stream of particles. The magnitude and
direction of the particle current are found to be controlled by a combination
of the zigzag angle and the distance of the colloids from the substrate.
Metropolis Monte Carlo and Brownian dynamics simulations predict results that
are in good agreement with each other. Using kinetic Monte Carlo we find that
at high density the particle transport is hindered by jamming.Comment: 8 pages, 9 figure
Crystallization and gelation in colloidal systems with short-ranged attractive interactions
We systematically study the relationship between equilibrium and
non-equilibrium phase diagrams of a system of short-ranged attractive colloids.
Using Monte Carlo and Brownian dynamics simulations we find a window of
enhanced crystallization that is limited at high interaction strength by a
slowing down of the dynamics and at low interaction strength by the high
nucleation barrier. We find that the crystallization is enhanced by the
metastable gas-liquid binodal by means of a two-stage crystallization process.
First, the formation of a dense liquid is observed and second the crystal
nucleates within the dense fluid. In addition, we find at low colloid packing
fractions a fluid of clusters, and at higher colloid packing fractions a
percolating network due to an arrested gas-liquid phase separation that we
identify with gelation. We find that this arrest is due to crystallization at
low interaction energy and it is caused by a slowing down of the dynamics at
high interaction strength. Likewise, we observe that the clusters which are
formed at low colloid packing fractions are crystalline at low interaction
energy, but glassy at high interaction energy. The clusters coalesce upon
encounter.Comment: 8 pages, 8 figure
Evidence from multivariate morphometric study of the quercus pubescens complex in southeast Italy
The name Quercus pubescens s.l. encompasses a complex of deciduous oak taxa with mainly southeastEuropean
distribution and a large ecological niche. As the easternmost region of Italy, Apulia is
rather isolated from a geographical and physiographical viewpoint and counts the highest number
of oak species (10). In the taxonomic and phytosociological literature, the occurrence of several
species belonging to the Quercus pubescens collective group is reported for this region. In order to
verify if different sets of morphological characters are associated with different taxa, 24 populations
of Quercus pubescens s.l. located in different ecological-geographical areas of Apulia were sampled.
A total of 367 trees, 4254 leaves and 1120 fruits were collected and morphologically analysed.
Overall, 25 morphological characters of oak leaves and fruits were statistically treated using both
univariate and multivariate analysis. Nested ANOVA showed that leaves collected from a single tree
exhibited a degree of morphological variability higher than that observed when comparing leaves
coming from different trees of the same population and from different trees of different populations
as well. Almost all the morphological characters analysed exhibited a continuous trend of variation
so that none of them can be used as a character to discriminate between populations. Only leaf
and fruit “size” and fruit petiole length emerged as slightly discriminating characters. Our results
suggest that it is unlikely that more than one species belonging to the Quercus pubescens complex
occurs in the Apulia region. Comparison between the Apulian populations and a genetically
pure Q. pubescens population coming from a different area (the Molise region) strengthened the
assumption as to the existence of a single species that can provisionally be classified under the name
of Q. pubescens s.
Electromagnetic waves in gravitational wave spacetimes
We have considered the propagation of electromagnetic waves in a space-time representing an exact gravitational plane wave and calculated the induced changes on the four-potential field AÎĽ of a plane electromagnetic wave. By choosing a suitable photon round-trip in a Michelson interferometer, we have been able to identify the physical effects of the exact gravitational wave on the electromagnetic field, i.e. phase shift, change of the polarization vector, angular deflection and delay. These results have been exploited to study the response of an interferometric gravitational wave detector beyond the linear approximation of the general theory of relativity. A much more detailled examination of this problem can be found in our paper recently published in Classical and Quantum Gravity (28 (2011) 235007)
A comparison between matter wave and light wave interferometers for the detection of gravitational waves
We calculate and compare the response of light wave interferometers and
matter wave interferometers to gravitational waves. We find that metric matter
wave interferometers will not challenge kilometric light wave interferometers
such as Virgo or LIGO, but could be a good candidate for the detection of very
low frequency gravitational waves
The van Hove distribution function for Brownian hard spheres: dynamical test particle theory and computer simulations for bulk dynamics
We describe a test particle approach based on dynamical density functional
theory (DDFT) for studying the correlated time evolution of the particles that
constitute a fluid. Our theory provides a means of calculating the van Hove
distribution function by treating its self and distinct parts as the two
components of a binary fluid mixture, with the `self' component having only one
particle, the `distinct' component consisting of all the other particles, and
using DDFT to calculate the time evolution of the density profiles for the two
components. We apply this approach to a bulk fluid of Brownian hard spheres and
compare to results for the van Hove function and the intermediate scattering
function from Brownian dynamics computer simulations. We find good agreement at
low and intermediate densities using the very simple Ramakrishnan-Yussouff
[Phys. Rev. B 19, 2775 (1979)] approximation for the excess free energy
functional. Since the DDFT is based on the equilibrium Helmholtz free energy
functional, we can probe a free energy landscape that underlies the dynamics.
Within the mean-field approximation we find that as the particle density
increases, this landscape develops a minimum, while an exact treatment of a
model confined situation shows that for an ergodic fluid this landscape should
be monotonic. We discuss possible implications for slow, glassy and arrested
dynamics at high densities.Comment: Submitted to Journal of Chemical Physic
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