18,212 research outputs found
The space of unipotently supported class functions on a finite reductive group
We determine the Lusztig restrictions on the space of class functions with a
unipotent support on a finite reductive group. In particular we give a simple
expression for the Lusztig restrictions of the generalized Green functions and
we describe the Lusztig restrictions of the generalized Gelfand-Graev
representations. We make explicit computations for the Gelfand-Graev
representations associated to the subregular unipotent class. In the case of
SLn we show that the computations can be reduced to the case of GLn' for
various n'.Comment: 21 page
Observation of noise correlated by the Hawking effect in a water tank
We measured the power spectrum and two-point correlation function for the
randomly fluctuating free surface on the downstream side of a stationary flow
with a maximum Froude number reached above a
localised obstacle. On such a flow the scattering of incident long wavelength
modes is analogous to that responsible for black hole radiation (the Hawking
effect). Our measurements of the noise show a clear correlation between pairs
of modes of opposite energies. We also measure the scattering coefficients by
applying the same analysis of correlations to waves produced by a wave maker.Comment: 11 pages, 14 figures. several points clarified; two new subsections
in the Supplemental Material on the wave equation and the links with
experiments in BEC
Depolarization volume and correlation length in the homogenization of anisotropic dielectric composites
In conventional approaches to the homogenization of random particulate
composites, both the distribution and size of the component phase particles are
often inadequately taken into account. Commonly, the spatial distributions are
characterized by volume fraction alone, while the electromagnetic response of
each component particle is represented as a vanishingly small depolarization
volume. The strong-permittivity-fluctuation theory (SPFT) provides an
alternative approach to homogenization wherein a comprehensive description of
distributional statistics of the component phases is accommodated. The
bilocally-approximated SPFT is presented here for the anisotropic homogenized
composite which arises from component phases comprising ellipsoidal particles.
The distribution of the component phases is characterized by a two-point
correlation function and its associated correlation length. Each component
phase particle is represented as an ellipsoidal depolarization region of
nonzero volume. The effects of depolarization volume and correlation length are
investigated through considering representative numerical examples. It is
demonstrated that both the spatial extent of the component phase particles and
their spatial distributions are important factors in estimating coherent
scattering losses of the macroscopic field.Comment: Typographical error in eqn. 16 in WRM version is corrected in arxiv
versio
A Sagittarius-Induced Origin for the Monoceros Ring
The Monoceros ring is a collection of stars in nearly-circular orbits at
roughly 18 kpc from the Galactic center. It may have originated (i) as the
response of the disc to perturbations excited by satellite companions or (ii)
from the tidal debris of a disrupted dwarf galaxy. The metallicity of Monoceros
stars differs from that of disc stars at comparable Galactocentric distances,
an observation that disfavours the first scenario. On the other hand, circular
orbits are difficult to accommodate in the tidal-disruption scenario, since it
requires a satellite which at the time of disruption was itself in a nearly
circular orbit. Such satellite could not have formed at the location of the
ring and, given its low mass, dynamical friction is unlikely to have played a
major role in its orbital evolution. We search cosmological simulations for
low-mass satellites in nearly-circular orbits and find that they result, almost
invariably, from orbital changes induced by collisions with more massive
satellites: the radius of the circular orbit thus traces the galactocentric
distance of the collision. Interestingly, the Sagittarius dwarf, one of the
most luminous satellites of the Milky Way, is in a polar orbit that crosses the
Galactic plane at roughly the same Galactocentric distance as Monoceros. We use
idealized simulations to demonstrate that an encounter with Sagittarius might
well have led to the circularization and subsequent tidal demise of the
progenitor of the Monoceros ring.Comment: 6 pages, 4 figures, to match version published in MNRAS Letters
(http://onlinelibrary.wiley.com/doi/10.1111/j.1745-3933.2011.01035.x/abstract
A Methodology to Engineer and Validate Dynamic Multi-level Multi-agent Based Simulations
This article proposes a methodology to model and simulate complex systems,
based on IRM4MLS, a generic agent-based meta-model able to deal with
multi-level systems. This methodology permits the engineering of dynamic
multi-level agent-based models, to represent complex systems over several
scales and domains of interest. Its goal is to simulate a phenomenon using
dynamically the lightest representation to save computer resources without loss
of information. This methodology is based on two mechanisms: (1) the activation
or deactivation of agents representing different domain parts of the same
phenomenon and (2) the aggregation or disaggregation of agents representing the
same phenomenon at different scales.Comment: Presented at 3th International Workshop on Multi-Agent Based
Simulation, Valencia, Spain, 5th June 201
Mode stability in delta Scuti stars: linear analysis versus observations in open clusters
A comparison between linear stability analysis and observations of pulsation
modes in five delta Scuti stars, belonging to the same cluster, is presented.
The study is based on the work by Michel et al. (1999), in which such a
comparison was performed for a representative set of model solutions obtained
independently for each individual star considered. In this paper we revisit the
work by Michel et al. (1999) following, however, a new approach which consists
in the search for a single, complete, and coherent solution for all the
selected stars, in order to constrain and test the assumed physics describing
these objects. To do so, refined descriptions for the effects of rotation on
the determination of the global stellar parameters and on the adiabatic
oscillation frequency computations are used. In addition, a crude attempt is
made to study the role of rotation on the prediction of mode instabilities.The
present results are found to be comparable with those reported by Michel et al.
(1999). Within the temperature range log T_eff = 3.87-3.88 agreement between
observations and model computations of unstable modes is restricted to values
for the mixing-length parameter alpha_nl less or equal to 1.50. This indicates
that for these stars a smaller value for alpha_nl is required than suggested
from a calibrated solar model. We stress the point that the linear stability
analysis used in this work still assumes stellar models without rotation and
that further developments are required for a proper description of the
interaction between rotation and pulsation dynamics.Comment: 8 pages, 4 figures, 3 tables. (MNRAS, in press
Asteroseismology of delta Scuti stars in open clusters: Praesepe
The present paper provides a general overview of the asteroseismic potential
of delta Scuti stars in clusters, in particular focusing on convection
diagnostics. We give a summarise of the last results obtained by the authors
for the Praesepe cluster of which five delta Scuti stars are analysed. In that
work, linear analysis is confronted with observations, using refined
descriptions for the effects of rotation on the determination of the global
stellar parameters and on the adiabatic oscillation frequency computations. A
single, complete, and coherent solution for all the selected stars is found,
which lead the authors to find important restrictions to the convection
description for a certain range of effective temperatures. Furthermore, the
method used allowed to give an estimate of the global parameters of the
selected stars and constrain the cluster.Comment: 6 pages, 1 figure. Accepted for publication in Communications in
Asteroseismolog
Generation of the electrostatic field in the pulsar magnetosphere plasma
The behaviour of a relativistic electron-positron plasma in the pulsar
magnetosphere is investigated. The equation of the motion of the magnetospheric
plasma is discussed, from which it follows that, if the plasma particle radial
velocity (where c is the speed of light), the centrifugal
acceleration changes its sign and the particle braking begins. The stability of
the magnetospheric plasma with respect to the radially oriented potential
perturbations is discussed and the possibility of the electrostatic field
generation in this plasma along the pulsar magnetic field lines is shown.Comment: Plain LaTe
Neutron spin polarization in strong magnetic fields
The effects of strong magnetic fields on the inner crust of neutron stars are
investigated after taking into account the anomalous magnetic moments of
nucleons. Energy spectra and wave functions for protons and neutrons in a
uniform magnetic field are provided. The particle spin polarizations and the
yields of protons and neutrons are calculated in a free Fermi gas model.
Obvious spin polarization occurs when G for protons and
G for neutrons, respectively. It is shown that the neutron spin
polarization depends solely on the magnetic field strength.Comment: Replaced by the revised version; 10 pages, including 3 eps figure
Observation of resonant interactions among surface gravity waves
We experimentally study resonant interactions of oblique surface gravity
waves in a large basin. Our results strongly extend previous experimental
results performed mainly for perpendicular or collinear wave trains. We
generate two oblique waves crossing at an acute angle, while we control their
frequency ratio, steepnesses and directions. These mother waves mutually
interact and give birth to a resonant wave whose properties (growth rate,
resonant response curve and phase locking) are fully characterized. All our
experimental results are found in good quantitative agreement with four-wave
interaction theory with no fitting parameter. Off-resonance experiments are
also reported and the relevant theoretical analysis is conducted and validated.Comment: 11 pages, 7 figure
- …