4,189 research outputs found
Instabilities of rotating compact stars: a brief overview
Direct observations of gravitational waves will open in the near future new
windows on the Universe. Among the expected sources, instabilities of rotating
compact astrophysical objects are waited to be detected with some impatience as
this will sign the birth of ``gravitational waves asteroseismology'', a crucial
way to improve our knowledge of matter equation of state in conditions that
cannot be reproduced in a lab. However, the theoretical work needed to really
get informations from to-be-detected signals is still quite large, numerical
simulations having become a necessary key ingredient. This article tries to
provide a short overview of the main physical topics involved in this field
(general relativity, gravitational waves, instabilities of rotating fluids,
{\it etc.}), concluding with a brief description of the work that was done in
Paris-Meudon Observatory by Silvano Bonazzola and collaborators.Comment: 19 pages, Proceeding of Cargese School "Astrophysical fluid dynamics"
(May 2005) organized by B. Dubrulle and M. Rieutord in honour of J.-P. Zahn
and S. Bonazzola. Slightly upgraded version: references added, summary on
compact stars birth clarifie
1D Cahn-Hilliard dynamics : coarsening and interrupted coarsening
Many systems exhibit a phase where the order parameter is spatially
modulated. These patterns can be the result of a frustration caused by the
competition between interaction forces with opposite effects. In all models
with local interactions, these ordered phases disappear in the strong
segregation regime (low temperature). It is expected however that these phases
should persist in the case of long range interactions, which can't be correctly
described by a Ginzburg-Landau type model with only a finite number of spatial
derivatives of the order parameter. An alternative approach is to study the
dynamics of the phase transition or pattern formation. While, in the usual
process of Ostwald ripening, succession of doubling of the domain size leads to
a total segregation, or macro-segregation, C. Misbah and P. Politi have shown
that long-range interactions could cause an interruption of this coalescence
process, stabilizing a pattern which then remains in a micro-structured state
or super-crystal. We show that this is the case for a modified Cahn-Hilliard
dynamics due to Oono which includes a non local term and which is particularly
well suited to describe systems with a modulated phase
Phase transition of the three-dimensional chiral Ginzburg-Landau model -- search for the chiral phase
Nature of the phase transition of regularly frustrated vector spin systems in
three dimensions is investigated based on a Ginzburg-Landau-type effective
Hamiltonian. On the basis of the variational analysis of this model, Onoda et
al recently suggested the possible occurrence of a chiral phase, where the
vector chirality exhibits a long-range order without the long-range order of
the spin [Phys. Rev. Lett. 99, 027206 (2007)]. In the present paper, we
elaborate their analysis by considering the possibility of a first-order
transition which was not taken into account in their analysis. We find that the
first-order transition indeed occurs within the variational approximation,
which significantly reduces the stability range of the chiral phase, while the
chiral phase still persists in a restricted parameter range. Then, we perform
an extensive Monte Carlo simulation focusing on such a parameter range.
Contrary to the variational result, however, we do not find any evidence of the
chiral phase. The range of the chiral phase, if any, is estimated to be less
than 0.1% in the temperature width.Comment: 19 pages, 17 figure
Inertial modes in slowly rotating stars : an evolutionary description
We present a new hydro code based on spectral methods using spherical
coordinates. The first version of this code aims at studying time evolution of
inertial modes in slowly rotating neutron stars. In this article, we introduce
the anelastic approximation, developed in atmospheric physics, using the mass
conservation equation to discard acoustic waves. We describe our algorithms and
some tests of the linear version of the code, and also some preliminary linear
results. We show, in the Newtonian framework with differentially rotating
background, as in the relativistic case with the strong Cowling approximation,
that the main part of the velocity quickly concentrates near the equator of the
star. Thus, our time evolution approach gives results analogous to those
obtained by Karino {\it et al.} \cite{karino01} within a calculation of
eigenvectors. Furthermore, in agreement with the work of Lockitch {\it et al.}
\cite{lockandf01}, we found that the velocity seems to always get a
non-vanishing polar part.Comment: 36 pages, 27 figures, accepted for publication in Phys. Rev. D
(discussion added in the introduction
Study of Chirality in the Two-Dimensional XY Spin Glass
We study the chirality in the Villain form of the XY spin glass in
two--dimensions by Monte Carlo simulations. We calculate the chiral-glass
correlation length exponent and find that
in reasonable agreement with
earlier studies. This indicates that the chiral and phase variables are
decoupled on long length scales and diverge as with {\em different}
exponents, since the spin-glass correlation length exponent was found, in
earlier studies, to be about 1.0.Comment: 4 pages. Latex file and 4 embedded postscript files are included in a
self-unpacking compressed tar file. A postscript version is available at
ftp://chopin.ucsc.edu/pub/xysg.p
The Lamellar-Disorder Interface : One-Dimensional Modulated Profiles
We study interfacial behavior of a lamellar (stripe) phase coexisting with a
disordered phase. Systematic analytical expansions are obtained for the
interfacial profile in the vicinity of a tricritical point. They are
characterized by a wide interfacial region involving a large number of
lamellae. Our analytical results apply to systems with one dimensional symmetry
in true thermodynamical equilibrium and are of relevance to metastable
interfaces between lamellar and disordered phases in two and three dimensions.
In addition, good agreement is found with numerical minimization schemes of the
full free energy functional having the same one dimensional symmetry. The
interfacial energy for the lamellar to disordered transition is obtained in
accord with mean field scaling laws of tricritical points.Comment: 12 pages, 8 figure
Two snap-stabilizing point-to-point communication protocols in message-switched networks
A snap-stabilizing protocol, starting from any configuration, always behaves
according to its specification. In this paper, we present a snap-stabilizing
protocol to solve the message forwarding problem in a message-switched network.
In this problem, we must manage resources of the system to deliver messages to
any processor of the network. In this purpose, we use information given by a
routing algorithm. By the context of stabilization (in particular, the system
starts in an arbitrary configuration), this information can be corrupted. So,
the existence of a snap-stabilizing protocol for the message forwarding problem
implies that we can ask the system to begin forwarding messages even if routing
information are initially corrupted. In this paper, we propose two
snap-stabilizing algorithms (in the state model) for the following
specification of the problem: - Any message can be generated in a finite time.
- Any emitted message is delivered to its destination once and only once in a
finite time. This implies that our protocol can deliver any emitted message
regardless of the state of routing tables in the initial configuration. These
two algorithms are based on the previous work of [MS78]. Each algorithm needs a
particular method to be transform into a snap-stabilizing one but both of them
do not introduce a significant overcost in memory or in time with respect to
algorithms of [MS78]
Inertial modes in stratified rotating neutron stars : An evolutionary description
With (non-barotropic) equations of state valid even when the neutron, proton
and electron content of neutron star cores is not in beta equilibrium, we study
inertial and composition gravity modes of relativistic rotating neutron stars.
We solve the relativistic Euler equations in the time domain with a three
dimensional numerical code based on spectral methods, in the slow rotation,
relativistic Cowling and anelastic approximations. Principally, after a short
description of the gravity modes due to smooth composition gradients, we focus
our analysis on the question of how the inertial modes are affected by
non-barotropicity of the nuclear matter. In our study, the deviation with
respect to barotropicity results from the frozen composition of non-superfluid
matter composed of neutrons, protons and electrons, when beta equilibrium is
broken by millisecond oscillations. We show that already for moderatly fast
rotating stars the increasing coupling between polar and axial modes makes
those two cases less different than for very slowly rotating stars. In
addition, as we directly solve the Euler equations, without coupling only a few
number of spherical harmonics, we always found, for the models that we use, a
discrete spectrum for the inertial mode. Finally, we find that, for
non-barotropic stars, the frequency of this mode, which is our main focus,
decreases in a non-negligible way, whereas the time dependence of the energy
transfer between polar and axial modes is substantially different due to the
existence of low-frequencies gravity modes.Comment: 34 pages, 24 figures, published versio
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