6,427 research outputs found
Oscillations and instabilities of fast and differentially rotating relativistic stars
We study non-axisymmetric oscillations of rapidly and differentially rotating
relativistic stars in the Cowling approximation. Our equilibrium models are
sequences of relativistic polytropes, where the differential rotation is
described by the relativistic -constant law. We show that a small degree of
differential rotation raises the critical rotation value for which the
quadrupolar f-mode becomes prone to the CFS instability, while the critical
value of at the mass-shedding limit is raised even more. For softer
equations of state these effects are even more pronounced. When increasing
differential rotation further to a high degree, the neutral point of the CFS
instability first reaches a local maximum and is lowered afterwards. For stars
with a rather high compactness we find that for a high degree of differential
rotation the absolute value of the critical is below the corresponding
value for rigid rotation. We conclude that the parameter space where the CFS
instability is able to drive the neutron star unstable is increased for a small
degree of differential rotation and for a large degree at least in stars with a
higher compactness.Comment: 16 pages, 11 figures; paper accepted for publication in Phys. Rev. D
(81.084019
Measuring patchy reionisation with kSZ-21 cm correlations
We study cross-correlations of the kinetic Sunyaev-Zel'dovich effect (kSZ)
and 21 cm signals during the epoch of reionisation (EoR) to measure the effects
of patchy reionisation. Since the kSZ effect is proportional to the
line-of-sight velocity, the kSZ-21 cm cross correlation suffers from
cancellation at small angular scales. We thus focus on the correlation between
the kSZ-squared field (kSZ) and 21 cm signals. When the global ionisation
fraction is low (), the kSZ fluctuation is dominated by
rare ionised bubbles which leads to an anti-correlation with the 21 cm signal.
When , the correlation is dominated by small pockets of
neutral regions, leading to a positive correlation. However, at very high
redshifts when , the spin temperature fluctuations change the sign of
the correlation from negative to positive, as weakly ionised regions can have
strong 21 cm signals in this case. To extract this correlation, we find that
Wiener filtering is effective in removing large signals from the primary CMB
anisotropy. The expected signal-to-noise ratios for a 10-hour integration
of upcoming Square Kilometer Array data cross-correlated with maps from the
current generation of CMB observatories with 3.4~K arcmin noise and
1.7~arcmin beam over 100~deg are 51, 60, and 37 for , 0.5, and
0.9, respectively.Comment: 7pages, 7 figure
Leaf area index and topographical effects on turburlent diffusion in a deciduous forest
In order to investigate turbulent diffusion in a deciduous forest canopy, wind velocity
measurements were conducted from late autumn of 2009 to early spring of 2010, using an observation tower
20 m in height located in the campus of Kanazawa University. Four sonic anemometers mounted on the
tower recorded the average wind velocities and temperatures, as well as their fluctuations, at four different
heights simultaneously. Two different types of data sets were selected, in which the wind velocities, wind
bearings and atmospheric stabilities were all similar, but the Leaf Area Indexes (LAI's) were different.
Vertical profiles of average wind velocities were found to have an approximately exponential profile in each
case. The characteristic length scales of turbulence were evaluated by both von Karman's method and the
integral time scale deduced from the autocorrelation from time-series analyses. Both methods produced
comparable values of eddy diffusivity for the cases with some foliage during late autumn, but some
discrepancy in the upper canopy layer was observed when the trees did not have their leaves in early spring.
It was also found that the eddy diffusivities generally take greater values at higher positions, where the wind
speeds are large. Anisotropy of eddy diffusivities between the vertical and horizontal components was also
observed, particularly in the cases when the canopy does not have leaves, when the horizontal eddy
diffusivities are generally larger than the vertical ones. On the other hand, the anisotropy is less visible when
the trees have some foliage during autumn. The effects of topography on the turbulent diffusion were also
investigated, including evaluation of the non-zero time-averaged vertical wind velocities. The results show
that the effects are marginal for both cases, and can be neglected as far as diffusion in the canopy is
concerned
Position-dependent power spectra of the 21-cm signal from the epoch of reionization
The 21-cm signal from the epoch of reionization is non-Gaussian. Current
radio telescopes are focused on detecting the 21-cm power spectrum, but in the
future the Square Kilometre Array is anticipated to provide a first measurement
of the bispectrum. Previous studies have shown that the position-dependent
power spectrum is a simple and efficient way to probe the squeezed-limit
bispectrum. In this approach, the survey is divided into subvolumes and the
correlation between the local power spectrum and the corresponding mean density
of the subvolume is computed. This correlation is equivalent to an integral of
the bispectrum in the squeezed limit, but is much simpler to implement than the
usual bispectrum estimators. It also has a clear physical interpretation: it
describes how the small-scale power spectrum of tracers such as galaxies and
the 21-cm signal respond to a large-scale environment. Reionization naturally
couples large and small scales as ionizing radiation produced by galactic
sources can travel up to tens of Megaparsecs through the intergalactic medium
during this process. Here we apply the position-dependent power spectrum
approach to fluctuations in the 21-cm background from reionization. We show
that this statistic has a distinctive evolution in time that can be understood
with a simple analytic model. We also show that the statistic can easily
distinguish between simple "inside-out" and "outside-in" models of
reionization. The position-dependent power spectrum is thus a promising method
to validate the reionization signal and to extract higher-order information on
this process.Comment: 24 pages, 10 figures, accepted in JCA
Primordial Non-Gaussianity and Analytical Formula for Minkowski Functionals of the Cosmic Microwave Background and Large-scale Structure
We derive analytical formulae for the Minkowski Functions of the cosmic
microwave background (CMB) and large-scale structure (LSS) from primordial
non-Gaussianity. These formulae enable us to estimate a non-linear coupling
parameter, f_NL, directly from the CMB and LSS data without relying on
numerical simulations of non-Gaussian primordial fluctuations. One can use
these formulae to estimate statistical errors on f_NL from Gaussian
realizations, which are much faster to generate than non-Gaussian ones, fully
taking into account the cosmic/sampling variance, beam smearing, survey mask,
etc. We show that the CMB data from the Wilkinson Microwave Anisotropy Probe
should be sensitive to |f_NL|\simeq 40 at the 68% confidence level. The Planck
data should be sensitive to |f_NL|\simeq 20. As for the LSS data, the late-time
non-Gaussianity arising from gravitational instability and galaxy biasing makes
it more challenging to detect primordial non-Gaussianity at low redshifts. The
late-time effects obscure the primordial signals at small spatial scales.
High-redshift galaxy surveys at z>2 covering \sim 10Gpc^3 volume would be
required for the LSS data to detect |f_NL|\simeq 100. Minkowski Functionals are
nicely complementary to the bispectrum because the Minkowski Functionals are
defined in real space and the bispectrum is defined in Fourier space. This
property makes the Minksowski Functionals a useful tool in the presence of
real-world issues such as anisotropic noise, foreground and survey masks. Our
formalism can be extended to scale-dependent f_NL easily.Comment: 16 pages, 5 figures, accepted for publication in ApJ (Vol. 653, 2006
Relativistic stars with purely toroidal magnetic fields
We investigate the effects of the purely toroidal magnetic field on the
equilibrium structures of the relativistic stars. The master equations for
obtaining equilibrium solutions of relativistic rotating stars containing
purely toroidal magnetic fields are derived for the first time. To solve these
master equations numerically, we extend the Cook-Shapiro-Teukolsky scheme for
calculating relativistic rotating stars containing no magnetic field to
incorporate the effects of the purely toroidal magnetic fields. By using the
numerical scheme, we then calculate a large number of the equilibrium
configurations for a particular distribution of the magnetic field in order to
explore the equilibrium properties. We also construct the equilibrium sequences
of the constant baryon mass and/or the constant magnetic flux, which model the
evolution of an isolated neutron star as it loses angular momentum via the
gravitational waves. Important properties of the equilibrium configurations of
the magnetized stars obtained in this study are summarized as follows ; (1) For
the non-rotating stars, the matter distribution of the stars is prolately
distorted due to the toroidal magnetic fields. (2) For the rapidly rotating
stars, the shape of the stellar surface becomes oblate because of the
centrifugal force. But, the matter distribution deep inside the star is
sufficiently prolate for the mean matter distribution of the star to be
prolate. (3) The stronger toroidal magnetic fields lead to the mass-shedding of
the stars at the lower angular velocity. (4) For some equilibrium sequences of
the constant baryon mass and magnetic flux, the stars can spin up as they lose
angular momentum.Comment: 13 figures, 7 tables, submitted to PR
Temperature and Polarization CMB Maps from Primordial non-Gaussianities of the Local Type
The forthcoming Planck experiment will provide high sensitivity polarization
measurements that will allow us to further tighten the f_NL bounds from the
temperature data. Monte Carlo simulations of non-Gaussian CMB maps have been
used as a fundamental tool to characterize non-Gaussian signatures in the data,
as they allow us to calibrate any statistical estimators and understand the
effect of systematics, foregrounds and other contaminants. We describe an
algorithm to generate high-angular resolution simulations of non-Gaussian CMB
maps in temperature and polarization. We consider non-Gaussianities of the
local type, for which the level of non-Gaussianity is defined by the
dimensionless parameter, f_NL. We then apply the temperature and polarization
fast cubic statistics recently developed by Yadav et al. to a set of
non-Gaussian temperature and polarization simulations. We compare our results
to theoretical expectations based on a Fisher matrix analysis, test the
unbiasedness of the estimator, and study the dependence of the error bars on
f_NL. All our results are in very good agreement with theoretical predictions,
thus confirming the reliability of both the simulation algorithm and the fast
cubic temperature and polarization estimator.Comment: 14 pages, 9 figures, revised version accepted by PRD, minor changes
and acknowledgements adde
Cosmological perturbations in a family of deformations of general relativity
We study linear cosmological perturbations in a previously introduced family
of deformations of general relativity characterized by the absence of new
degrees of freedom. The homogeneous and isotropic background in this class of
theories is unmodified and is described by the usual Friedmann equations. The
theory of cosmological perturbations is modified and the relevant deformation
parameter has the dimension of length. Gravitational perturbations of the
scalar type can be described by a certain relativistic potential related to the
matter perturbations just as in general relativity. A system of differential
equations describing the evolution of this potential and of the stress-energy
density perturbations is obtained. We find that the evolution of scalar
perturbations proceeds with a modified effective time-dependent speed of sound,
which, contrary to the case of general relativity, does not vanish even at the
matter-dominated stage. In a broad range of values of the length parameter
controlling the deformation, a specific transition from the regime of modified
gravity to the regime of general relativity in the evolution of scalar
perturbations takes place during the radiation domination. In this case, the
resulting power spectrum of perturbations in radiation and dark matter is
suppressed on the comoving spatial scales that enter the Hubble radius before
this transition. We estimate the bounds on the deformation parameter for which
this suppression does not lead to observable consequences. Evolution of scalar
perturbations at the inflationary stage is modified but very slightly and the
primordial spectrum generated during inflation is not noticeably different from
the one obtained in general relativity.Comment: 45 pages, version published in JCAP; minor changes, one section moved
to the appendi
The bar-mode instability in differentially rotating neutron stars: Simulations in full general relativity
We study the dynamical stability against bar-mode deformation of rapidly
spinning neutron stars with differential rotation. We perform fully
relativistic 3D simulations of compact stars with , where is
the total gravitational mass and the equatorial circumferential radius. We
adopt an adiabatic equation of state with adiabatic index . As in
Newtonian theory, we find that stars above a critical value of (where is the rotational kinetic energy and the gravitational
binding energy) are dynamically unstable to bar formation. For our adopted
choices of stellar compaction and rotation profile, the critical value of
is , only slightly smaller than the
well-known Newtonian value for incompressible Maclaurin spheroids.
The critical value depends only very weakly on the degree of differential
rotation for the moderate range we surveyed. All unstable stars form bars on a
dynamical timescale. Models with sufficiently large subsequently form
spiral arms and eject mass, driving the remnant to a dynamically stable state.
Models with moderately large do not develop spiral
arms or eject mass but adjust to form dynamically stable ellipsoidal-like
configurations. If the bar-mode instability is triggered in supernovae collapse
or binary neutron star mergers, it could be a strong and observable source of
gravitational waves. We determine characteristic wave amplitudes and
frequencies.Comment: 17 pages, accepted for publication in AP
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