7,561 research outputs found
Boundary migration and disappearance of voids in Alpha-Al2O3 at 2000 deg C
A series of photographs taken with Osaka University's high temperature 3MV electron microscope of alpha-A1(z)O(3) at 2000 C is presented. The dynamic study shows grain boundary migration in progress and demonstrates that disappearance of voids is controlled by boundary migration
Measuring angular diameter distances of strong gravitational lenses
The distance-redshift relation plays a fundamental role in constraining
cosmological models. In this paper, we show that measurements of positions and
time delays of strongly lensed images of a background galaxy, as well as those
of the velocity dispersion and mass profile of a lens galaxy, can be combined
to extract the angular diameter distance of the lens galaxy. Physically, as the
velocity dispersion and the time delay give a gravitational potential ()
and a mass () of the lens, respectively, dividing them gives a physical
size () of the lens. Comparing the physical size with the image positions of
a lensed galaxy gives the angular diameter distance to the lens. A mismatch
between the exact locations at which these measurements are made can be
corrected by measuring a local slope of the mass profile. We expand on the
original idea put forward by Paraficz and Hjorth, who analyzed singular
isothermal lenses, by allowing for an arbitrary slope of a power-law spherical
mass density profile, an external convergence, and an anisotropic velocity
dispersion. We find that the effect of external convergence cancels out when
dividing the time delays and velocity dispersion measurements. We derive a
formula for the uncertainty in the angular diameter distance in terms of the
uncertainties in the observables. As an application, we use two existing strong
lens systems, B1608+656 () and RXJ11311231 (), to show that the uncertainty in the inferred angular diameter
distances is dominated by that in the velocity dispersion, , and its
anisotropy. We find that the current data on these systems should yield about
16% uncertainty in per object. This improves to 13% when we measure
at the so-called sweet-spot radius. Achieving 7% is possible if we
can determine with 5% precision.Comment: Accepted to JCA
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
Kompaneets equation for neutrinos: Application to neutrino heating in supernova explosions
We derive a `Kompaneets equation' for neutrinos, which describes how the
distribution function of neutrinos interacting with matter deviates from a
Fermi-Dirac distribution with zero chemical potential. To this end, we expand
the collision integral in the Boltzmann equation of neutrinos up to the second
order in energy transfer between matter and neutrinos. The distortion of the
neutrino distribution function changes the rate at which neutrinos heat matter,
as the rate is proportional to the mean square energy of neutrinos, .
For electron-type neutrinos the enhancement in over its thermal value
is given approximately by
where is the bulk velocity of nucleons, while for the other neutrino
species the enhancement is , where is the
kinetic energy of nucleons divided by the thermal energy. This enhancement has
a significant implication for supernova explosions, as it would aid
neutrino-driven explosions.Comment: 14 pages, 1 figure, matched to published versio
A General Relativistic study of the neutrino path and calculation of minimum photosphere for different stars
A detailed general relativistic (GR) calculation of the neutrino path for a
general metric describing a rotating star is studied. We have calculated the
neutrino path along a plane, with the consideration that the neutrino does not
at any time leave the plane. The expression for the minimum photosphere radius
(MPR) is obtained and matched with the Schwarzschild limit. The MPR is
calculated for the stars with two different equations of state (EOS) each
rotating with two different velocities. The results shows that the MPR for the
hadronic star is much greater than the quark star and the MPR increases as the
rotational velocity of the star decreases. The MPR along the polar plane is
larger than that along the equatorial plane.Comment: 13 pages, 5 figures and 1 tabl
Time-delay Cosmography: Increased Leverage with Angular Diameter Distances
Strong lensing time-delay systems constrain cosmological parameters via the
so-called time-delay distance and the angular diameter distance to the lens. In
previous studies, only the former information was used. In this paper, we show
that the cosmological constraints improve significantly when the latter
information is also included. Specifically, the angular diameter distance plays
a crucial role in breaking the degeneracy between the curvature of the Universe
and the time-varying equation of state of dark energy. Using a mock sample of
55 bright quadruple lens systems based on expectations for ongoing/future
imaging surveys, we find that adding the angular diameter distance information
to the time-delay distance information and the cosmic microwave background data
of Planck improves the constraint on the constant equation of state by 30%, on
the time variation in the equation of state by a factor of two, and on the
Hubble constant in the flat CDM model by a factor of two. Therefore,
previous forecasts for the statistical power of time-delay systems were
significantly underestimated, i.e., time-delay systems are more powerful than
previously appreciated.Comment: [v2] 18 pages, 12 figures, submitted to JCAP. An error in the fisher
matrix for SNIa fixed; conclusions unchange
Revising the multipole moments of numerical spacetimes, and its consequences
Identifying the relativistic multipole moments of a spacetime of an
astrophysical object that has been constructed numerically is of major
interest, both because the multipole moments are intimately related to the
internal structure of the object, and because the construction of a suitable
analytic metric that mimics a numerical metric should be based on the multipole
moments of the latter one, in order to yield a reliable representation. In this
note we show that there has been a widespread delusion in the way the multipole
moments of a numerical metric are read from the asymptotic expansion of the
metric functions. We show how one should read correctly the first few multipole
moments (starting from the quadrupole mass-moment), and how these corrected
moments improve the efficiency of describing the metric functions with analytic
metrics that have already been used in the literature, as well as other
consequences of using the correct moments.Comment: article + supplemental materia
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
Improved Method for Detecting Local Discontinuities in CMB data by Finite Differencing
An unexpected distribution of temperatures in the CMB could be a sign of new
physics. In particular, the existence of cosmic defects could be indicated by
temperature discontinuities via the Kaiser-Stebbins effect. In this paper, we
show how performing finite differences on a CMB map, with the noise regularized
in harmonic space, may expose such discontinuities, and we report the results
of this process on the 7-year Wilkinson Microwave Anisotropy Probe data.Comment: 5 pages, 6 figures; Text has been edited, in line with the PRD
articl
Relativistic stars in differential rotation: bounds on the dragging rate and on the rotational energy
For general relativistic equilibrium stellar models (stationary axisymmetric
asymptotically flat and convection-free) with differential rotation, it is
shown that for a wide class of rotation laws the distribution of angular
velocity of the fluid has a sign, say "positive", and then both the dragging
rate and the angular momentum density are positive. In addition, the "mean
value" (with respect to an intrinsic density) of the dragging rate is shown to
be less than the mean value of the fluid angular velocity (in full general,
without having to restrict the rotation law, nor the uniformity in sign of the
fluid angular velocity); this inequality yields the positivity and an upper
bound of the total rotational energy.Comment: 23 pages, no figures, LaTeX. Submitted to J. Math. Phy
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