30,812 research outputs found
Non-axisymmetric oscillations of rapidly rotating relativistic stars by conformal flatness approximation
We present a new numerical code to compute non-axisymmetric eigenmodes of
rapidly rotating relativistic stars by adopting spatially conformally flat
approximation of general relativity. The approximation suppresses the radiative
degree of freedom of relativistic gravity and the field equations are cast into
a set of elliptic equations. The code is tested against the low-order f- and
p-modes of slowly rotating stars for which a good agreement is observed in
frequencies computed by our new code and those computed by the full theory.
Entire sequences of the low order counter-rotating f-modes are computed, which
are susceptible to an instability driven by gravitational radiation.Comment: 3 figures. To appear in Phys.Rev.
Rossby-Haurwitz waves of a slowly and differentially rotating fluid shell
Recent studies have raised doubts about the occurrence of r modes in
Newtonian stars with a large degree of differential rotation. To assess the
validity of this conjecture we have solved the eigenvalue problem for
Rossby-Haurwitz waves (the analogues of r waves on a thin-shell) in the
presence of differential rotation. The results obtained indicate that the
eigenvalue problem is never singular and that, at least for the case of a
thin-shell, the analogues of r modes can be found for arbitrarily large degrees
of differential rotation. This work clarifies the puzzling results obtained in
calculations of differentially rotating axi-symmetric Newtonian stars.Comment: 8pages, 3figures. Submitted to CQ
Precise subtyping for synchronous multiparty sessions
The notion of subtyping has gained an important role both in theoretical and applicative domains: in lambda and concurrent calculi as well as in programming languages. The soundness and the completeness, together referred to as the preciseness of subtyping, can be considered from two different points of view: operational and denotational. The former preciseness has been recently developed with respect to type safety, i.e. the safe replacement of a term of a smaller type when a term of a bigger type is expected. The latter preciseness is based on the denotation of a type which is a mathematical object that describes the meaning of the type in accordance with the denotations of other expressions from the language. The result of this paper is the operational and denotational preciseness of the subtyping for a synchronous multiparty session calculus. The novelty of this paper is the introduction of characteristic global types to prove the operational completeness
A numerical study of the r-mode instability of rapidly rotating nascent neutron stars
The first results of numerical analysis of classical r-modes of {\it rapidly}
rotating compressible stellar models are reported. The full set of linear
perturbation equations of rotating stars in Newtonian gravity are numerically
solved without the slow rotation approximation. A critical curve of
gravitational wave emission induced instability which restricts the rotational
frequencies of hot young neutron stars is obtained. Taking the standard cooling
mechanisms of neutron stars into account, we also show the `evolutionary
curves' along which neutron stars are supposed to evolve as cooling and
spinning-down proceed. Rotational frequencies of stars suffering
from this instability decrease to around 100Hz when the standard cooling
mechanism of neutron stars is employed. This result confirms the results of
other authors who adopted the slow rotation approximation.Comment: 4 pages, 2 figures; MNRAS,316,L1(2000
Relativistic r-modes in Slowly Rotating Neutron Stars: Numerical Analysis in the Cowling Approximation
We investigate the properties of relativistic -modes of slowly rotating
neutron stars by using a relativistic version of the Cowling approximation. In
our formalism, we take into account the influence of the Coriolis like force on
the stellar oscillations, but ignore the effects of the centrifugal like force.
For three neutron star models, we calculated the fundamental -modes with
and 3. We found that the oscillation frequency of the
fundamental -mode is in a good approximation given by , where is defined in the corotating frame at the
spatial infinity, and is the angular frequency of rotation of the
star. The proportional coefficient is only weakly dependent on
, but it strongly depends on the relativistic parameter ,
where and are the mass and the radius of the star. All the fundamental
-modes with computed in this study are discrete modes with distinct
regular eigenfunctions, and they all fall in the continuous part of the
frequency spectrum associated with Kojima's equation (Kojima 1998). These
relativistic -modes are obtained by including the effects of rotation higher
than the first order of so that the buoyant force plays a role, the
situation of which is quite similar to that for the Newtonian -modes.Comment: 22 pages, 8 figures, accepted for publication in Ap
The R-Mode Oscillations in Relativistic Rotating Stars
The axial mode oscillations are examined for relativistic rotating stars with
uniform angular velocity. Using the slow rotation formalism and the Cowling
approximation, we have derived the equations governing the r-mode oscillations
up to the second order with respect to the rotation. In the lowest order, the
allowed range of the frequencies is determined, but corresponding spatial
function is arbitrary. The spatial function can be decomposed in non-barotropic
region by a set of functions associated with the differential equation of the
second-order corrections. The equation however becomes singular in barotropic
region, and a single function can be selected to describe the spatial
perturbation of the lowest order. The frame dragging effect among the
relativistic effects may be significant, as it results in rather broad spectrum
of the r-mode frequency unlike in the Newtonian first-order calculation.Comment: 19 pages, 4 figures, AAS LaTeX, Accepted for publication in The
Astrophysical Journa
V-V Bond-Length Fluctuations in Vox
We report a significantly stronger suppression of the phonon contribution to
the thermal conductivity in VOx than can be accounted for by disorder of the 16
% atomic vacancies present in VO. Since the transition from localized to
itinerant electronic behavior is first-order and has been shown to be
characterized by bond-length fluctuations in several transition-metal oxides
with the perovskite structure, we propose that cooperative V-V bond-length
fluctuations play a role in VO similar to the M-O bond-length fluctuations in
the perovskites. This model is able to account for the strong suppression of
the thermal conductivity, the existence of a pseudogap confirmed by
thermoelectric power, an anomalously large Debye-Waller factor, the temperature
dependence of the magnetic susceptibility, and the inability to order atomic
vacancies in VO.Comment: 5 pages, 5 figure
An Improved Method for Preparing Microvascular Corrosion Casts of Rat Embryos
This paper presents an improved method for preparing microvascular corrosion casts of rat embryos (day 16.5-21.5). Special attention was paid on the viscosity of the casting material, the method of mechanical restraint, and the subsequent drying of the casts. The embryos were perfused with Mercox resin diluted with 25% methyl methacrylate monomer followed by undiluted Mercox resin to avoid outflow of resin from the vasculature after perfusion. The specimens were mounted on a stainless steel plate with self-curing resin to prevent flotation, mechanical damage, and collapse of the cast specimens following preparation. After digestion and washing, the cast specimens were freeze dried to prevent deformation of the casts.
This specimen preparation resulted in much better scanning electron microscopic images of the embryonic microvasculature of various oral tissues, free from leakage and insufficient filling
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