2,021 research outputs found
Quasinormal modes of D-dimensional de Sitter spacetime
We calculate the exact values of the quasinormal frequencies for an
electromagnetic field and a gravitational perturbation moving in
-dimensional de Sitter spacetime (). We also study the quasinormal
modes of a real massive scalar field and we compare our results with those of
other references.Comment: 26 pages, 1 table. Some changes made according to referee's
suggestions. Matches published version in GR
Electromagnetic quasinormal modes of D-dimensional black holes II
By using the sixth order WKB approximation we calculate for an
electromagnetic field propagating in D-dimensional Schwarzschild and
Schwarzschild de Sitter black holes its quasinormal frequencies for the
fundamental mode and first overtones. We study the dependence of these QN
frequencies on the value of the cosmological constant and the spacetime
dimension. We also compare with the known results for the gravitational
perturbations propagating in the same background. Moreover we exactly compute
the QN frequencies of the electromagnetic field propagating in D-dimensional
massless topological black hole and for charged D-dimensional Nariai spacetime
we exactly calculate the QN frequencies of the coupled electromagnetic and
gravitational perturbations.Comment: 34 pages, 14 figures, 6 table
Converging shocks in elastic-plastic solids
We present an approximate description of the behavior of an elastic-plastic material processed by a cylindrically or spherically symmetric converging shock, following Whitham's shock dynamics theory. Originally applied with success to various gas dynamics problems, this theory is presently derived for solid media, in both elastic and plastic regimes. The exact solutions of the shock dynamics equations obtained reproduce well the results obtained by high-resolution numerical simulations. The examined constitutive laws share a compressible neo-Hookean structure for the internal energy e = e_(s)(I_1)+e_(h)(Ï,Ï), where e_(s) accounts for shear through the first invariant of the CauchyâGreen tensor, and e_(h) represents the hydrostatic contribution as a function of the density Ï and entropy Ï. In the strong-shock limit, reached as the shock approaches the axis or origin r=0, we show that compression effects are dominant over shear deformations. For an isothermal constitutive law, i.e., e_(h) = e_(h)(Ï), with a power-law dependence e_(h) â Ï_(α), shock dynamics predicts that for a converging shock located at r=R(t) at time t, the Mach number increases as M â [log(1/R)]^α, independently of the space index s, where s=2 in cylindrical geometry and 3 in spherical geometry. An alternative isothermal constitutive law with p(Ï) of the arctanh type, which enforces a finite density in the strong-shock limit, leads to M â R^(â(sâ1)) for strong shocks. A nonisothermal constitutive law, whose hydrostatic part eh is that of an ideal gas, is also tested, recovering the strong-shock limit MâR^(â(sâ1)/n(Îł)) originally derived by Whitham for perfect gases, where Îł is inherently related to the maximum compression ratio that the material can reach, (Îł+1)/(Îłâ1). From these strong-shock limits, we also estimate analytically the density, radial velocity, pressure, and sound speed immediately behind the shock. While the hydrostatic part of the energy essentially commands the strong-shock behavior, the shear modulus and yield stress modify the compression ratio and velocity of the shock far from the axis or origin. A characterization of the elastic-plastic transition in converging shocks, which involves an elastic precursor and a plastic compression region, is finally exposed
Quasinormal frequencies of the dimensionally reduced BTZ black hole
We calculate numerically the quasinormal frequencies of the Klein-Gordon and
Dirac fields moving in the two-dimensional dimensionally reduced BTZ black
hole. Our work extends results previously published on the damped oscillations
of this black hole. First, we compute the quasinormal frequencies of the
minimally coupled Klein-Gordon field for a range of the dimensionally reduced
BTZ black hole physical parameters that is not previously analyzed. Furthermore
we determine, for the first time, the quasinormal frequencies of the Dirac
field propagating in the dimensionally reduced BTZ black hole. For the Dirac
field we use the Horowitz-Hubeny method and the asymptotic iteration method,
while for the Klein-Gordon field the extension of the previous results is based
on the asymptotic iteration method. Finally we discuss our main results.Comment: 11 figures, 11 tables, 28 pages. Already published in General
Relativity and Gravitatio
On the quasinormal modes of the de Sitter spacetime
Modifying a method by Horowitz and Hubeny for asymptotically anti-de Sitter
black holes, we establish the classical stability of the quasinormal modes of
the de Sitter spacetime. Furthermore using a straightforward method we
calculate the de Sitter quasinormal frequencies of the gravitational
perturbations and discuss some properties of the radial functions of these
quasinormal modes.Comment: 11 pages, 4 figure
The molecular environment of the pillar-like features in the HII region G46.5-0.2
At the interface of HII regions and molecular gas peculiar structures appear,
some of them with pillar-like shapes. Understanding their origin is important
for characterizing triggered star formation and the impact of massive stars on
the interstellar medium. In order to study the molecular environment and the
influence of the radiation on two pillar-like features related to the HII
region G46.5-0.2, we performed molecular line observations with the Atacama
Submillimeter Telescope Experiment, and spectroscopic optical observations with
the Isaac Newton Telescope. From the optical observations we identified the
star that is exciting the HII region as a spectral type O4-6. The molecular
data allowed us to study the structure of the pillars and a HCO+ cloud lying
between them. In this HCO+ cloud, which have not any well defined 12CO
counterpart, we found direct evidence of star formation: two molecular outflows
and two associated near-IR nebulosities. The outflows axis orientation is
perpendicular to the direction of the radiation flow from the HII region.
Several Class I sources are also embedded in this HCO+ cloud, showing that it
is usual that the YSOs form large associations occupying a cavity bounded by
pillars. On the other hand, it was confirmed that the RDI process is not
occurring in one of the pillar tips.Comment: Accepted in MNRAS (2017 June 13
Osteonecrosis of the jaw in a patient under treatment of osteoporosis with oral bisphosphonate
Although uncommon in patients under oral therapy, bisphosphonate-related osteonecrosis of the jaw (BRONJ) can be a very severe issue. Early intervention with surgical resection should be the preferable method of treating any stage of the disease, resulting in better outcomes and decreasing the morbidity of this condition. A 77-year-old female patient attended the Special Care Dentistry Centre of the University of SĂŁo Paulo Faculty of Dentistry (CAPE FOUSP) complaining mainly of âan exposed bone that appeared after tooth extraction performed six months earlierâ. The patient was diagnosed with osteonecrosis associated with bisphosphonate (sodium ibandronate) and surgically treated with removal of bone sequestration and antibiotic therapy. The patient was followed up for six years (a total of 6 appointments), presenting good general health and no sign of bone exposure. Imaging findings showed no changes related to BRONJ either
Numerical simulations of the Richtmyer-Meshkov instability in solid-vacuum interfaces using calibrated plasticity laws
The Richtmyer-Meshkov instability of interfaces separating elastic-plastic materials from vacuum (heavy-light configuration) is studied by means of computational techniques. A fully Eulerian multimaterial algorithm that solves consistently the Euler equations and the time evolution of the deformations in the material is applied to three distinct materials (copper, aluminum, and stainless steel). If a perfectly plastic constitutive relation is considered, an empirical law is computed that relates the long-term perturbation amplitude of the interface, its maximum growth rate, the initial density, and the yield stress of the material. It is shown that this linear relation can be extended to materials that follow more complex plastic behavior which can account for rate dependency, hardening, and thermal softening, and to situations in which small-perturbation theory is no longer valid. In effect, the yield stress computed from measurements of the long-term amplitude and maximum growth rate closely matches the von Mises stress found at the interface of solid materials for a wide range of cases with different initial parameters
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