共查询到20条相似文献，搜索用时 31 毫秒
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The scalar field theory on the background of cosmological models with n(n ≥ 1) spaces of constant curvature is considered. We take the integrable case of Ricci flat internal spaces. The coupling between the scalar and the gravitational fields includes the minimal coupling as well as the conformal case. In the ground state of the scalar field we find the conditions for vacuum instability realized for most of the possible solutions to Einstein's equations if the coupling parameter takes appropriate values. For the excited states of the scalar field we show the induction of massive modes and discuss their properties. 相似文献
3.
A study is made of axisymmetric, low sonic-Mach-number flows of a viscous fluid with angular momentum outside of a black-hole. The viscosity is an eddy viscosity due to turbulence in the sheared flows. Self-similar solutions arise naturally, reducing the Navier-Stokes equations to a set of nonlinear ordinary differential equations. These equations are solved analytically for flows of constant specific angular momentum and numerically for more general flows. For flows with non-constant specific angular momentum, the momentum flux density includes a planar discontinuity which is interpreted as an accretion disc. In general, two flow regions appear on each side of the disk, corresponding to accretion onto the disk and jet-like outflows along the ±z-axes. Physical interpretations of the solutions show that these flows arise in response to point sources of axial momentum at the origin directed in the ±z-directions. The power needed to maintain this momentum input is assumed to come from the mass accretion onto the black hole.The hydrodynamic flows are generalized to include a magnetic field. In the limit of infinite electrical conductivity, the possible types of flow patterns are the same as in hydrodynamic case. The magnetic field alters the relative amounts of reversible and irreversible momentum and angular momentum transport by the flow. For a flow with turbulent viscosity, the magnetic field acts to reduce the level of the turbulence and the effective value of the eddy viscosity. 相似文献
4.
The observed association of Long Gamma-Ray Bursts (LGRBs) with peculiar Type Ic supernovae gives support to Woosley‘s collapsar/hypernova
model, in which the GRB is produced by the collapse of the rapidly rotating core of a massive star to a black hole. The association
of LGRBs with small star-forming galaxies suggests low-metallicity to be a condition for a massive star to evolve to the collapsar
stage. Both completely-mixed single star models and binary star models are possible. In binary models the progenitor of the
GRB is a massive helium star with a close companion. We find that tidal synchronization during core-helium burning is reached
on a short timescale (less than a few millennia). However, the strong core-envelope coupling in the subsequent evolutionary
stages is likely to rule out helium stars with main-sequence companions as progenitors of hypernovae/GRBs. On the other hand,
helium stars in close binaries with a neutron-star or black-hole companion can, despite the strong core-envelope coupling
in the post-helium burning phase, retain sufficient core angular momentum to produce a hypernova/GRB. 相似文献
5.
Cristian R. Ghezzi 《Astrophysics and Space Science》2011,333(2):437-447
A model of compact object coupled to inhomogeneous anisotropic dark energy is studied. It is assumed a variable dark energy
that suffers a phase transition at a critical density. The anisotropic Λ-Tolman-Oppenheimer-Volkoff equations are integrated
to know the structure of these objects. The anisotropy is concentrated on a thin shell where the phase transition takes place,
while the rest of the star remains isotropic. The family of solutions obtained depends on the coupling parameter between the
dark energy and the fermionic matter. The solutions share several features in common with the gravastar model. There is a
critical coupling parameter that gives non-singular black hole solutions. The mass-radius relations are studied as well as
the internal structure of the compact objects. The hydrodynamic stability of the models is analyzed using a standard test
from the mass-radius relation. For each permissible value of the coupling parameter there is a maximum mass, so the existence
of black holes is unavoidable within this model. 相似文献
6.
P. Casella G. Ponti A. Patruno T. Belloni G. Miniutti L. Zampieri 《Monthly notices of the Royal Astronomical Society》2008,387(4):1707-1711
We describe a new method to estimate the mass of black holes in Ultraluminous X-ray Sources (ULXs). The method is based on the recently discovered 'variability plane', populated by Galactic stellar-mass black-hole candidates (BHCs) and supermassive active galactic nuclei (AGNs), in the parameter space defined by the black-hole mass, accretion rate and characteristic frequency. We apply this method to the two ULXs from which low-frequency quasi-periodic oscillations have been discovered, M82 X-1 and NGC 5408 X-1. For both sources we obtain a black-hole mass in the range 100–1300 M⊙ , thus providing evidence for these two sources to host an intermediate-mass black hole. 相似文献
7.
By introducing an asymmetry between the two hemispheres, we study whether the solar dynamo solutions in the two hemispheres
remain coupled with each other. Our calculations are based on the solar dynamo code SURYA, which incorporates the helioseismically-determined
solar-rotation profile, a Babcock–Leighton α effect concentrated near the surface, and a meridional circulation. When the magnetic coupling between the hemispheres is
enhanced by either increasing the diffusion or introducing an α effect distributed throughout the convection zone, we find that the solutions in the two hemispheres evolve together with
a single period even when we make the meridional circulation or the α effect different in the two hemispheres. On the other hand, when the hemispheric coupling is weaker for other values of parameters,
an asymmetry between the hemispheres can make solutions in the two hemispheres evolve independently with different periods. 相似文献
8.
The exact closed-form mathematical solution for Schwarzschild black-hole fed by the Cosmic Micro-wave Background Radiance (CMBR) is obtained analytically. This unique solution holds for both growth (evolution) and decay, which are bifurcated by the critical initial mass, which depends on the temperature of CMBR. Results are presented to illustrate this solution in terms of decay/growth of black-hole, and the variation of rate of growth/decay with CMBR. 相似文献
9.
A. P. Trofimenko 《Astrophysics and Space Science》1991,180(2):185-200
The phenomenon of pulsars is considered as the evidence for existence of black holes in neutron and quark stars. Within the framework of the degenerated star model with black-hole interior the existence of millisecond pulsars withP<0.5 ms and single pulsars with negative derivative of the period were predicted. The anisotropic accretion of neutron (or quark) star matter on to a rotating black hole leads to the formation of directed radiation (projector), which makes heat spots at surface (volcanos), that explains the nature of pulsating radiation and the complicated structure of impulses. This model gives both the mechanism of self-acceleration of degenerated star rotation (mass accretion on to the internal black hole) producing millisecond pulsars and also the mechanism of significant deceleration of rotation (ejection of neutral mass through a volcanic crater), leading to long-periodic X-ray pulsars. The black hole produces high densities and temperatures of the degenerated star mass that transforms gradually the neutron star into quark star (Cygnus X-3). 相似文献
10.
S. V. Vorontsov 《Monthly notices of the Royal Astronomical Society》2007,378(4):1499-1506
Accurate measurements of solar p-mode frequencies and frequency splittings at high degree l require an adequate theoretical knowledge of the effects of mode coupling, induced by the variation with latitude of the angular velocity of the solar internal rotation. Earlier results for expansion coefficients of composite solutions (coupling coefficients) are due to Woodard. In this paper, the analysis is extended to allow for the dependence of the differential rotation on depth, and the result is expressed in terms of measurable quantities (the rotational splitting coefficients), which makes it convenient for diagnostic purposes. The analysis is based on the approach of quasi-degenerate perturbation theory, and is extended further to address possible effects of mode coupling in the observational line profiles. It is shown, using approximations applicable at high degree l , that the expected line profiles of composite modes in the observational power spectra are not distorted by mode coupling. 相似文献
11.
None of the tentative black-hole identifications has remained without serious difficulties. As a class, the black-hole candidates
differ in no obvious property from neutron-star binaries other than in their estimated high mass. We favour an interpretation
of the black-hole candidates as binary neutron stars surrounded by a more or less massive accretion disc 相似文献
12.
We have calculated the spectrum of Cyg X-1 under the assumption that the radiation orginates in a disk around a 11M
black-hole. Supersonic turbulence prevails in the outer parts of the disk and electron-electron bremsstrahlung appears to be resonsible for the maintenance of the temperature at a level less than 1010 K near the inner edge of the disk. The theoretical spectrum gives the best fit with the observations if the Reynolds number is about 1200. 相似文献
13.
The recent detection of very-high-energy (GeV – TeV) γ-ray emission from the Galactic black-hole candidate and microquasar LS 5039 has sparked renewed interest in jet models for
the high-energy emission in those objects. In this work, we have focused on models in which the high-energy emission results
from synchrotron and Compton emission by relativistic electrons in the jet (leptonic jet models). Particular attention has
been paid to a possible orbital modulation of the high-energy emission due to azimuthal asymmetries caused by the presence
of the companion star. Both orbital-phase dependentγγ absorption and Compton scattering of optical/UV photons from the companion star may lead to an orbital modulation of the
gamma-ray emission. We make specific predictions which should be testable with refined data from HESS and the upcoming GLAST
mission. 相似文献
14.
A general-purpose melon-seed-overpressure-expansion (MSOE) model is used to examine the dependence of the expansion and propagation
of fast CMEs on average initial values of the magnetic field inside the CME, the magnetic field outside the CME, and the CME
mass. The MSOE model extracts from the real situation only features that are essential in determining the expansion and propagation
processes and idealizes them to obtain an adaptable “minimalist” set of equations that governs these processes yet retains
enough relevant physics to give useful results. This minimalist set is used to carry out a systematic comparison of solutions
against four sets of observed correlations between CME and ICME parameters in which the input parameters to the MSOE model
are varied to achieve simultaneous fits to all four correlations. The fits impose relations between the three independent
input variables: initial internal field strength, initial external field strength, and initial density. For example, light
CMEs (e.g., those with no prominences) require the external field strength to nearly equal the internal field strength. However, heavy
CMEs (e.g., those with prominences) require the external field strength to be much weaker than the internal field strength. 相似文献
15.
本文用广义相对论讨论了黑洞吸积盘内边缘半径ｒ（ｍｓ）的演化规律。结果表明，吸积盘的中心黑洞在由Ｓｃｈｗａｒｚｓｃｈｉｌｄ型向极端Ｋｅｒｒ型演化的过程中，黑洞的角动量变化对ｒ（ｍｓ）的影响始终比黑洞的质量变化对ｒ（ｍｓ）的影响大。在此过程中ｒ（ｍｓ）始终是连续、单调减小的。本文得出ｒ（ｍｓ）对时间变化率的取值范围，并对其物理意义作了讨论。 相似文献
16.
Robert J. Stéfant 《Astrophysics and Space Science》1981,76(2):301-328
The linear coupling between the different kinds of waves propagating in a warm plasma inhomogeneous along thex direction is investigated in order to locate the regions (,k) space where two of the roots of the characteristic equation coalesce. Firstly, using the approximation of geometrical optics the differential equation is derived and wave propagation at fixed wave numberk
z
is studied in these special cases for which the characteristic equation reduces to a biquadratic. When the density gradient is parallel to the magnetic field, a detailed analysis of the different properties of the waves shows that the mechanism proposed by Gurnett and others to explain the characteristics of the proton whistler is unlikely to operate, even if a wave coupling occurs at the so called cross over frequency for small incidence angles. The only relevant process occurs when the density gradient is perpendicular to the magnetic field for waves propagating at small incidence angles. It is shown that, close to a coalescence point, but within the limit of the geometrical optics approximation, one of the WKB solutions is a mixed (transverse-longitudinal) mode which becomes purely longitudinal in the limit of large wave numbers. Consequently, as this wave has E almost parallel tok, coalescence implies that the waves are nearly longitudinal at the singular point, in agreement with other results. Next, application of the theory is made to some relevant space observations. It is shown that the proton whistler could be the result of a linear coupling between the extraordinary and the slow ion cyclotron waves close to the Buchsbaum resonance in ionospheric regions above 300 to 400 km where the H+ density begins to grow. Transformation conditions are given which favour the coupling mechanism in regions of strong latitudinal gradients. Finally, a comparison is made with experiment which confirms the principal features of the present theory. 相似文献
17.
The equations determining cosmic nucleosynthesis up to4He are modified to take account of black-hole evaporation during nucleosynthesis. Numerical calculation shows that deuterium is enhanced by the presence of these micro-mini black holes (mBH); but very high-density universes are difficult to reconcile with observations, even with a very large abundance of mBH. 相似文献
18.
Peter Höflich 《Astrophysics and Space Science》2005,298(1-2):87-92
We discuss the current status of our hydrodynamical radiation (HYDRA) code for rapidly expanding, low-density envelopes commonly
found in core collapse and thermonuclear supernovae. In supernovae, one of the main issues is the coupling between a radiation
field and properties of the matter. Due to the low densities, nonthermal excitation by high-energy photons from radioactive
decays and the time dependence of the problem, significant departures from local thermodynamical equilibrium (LTE) are common
throughout the envelope even at large optical depths. This effect must be taken into account to simulate the evolution of
spectra and light curves which are the basic tools to link between explosion physics and observations.
The large velocity fields and the non-LTE problem result in a coupling of spatial, frequency space and the level population.
This physical system can be described by a large system of coupled integro-differential equations for which the spatial and
energy discretization (and its errors) are coupled. For the numerical solution, we use variable separation, analytic solutions
and approximations, and iterative schemes. The need for adaptive mesh refinement (AMR) is demonstrated. As example, we show
detailed spectra and light curves for the thermonuclear Supernova SN99by. 相似文献
19.
N. Ibotombi Singh S. Surendra Singh S. Romaleima Devi A. Sumati Devi 《Astrophysics and Space Science》2009,324(1):67-70
We consider cosmology with the gravitational and cosmological constants generalized as coupling scalars in Einstein’s theory.
A general method of solving the field equations is given. We study here the exact solutions for negative pressure models satisfying
G=G
0(R/R
0)
n
. 相似文献
20.
A simple idealized nonlinear model applicable to long period variable stars has been formulated that assumes the convective envelope ofM giants is composed of giant convection cells, which are comparable in size to the stellar radius. The simplicity of this model essentially constitutes a physical analog to the strong dynamic coupling that occurs if the convective envelope of the star undergoes both modes of motion. As shown implicitly in the time scales associated with these motions, the coupling produces asymmetrical fluctuations of the entire star, the mean velocity of which is comparable to the escape velocity of the star at particular values of the ratio of the pulsation and convection time scales. It is suggested that this can account for the mass loss from late type stars, and the circumstellar dust shells that are associated extensively with long period variables.For critical values of the pulsation and convection time scales, the solutions correspond to the rapid expansion of the entire convective envelope, and is the basis of a new mechanism that simulates the manner in which pulsating stars ballistically accelerate their convective shells to form planetary nebulae. 相似文献