共查询到20条相似文献，搜索用时 234 毫秒
1.
Within the framework of Einstein-Cartan theory with Weyssenhoff fluid, we investigate an inflation model for an anisotropic Bianchi type-IX cosmological model. The system of field equations is solved numerically and an inflation epoch is achieved. The general condition for the inflation to occur is also discussed. This anisotropic model evolves quickly towards to an isotropic one. 相似文献
2.
John E. Gizis David G. Monet I. Neill Reid J. Davy Kirkpatrick Adam J. Burgasser 《Monthly notices of the Royal Astronomical Society》2000,311(2):385-388
We report the discovery of two binary M dwarf systems in the immediate solar neighbourhood using the Two Micron All Sky Survey (2MASS). The first is an M6.5 companion to the nearby G star HD 86728 (Gl 376). The known properties of HD 86728 indicate that the M dwarf (Gl 376B) is old, metal-rich and only 14.9 parsec away. The M dwarf is highly active, with both H α and X-ray emission. Thus, Gl 376B offers the opportunity to study an old, bright, active M dwarf with known metallicity, age and luminosity. We show that it is probable that Gl 376B is itself an unresolved pair. The other system consists of an M6.5 and an M8 dwarf with 14.5 arcsec separation. We estimate a distance of ∼16 parsec for this very low-mass pair. Stronger activity is observed in the M6.5 dwarf, supporting evidence that chromospheric activity is weakening near the hydrogen-burning limit. 相似文献
3.
Marcello Romano 《Celestial Mechanics and Dynamical Astronomy》2008,100(3):181-189
The exact analytic solution is introduced for the rotational motion of a rigid body having three equal principal moments of
inertia and subjected to an external torque vector which is constant for an observer fixed with the body, and to arbitrary
initial angular velocity. In the paper a parametrization of the rotation by three complex numbers is used. In particular,
the rows of the rotation matrix are seen as elements of the unit sphere and projected, by stereographic projection, onto points
on the complex plane. In this representation, the kinematic differential equation reduces to an equation of Riccati type,
which is solved through appropriate choices of substitutions, thereby yielding an analytic solution in terms of confluent
hypergeometric functions. The rotation matrix is recovered from the three complex rotation variables by inverse stereographic
map. The results of a numerical experiment confirming the exactness of the analytic solution are reported. The newly found
analytic solution is valid for any motion time length and rotation amplitude. The present paper adds a further element to
the small set of special cases for which an exact solution of the rotational motion of a rigid body exists. 相似文献
4.
Shinpei Shibata 《Astrophysics and Space Science》1989,161(1):145-158
A numerical method to determine the electromagnetic field of a steadily rotating magnetosphere with an inclined magnetic moment under a given boundary condition on an arbitrary shaped boundary surface is presented. The region may include the light cylinder. The present method, together with a companion method giving particle motion and creation, makes an iterative scheme to obtain a global model of the pulsar magnetosphere. A key problem for explaining the particle acceleration in pulsars is to solve field-aligned electric field in an accelerating region bounded by an ideal-MHD region. The present method is fit to connect a solution for the non-ideal-MHD region with another solution for the ideal-MHD region on a boundary surface whose location should also be solved (i.e., a floating boundary). The integration scheme is based on the boundary element method and it has great advantage as compared with other methods like the finite difference method and the Fourier transformation method. 相似文献
5.
The equilibrium shape of a slender flux tube in the stratified solar atmosphere is studied. The path is determined by a balance between the downwards magnetic tension, which depends on the curvature of the loop, and the upwards buoyancy force. Previous results for untwisted slender tubes are extended to include twisted tubes embedded in an external magnetic field.The path of an untwisted tube in an atmosphere with an ambient magnetic field is calculated. For a given footpoint separation, the height of the tube is lowered by increasing the strength of the external magnetic field. If the footpoints are slowly moved apart, the tube rises, until a threshold separation is reached beyond which there is no possible equilibrium height. This threshold width does not depend on the strength of the external field.The effects of twisting up a curved loop are studied, using an extension of results obtained for slender curved tubes with a straight axis. It is shown that for a twisted tube of given width, there can be two possible values of the equilibrium height. If, however, the tube is twisted more than a certain amount or if the footpoints are too widely separated there is no equilibrium. The critical footpoint separation for non-equilibrium is smaller for a twisted tube that an untwisted one.Twisting a tube or moving its feet apart is thus likely to result in non-equilibrium, causing the tube to rise indefinitely under the influence of the unbalanced buoyant force. It is suggested that this phenomenon could be important in the preflare stage of a large two-ribbon solar flare, by causing the initial slow rise of an active region filament. As well as being involved in the onset of an erupting prominence, this non-equilibrium may also be relevant to the formation of coronal loop transients. 相似文献
6.
C. C. Kuranz R. P. Drake M. J. Grosskopf H. F. Robey B. A. Remington J. F. Hansen B. E. Blue J. Knauer 《Astrophysics and Space Science》2009,322(1-4):49-55
This paper discusses high-energy-density laboratory astrophysics experiments exploring the Rayleigh-Taylor instability under conditions similar to the blast wave driven, outermost layer in a core-collapse supernova. The planar blast wave is created in an experimental target using the Omega laser. The blast wave crosses an unstable interface with a seed perturbation machined onto it. The perturbation consists of a 3D “egg crate” pattern and, in some cases, an additional longer wavelength mode is added to this 3D, single-mode pattern. The main diagnostic of this experiment is x-ray radiography. This paper explores an image processing technique to improve the identification and characterization of structure in the radiographic data. 相似文献
7.
Madeleine Pascal 《Celestial Mechanics and Dynamical Astronomy》1987,41(1-4):253-274
The subject of this work is the dynamics of flexible space vehicles modelled by a chain of rigid and elastic bodies with tree structure. The aim of this investigation is to obtained an impedance matrix giving in frequency domain the response of the structure to external forces. We show that it is possible to obtain an expansion of this impedance matrix in terms of an infinite set of modal frequencies termed constrained modes and an expansion of the inverse of this impedance matrix in terms of an other set of modes. The work is a generalization of the theory made by Hugues [3] and others for systems with star structure. 相似文献
8.
P. Anagnostopoulos G. A. Georgantopoulos C. Filandros V. Ifantis 《Astrophysics and Space Science》1988,147(2):287-293
The effects of heat and mass transfer on the flow field of a laminar boundary layer is considered. The flow is that of an incompressible viscous fluid past an impulsively started permeable vertical limiting surface with constant heat flux. The solution of the problem was obtained numerically, using an implicit finite difference scheme. The solution is given in a number of diagrams, which depict the influence ofG on velocity,P on temperature and Sc on concentration. The influence ofG on skin friction is also given. 相似文献
9.
J. J. Podesta 《Solar physics》2005,232(1-2):1-23
The solar convection zone is modeled as a horizontally stratified atmosphere with a constant gravitational field and an adiabatic
temperature gradient (a neutrally stratified polytrope). At equilibrium, the gas pressure and density decreases to zero at
the solar surface so that the solar surface is treated as a free surface which is bounded by vacuum. The evolution of small
amplitude perturbations about the equilibrium state is described by the linearized Euler equations for an inviscid compressible
fluid with an adiabatic equation of state. A sunquake is initiated at time zero by means of an initial perturbation with a
Gaussian velocity profile and the exact solution of the initial value problem is obtained in terms of a Fourier integral.
Comparisons between theory and observations indicate that this highly simplified model is able to predict the propagation
of sunquake waves across the solar surface with an error of approximately 10% or 20%. 相似文献
10.
The three-dimensional current system over an enhanced conductivity strip identified with an auroral arc is calculated for the case of the magnetospheric plasma convection across this strip. The strip produces a stationary Alfvén wave which propagates along magnetic field lines and is carried simultaneously by the convecting plasma. The Alfvén wave generation corresponds to an appearance of field-aligned currents over the arc. The three-dimensional current system generated over the arc is studied, taking into account reflection of the waves from the ionosphere of the opposite hemisphere. The correspondence of the theory with the experimental results is found. 相似文献
11.
R. Chan M. F. A. da Silva Jaime F. Villas?da?Rocha 《Astrophysics and Space Science》2012,337(1):185-191
Using the conventional gravastar model, that is, an object constituted by two components where one of them is a massive infinitely
thin shell and the other one is a de Sitter interior spacetime, we physically interpret a solution characterized by a zero
Schwarzschild mass. No stable gravastar is formed and it collapses without forming an event horizon, originating what we call
a massive non-gravitational object. The most surprise here is that the collapse occurs with an exterior de Sitter vacuum spacetime.
This creates an object which does not interact gravitationally with an outside test particle and it may evolve to a point-like
topological defect. 相似文献
12.
The problem of a one-dimensional, nonlinear, circularly polarized wave in a collisionless plasma is solved for the case in which the group velocity of the localized wave is zero relative to the plasma frame.For an electron-positron plasma the ponderomotive potential is repulsive, and both small and large amplitude waves in the presence of reflected and transmitted particles is discussed.The inclusion of an external magnetic field such that the cyclotron frequency is smaller than the wave frequency does not alter qualitatively the results of the fieldless case, except for the fact that an electrostatic potential is generated.For the case in which the cyclotron frequency is larger than the wave frequency, the resulting potential is attractive, and it is shown that no localized waves can coexist with confined and transmitted particles. If, however, an ionic component is presnt in the electron-positron plasma, such as may be the case in pulsar magnetospheres, localized waves may again be possible. 相似文献
13.
Vladimir Martinusi Lamberto Dell’Elce Gaëtan Kerschen 《Celestial Mechanics and Dynamical Astronomy》2017,127(4):451-476
The paper offers the fully analytic solution to the motion of a satellite orbiting under the influence of the two major perturbations, due to the oblateness and the atmospheric drag. The solution is presented in a time-explicit form, and takes into account an exponential distribution of the atmospheric density, an assumption that is reasonably close to reality. The approach involves two essential steps. The first one concerns a new approximate mathematical model that admits a closed-form solution with respect to a set of new variables. The second step is the determination of an infinitesimal contact transformation that allows to navigate between the new and the original variables. This contact transformation is obtained in exact form, and afterwards a Taylor series approximation is proposed in order to make all the computations explicit. The aforementioned transformation accommodates both perturbations, improving the accuracy of the orbit predictions by one order of magnitude with respect to the case when the atmospheric drag is absent from the transformation. Numerical simulations are performed for a low Earth orbit starting at an altitude of 350 km, and they show that the incorporation of drag terms into the contact transformation generates an error reduction by a factor of 7 in the position vector. The proposed method aims at improving the accuracy of analytic orbit propagation and transforming it into a viable alternative to the computationally intensive numerical methods. 相似文献
14.
The influence of the internal heat generation and radiation absorption in the free-convection flow of an incompressible viscous fluid past an impulsively started infinite vertical porous plate is considered. The set of the governing equations is solved by incorporating a finite-difference, algorithm. The influence of the vairous parameters for two fluids (air and water) is discussed with the aid of graphs. 相似文献
15.
The free-convection flow of an incompressible viscous fluid past an infinite, vertical plate for impulsive as well as uniformly accelerated motion of the plate is discussed when the plate temperatuve varies as the square root of time. The Laplace transform technique is used to obtain the expressions for velocity and skin-friction. The influence of the various parameters, entering into problem, on the velocity field and skin-friction is extensively discussed. 相似文献
16.
George H. Born 《Celestial Mechanics and Dynamical Astronomy》1970,2(1):103-113
In this paper, a combination analytical-numerical integration method for solving the differential equations of a modified set of Lagrange's planetary equations is described. The integration method is an Encke-type method because it involves integrating the deviations between the actual trajectory and a reference trajectory. The reference trajectory is obtained from an analytical solution containing the dominant secular and periodic effects of the gravitational field of the primary body. A set of nonsingular elements is used so that the method will be valid for all circular and elliptical motions. It is shown that the method is an accurate and efficient means of satellite ephemeris generation.This paper was presented at the AIAA/AAS Meeting, Princeton University, August 1969. 相似文献
17.
The smallness parameter of the approximation method is defined in terms of the non-dimensional initial distance between target and chaser satellite. In the case of a circular target orbit, compact analytical expressions are obtained for the interception travel time up to third order. For eccentric target orbits, an explicit result is worked out to first order, and the tools are prepared for numerical evaluation of higher order contributions. The possible transfer orbits are examined within Lambert’s theorem. For an eventual rendezvous it is assumed that the directions of the angular momenta of the two orbits enclose an acute angle. This assumption, together with the property that the travel time should vanish with vanishing initial distance, leads to a condition on the admissible initial positions of the chaser satellite. The condition is worked out explicitly in the general case of an eccentric target orbit and a non-coplanar transfer orbit. The condition is local. However, since during a rendezvous maneuver, the chaser eventually passes through the local space, the condition propagates to non-local initial distances. As to quantitative accuracy, the third order approximation reproduces the elements of Mars, in the historical problem treated by Gauss, to seven decimals accuracy, and in the case of the International Space Station, the method predicts an encounter error of about 12 m for an initial distance of 70 km. 相似文献
18.
M. R. E. Proctor 《Astronomische Nachrichten》1977,298(1):19-25
The kinematic α-effect dynamo problem is investigated in the case of an exterior perfect conductor. It is shown that certain approximate symmetries discovered in the numerical analysis of ROBERTS (1972) are exact for this case. As an illustration, an exact solution is given in a cylindrical geometry, where the equations can be written in terms of one variable. The implications for the earth's dynamo are discussed. 相似文献
19.
The secular effect of YORP torque on the rotational dynamics of an asteroid in non-principal axis rotation is studied. The
general rotational equations of motion are derived and approximated with an illumination function expanded up to second order.
The resulting equations of motion can be averaged over the fast rotation angles to yield secular equations for the angular
momentum, dynamic inertia and obliquity. We study the properties of these secular equations and compare results to previous
research. Finally, an application to several real asteroid shapes is made, in particular we study the predicted rotational
dynamics of the asteroid Toutatis, which is known to be in a non-principal axis state. 相似文献
20.
L.J. King N. Jackson R.D. Blandford M.N. Bremer I.W.A. Browne A.G. de Bruyn C. Fassnacht L. Koopmans D. Marlow P.N. Wilkinson 《Monthly notices of the Royal Astronomical Society》1998,295(2):L41-L44
We report the discovery, using NICMOS on the Hubble Space Telescope , of an arcsec-diameter Einstein ring in the gravitational lens system B1938+666. The lensing galaxy is also detected, and is most likely an early-type galaxy. Modelling of the ring is presented and compared with the radio structure from MERLIN maps. We show that the Einstein ring is consistent with the gravitational lensing of an extended infrared component, centred between the two radio components. 相似文献