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1.
We obtain a well behaved class of charge analogues of neutral superdense star model due to Kuchowicz, by using a particular electric field, which involves a parameter K and vanishes when K=0. The members of this class are seen to satisfy the various physical conditions e.g. c 2 ρ≥3p≥0, dp/dr<0, dρ/dr<0, along with the velocity of sound, dp/c 2 dρ<1 and the adiabatic index ((p+c 2 ρ)/p)(dp/(c 2 dρ))>1, for the interval 0<K<1 with the maximum mass 6.8374M Θ and the radius 23.4679 km with the central red shift Z c =0.75364. In the interval, 0<K≤0.1179, the velocity of sound and the ratio p/c 2 ρ are found monotonically decreasing towards the pressure free interface, which presents a relevant model for massive star like Neutron star or pulsar with the maximum mass as 4.1474M Θ and the radius 20.5481 km with the central red shift Z c =0.6654.  相似文献   

2.
The stability of strange dwarfs for quark cores with M 0core /M ⨀ = 10−4, has been studied by calculating, in each individual case, a series of strange dwarfs with configurations in which 5 ⋅ 10−4, 10−3, 5 ⋅ 10−3, 10−2, 1.31 ⋅ 10−2, 1.6 ⋅ 10−2, 1.7 ⋅ 10−2, 2 ⋅ 10−2, ranges from the values in white dwarfs to ρ drip = 4.3 ⋅ 1011 g/cm3, at which free neutrons are produced in the crust. For the series with M 0core /M ⨀ < 0.0131, stability is lost when ρ tr < ρ drip . For the series with M 0core /M ⨀ > 0.0131, the equality ρ tr = ρ drip is reached before the strange dwarf attains its maximum mass. Although the frequency of the radial pulsations in the fundamental mode obeys ω02 > 0 for these configurations, they are unstable with respect to transitions into a strange star state with the same total number of baryons and a radius on the order of that of neutron stars. An energy on the order of the energy in a supernova explosion is released during these transitions. It is shown that the gravitational red shift of white and strange dwarfs are substantially different for low and limiting (high) masses.  相似文献   

3.
This is a study of the stability of strange dwarfs, superdense stars with a small quark core (M 0core /M ⨀ < 0.017) and an extended crust consisting of atomic nuclei and a degenerate electron gas where the density may be two orders of magnitude greater than the maximum density for white dwarfs. For a given equation of state, the mass, total number of baryons, and radius of strange dwarfs are uniquely determined by the central energy density ρ c and the energy density ρ tr of the crust at the surface of the quark core. Thus, the entire range of variation of ρ c and ρ tr must be taken into account in studying the stability of these configurations. This can be done by examining a series of configurations with a fixed rest mass M 0 (total baryon number) of the quark core and different masses of the crust. In each series, ρ tr ranges from the value for white dwarfs to ρ drip = 4.3∙1011 g/cm3, at which free neutrons are created in the crust. According to the static criterion for stability, stability is lost in an individual series when the mass of the strange dwarf reaches a maximum as a function of ρ tr . Translated from Astrofizika, Vol. 52, No. 2, pp. 325–332 (May 2009).  相似文献   

4.
Some Bianchi type-I viscous fluid string cosmological models with magnetic field are investigated. The viscosity coefficient of bulk viscous fluid is assumed to be a power function of mass density ξ(t)=ξ 0 ρ m , where ξ 0 and m are constants. To get a determinate model, we assume conditions ρ=(1+ω)λ, where ρ is rest energy density, ω a positive constant and λ the string tension density and expansion θ is proportional to eigen value σ 11 of the shear tensor σ j i . The behaviour of the models from physical and geometrical aspects in presence and absence of magnetic field is discussed.   相似文献   

5.
In this work we propose cyclical reversible transitions as the scenario in which the universe evolves, through a series consisting of reversible expansion, temporary stability, and contraction. Our model is based on the comparison between local and global time-dependent densities {ρ 0(τ 0),ρ(τ)} instead of the critical density ρ c, local and global time-dependent Hubble parameters {H 0(τ 0),H(τ)}, and the variations {Δρ(τ),ΔH(τ)} due to cosmological chaotic fluctuations, which are generally ignored in certain oscillating models. By taking into account all these factors, a rate equation in the form of (H 0/H)2 ∝(ρ 0/ρ) has been established, and from it we derive some others, to provide a mechanism that is responsible for the cyclical reversible transitions. Also, the problems of singularities, black hole overproduction, and the second law of thermodynamics arising in oscillating universe models are conceptually resolved.  相似文献   

6.
The density of the white dwarf stars is reconsidered from the point of view of the theory of the poly tropic gas spheres, and gives for themean density of a white dwarf (under ideal conditions) the formula ρ=2.162 × 106 × (M/⊙)2. The above formula is derived on considerations which are a much nearer approximation to the conditionsactually existent in a white dwarf than the previous calculations of Stoner based on uniform density distribution in the star and which gave for the limiting density the formula ρ=3.977 × 106 × (M/⊙)2.  相似文献   

7.
We assume the four dimensional induced matter of the 5D Ricci flat bouncing cosmological solution contains a perfect fluid. The big bounce singularity of simple 5D cosmological model is studied with the cosmological term Λ=α ρ and Λ=β H 2 where α and β are constants and ρ and H are respectively energy density and Hubble parameter. This big bounce singularity is found to be an event horizon at which the scale factor and mass density of the universe are finite, while the pressure is infinite.   相似文献   

8.
We present the results of 3489 astrometric observations for 361 visual double stars performed in 2003–2007 with the 26-inch refractor of the Pulkovo Observatory. The angular separations between the components (ρ) and the position angles (θ) are given. The errors in these quantities are, on average, 0″.009 for ρ and 0°.40/ρ for θ, where ρ is the separation in arcseconds.  相似文献   

9.
The Bianchi type-V cosmological model with variable modified Chaplygin gas having the equation of state p=Aρ−B/ρ α , where 0≤α≤1, A is a positive constant and B is a positive function of the average scale factor a(t) of the universe [i.e. B=B(a)] has been studied. While studying its role in accelerated phase of the universe, it is observed that the equation of state of the variable modified Chaplygin gas interpolates from radiation dominated era to quintessence dominated era. The statefinder diagnostic pair {r,s} is adopted to characterize different phases of the universe.  相似文献   

10.
A global iteration method to determine the self-consistent structure of steady plane-parallel radiative shock waves is shown to converge to the stable solution with upstream front velocities of 15 km/s ≤ U 1≤ 60 km/s and for hydrogen gas of unperturbed temperature T= 3000 K and density ρ = 10−10gcm−3. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

11.
Some locally rotationally symmetric (LRS) Bianchi type I cosmological models for a cloud string with bulk viscosity and magnetic field are presented. Where an equation of state ρ = kλ and a relation between metric potential R = AS n are considered. The solution describes a shearing and nonrotating model with a big bang start. In the absence of magnetic field it reduces to a string model with bulk viscosity, where the relation between the coefficient of bulk viscosity and energy density is ζ ∝ ρ1/2. After choosing k = , it further reduces to a string model without viscosity and magnetic field. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

12.
In this paper, it is shown that five dimensional LRS Bianchi type-I string cosmological models do not survive for Geometric and Takabayasi string whereas Barotropic string i.e. ρ=ρ(λ) survives and degenerates string with ρ+λ=0 in scalar tensor theory of gravitation proposed by Saez and Ballester (Phys. Lett. A 113:467, 1986). Further we studied some physical and geometrical properties of the model.  相似文献   

13.
The energy density of Vaidya-Tikekar isentropic superdense star is found to be decreasing away from the center, only if the parameter K is negative. The most general exact solution for the star is derived for all negative values of K in terms of circular and inverse circular functions. Which can further be expressed in terms of algebraic functions for K = 2-(n/δ)2 < 0 (n being integer andδ = 1,2,3 4). The energy conditions 0 ≤ p ≤ αρc 2, (α = 1 or 1/3) and adiabatic sound speed conditiondp dρ ≤ c 2, when applied at the center and at the boundary, restricted the parameters K and α such that .18 < −K −2287 and.004 ≤ α ≤ .86. The maximum mass of the star satisfying the strong energy condition (SEC), (α = 1/3) is found to be3.82 Mq· at K=−2/3, while the same for the weak energy condition (WEC), (α =1) is 4.57 M_ ⊙ atK=−>5/2. In each case the surface density is assumed to be 2 × 1014 gm cm-3. The solutions corresponding to K>0 (in fact K>1) are also made meaningful by considering the hypersurfaces t= constant as 3-hyperboloid by replacing the parameter R 2 by −R2 in Vaidya-Tikekar formalism. The solutions for the later case are also expressible in terms of algebraic functions for K=2-(n/δ2 > 1 (n being integer or zero and δ =1,2,3 4). The cases for which 0 < K < 1 do not possess negative energy density gradient and therefore are incapable of representing any physically plausible star model. In totality the article provides all the physically plausible exact solutions for the Buchdahl static perfect fluid spheres. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

14.
A new class of exact solutions of Einstein’s field equations with a bulk viscous fluid for an LRS Bianchi type-Ia obtained by using a time dependent deceleration parameter and cosmological term Λ. The coefficient of bulk viscosity is assumed to be a power function of mass density (ξ=ξ 0 ρ n ). We have obtained a general solution of the field equations from which six models of the universe are derived: exponential, polynomial and sinusoidal form respectively. The behaviour of these models of the universe are also discussed in the frame of reference of recent supernovae Ia observations.   相似文献   

15.
It is shown that quantum vacuum fluctuations give rise to a curvature of space-time equivalent to a cosmological constant, that is a homogeneous energy density ρ and pressure p fulfilling −p=ρ>0. The fact that the fluctuations produce curvature, even if the vacuum expectation of the energy vanishes, is a consequence of the non-linear character of the Einstein equation. A calculation is made, involving plausible hypotheses within quantized gravity, which establishes a relation between the two-point correlation of the vacuum fluctuations and the space-time curvature. Arguments are given which suggest that the density ρ might be of order the “dark energy” density currently assumed to explain the observed accelerated expansion of the universe.  相似文献   

16.
The dynamical masses of dwarf-spheroidals, spiral and elliptical galaxies, dwarf irregular binaries, groups of galaxies and clusters are shown to lie in a band about the M ∼ ρR3 line. The value of ρ is approximately the same as that estimated for unseen matter in the solar neighbourhood. The clusters themselves lie about theM ∼ R -3 line derived for a self-gravitating neutrino gas; their masses are distributed around the maximum Jeans-mass, MJmax. corresponding to mv - 10 eV in an expanding universe. The present day length scales of clusters and the dispersion in the velocities observed within them are understood in terms of a 100-fold expansion subsequent to the initial growth of the fluctuations at MJmax. These systematics on theR-M plane imply that the initial condensations in the expanding universe are on the scale of the rich clusters of galaxies, these condensations were triggered dominantly by the gravitation of the neutrinos and the constant density of al systems arises naturally due to the embedding of these systems in the large scale neutrino condensations. If the neutrino density falls off asr -2 beyond the cluster edge till the distributions from different clusters overlap, then the mean density of the neutrinos approximately equals the closure density of the universe.  相似文献   

17.
We model thermal evolution of magnetars with a phenomenological heat source in a spherical internal layer and compare the results with observations of persistent thermal radiation from magnetars. We show that the heat source should be located in the outer magnetar’s crust, at densities ρ≲5×1011 g cm−3, and the heating rate should be ∼1020 erg cm−3 s−1. Heating deeper layers is extremely inefficient because the thermal energy is mainly radiated away by neutrinos and does not warm up the surface to the magnetar’s level. This deep heating requires too much energy; it is inconsistent with the energy budget of neutron stars.   相似文献   

18.
The effect of time dependent bulk viscosity on the evolution of Friedmann models with zero curvature in Brans-Dicke theory is studied. The solutions of the field equations with ‘gamma-law’ equation of state p = (γ-1) ρ, where γ varies continuously as the Universe expands, are obtained by using the power-law relation φ = bR n , which lead to models with constant deceleration parameter. We obtain solutions for the inflationary period and radiation dominated era of the universe. The physical properties of cosmological solutions are also discussed. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

19.
In the problem of 2+2 bodies in the Robe’s setup, one of the primaries of mass m*1m^{*}_{1} is a rigid spherical shell filled with a homogeneous incompressible fluid of density ρ 1. The second primary is a mass point m 2 outside the shell. The third and the fourth bodies (of mass m 3 and m 4 respectively) are small solid spheres of density ρ 3 and ρ 4 respectively inside the shell, with the assumption that the mass and the radius of third and fourth body are infinitesimal. We assume m 2 is describing a circle around m*1m^{*}_{1}. The masses m 3 and m 4 mutually attract each other, do not influence the motion of m*1m^{*}_{1} and m 2 but are influenced by them. We also assume masses m 3 and m 4 are moving in the plane of motion of mass m 2. In the paper, the equations of motion, equilibrium solutions, linear stability of m 3 and m 4 are analyzed. There are four collinear equilibrium solutions for the given system. The collinear equilibrium solutions are unstable for all values of the mass parameters μ,μ 3,μ 4. There exist an infinite number of non collinear equilibrium solutions each for m 3 and m 4, lying on circles of radii λ,λ′ respectively (if the densities of m 3 and m 4 are different) and the centre at the second primary. These solutions are also unstable for all values of the parameters μ,μ 3,μ 4, φ, φ′. Such a model may be useful to study the motion of submarines due to the attraction of earth and moon.  相似文献   

20.
Scalar field as dark energy accelerating expansion of the Universe   总被引：1，自引：1，他引：0  
The features of a homogeneous scalar field ϕ with classical Lagrangian L = ϕ;i ϕ;i /2 − V(ϕ) and tachyon field Lagrangian L = −V(ϕ)√1 − ϕ;i ϕ;i causing the observable accelerated expansion of the Universe are analyzed. The models with constant equation-of-state parameter w de = p de/ρde < −1/3 are studied. For both cases the fields ϕ(a) and potentials V(a) are reconstructed for the parameters of cosmological model of the Universe derived from the observations. The effect of rolling down of the potential V(ϕ) to minimum is shown. Published in Ukrainian in Kinematika i Fizika Nebesnykh Tel, 2008, Vol. 24, No. 5, pp. 345–359. The article was translated by the authors.  相似文献   

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