共查询到20条相似文献，搜索用时 15 毫秒
1.
Koijam Manihar Singh 《Astrophysics and Space Science》2009,323(3):297-306
Presenting some interesting new solutions, rotating models of anisotropic two-fluid universes coupled with a magnetic field
are investigated and studied, where the anisotropic pressure is generated by the presence of two non-interacting perfect fluids
which are in relative motion with respect to each other. Here special discussion is made of the physically interesting class
of models in which one fluid is a comoving radiative perfect fluid which is taken to model the cosmic microwave background
and the second a non-comoving perfect fluid which will model the observed material content of the universe. Besides studying
their physical and dynamical properties the effects of rotation on these models are studied and the reactions of the magnetic
and gravitational fields with respect to the rotational motion are discussed. Analysis on the rotational perturbations are
also made, in the course of which the amount of anisotropy induced in pressure distribution by a small deviation from the
Friedmann metric is also investigated. The models obtained here are found to be theoretically satisfactory and thereby substantiates
the possibilities of existence of such astrophysical objects in this Universe and may be taken as good examples of real astrophysical
situations. 相似文献
2.
A set of smoothed temperature gradient profiles around overshooting layers at the solar convective zone bottom is considered. In classical local theories of convection the one point defined according to the Schwarzschild criterion is enough to describe a convective boundary. To get a sophisticated picture of the overshooting we use four points to compute the transition overshooting functions. Analyzing the transition gradient profiles we found that the overshooting convective flux may be either positive or negative. A negative overshooting flux appears in nonlocal convective theories and causes a steep temperature gradient profile. But we propose an evenly smoothed gradient which corresponds to a convective flux positive everywhere. To outline the effect of the temperature gradient on the solar oscillations the squared Brunt–Väisälä frequency N 2 is calculated. In local convective theories the N 2 profile shows the discontinuity of the first derivative at the convective boundary, while all smoothed profiles eliminate the break. 相似文献
3.
In this contribution we examine the problem of inferring ages and initial cluster masses from synthesis models at the limit of low-mass clusters (M≤ a few ×104 M⊙). We show that it is not possible to apply directly synthesis models using standard methods to such clusters, since the basic hypothesis implicit in the models (a fixed proportionality between the number of stars in different evolutionary phases) is not fulfilled due to an insufficient number of stars for a reliable sampling of the stellar initial mass function. The consequence of this incomplete sampling is a non-Gaussian distribution of the mass–luminosity relation for clusters that share the same evolutionary conditions (age, metallicity and stellar initial mass distribution function). We review some tests, that can be performed before the start of the analysis, to estimate if the observed cluster can be analyzed with synthesis models following traditional procedures (like χ 2 minimization) or if it is necessary make use of synthesis models in a probabilistic framework. Finally, we show the implications of these results for estimating the low-mass tail in the initial cluster mass distribution function. 相似文献
4.
We have studied the evolution of homogeneous and anisotropic Bianchi type-I cosmological models filled with perfect fluid
in Barber second self-creation theory by assuming a special law of variation for Hubble’s parameter that yield a constant
value of deceleration parameter. Some physical consequences of the models have been discussed in case of Zel’dovich fluid
and radiation dominated fluid. 相似文献
5.
We investigate a class of solutions of Einstein equations for the plane symmetric perfect fluid case. If these solutions have
shear, they must necessarily be non-static. Some physical and geometric properties of the models are also discussed.
相似文献
6.
In the previous years, p-mode oscillations (pressure oscillations stochastically excited by convection) have been detected in several solar-like stars thanks to the ground-based spectroscopic and space spectroscopic and photometric observations. We study the importance of seismic constraints on stellar modeling and the impact of their accuracy on reducing the uncertainties of global stellar parameters (i.e. mass, age, etc.). We use the Singular Value Decomposition (SVD) method to analyze the sensitivity of stellar models to seismic constraints. In this context, we construct a grid of evolutionary sequences for solar-like stars with varying age and mass. Around each model of this grid, we evaluate the partial derivatives with respect to a large set of free parameters: mass ?, age τ, mixing-length parameter α, initial helium abundance Y 0, and initial metallicity Z/X 0. Masses between 0.9 and 1.55 M ⊙ and central hydrogen abundances from Xc=0.7 to 0.05 have been considered in this study. 相似文献
7.
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
8.
9.
Sanjay Oli 《Astrophysics and Space Science》2008,314(1-3):89-94
This paper presents anisotropic, homogeneous two-fluid cosmological models in a Bianchi type I space–time with a variable
gravitational constant G and cosmological constant Λ. In the two-fluid model, one fluid represents the matter content of the universe and another fluid is chosen to model the
CMB radiation. We find a variety of solutions in which the cosmological parameter varies inversely with time t. We also discuss in detail the behavior of associated fluid parameters and kinematical parameters. This paper pictures cosmic
history when the radiation and matter content of the universe are in an interactive phase. Here, Ω is closing to 1 throughout
the cosmic evolution.
相似文献
10.
Exact Bianchi type-II, VIII and IX cosmological models are obtained in a scalar tensor theory proposed by Saez and Ballester (Phys. Lett. A 113:467, 1986) with perfect fluid as a source. Some physical and geometrical properties of the models are studied. It is observed that the models are free from initial singularities and they are expanding with time. 相似文献
11.
Bianchi type-I string cosmological models are obtained in bimetric theory of gravitation proposed by Rosen (Gen. Relativ. Gravit. 4:435, 1973). Established the existence of string cosmological models, unlike the earlier authors, in this theory and studied some physical and geometrical properties. 相似文献
12.
We have studied cosmological model generated by perfect fluid coupled with mass less scalar field for Kantowski–Sachs space–time in general theory of relativity. Two different physically viable models of the universe are obtained by using a special law of variation for Hubble’s parameter that yields a constant value of deceleration parameter. Some physical consequences of the models have been discussed in case of Zel’dovich fluid. 相似文献
13.
Five dimensional LRS Bianchi type-I effective stiff fluid cosmological models in scalar tensor theory of gravitation proposed by Saez and Ballester (Phys. Lett. A 113:467, 1986) are constructed. Further, some physical and geometrical features of these models are discussed. 相似文献
14.
In this paper, we have attempted to construct five dimensional string cosmological models in Riemannian and Lyra geometries.
It is found that cosmic string models do not survive in both the theories. Subsequently, the vacuum cosmological models are
constructed and discussed. 相似文献
15.
Einstein’s field equations with variable gravitational and cosmological “constant” are considered in presence of perfect fluid
for Bianchi type-I space-time. Consequences of the four cases of the phenomenological decay of Λ have been discussed which
are consistent with observations. The physical significance of the cosmological models have also been discussed.
相似文献
16.
V. U. M. Rao T. Vinutha M. Vijaya Shanthi K. V. S. Sireesha 《Astrophysics and Space Science》2008,315(1-4):211-214
Exact Bianchi type-V cosmological models are obtained in a scalar-tensor theory of gravitation proposed by Brans and Dicke (Phys. Rev. 124:925, 1961) in the presence of perfect fluid distribution. Some physical and geometrical properties of the models are also discussed. 相似文献
17.
Yveline Lebreton Josefina Montalbán Jørgen Christensen-Dalsgaard Ian W. Roxburgh Achim Weiss 《Astrophysics and Space Science》2008,316(1-4):187-213
We compare stellar models produced by different stellar evolution codes for the CoRoT/ESTA project, comparing their global quantities, their physical structure, and their oscillation properties. We discuss the differences between models and identify the underlying reasons for these differences. The stellar models are representative of potential CoRoT targets. Overall we find very good agreement between the five different codes, but with some significant deviations. We find noticeable discrepancies (though still at the per cent level) that result from the handling of the equation of state, of the opacities and of the convective boundaries. The results of our work will be helpful in interpreting future asteroseismology results from CoRoT. 相似文献
18.
In this study we present the new R and I light curve solutions for the eclipsing binary systems RW CrB and VZ Leo, which for
VZ Leo is the first one in the literature. Our new observations have been analyzed using the Wilson-Devinney code from which
new geometric and photometric elements are derived. The geometry of both stars is that of a semi-detached binary system where
the secondary component fills its Roche lobe while the primary component is well inside. In the case of RW CrB, asymmetry
in the light curve was explained by a spot model. The orbital period changes of both systems were studied and the results
indicated a period decrease which can be explained by angular momentum loss. We also investigated the possibility of pulsations
of the primary components since these systems are mentioned as candidates of δ Sct type pulsation. However, a time-series analysis of the residual curves in the filter I does not indicate any evidence
of periodic light variation for both systems. Finally, we compared the results obtained for both binary stars to those of
similar systems. 相似文献
19.
Exact Bianchi type II, VIII and IX String cosmological models are obtained in the Saez-Ballester theory of gravitation. Some
physical and geometrical properties of the models are studied. 相似文献
20.
In this paper, we constructed some cosmological models in five dimensional LRS Bianchi type-V space time based on general theory of relativity. Further, it is shown that source density of the meson field does not survive either in massive scalar field or in mass less scalar field. Some physical and geometrical properties of the models are discussed. 相似文献