共查询到20条相似文献，搜索用时 31 毫秒
1.
We derive a new class of inhomogeneous cosmological models to verify that in the limit of large cosmological time, some models approach the Friedmann-like universe. We investigated also the contribution of the viscosity to the production entropy. Finally, we estimated the anisotropy present in the models in the asymptotic limits and comparated the results obtained with the corresponding models without viscosity. 相似文献
2.
The present study deals with the inhomogeneous plane symmetric models in scalar-tensor theory of gravitation. We used symmetry group analysis method to solve the field equations analytically. A new class of similarity solutions have been obtained by considering the inhomogeneous nature of metric potential. The physical behavior and geometrical aspects of the derived models are also discussed. 相似文献
3.
We present new values of the structural coefficients η j , and related quantities, for realistic models of distorted stars in close binary systems. Our procedure involves numerical integration of Radau’s equation for detailed structural data and we verified our technique by referring to the 8-digit results of Brooker & Olle (Mon. Not. R. Astron. Soc. 115:101, 1955) for purely mathematical models. We provide tables of representative values of η j , and related quantities, for j=2,3,…,7 for a selection of Zero Age Stellar Main Sequence (ZAMS) stellar models taken from the EZWeb compilation of the Dept. of Astronomy, University of Wisconsin-Madison. We include also some preliminary comparisons of our findings with the results of Claret and Gimenez (Astron. Astrophys. 519:A57 2010) for some observed stars. 相似文献
4.
G. C. Samnata 《Astrophysics and Space Science》2014,353(2):731-736
The dark energy model with the equation of state \(p_{\mathit{DE}} = {-} \rho_{\mathit{DE}} - A\rho_{\mathit{DE}}^{\alpha} \) is studied in Kaluza-Klein space time. The model comprises and provides realization of several types of singularities in different parameter regimes. We discuss the finite-time singularities into four classes and explicitly present the models which give rise to these singularities by assuming the form of the equation of state of dark energy. Also, we discussed the models in terms of the cosmological redshift and some observational parameters. 相似文献
5.
I. Horváth B. G. Tóth J. Hakkila L. V. Tóth L. G. Balázs I. I. Rácz S. Pintér Z. Bagoly 《Astrophysics and Space Science》2018,363(3):53
GRB 170817A, associated with the LIGO-Virgo GW170817 neutron-star merger event, lacks the short duration and hard spectrum of a Short gamma-ray burst (GRB) expected from long-standing classification models. Correctly identifying the class to which this burst belongs requires comparison with other GRBs detected by the Fermi GBM. The aim of our analysis is to classify Fermi GRBs and to test whether or not GRB 170817A belongs—as suggested—to the Short GRB class. The Fermi GBM catalog provides a large database with many measured variables that can be used to explore gamma-ray burst classification. We use statistical techniques to look for clustering in a sample of 1298 gamma-ray bursts described by duration and spectral hardness. Classification of the detected bursts shows that GRB 170817A most likely belongs to the Intermediate, rather than the Short GRB class. We discuss this result in light of theoretical neutron-star merger models and existing GRB classification schemes. It appears that GRB classification schemes may not yet be linked to appropriate theoretical models, and that theoretical models may not yet adequately account for known GRB class properties. We conclude that GRB 170817A may not fit into a simple phenomenological classification scheme. 相似文献
6.
George S. Greenstein 《Astrophysics and Space Science》1968,2(2):155-165
We discuss the implications of the Brans-Dicke scalar-tensor theory for cosmology with particular emphasis on the primordial element abundances that would obtain. Two general classes of models are found. Models of one class expand through the nuclear burning stage slightly more rapidly than the general relativistic case: models of the other class may expand at any rate whatsoever. The first class of models yeilds primordial abundances of D, He3 and He4 in agreement with their general relativistic values if the present mass density is low. High-density cosmologies, however, would produce too much He4. The second class of models yields element abundances which are far too high unless the expansion rate was quite large: in this case no He4 at all is produced. Finally, we determine the rate of change of the constant of gravitationG at the present epoch. For all but a very small class of models is negative at the present epoch. Models with positive values of at the present epoch produce no primordial He4 whatsoever, and have ages significantly lower than the corresponding general relativistic ages. 相似文献
7.
D. Lorenz-Petzold 《Astrophysics and Space Science》1989,151(2):343-347
Within the framework of no-scale supergravity we investigate the isotropic FRM-model filled with conventional perfect fluid matter. We present a wide class of exact solutions which avoid the singularities that appear in cosmological models based on Einstein's theory of gravitation. 相似文献
8.
This work is devoted to the investigation of new holographic dark energy (infrared cutoff is the Hubble radius) in locally rotationally symmetric Bianchi type-\(I\) universe within the framework of Saez–Ballester (Phys. Lett. A 113:467, 1986) scalar–tensor theory of gravitation. We construct interacting and non-interacting dark energy models by solving the field equations using a relationship between the metric potentials. This leads to a variable deceleration parameter model which exhibits a transition of the universe from deceleration to acceleration. We evaluate various cosmological parameters of our models. We have observed that the energy density parameters, equation of state and important cosmological planes (\(\omega _{\mathit{de}} - \omega _{\mathit{de}}'\) and \(r - s\)) yield the results compatible with the modern observational data. We have, also, discussed the stability analysis of our models. 相似文献
9.
10.
We present grids of stellar models and their associated oscillation frequencies that have been used by the CoRoT Seismology Working Group during the scientific preparation of the CoRoT mission. The stellar models have been calculated with the CESAM stellar internal structure and evolution code while the oscillation frequencies have been obtained from the CESAM models by means of the ADIPLS adiabatic oscillation programme. The grids cover a range of masses, chemical compositions and evolutionary stages corresponding to those of the CoRoT primary targets. The stellar models and oscillation frequencies are available on line through the Evolution and Seismic Tools Activity (ESTA) web site. 相似文献
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12.
Solar tomography has progressed rapidly in recent years thanks to the development of robust algorithms and the availability of more powerful computers. It can today provide crucial insights in solving issues related to the line-of-sight integration present in the data of solar imagers and coronagraphs. However, there remain challenges such as the increase of the available volume of data, the handling of the temporal evolution of the observed structures, and the heterogeneity of the data in multi-spacecraft studies. We present a generic software package that can perform fast tomographic inversions that scales linearly with the number of measurements, linearly with the length of the reconstruction cube (and not the number of voxels), and linearly with the number of cores and can use data from different sources and with a variety of physical models: TomograPy ( http://nbarbey.github.com/TomograPy/ ), an open-source software freely available on the Python Package Index. For performance, TomograPy uses a parallelized-projection algorithm. It relies on the World Coordinate System standard to manage various data sources. A variety of inversion algorithms are provided to perform the tomographic-map estimation. A test suite is provided along with the code to ensure software quality. Since it makes use of the Siddon algorithm it is restricted to rectangular parallelepiped voxels but the spherical geometry of the corona can be handled through proper use of priors. We describe the main features of the code and show three practical examples of multi-spacecraft tomographic inversions using STEREO/EUVI and STEREO/COR1 data. Static and smoothly varying temporal evolution models are presented. 相似文献
13.
O. A. Merkulova L. V. Shalyapina V. A. Yakovleva G. M. Karataeva 《Astronomy Letters》2008,34(8):542-550
We present the results of our study of the stellar kinematics in the elliptical galaxy UGC 5119, which has previously been suspected to be a polar-ring galaxy. We have detected a rapidly rotating disk in the central region (r ≤ 3.2 kpc) of the galaxy’s main body and found a radial velocity gradient along its minor axis (in the putative ring). We conclude that UGC 5119 is a medium-luminosity elliptical galaxy with a rapidly rotating disk component and a stellar (probably polar) ring. We have calculated the Lick indices of the Hβ, Mggb, Fe 5270, and Fe 5335 absorption lines and compared them with evolutionary synthesis models. Differences in the [Mg/Fe] ratios, metallicities, and ages of the stars have been found: the young stellar population with a solar [Mg/Fe] ratio and a high metallicity dominates in the circumnuclear region (r ≤ 1 kpc), while the old one with a low metal abundance dominates in the ring. 相似文献
14.
Over the past thirty years, two bodies of literature have developed in parallel presenting mutually exclusive views of the
Sun’s upper transition region. One model holds that the Sun’s upper-transition-region plasmas are confined primarily in hydrostatic
funnels with a substantial backheating component. The other model holds that discrete structures, which are effectively isolated
from the corona, predominate in the Sun’s upper transition region. Purveyors of the latter position have recently begun to
present near-resolved observations of discrete structures. The funnel scenario, in contrast, has only been addressed by modeling
unresolved upper transition region emission. To address this paradox we have constructed hydrostatic funnel models and tested
them against a wider set of solar observations than previously performed. We reproduce the results of the previous analyses,
yet find that the hydrostatic funnels are unable to self-consistently match the wider set of observations against which we
test the models. We show that it is not possible for a class of funnels having peak temperatures in the transition region
or in the corona to match the observations. We conclude that it is implausible that a class of hydrostatic funnels constitutes
the dominant emitting component of the Sun’s upper-transition-region plasmas as has been suggested. 相似文献
15.
We present results on the ability of microturbulence, LTE line formation, and a homogeneous thermal model to realistically represent the center-limb variation of temporally and spacially averaged solar line profiles. We have used three somewhat similar semi-empirical thermal models in combination with five current microturbulence models which cover the gamut of homogeneous-isotropic to nonhomogeneous-anisotropic. From high resolution photoelectric data for λ λ5000–6000 at five μ-values (1, 0.63, 0.4, 0.25, 0.16) obtained at KPNO, we selected 17 Fe i and 5 Fe ii line profiles to unblend and symmetrize for study. An iterative scheme has been developed to calculate theoretical profiles for the various combination of models and compare them to the observed profile using the abundance at each limb position and the magnitude of the pressure broadening at the center of the disc as parameters. We find that a microturbulence model, for which the radial and tangential components increase into deeper layers with ξtan>ξrad, produces a reasonable good center-limb fit for lines less than 100 mÅ. However, for lines stronger than 140 mÅ, microturbulence models with no depth dependence produce the best match between theory and observation. Thus, there is reason to question the uniqueness of the microturbulence concept. 相似文献
16.
We present models of the shapes and rotational states of selected asteroids based on data from the Uppsala Asteroid Photometric Catalogue. The results show a wide variety of shapes especially among the smaller asteroids. Few asteroids show clear signs of significant albedo variegation. Most rotational states are in reasonable agreement with those previously estimated with rough models. We discuss some practical aspects of photometric analysis and present a simple way of building one-spot models. 相似文献
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18.
The high quality of the asteroseismic data provided by space missions such as CoRoT (Michel et al. in The CoRoT Mission, ESA Spec. Publ. vol. 1306, p. 39, 2006) or expected from new operating missions such as Kepler (Christensen-Dalsgaard et al. in Commun. Asteroseismol. 150:350, 2007) requires the capacity of stellar evolution codes to provide accurate models whose numerical precision is better than the expected observational errors (i.e. below 0.1 μHz on the frequencies in the case of CoRoT). We present a review of some thorough comparisons of stellar models produced by different evolution codes, involved in the CoRoT/ESTA activities (Monteiro in Evolution and Seismic Tools for Stellar Astrophysics, 2009). We examine the numerical aspects of the computations as well as the effects of different implementations of the same physics on the global quantities, physical structure and oscillations properties of the stellar models. We also discuss a few aspects of the input physics. 相似文献
19.
In this paper we present a new class of exact reconnection solutions in cylindrical geometry. We point out that in the case of planar reconnection there is a natural cylindrical analog to the Cartesian Dawson function model for the magnetic field. Although the resistive energy release scalings of these solutions mimic the Cartesian models an important new feature is the presence of curvature in the current sheet. We go on to show that these solutions can be generalized to three dimensions. 相似文献
20.
In the present work we consider the questions of star formation and evolution of nearby dwarf galaxies. We describe the method of star formation history determination based on multicolor photometry of resolved stars and models of color-magnitude diagrams of the galaxies. We present the results of star formation rate determination and its dependence on age and metallicity for dwarf irregular and dwarf spheroidal galaxies in the two nearby galaxy groups M81 and Cen A. Similar age of the last episode of star formation in the central part of the M81 group and also unusually high level of metal enrichment in the several galaxies of the Cen A group are mentioned. We pay special attention to the consideration of perspectives of star formation study in nearby dwarf galaxies with he new WSO-UV observatory. 相似文献