共查询到20条相似文献，搜索用时 15 毫秒
1.
Gravitational instabilities with respect to warp modes of vertical oscillations are examined for nonlinearly nonequilibrium
disk models with isotropic and anisotropic velocity diagrams. Nonstationary analogs of the dispersion relations for vertical
oscillations in these models are derived in a general form. A detailed study is made of the major large scale oscillatory
modes, which correspond primarily to the most common type of warp in the form of an integral sign, as well as to dome-shaped,
U-shaped, and precessional warps. Critical diagrams showing the initial virial relation as a function of the rotation parameter
for the nonstationary model are constructed for each of these vertical oscillation modes. A comparative analysis is made of
the growth rates of the instabilities for these modes in order to determine the dependence of the characteristic times for
their appearance on the basic physical parameters of the two models. 相似文献
2.
Przemysław Bartczak Sławomir Breiter Przemysław Jusiel 《Celestial Mechanics and Dynamical Astronomy》2006,96(1):31-48
Simple models of potential based on material points and material segments are confronted with a homogeneous ellipsoid potential.
A spheroid is approximated with a pair of material points or with one material segment. The segment model proves to be more
accurate. For a triaxial ellipsoid, two models are considered: one with five material points and one with two material segments
and a point. When their parameters are determined with the fourth sectorial harmonic ignored, both simple models have a similar
approximation error. Numerical tests indicate that the approximate models of a triaxial ellipsoid are 5 to 10 times faster
than exact formulation, whereas for a spheroid the approximate models are at most twice as fast as the exact formulae. 相似文献
3.
4.
R. F. Sisteró 《Astrophysics and Space Science》1972,17(1):150-160
A further extension of the theory of interacting matter-radiation cosmological models is presented. The neutrino contribution to the radiation field is explicitly included. A discussion and interpretation of the observables within the theory is given. Mean evolutionary corrections for galaxies are shown to be implied by these models. Finally, as an example we present a cosmological interpretation of quasars. 相似文献
5.
G. Chanmugam 《Astrophysics and Space Science》1976,42(1):83-87
A number of models have been proposed for the observed cosmic gamma-ray bursts. A class of such models involves the use of magnetic energy as the principal source of energy required for the bursts. In this case, arguments are presented to show that degenerate stars are favored. Mechanisms for magnetohydrodynamic instabilities in white dwarfs and neutron stars are discussed. Preliminary work indicates that magnetic white dwarfs can (but neutron stars probably cannot) account for many of the observed features of the bursts.Paper presented at the COSPAR Symposium on Fast Transients in X- and Gamma-Rays, held at Varna, Bulgaria, 29–31 May, 1975. 相似文献
6.
Vincenzo Zappalà 《Earth, Moon, and Planets》1980,23(3):345-353
This paper presents some simple geometrical models of asteroids with theoretical light curves similar to the observed ones. In some cases the results suggest rotation periods to be double those one can obtain adopting two- or three-axial ellipsoids as models.A possible model, not in terms of a binary system, for asteroids with light curves like those of eclipsing binary stars, is also given.It should be stressed that the models studied in this paper are probably not very similar to real asteroids, but the principal conclusions should not be changed when more sophisticated models are considered.The work is to be a starting point for future researches on laboratory models of asteroids, in order to define, in a quantitative way, how the light curves are affected by the surface roughness and/or the large scale irregularities of the shape of an asteroid. 相似文献
7.
A class of non-singular bouncing FRW models are obtained by constraining the deceleration parameter in the presence of an
interacting dark energy represented by a time-varying cosmological constant. The models being geometrically closed, initially
accelerate for a certain period of time and decelerate thereafter and are also free from the entropy and cosmological constant
problems. Taking a constant of integration equal to zero one particular model is discussed in some detail and the variation
of different cosmological parameters are shown graphically for specific values of the parameters of the model. For some specific
choice of the parameters of the model the ever expanding models of Ozer & Taha and Abdel-Rahman and the decelerating models
of Berman and also the Einstein de-Sitter model may be obtained as special cases of this particular model. 相似文献
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9.
M. A. Falkenberg R. Kotulla U. Fritze 《Monthly notices of the Royal Astronomical Society》2009,397(4):1954-1965
In a previous paper, we have shown that the classical definition of E+A galaxies excludes a significant number of post-starburst galaxies. We suggested that analysing broad-band spectral energy distributions (SEDs) is a more comprehensive method to select and distinguish post-starburst galaxies than the classical definition of measuring equivalent widths of (Hδ) and [O ii ] lines.
In this paper, we will carefully investigate this new method and evaluate it by comparing our model grid of post-starburst galaxies to observed E+A galaxies from the MORPHS catalogue.
In the first part, we investigate the UV-optical-NIR (near-infrared) SEDs of a large variety in terms of progenitor galaxies, burst strengths and time-scales of post-starburst models and compare them to undisturbed spiral, S0 and E galaxies as well as to galaxies in their starburst phase. In the second part, we compare our post-starburst models with the observed E+A galaxies in terms of Lick indices, luminosities and colours. We then use the new method of comparing the model SEDs with SEDs of the observed E+A galaxies.
We find that the post-starburst models can be distinguished from undisturbed spiral, S0 and E galaxies and galaxies in their starburst phase on the basis of their SEDs. It is even possible to distinguish most of the different post-starbursts by their SEDs. From the comparison with observations, we find that all observed E+A galaxies from the MORPHS catalogue can be matched by our models. However, only models with short decline time-scales for the star formation rate are possible scenarios for the observed E+A galaxies in agreement with our results from the first paper. 相似文献
In this paper, we will carefully investigate this new method and evaluate it by comparing our model grid of post-starburst galaxies to observed E+A galaxies from the MORPHS catalogue.
In the first part, we investigate the UV-optical-NIR (near-infrared) SEDs of a large variety in terms of progenitor galaxies, burst strengths and time-scales of post-starburst models and compare them to undisturbed spiral, S0 and E galaxies as well as to galaxies in their starburst phase. In the second part, we compare our post-starburst models with the observed E+A galaxies in terms of Lick indices, luminosities and colours. We then use the new method of comparing the model SEDs with SEDs of the observed E+A galaxies.
We find that the post-starburst models can be distinguished from undisturbed spiral, S0 and E galaxies and galaxies in their starburst phase on the basis of their SEDs. It is even possible to distinguish most of the different post-starbursts by their SEDs. From the comparison with observations, we find that all observed E+A galaxies from the MORPHS catalogue can be matched by our models. However, only models with short decline time-scales for the star formation rate are possible scenarios for the observed E+A galaxies in agreement with our results from the first paper. 相似文献
10.
R. Caimmi 《Astrophysics and Space Science》1980,71(1):75-85
This paper deals with the question of whether elliptical galaxies can be considered as equilibrium systems (i.e., the gravitational+centrifugal potential is constant on the external surface). We find that equilibrium models such as Emden-Chandrasekhar polytropes and Roche polytropes withn=0 can account for the main part of observations relative to the ratio of maximum rotational velocity to central velocity dispersion in, elliptical systems. More complex models involving, for example, massive halos could lead to a more complete agreement. Models that are a good fit to the observed data are characterized by an inner component (where most of the mass is concentrated) and a low-density outer component. A comparison is performed between some theoretical density distributions and the density distribution observed by Younget al. (1978) in NGC 4473, but a number of limitations must be adopted. Alternative models, such as triaxial oblate non-equilibrium configurations with coaxial shells, involve a number of problems which are briefly discussed. We conclude that spheroidal oblate models describing elliptical glaxies cannot be ruled out until new analyses relative to more refined theoretical equilibrium models (involving, for example, massive halos) and more detailed observations are performed. 相似文献
11.
Shyamal K. Banerjee Jayant V. Narlikar 《Monthly notices of the Royal Astronomical Society》1999,307(1):73-78
The data on angular sizes and redshifts of ultracompact radio sources used by Jackson & Dodgson in a recent paper have been applied to the various theoretical models in the framework of the quasi-steady-state cosmology proposed by Hoyle, Burbidge & Narlikar. It is found that although acceptable fits to the data are available for the flat models, those with a negative curvature of spatial sections provide a better fit. These latter models require low densities of matter and as such do not demand too high a proportion of dark matter. A comparison is made with the work of Jackson & Dodgson and theoretical implications of the test are discussed. 相似文献
12.
Singularity-free Robertson-Walker cosmological models (RWCM) are developed by considering the cosmic matter as composed of an interacting viscous fluid (with zero bulk viscosity) and a massive scalar (meson) field. Solutions are obtained for two different cases, viz., when the Hubble's parameterH is epoch independent and whenH is epoch dependent. A solution corresponding to RWCM with only the massive scalar field as the matter content is also presented. The essential physical behaviour of the models developed are discussed in detail. 相似文献
13.
A. F. Heavens 《Monthly notices of the Royal Astronomical Society》1998,299(3):805-808
The bispectrum of the microwave background sky is a possible discriminator between inflationary and defect models of structure formation in the Universe. The bispectrum, which is the analogue of the temperature three-point correlation function in harmonic space, is zero for most inflationary models, but non-zero for non-Gaussian models. The expected departures from zero are small, and easily masked by noise, so it is important to be able to estimate the bispectrum coefficients as accurately as possible, and to know the errors and correlations between the estimates so that they may be used in combination as a diagnostic to rule out non-Gaussian models. This paper presents a method for estimating in an unbiased way the bispectrum from a microwave background map in the near-Gaussian limit. The method is optimal, in the sense that no other method can have smaller error bars, and, in addition, the covariances between the bispectrum estimates are calculated explicitly. The method deals automatically with partial sky coverage and arbitrary noise correlations without modification. A preliminary application to the Cosmic Background Explorer 4-yr data set shows no evidence for non-Gaussian behaviour. 相似文献
14.
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
15.
以太阳系质心参考系为基础，根据太阳系的质心参考系和非旋转地球质心参考系的坐标转换关系，推导了太阳系天体地面VLBI观测的相对论时间延迟模型，给出了一个通用的解析表达式．根据这一公式可以得到平劲松博士所采用的公式，以及当地心与源的距离无限大时，可得河外射电源VLBI观测的Zhu—Groten模型、Shapiro模型和IERS(92，96)推荐模型．所推导的公式严格解析且无误差，在实际应用中建议采用这一公式．同时详细地讨论了所推导公式的实用范围和各种舍掉项的量级估计，并详细给出了时间延迟理论模型的计算步骤． 相似文献
16.
《天文和天体物理学研究(英文版)》2017,(4)
A multi-model integration method is proposed to develop a multi-source and heterogeneous model for short-term solar flare prediction.Different prediction models are constructed on the basis of extracted predictors from a pool of observation databases.The outputs of the base models are normalized first because these established models extract predictors from many data resources using different prediction methods.Then weighted integration of the base models is used to develop a multi-model integrated model(MIM).The weight set that single models assign is optimized by a genetic algorithm.Seven base models and data from Solar and Heliospheric Observatory/Michelson Doppler Imager longitudinal magnetograms are used to construct the MIM,and then its performance is evaluated by cross validation.Experimental results showed that the MIM outperforms any individual model in nearly every data group,and the richer the diversity of the base models,the better the performance of the MIM.Thus,integrating more diversified models,such as an expert system,a statistical model and a physical model,will greatly improve the performance of the MIM. 相似文献
17.
D. Lorenz-Petzold 《Journal of Astrophysics and Astronomy》1986,7(3):155-170
We present a detailed study of the inhomogeneous Stephani-Krasinski solution with time-dependent curvature index. In general,
the cosmological behaviour of the models depends on six arbitrary functions of time. Such models are termed ‘private universes’
and cannot be in accord with observation in the most general case. Two simple models with changing topology are considered
as illustrating examples. In one of these models the pressure turns out to be negative and hence a violation of the weak energy
condition in the singularity theorems is possible. A brief review of other inhomogeneous cosmologies is included for the sake
of clarity. It is shown that the geodesic equation can be reduced to a complicated differential equation, which depends on
the three arbitrary functions involved. Therefore, it is difficult to obtain explicit formulas for the various observational
relations. 相似文献
18.
V. N. Zharkov 《Earth, Moon, and Planets》1983,29(2):139-175
A survey is made of the physics of the interiors of Venus. The introduction explains the main concepts used in the construction of models of Venus and the history of the question; observational data are gathered and analyzed. The method of constructing the models of the planet is explained and earth-like models of Venus and parametrically simple PVM models are discussed. Within the compass of a physical nodel of Venus, the thermodynamics of the mantle and core is constructed and questions are discussed concerning the heat conduction, temperature distribution in the lithosphere and the thermal flux from the interior of Venus, the electrical conduction and mechanical quality, and large-scale steady stresses in the mantle of Venus. A rheological model of the crust and mantle is constructed. In conclusion, the question as to the distribution of radioactivity and convection in the interior of the planet is discussed. 相似文献
19.
A. A. Saharian 《Astrophysics》1996,39(2):153-168
Multidimensional, anisotropic cosmological models with Ricci-flat subspaces are investigated within the framework of low-energy string theory. The main properties of these models, their behavior at early and late stages of evolution, in particular, are determined on the basis of a qualitative theory of dynamical systems. The conditions for dynamical compactification of extra dimensions are found. A concrete model with a Kalb — Ramond field as the source is considered as an illustration.Translated from Astrofizika, Vol. 39, No. 2, pp. 287–311, April–June, 1996. 相似文献
20.