共查询到20条相似文献，搜索用时 31 毫秒
1.
B. Vršnak 《Solar physics》1990,129(2):295-312
The stability of prominences and the dynamics of an eruption are studied. The prominence is represented by an uniformly twisted, curved, magnetic tube, anchored at both ends in the photosphere. Several stages of the eruption are analyzed, from the pre-eruptive phase and the onset of the instability, up to the late phases of the process. Before the eruption, the prominence evolves through a series of equilibrium states, slowly ascending either due to an increase of the electric current or to mass loss. The eruption starts when the ratio of the current to the total mass attains a critical value after which no neighbouring equilibrium exists. The linearized equation of motion was used to obtain the instability threshold, which is presented in a form enabling comparison with the observations. The height at which the prominence erupts depends on the twist, and is typically comparable with the footpoint half-separation. Low-lying prominences are stable even for large twists. The importance of the external field reconnection below the filament, and the mass loss through the legs in the early phases of the eruption is stressed. The oscillations of stable prominences with periods on the Alfvén time-scale are discussed. The results are compared with the observations. 相似文献
2.
We found another critical mass ratio value μ between μ4 and μ5 concerning the genealogy of the long period family around the equilateral equilibrium point L4 in the re-stricted three-body problem. This value has not been pointed out before. We used numerical computations to show how the long period family evolves around this critical value. The case is similar to that of the critical values between μ2 and μ4, with slight difference in evolution details. 相似文献
3.
We present an extensive study of the double β model for the X-ray surface brightness profiles of clusters, and derive analytically the gas density and total masses of clusters under the hydrostatic equilibrium hypothesis. It is shown that the employment of the double β model instead of the conventional single β model can significantly improve the goodness-of-fit to the observed X-ray surface brightness profiles of clusters, which will in turn lead to a better determination of the gas and total mass distributions in clusters. In particular, the observationally fitted β parameter for the extended component in a double β model may become larger. This opens a new possibility of resolving the long-standing β discrepancy for clusters. Using an ensemble of 33 ROSAT PSPC observed clusters drawn from the Mohr, Mathiesen & Evrard sample, we find that the asymptotic value of β fit is 0.83±0.33 at large radii, consistent with both the average spectroscopic parameter β spec =0.78±0.37 and the result given by numerical simulations. 相似文献
4.
《Icarus》1986,67(3):391-408
The evolution of the giant planets is calculated under the general hypothesis that the solid cores formed first, by accretion of small particles, and that these cores later gravitationally attracted their gaseous envelopes from the solar nebula. The evolution passes through the following phases. (1) Planetesimals accrete to form a core of rocky and icy material. (2) When the core mass has grown to a few tenths of an Earth mass, a gaseous envelope in hydrostatic equilibrium begins to form around the core. (3) The core and envelope continue to grow until the “critical” core mass is reached, beyond which point the envelope increases in mass much more rapidly than the core. (4) The envelope mass increases quickly to its present value and prodices a relatively high luminosity, derived from gravitational contraction. (5) Accretion of both core and envelope terminates, and the planet contracts and cools to its present state on a time scale of 5 × 109 years. Evolutionary calculations of phases (2) through (5) are presented, based on solutions of the time-dependent stellar structure equations in spherical symmetry. The physical considerations that determine the critical core mass are discussed; its value is found to depend strongly on the core accretion rate but only weakly on surface boundary conditions. Evolutionary tracks up to the present state are presented for objects of Uranus and Saturn mass. 相似文献
5.
Two possible limiting scenarios are proposed for the production of a coronal mass ejection. In the first the magnetic field around a prominence evolves until it loses equilibrium and erupts, which drives reconnection below the prominence and an eruption of the overlying magnetic arcade. In the second a large-scale magnetic arcade evolves until it loses equilibrium and erupts, thereby causing a prominence to erupt. In general it is likely to be the non-equilibrium of the coupled system which creates the eruption. Furthermore, large quiescent prominences are expected to be centred within the magnetic bubble of a coronal mass ejection whereas when active-region prominences erupt they are likely to be located initially to one side of the bubble.A model is set up for the eruption of a magnetically coupled prominence and coronal mass ejection. This represents a development of the Anzer and Pneuman (1982) model by overcoming two limitations of it, namely that: it is not globally stable initially and so one wonders how it can be set up in a stable way before the eruption; it has reconnection driving the CME whereas recent observations suggest that the reverse may be happening. In our model we assume that magnetic reconnection below the prominence is driven by the eruption and the driver is magnetic non-equilibrium in the coupled prominence-mass ejection system. The prominence is modelled as a twisted flux tube and the mass ejection as an overlying void and magnetic bubble. Two different models of the prominence are considered. In one a globally stable equilibrium becomes unstable when a threshold magnetic flux below the prominence is exceeded and, in the other, equilibrium ceases to exist. In both cases, the prominence and mass-ejection accelerate upwards before reaching constant velocities in a manner that is consistent with observations. It is found that the greater the reconnection that is driven by the eruption, the higher is the final speed. 相似文献
6.
S. W. Allen R. W. Schmidt A. C. Fabian 《Monthly notices of the Royal Astronomical Society》2002,334(2):L11-L15
We present precise measurements of the X-ray gas mass fraction for a sample of luminous, relatively relaxed clusters of galaxies observed with the Chandra observatory, for which independent confirmation of the mass results is available from gravitational lensing studies. Parametrizing the total (luminous plus dark matter) mass profiles using the model of Navarro, Frenk & White, we show that the X-ray gas mass fractions in the clusters asymptote towards an approximately constant value at a radius r 2500 , where the mean interior density is 2500 times the critical density of the Universe at the redshifts of the clusters. Combining the Chandra results on the X-ray gas mass fraction and its apparent redshift dependence with recent measurements of the mean baryonic matter density in the Universe and the Hubble constant determined from the Hubble Key Project, we obtain a tight constraint on the mean total matter density of the Universe, , and measure a positive cosmological constant, . Our results are in good agreement with recent, independent findings based on analyses of anisotropies in the cosmic microwave background radiation, the properties of distant supernovae, and the large-scale distribution of galaxies. 相似文献
7.
S. Nesslinger H. Drechsel R. Lorenz P. Harmanec P. Mayer M. Wolf 《Astrophysics and Space Science》2006,301(1-4):47-49
A Cosmological model with a viscous fluid in Kaluza-Klein metric is obtained assuming a time-dependent equation of state.
The solution is in fact a generalization of an earlier work by Hajj and Boutros for a perfect fluid. It is also found that
dimensional reduction of the extra space takes place such that the five-dimensional universe naturally evolves into an effective
four-dimensional one. The dynamical behavior of the model is examined and it is also found that with a decrease in extra space
the observable 3D space entropy increases thus accounting for the large value of entropy observable at present. 相似文献
8.
W. van Hamme 《Astrophysics and Space Science》1979,64(1):239-248
In this paper we examine the possible outcome of the tidal evolution of a close binary system using a method from which the outline has already been given by Counselman (1973). If the value of the total angular momentum of the system is sufficiently large, two equilibrium states corresponding to synchronism between stellar rotation and orbital motion are possible. In one of these states the total energy attains no extreme value. The considered evolution can be visualized geometrically by the motion of a point along a hyperbolic cylinder in three-dimensional space. A comparison with some observational data reveals that most of the synchronously rotating detached systems have attained a stable equilibrium state of minimum total energy for the given value of total angular momentum. 相似文献
9.
Reduction the secular solution to periodic solution in the generalized restricted three-body problem
Elbaz I. Abouelmagd M. E. Awad E. M. A. Elzayat Ibrahim A. Abbas 《Astrophysics and Space Science》2014,350(2):495-505
The aim of the present work is to find the secular solution around the triangular equilibrium points and reduce it to the periodic solution in the frame work of the generalized restricted thee-body problem. This model is generalized in sense that both the primaries are oblate and radiating as well as the gravitational potential from a belt. We show that the linearized equation of motion of the infinitesimal body around the triangular equilibrium points has a secular solution when the value of mass ratio equals the critical mass value. Moreover, we reduce this solution to periodic solution, as well as some numerical and graphical investigations for the effects of the perturbed forces are introduced. This model can be used to examine the existence of a dust particle near the triangular points of an oblate and radiating binary stars system surrounded by a belt. 相似文献
10.
We analyze the R-and K s-band photometric profiles for two independent samples of edge-on galaxies. The thickness of old stellar disks is shown to be related to the relative masses of the spherical and disk components of galaxies. The radial-to-vertical scale length ratio for galactic disks increases (the disks become thinner) with increasing total mass-to-light ratio of the galaxies, which reflects the relative contribution of the dark halo to the total mass, and with decreasing central deprojected disk brightness (density). Our results are in good agreement with numerical models of collisionless disks that evolved to a marginally stable equilibrium state. This suggests that, in most galaxies, the vertical stellar-velocity dispersion, on which the equilibrium-disk thickness depends, is close to a minimum value that ensures disk stability. The thinnest edge-on disks appear to be low-brightness galaxies in which the dark-halo mass far exceeds the stellar-disk mass. 相似文献
11.
We compare two different N-body models simulating elliptical galaxies. Namely, the first model is a non-rotating triaxial N-body equilibrium model with smooth center, called SC model. The second model, called CM model, is derived from the SC by inserting a central mass in it, so that all possible differences between the two models are due to the effect of the central mass. The central mass is assumed to be mainly due to a massive central black hole of mass about 1% of the total mass of the galaxy. By using the fundamental frequency analysis, the two systems are thoroughly investigated as regards the types of orbits described either by test particles, or by the real particles of the systems at all the energy levels. A comparison between the orbits of test particles and the orbits of real particles at various energy levels is made on the rotation number plane. We find that extensive stable regions of phase space, detected by test particles remain empty, i.e. these regions are not occupied by real particles, while many real particles move in unstable regions of phase space describing chaotic orbits. We run self-consistently the two models for more than a Hubble time. During this run, in spite of the noise due to small variations of the potential, the SC model maintains (within a small uncertainly) the number of particles moving on orbits of each particular type. In contrast, the CM model is unstable, due to the large amount of mass in chaotic motion caused by the central mass. This system undergoes a secular evolution towards an equilibrium state. During this evolution it is gradually self-organized by converting chaotic orbits to ordered orbits mainly of the short axis tube type approaching an oblate spheroidal equilibrium. This is clearly demonstrated in terms of the fundamental frequencies of the orbits on the rotation number plane and the time evolution of the triaxiality index. 相似文献
12.
《Planetary and Space Science》2007,55(10):1299-1309
We investigate a planetary model in spherical symmetry, which consists of a solid core and an envelope of ideal and isothermal gas, embedded in a gaseous nebula. The model equations describe equilibrium states of the envelope. So far, no analytical expressions for their solutions exist, but of course, numerical results have been computed. The point of critical mass, above which no more static solutions for the envelope exist, could not be determined analytically until now. We derive explicit formulas for the core mass and the gas density at the core surface, for the point of critical mass. The critical core mass is also an indicator for the ability of a core to keep its envelope when the surrounding nebula is removed, because at this point, the core's influence extends up to the outer boundary at the Hill radius. 相似文献
13.
根据球状星团动力学演化理论,本文探讨了球状星团致密度的演化与球状星用质量和位置的关系。结果表明,银心距和球状星团质量都与致密度的演化紧密相关。一般来说,银心距很大时致密度演化极少,球状星团质量愈大致密度演化愈缓慢。对现有球状星团致密度的分布作了统计研究,结果表明,在银心距较小的区域,致密度的分布与理论分析结果一致。在大银心距处发现,质量不同的球状星团其致密度分布有明显不同,它可能反映了球状星团形成阶段其致密度与质量紧密相关。 相似文献
14.
Galaxies with supermassive binary black holes: (I) a possible model for the centers of core galaxies
The dynamics of galactic systems with central binary black holes is studied. The model is a modification from the restricted three body problem, in which a galactic potential is added as an external potential. Considering the case with an equal mass binary black holes, the conditions of existence of equilibrium points, including Lagrange Points and additional new equilibrium points, i.e. Jiang-Yeh Points, are investigated. A critical mass is discovered to be fundamentally important. That is, Jiang-Yeh Points exist if and only if the galactic mass is larger than the critical mass. The stability analysis is performed for all equilibrium points. The results that Jiang-Yeh Points are unstable could lead to the core formation in the centers of galaxies. 相似文献
15.
Xiang-Ping Wu Tzihong Chiueh Li-Zhi Fang & Yan-Jie Xue 《Monthly notices of the Royal Astronomical Society》1998,301(3):861-871
Rich and massive clusters of galaxies at intermediate redshift are capable of magnifying and distorting the images of background galaxies. A comparison of different mass estimators among these clusters can provide useful information about the distribution and composition of cluster matter and its dynamical evolution. Using the hitherto largest sample of lensing clusters drawn from the literature, we compare the gravitating masses of clusters derived from the strong/weak gravitational lensing phenomena, from the X-ray measurements based on the assumption of hydrostatic equilibrium, and from the conventional isothermal sphere model for the dark matter profile characterized by the velocity dispersion and core radius of galaxy distributions in clusters. While there is excellent agreement between the weak lensing, X-ray and isothermal sphere model-determined cluster masses, these methods are likely to underestimate the gravitating masses enclosed within the central cores of clusters by a factor of 2–4 as compared with the strong lensing results. Such a mass discrepancy has probably arisen from the inappropriate applications of the weak lensing technique and the hydrostatic equilibrium hypothesis to the central regions of clusters, as well as from assuming an unreasonably large core radius for both luminous and dark matter profiles. Nevertheless, it is pointed out that these cluster mass estimators may be safely applied on scales greater than the core sizes. Namely, the overall clusters of galaxies at intermediate redshift can still be regarded as the dynamically relaxed systems, in which the velocity dispersion of galaxies and the temperature of X-ray emitting gas are good indicators of the underlying gravitational potentials of clusters. 相似文献
16.
17.
《New Astronomy》2021
Magnetic fields have been observed in galaxy clusters with strengths of the order of ~ μG. The non-thermal pressure exerted by magnetic fields also contributes to the total pressure in galaxy clusters and can in turn affect the estimates of the gas mass fraction, fgas. In this paper, we have considered a central magnetic field strength of 5μG, motivated by observations and simulations of galaxy clusters. The profile of the magnetic field has also been taken from the results obtained from simulations and observations. The role of magnetic field has been taken into account in inferring the gas density distribution through the hydrostatic equilibrium condition (HSE) by including the magnetic pressure. We have found that the resultant gas mass fraction is smaller with magnetic field as compared to that without magnetic field. However, this decrease is dependent on the strength and the profile of the magnetic field. We have also determined the total mass using the NFW profile to check for the dependency of fgas estimates on total mass estimators. From our analysis, we conclude that for the magnetic field strength that galaxy clusters seem to possess, the non-thermal pressure from magnetic fields has an impact of ≈ 1 % on the gas mass fraction of galaxy clusters. However, with upcoming facilities like Square Kilometre Array (SKA), it can be further expected to improve with more precise observations of the magnetic field strength and profile in galaxy clusters, particularly in the interior region. 相似文献
18.
We study the evolution of globular clusters with mass spectra under the influence of the steady Galactic tidal field, including the effects of velocity anisotropy. Similarly to single-mass models, velocity anisotropy develops as the cluster evolves, but the degree of anisotropy is much smaller than in isolated clusters. Except for very early epochs of the cluster evolution, the velocity distributions of nearly all mass components become tangentially anisotropic at the outer parts. We examine how the mass function (MF) changes in time. Specifically, we find that the power-law index of the MF decreases monotonically with the total mass of the cluster, in agreement with previous findings based on isotropic models or N -body studies. This is also consistent with the behaviour of the observed slopes of MFs for a limited number of clusters. We attempt to compare our results with multimass King models, although it is almost impossible to fit the entire density profiles for all mass components. When the MF is fixed, the central densities of individual components show significant differences between Fokker–Planck and King models. We obtain 'best-fitting' multimass King models, for which the central density of individual components as well as the total density distribution agrees with the Fokker–Planck models by adjusting the MF. The MFs obtained in this way closely resemble the MF within the half-mass radius of the Fokker–Planck result. Also, we find that the local MFs predicted by Fokker–Planck calculations vary more rapidly with radius than best-fitting multimass King models. The projected velocity profiles for anisotropic models show significant flattening toward the tidal radius compared with the isotropic model. This is caused by the fact that the tangential velocity dispersion becomes dominant at the outer parts. Such a behaviour of velocity profile appears to be consistent with the observed profiles of the collapsed cluster M15. 相似文献
19.
20.
P. A. Thomas Orrarujee Muanwong F. R. Pearce H. M. P. Couchman A. C. Edge A. Jenkins L. Onuora 《Monthly notices of the Royal Astronomical Society》2001,324(2):450-462
We have extracted over 400 clusters, covering more than two decades in mass, from three simulations of the τ CDM cosmology. This represents the largest uniform catalogue of simulated clusters ever produced. The clusters exhibit a wide variety of density profiles. Only a minority are well-fitted in their outer regions by the widely used density profile of Navarro, Frenk & White (NFW), which is applicable to relaxed haloes. Others have steeper outer density profiles, show sharp breaks in their density profiles, or have significant substructure. If we force a fit to the NFW profile, then the best-fitting concentrations decline with increasing mass, but this is driven primarily by an increase in substructure as one moves to higher masses. The temperature–mass relations for properties measured within a sphere enclosing a fixed overdensity all follow the self-similar form, T ∝ M 2/3 ; however, the normalization is lower than the value inferred for observed clusters. The temperature–mass relations for properties measured within a fixed physical radius are significantly steeper then this. Both can be accurately predicted using the NFW model. 相似文献