共查询到20条相似文献，搜索用时 31 毫秒
1.
The nature of scalar field potentials plays a dominant role in the cosmological dynamics of scalar-tensor gravity. The superpotential reconstruction technique is an interesting way to determine the nature of scalar field potentials by taking the Hubble parameter as a function of scalar field. The present study is an application of this technique in the gravitational framework of scalar-tensor gravity using LRS Bianchi type I universe. We explore the nature of scalar field potentials for some particular cases. It is found that the potentials in all cases turn out to be of polynomial nature and the anisotropy parameter m classifies its degree. The graphical behavior of the directional Hubble parameter shows monotonic behavior which is in contrast to the FRW case. 相似文献
2.
Manuele Santoprete 《Celestial Mechanics and Dynamical Astronomy》2006,94(1):17-35
In this paper, we study the linear stability of the relative equilibria for homogeneous and quasihomogeneous potentials. First,
in the case the potential is a homogeneous function of degree −a, we find that any relative equilibrium of the n-body problem with a>2 is spectrally unstable. We also find a similar condition in the quasihomogeneous case. Then we consider the case of three
bodies and we study the stability of the equilateral triangle relative equilibria. In the case of homogeneous potentials we
recover the classical result obtained by Routh in a simpler way. In the case of quasihomogeneous potentials we find a generalization
of Routh inequality and we show that, for certain values of the masses, the stability of the relative equilibria depends on
the size of the configuration. 相似文献
3.
We study the viscosity of a differentially rotating particle disk in the limiting case where the particles are densely packed and their collective behavior resembles that of a liquid. The pressure tensor is derived from the equations of hydrodynamics and from a simple kinetic model of collisions described by Haff (1983). We find that density waves and narrow circular rings are unstable if the liquid approximation applies. The resulting development of nonlinear perturbations may give rise to “splashing” of the ring material in the vertical direction. These results may help in understanding the origin of the ellipticities of ringlets, the nonaxisymmetric features near the outer edge of the Saturnian B ring, and the unexplained residuals in kinematic models of the Saturnian and Uranian rings. 相似文献
4.
In this paper we establish the possibility of a new mechanism for the formation of ring-like structures in barred galaxies. We study the stability of periodic orbits with respect to perturbations perpendicular (vertical stability) or tangent (plane stability) to the plane of motion and we find that matter can be trapped in the case of a relatively strong and fast bar, in a narrow annular region around the bar. The periodic orbits inside the annulus are totally stable—i.e., both vertically and plane. The region outside the annulus is depleted of matter by the action of vertical or plane instability. The resulting ring-like structure has the geometrical characteristics of the true inner rings observed in barred galaxies. In this way the formation of inner rings seems to be a result of combined action ofvertical and plane instability. A similar study of stability in the outer parts of the Galaxy, where outer rings are observed, shows that the above mechanism is absent there. 相似文献
5.
Yves Grandati Alain Bérard Hervé Mohrbach 《Celestial Mechanics and Dynamical Astronomy》2010,106(2):109-116
We show that every generalized Gorringe-Leach equation admits an associated Fradkin-Bacry-Ruegg-Souriau’s vector which, in general, is only a piecewise conserved quantity. In the case of dualizable generalized Gorringe-Leach equations, which include the case of conservative motions in central power law potentials, the image sets of the FBRS vectors for dual classes are dual images of each other. 相似文献
6.
We address the occurrence of narrow planetary rings and some of their structural properties, in particular when the rings
are shepherded. We consider the problem as Hamiltonian scattering of a large number of non-interacting massless point particles in an effective potential. Using the existence of stable motion
in scattering regions in this set up, we describe a mechanism in phase space for the occurrence of narrow rings and some consequences
in their structure. We illustrate our approach with three examples. We find eccentric narrow rings displaying sharp edges,
variable width and the appearance of distinct ring components (strands) which are spatially organized and entangled (braids).
We discuss the relevance of our approach for narrow planetary rings. 相似文献
7.
Keiji Ohtsuki 《Icarus》2004,172(2):432-445
We examine the rotation of a small moonlet embedded in planetary rings caused by impacts of ring particles, using analytic calculation and numerical orbital integration for the three-body problem. Taking into account the Rayleigh distribution of particles' orbital eccentricities and inclinations, we evaluate both systematic and random components of rotation, where the former arises from an average of a large number of small impacts and the latter is contribution from large impacts. Calculations for parameter values corresponding to inner parts of Saturn's rings show that a moonlet would spin slowly in the prograde direction if most impactors are small particles whose velocity dispersion is comparable to or smaller than the moonlet's escape velocity. However, we also find that the effect of the random component can be significant, if the velocity dispersion of particles is larger and/or impacts of large particles comparable to the moonlet's size are common: in this case, both prograde and retrograde rotations can be expected. In the case of a small moonlet embedded in planetary rings of equal-sized particles, we find that the systematic component dominates the moonlet rotation when m/M?0.1 (m and M are the mass of a particle and a moonlet, respectively), while the random component is dominant when m/M?0.3. We derive the condition for the random component to dominate moonlet rotation on the basis of our results of three-body orbital integration, and confirm agreement with N-body simulation. 相似文献
8.
Samiran Ghosh Tushar K. Chaudhuri S. Sarkar Manoranjan Khan M.R. Gupta 《Astrophysics and Space Science》2001,278(4):465-477
Nonlinear properties of small amplitude dust acoustic waves, incorporatingboth the ion inertial effect and dust drift effect have been studied.The effect of dust charge variation is also incorporated. It is seen thatdue to the dust charge variation, a Korteweg-de Vries (KdV) equationwith positive or negative damping term depending on the wave velocityand the ring parameters governes the nonlinear dust acoustic wave. It isseen that the damping or growth arises due to the assumption that dusthydrodynamical time scale is much smaller than that of the dust chargingscale. This assumption is valid only for planetary rings such as Saturn'sF, G and E rings. Numerical investigations reveal thatall the three rings in F, G and E, dust acoustic solitary wave admits both negative and positive potentials. Instability arises from the available freeenergy of drift motion of dust grains only for the wave with wave velocity 0, the drift velocity of the dust. 相似文献
9.
Frank C. van den Bosch & Eric Emsellem 《Monthly notices of the Royal Astronomical Society》1998,298(1):267-274
We present circumstantial evidence that the central region of the edge-on S0 galaxy NGC 4570, which harbours a 150-pc scale nuclear disc in addition to its main outer disc, has been shaped under the influence of a small (∼ 500 pc) bar. This is based on the discovery of two edge-on rings, the locations of which are consistent with the inner Lindblad and ultraharmonic resonances of a rapidly tumbling triaxial potential. Observed features in the photometry and rotation curve correspond nicely with the positions of the main resonances, strengthening the case for a tumbling bar potential. The relative blue colour of the ILR ring, and the complete absence of any detected ISM, indicates that the nuclear ring is made of relatively young (≲ 2 Gyr) stars. We discuss a possible secular evolution scenario for this complex multicomponent galaxy, which may also apply to many other S0 galaxies with observed rings and/or multiple disc components. 相似文献
10.
Stefan Horatschek David Petroff 《Monthly notices of the Royal Astronomical Society》2009,392(3):1211-1216
A Roche model for describing uniformly rotating rings is presented, and the results are compared with the numerical solutions to the full problem for polytropic rings. In the thin ring limit, the surfaces of constant pressure including the surface of the ring itself are given in analytical terms, even in the mass-shedding case. 相似文献
11.
《天文和天体物理学研究(英文版)》2016,(2)
We use a simple dynamical model which consists of a harmonic oscillator and a spherical component, in order to investigate the regular or chaotic character of orbits in a barred galaxy with a central spherically symmetric nucleus. Our aim is to explore how the basic parameters of the galactic system influence the nature of orbits, by computing in each case the percentage of chaotic orbits, as well as the percentages of different types of regular orbits. We also give emphasis to the types of regular orbits that support either the formation of nuclear rings or the barred structure of the galaxy. We provide evidence that the traditional x1 orbital family does not always dominate in barred galaxy models since we found several other types of resonant orbits which can also support the barred structure. We also found that sparse enough nuclei, fast rotating bars and high energy models can support the galactic bars. On the other hand, weak bars, dense central nuclei, slowly rotating bars and low energy models favor the formation of nuclear rings.We also compare our results with previous related work. 相似文献
12.
Giuseppe Pucacco 《Celestial Mechanics and Dynamical Astronomy》2004,90(1-2):109-123
We study strongly and weakly integrable 2-dimensional Hamiltonian systems with velocity dependent potentials. We determine
the set of conditions which must be satisfied in order to allow the existence of an independent second invariant polynomial
in the momenta. We then investigate the linear case for which a complete solution of the problem can be obtained. We recover
the classical set of linear strongly integrable systems and provide several new examples of weakly integrable systems whose
equations of motion can be explicitly solved at a fixed value of the energy. 相似文献
13.
《Icarus》1987,70(1):124-137
The sharp, about 100-km-wide, transition between Saturn's C and B rings is at the inner stability limit of small (micrometer or less) highly charged debris from micrometeorite bombardment of the main ring bodies. The latter vary from about 1 cm to 5 m in radius. In the C ring this charged debris escapes from the ring plane to Saturn along magnetic field lines because of gravitational pull, thus producing a net mass loss. But in the B ring the debris oscillates stably back and forth through the ring plane until reabsorbed by a large ring body. In this model we assume that what is now the B and C rings was initially formed as one ring with the optical thickness of the present B ring. We estimate the C ring net erosion rate and determine the ring age, assuming that the mass influx is small compared with the erosion flux. The erosion rate has been calculated with the use of presently observed micrometeorite fluxes. We also use the best present estimates of the size distribution and total mass eroded by a micrometeorite of a given size and energy. We find that the ring age is between 4.4 and 67 myr. In either case the age is much younger than the 4.5 byr of the solar system. The sharpness of the transition between the B and C rings indicates that the principal mass loss is carried by particles moving at a few meters per second with respect to the parent bodies from which they were eroded. 相似文献
14.
Gene G. Byrd Derrick Ousley & Chris Dalla Piazza 《Monthly notices of the Royal Astronomical Society》1998,298(1):78-86
Many barred disc galaxies show rings of gas clouds and young stars thought to be in periodic orbits near the two-fold inner and outer Lindblad resonances (ILR and OLR) plus a four-fold ultraharmonic resonance (UHR) of the turning bar with oscillations about the disc orbital motion. To confirm and extend simulations by Schwarz and by Byrd et al. of resonance ring formation, we present an analytical formulation of the clouds' orbital motion which includes dissipative damping of oscillations relative to the local interstellar medium plus the rotation curve, bar pattern speed, and strength. Observed ring morphology matches our plots of periodic orbits where the density is enhanced but clouds do not collide violently. Pairs of 'outer rings' bracket the OLR. Dimpled outer rings like that of ESO 507-16 can be matched by plots with strong bars. Slightly dimpled outer rings like that of ESO 509-98 can be matched by weak bar plots. For flat rotation curves, a pair of two-fold rings bracket the ILR; the smaller can be identified with the tiny 'nuclear rings'. We find narrow UHR rings just outside this pair as well as just inside the OLR pair. We confirm the identification of the larger ILR ring and the inner UHR ring with 'inner rings'. Disagreeing with the common identification, we associate the dimpled outer rings with the UHR just inside the OLR. See ESO 507-16 as an example. We predict that damping can misalign the ILR and OLR rings relative to the bar as seen in our match to ESO 507-16. We find that for weak bars, if the linearly rising portion of the rotation curve is a significant fraction of the corotation radius, nuclear and inner rings are absent with outer rings still present. We show this in a match to ESO 509-98. Success of the matches to ESO 507-16 and 509-98 shows how the analytic formulation can be used to estimate disc orientation and pattern speed if rotation curve observations are available. 相似文献
15.
Librations of central configurations and braided Saturn rings 总被引：1，自引：1，他引：0
We give a simple mathematical model for braided rings of a planet based on Maxwell's model for the rings of Saturn. 相似文献
16.
J. J. Monaghan 《Earth, Moon, and Planets》1995,71(1-2):73-84
The Modern Laplacian Theory of the origin of the solar system (Prentice 1978) assumes a non-dissipative model of supersonic turbulence, and the existence of stable rings left behind during the contraction of the proto-sun. We show by numerical simulation that the turbulence is highly dissipative and the rings unstable. As a result of the instability, the rings spread and interact with the proto-sun. The rings therefore cannot form in the way Prentice proposed. 相似文献
17.
We investigate the spin rates of moonlets embedded in planetary rings, subject to collisions with surrounding small particles, using three-body integrations including friction and spins. All successive impacts of the particle with the moonlet are followed, including a possible sliding phase after the initial inelastic rebounds. Two methods for treating impacts, (1) as instantaneous velocity changes and (2) using an impact force model, are applied after Salo (1995, Icarus 117, 287). Conducting a series of integrations with various initial summed spin velocity of the moonlet and the particle, we determine the equilibrium spin rate for which the averaged torque vanishes. This equilibrium spin rate corresponds to the final spin rate of the moonlet if the moonlet is much larger than the surrounding particles; it also corresponds to the mean spin rate for a ring composed of identical particles. We find that the equilibrium spin rate is enhanced by sliding orbits as compared with the spin rate determined by considering only the first impacts of the particles with the moonlet. If the random velocities of incident particles are small enough, the resulting equilibrium spin rate of the moonlet can be larger than the synchronous rotation rate, for rp∼1, where rp denotes the sum of radii of the colliding pair normalized by their mutual Hill radius. In this special case aggregates without internal strength may become rotationally unstable. However, the equilibrium spin rate decreases with increasing random velocity, and aggregates are always rotationally stable in the more likely case where the relative velocities are comparable to the mutual escape velocity. We also compare our results with the mean spin rates found in previous N-body simulations, and find a good agreement for optically thin rings; however the spin rates for optically thick rings are significantly larger than those predicted by our three-body calculations. 相似文献
18.
Alberto Flandes Linda Spilker Stuart Pilorz Nicolas Altobelli Scott G. Edgington 《Planetary and Space Science》2010,58(13):1758-1765
Early ground-based and spacecraft observations suggested that the temperature of Saturn's main rings (A, B and C) varied with the solar elevation angle, B′. Data from the composite infrared spectrometer (CIRS) on board Cassini, which has been in orbit around Saturn for more than five years, confirm this variation and have been used to derive the temperature of the main rings from a wide variety of geometries while B′ varied from near −24° to 0° (Saturn's equinox).Still, an unresolved issue in fully explaining this variation relates to how the ring particles are organized and whether even a simple mono-layer or multi-layer approximation describes this best. We present a set of temperature data of the main rings of Saturn that cover the ∼23°—range of B′ angles obtained with CIRS at low (α∼30°) and high (α≥120°) phase angles. We focus on particular regions of each ring with a radial extent on their lit and unlit sides. In this broad range of B′, the data show that the A, B and C rings’ temperatures vary as much as 29-38, 22-34 and 18-23 K, respectively. Interestingly the unlit sides of the rings show important temperature variations with the decrease of B′ as well. We introduce a simple analytical model based on the well known Froidevaux monolayer approximation and use the ring particles’ albedo as the only free parameter in order to fit and analyze this data and estimate the ring particle's albedo. The model considers that every particle of the ring behaves as a black body and warms up due to the direct energy coming from the Sun as well as the solar energy reflected from the atmosphere of Saturn and on its neighboring particles. Two types of shadowing functions are used. One analytical that is used in the latter model in the case of the three rings and another, numerical, that is applied in the case of the C ring alone. The model lit side albedo values at low phase are 0.59, 0.50 and 0.35-0.38 for the A, B and C rings, respectively. 相似文献
19.
《天文和天体物理学研究(英文版)》2020,(2)
Using a sample of 13 early-type spiral galaxies hosting nuclear rings,we report remarkable correlations between the properties of the nuclear rings and the central intensity ratio(CIR) of their host galaxies.The CIR,a function of intensity of light within the central 1.5 and 3 arcsec region,is found to be a vital parameter in galaxy evolution,as it shares strong correlations with many structural and dynamical properties of early-type galaxies,including mass of the central supermassive black hole(SMBH).We use archival HST images for aperture photometry at the centre of the galaxy image to compute the CIR.We observe that the relative sizes of nuclear rings and ring cluster surface densities strongly correlate with the CIR.These correlations suggest reduced star formation in the centres of galaxies hosting small and dense nuclear rings.This scenario appears to be a consequence of strong bars as advocated by the significant connection observed between the CIR and bar strengths.In addition,we observe that the CIR is closely related with the integrated properties of the stellar population in the nuclear rings,associating the rings hosting older and less massive star clusters with low values of CIR.Thus,the CIR can serve as a crucial parameter in unfolding the coupled evolution of bars and rings as it is intimately connected with both their properties. 相似文献
20.
It is shown in this paper that the only potentials corresponding to central force for which all the bounded orbits are periodic are the potential of the harmonic oscillator and of the two body problem. A discussion is given in the case where a circular orbit exists and when the orbits near the circular orbit are periodic.We calculate in these cases the angle between pericentre and apocentre.Celestial Mechanics 相似文献