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1.
V. M. Danilov 《Astronomy Reports》2011,55(6):473-486
The kinematics of stars in open-cluster models are investigated. The central regions of these model clusters are fairly “cool,
” leading to gravitational instability in the cluster cores. A temporary virilization is observed in the model clusters, during
which an appreciable fraction of the fluctuation energy of the cluster is temporarily transformed into the kinetic energy
of the peculiar motions of stars. The duration of this stage can reach ∼108 yr. The origins of this temporary virialization of the model clusters are discussed. The instability of the intrinsic fluctuations
of the phase density in the centers of six clusters and model clusters is investigated. Several new regions of unstable phase-density
fluctuations are found, in addition to those already known. The observed and model clusters can move toward a stable equilibrium
state when the density of the cluster core both decreases and increases. The structure of regions of instability of the phase-density
fluctuations in the centers of the six clusters is studied. Resonance curves for the amplitudes of steady-state phase-density
fluctuations in the center of NGC 6705 are constructed. Analysis of the structure of the regions of instability indicates
an appreciable rate of fluctuation-energy loss due to relaxation effects. The instability increments and widths of regions
of instability fall off with increasing distance of the cluster from the Galactic center. An important role in the formation
of regions of instability may be played by resonances between the frequency of the orbital motion of the cluster in the Galaxy
and the frequencies of intrinsic phase-density fluctuations in the centers of the clusters considered. 相似文献
2.
V. M. Danilov 《Astronomy Reports》2010,54(6):514-527
Instability of proper fluctuations of the phase density at the centers of six open clusters is demonstrated. The boundaries
of the region of instability in terms of the mean mass densities of the cluster cores are determined. Estimates for a number
of dynamical parameters (the potential, velocity dispersion of the stellar motions, mass density, phase-density fluctuation
period, time for the development of instability of the fluctuations, etc.) are obtained for the central parts of these clusters,
as well as for numerical dynamical models for open clusters. Equation that can be used to estimate the dynamical mass of a
cluster taking into account the influence of the external field of the Galaxy and the non-stationarity of the cluster is derived.
Allowance for these effects makes it possible to decrease the dynamical masses of clusters compared to the virial masses assuming
isolated systems. Astrophysical applications of the results are discussed. 相似文献
3.
V. M. Danilov 《Astronomy Reports》2002,46(6):443-450
Models of open clusters in a state of equilibrium in the space of the three parameters of the stellar motion and simultaneously far from virial equilibrium are analyzed. A formula for the phase-space stellar number density in such cluster models is derived, as well as formulas for the integrated and differential energy distributions of the phase-space volume occupied by cluster stars per unit stellar mass. These three quantities are computed for several times exceeding the time for violent relaxation of the cluster model. The phase-space density function obtained is used to compute the distribution of the absolute values of stellar velocities for the cluster model considered. 相似文献
4.
V. M. Danilov 《Astronomy Reports》2002,46(11):887-899
Fluxes of stars in models of open clusters that are nonstationary in the regular field of the cluster are analyzed. An equilibrium distribution function describes the state of these models from the beginning of their dynamical evolution. Violent relaxation in the open-cluster models proceeds under equilibrium conditions in phase space and does not result in virial equilibrium. The cluster relaxation times in the one-dimensional spaces of a number of stellar-motion parameters are estimated. The stellar fluxes are anisotropic in some two-dimensional parameter spaces. Such open-cluster models exhibit a number of manifestations of self-organization (an energy flux toward the center of the cluster and a transfer of energy from large-scale to small-scale motions, periodic decreases in the entropies of the cluster models with a period equal to that of oscillations of the regular field of the cluster, etc.). It is concluded that violent relaxation represents one form of self-organization in such systems. 相似文献
5.
A. V. Loktin 《Astronomy Reports》2005,49(9):693-700
We report an analysis of the kinematic properties of stars in the old open cluster M67 based on proper motions taken from a master catalog made up of nine different catalogs containing proper motions of stars in the cluster field. A modified Sanders method is used to identify 511 probable cluster members. The dependence of the mean radial components of the proper motions of cluster members on clustercentric radius indicates that the cluster core is expanding with a velocity of 0.4 km/s. The radial dependence of the mean tangential components of the proper motions suggests the possible rotation of the cluster core. The dispersions of the velocity components of the cluster members show no evidence for the dominance of elongated stellar orbits in the cluster. The kinetic energy of the cluster stars depends strongly on their mass. 相似文献
6.
Stellar trajectories in models of open star clusters that are nonstationary in the regular field of the cluster are analyzed. The maximum characteristic Lyapunov exponents λ of the trajectories of the stellar motions in the open cluster are estimated. The mean λ in the open-cluster models considered are \(\bar \lambda \simeq ({\rm M}yr)^{ - 1} \). Cluster cores and halos are regions of highly stochastic and more ordered stellar motions, respectively. The mean Lyapunov exponent, \(\bar \lambda \), increases with the cluster density, as does the size of the highly stochastic region in the cluster core. The stellar trajectories in phase space are “glued” to a domain with a given λ. A Fourier analysis of the stellar trajectories in the open-cluster models is performed. The distributions of the periods of the stellar trajectories with the highest power-spectrum levels are constructed. The distributions of the periods corresponding to the most significant oscillations of the stellar trajectories exhibit peaks with periods commensurable with (or close to) those of the most significant oscillations of the regular field of the system. Specific features of the distributions of the periods of the most significant oscillations of the stellar trajectories and the origins of the formation of these features in the open-cluster models are discussed. 相似文献
7.
A catalog of Galactic globular clusters has been compiled and used to analyze relations between the chemical and kinematic parameters of the clusters. The catalog contains positions, distances, luminosities, metallicites, and horizontal-branch morphology indices for 157 globular clusters, as well as space velocities for 72 globular clusters. For 69 globular clusters, these data are suppleented with the relative abundances of 28 chemical elements produced in various nuclear-synthesis processes, taken from 101 papers published between 1986 and 2018. The tendency for redder horizontal branches in lowmetallicity accreted globular clusters is discussed. The discrepancy between the criteria for cluster membership in the thick-disk and halo subsystems based on chemical and kinematic properties is considered. This is manifest through the fact that all metal-rich ([Fe/H] > ?1.0) clusters are located close to the center and plane of the Galaxy, regardless of their kinematic membership in particular Galaxy subsystems. An exception is three accreted clusters lost by a dwarf galaxy in Sagittarius. At the same time, the fraction of more distant clusters is high among metal-poorer clusters in any kinematically selected Galactic subsystem. In addition, all metal-rich clusters whose origins are related to the same protogalactic cloud are located in the [Fe/H]–[α/Fe] diagram considerably higher than the strip populated with field stars. All metal-poor clusters (most of them accreted) populate the entire width of the strip formed by high-velocity (i.e., presumably accreted) field stars. Stars of dwarf satellite galaxies (all of them being metal-poor) are located in this diagram much lower than accreted field stars. These facts suggest that all stellar objects in the accreted halo are remnants of galaxies with higher masses than those in the current environment of the Galaxy. Differences in the relative abundances of α-process elements among stellar objects of the Galaxy and surrounding dwarf satellite galaxies confirmthat the latter have left no appreciable stellar traces in the Galaxy, with the possible exception of the low-metallicity cluster Rup 106, which has low relative abundances of α-process elements.
相似文献8.
Robert Shcherbakov Jordan Van Aalsburg John B. Rundle Donald L. Turcotte 《Tectonophysics》2006,413(1-2):53
Correlations in space and time play a fundamental role in earthquake processes. One direct manifestation of the effects of correlations is the occurrence of aftershocks due to the stress transfer in the vicinity of a main shock. Less obvious and more speculative changes in correlations may occur in the background seismicity before large earthquakes. Using statistical physics it is possible to introduce a measure of spatial correlations through a correlation length. This quantity characterizes how local fluctuations can influence the occurrence of earthquakes over distances comparable with the correlation length. In this work, the physical basis of spatial correlations of earthquakes is discussed in the context of critical phenomena and the percolation problem. The method of two-point correlation function is applied to the seismicity of California. Well defined variations in time of the correlation length are found for aftershock sequences and background seismicity. The scaling properties of our obtained distributions are analyzed with respect to changes in several scaling parameters such as lower magnitude cutoff of earthquakes, the maximum time interval between earthquakes, and the spatial size of the area considered. This scaling behavior can be described in a unified manner by utilizing the multifractal fit. Utilizing the percolation approach the time evolution of clusters of earthquakes is studied with the correlation length defined in terms of the radius of gyration of clusters. This method is applied to the seismicity of California. 相似文献
9.
A. V. Loktin 《Astronomy Reports》2006,50(9):714-721
A master catalog of stellar proper motions in the field of the Pleiades young open cluster has been created based on data adopted from 12 catalogs. A total of 340 likely cluster members are identified, 80% with linear-velocity components in the plane of the sky estimated to better than 0.7 km/s. The average radial and tangential motions are analyzed. No rotation of the cluster or expansion or compression of the cluster as a whole or of its parts was found. The dispersion of the residual velocities does not depend on the masses of the stars, as is demonstrated by an absence of mass segregation in the spatial distribution of the stars in the cluster. Most of the stellar orbits in the cluster are strongly elongated. 相似文献
10.
The spatial structure of the intermediate and outer regions of the old open cluster NGC 2420 is analyzed using data from the catalog of Paparó. The differential and integrated distributions of the projected [ΔF(r) and F(r)] and spatial [Δf(r) and f(r)] stellar densities are obtained for various subsystems of cluster stars using Kholopov’s star-count method. Analysis of these curves shows that: (1) the cluster has at least three distinct spatial zones with different stellar-density gradients, (2) the each cluster subsystem has a layered structure, and (3) the spatial structure of the cluster changes systematically in the transition to subsystems containing fainter main-sequence stars. Empirical relations describing the structure of the cluster are also derived. Similar structural features can also be found in other globular and open clusters. 相似文献
11.
Z. I. Peikov 《Astronomy Reports》2002,46(5):352-359
Lower limits for the percentages of stars with various luminosities in the cores of six globular clusters are derived using stellar spatial density distributions f(r) to deep limiting B magnitudes obtained earlier. For NGC 6535 and NGC 5466, the logarithmic density range and Kholopov parameters Df and Dr are also determined. These two parameters are correlated with the mean masses of stars of various subsystems and the total mass (number) of stars in the cluster. 相似文献
12.
The region of Orion’s Sword with coordinates α = 83.79°, δ = −5.20° and a size of 1.0° × 2.5° is analyzed. We compiled a master
catalog of stars observed in the optical, containing positions, proper motions, and UBV photometry for 1634 stars. Using the nearest-neighbor-distance technique, we subdivided the region into stellar groups with
different numbers of members. The positions of five groups coincide with known clusters, and two groups coincide with aggregates
of stars with Hα emission. We have identified groups with low membership that are moving away from the system. We also considered
the kinematic structure of the groups using the AD-diagram method we developed earlier. Most of the stellar clusters and groups display similar kinematics, with the exception
of the group OMC-2, which is moving toward its own apex. We also confirmed the existence of the kinematic star group 189 discovered
earlier; its position is close to the cluster NGC 1977, and it is probably a component of its corona. 相似文献
13.
Deep stellar photometry of one of the most distant Galactic globular clusters, Palomar 3, based on frames taken with the VLT in Johnson–Cousins broadband V and I filters is presented, together with medium-resolution stellar spectroscopy in the central region of the cluster obtained with the CARELEC spectrograph of the Observatoire de Haute Provence and measurements of the Lick spectral indices for the integrated spectrum. Computations of the orbital parameters of Palomar 3 and nine Galactic globular clusters with similar metallicities and ages are also presented. The orbital parameters, age, metallicity, and distance of Palomar 3 are estimated. The interstellar absorption is consistent with and supplements values fromthe literature. The need to obtainmore accurate data on the propermotions, ages, and chemical compositions of the cluster stars to elucidate the origin of this globular cluster is emphasized. 相似文献
14.
V. M. Danilov 《Astronomy Reports》2005,49(8):604-610
Equations of motion containing a small parameter μ are derived for stars at the peripheries of open star clusters. The parameter μ is obtained for six numerical open-cluster models. The general analytical solution of these equations of motion for μ = 0 is found. An iterative method is used to derive the frequencies of the stellar motions for first-order expansions in μ of the solutions of the equations of motion for stars at the cluster periphery. Applications of the results are discussed. 相似文献
15.
The paper analyzes possible origins of stars located in intergalactic space that are not bound to specific galaxies, which
comprise 15–50% of all stars in galaxy clusters. Some such stars can form in streams of intergalactic gas flowing around gas-rich
disk galaxies moving in the cluster. Others may be the products of the decay of young, low-mass, spheroidal galaxies after
the loss of their gaseous components during an initial burst of star formation. The decay of low-mass disk galaxies moving
at high speeds after they have lost their gaseous components due to the pressure of the incident flow of dense intergalactic
gas is possible in the cluster core. The largest fraction of intergalactic stars are probably produced by the partial disruption
of galaxies as a result of close passages, collisions, or mergers. Collisions of low-mass, gas-rich galaxies are especially
good suppliers of intergalactic stars. Both stars from decaying stellar components of galaxies and stars arising in the gaseous
components of colliding galaxies can be supplied to the intergalactic medium. The merger of galaxies harboring supermassive
black holes in their nuclei could lead to the partial or total disruption of these galaxies during the deceleration of the
binary black hole that is formed during the merger. An enhanced density of intergalactic stars is observed in the cores of
galaxy clusters, underscoring the role of galaxy collisions in the formation of the intergalactic stellar population, since
the frequency of galaxy collisions grows with their density. 相似文献
16.
V. M. Danilov 《Astronomy Reports》2006,50(5):346-357
A model for a uniform, gravitating, ellipsoidal star cluster moving in a circular orbit around the Galactic center is considered. Three independent isolated integrals of stellar motion are written for this model. The characteristic features of the motion of a cluster star according to these three integrals are analyzed. Retrograde stellar motions dominate at the periphery of the model cluster, and the distribution of the stellar velocities is elongated along the direction of the cluster motion. A phase-space density function that depends on two of the integrals of motion is constructed. The distribution of the stellar velocities is constructed for the case of a three-integral phase-space density. Possible applications of the results are discussed. 相似文献
17.
The wide neighborhoods of 401 open clusters are analyzed using the modern, high-precision, homogeneous ASCC-2.5 all-sky catalog. More than 28000 possible cluster members (including about 12500 most probable members) are identified using kinematic and photometric criteria. Star counts with the ASCC-2.5 and USNO-A2.0 catalogs are used to determine the angular and linear radii of the cluster cores and coronas, which exceed the previously published values by factors of two to three. The segregation (differing central concentration) of member stars by magnitude is observed. The mean proper motions are determined directly in the Hipparcos system for 401 clusters, for 183 of them for the first time. The heliocentric distances of 118 clusters are determined for the first time based on color-magnitude diagrams for their identified members. 相似文献
18.
V. M. Danilov 《Astronomy Reports》2000,44(5):298-308
In dynamical models for open clusters, virial equilibrium is not achieved over the violent relaxation time scale τvr. The stars form an equilibrium distribution in (?, ?ζ, l) space, where ? and l are the energy and angular momentum per unit stellar mass in the combined field of the Galaxy and cluster and ?ζ is the energy of motion perpendicular to the Galactic plane per unit mass of cluster stars in the gravitational field of the Galaxy. This distribution of stars changes little when t>τvr. The stellar phase-space distribution corresponding to this type of equilibrium and the regular cluster potential vary periodically (or quasi-periodically) with time. This phase-space equilibrium is probably possible due to an approximate balance in the stellar transitions between phase-space cells over times equal to the oscillation period for the regular cluster field. 相似文献
19.
Relaxation times in the spaces of several stellar-motion parameters are obtained for a number of open-cluster models. The differences between the relaxation times in these spaces increase with the degree of nonstationarity of the cluster models. In the course of the cluster’s evolution, the relaxation times increase in all the spaces considered. During violent relaxation, the stars occupy all domains accessible to them, first in absolute velocity and then in clustercentric distance. The dependence of the coarse-grained phase-space density of the cluster on small initial perturbations of the phase-space coordinates of its constituent stars tends to decrease at times exceeding the time scales for violent and “collisional” relaxation. 相似文献
20.
Yu. N. Efremov 《Astronomy Reports》2003,47(12):1000-1012
We have made a new comparison of the positions of Cepheids and clusters in the LMC and constructed a new empirical period-age relation taking into account all available data on Cepheids in the LMC bar provided by the OGLE project. The most probable relation is logT=8.50?0.65 logP, in reasonably good agreement with theoretical expectations. Numerous Cepheids in rich clusters of the LMC provide the best data for comparing theories of stellar evolution and pulsation and the dynamical evolution of clusters with observations. These data suggest that stars undergoing their first crossing of the instability strip are first-overtone pulsators, though the converse is true of only a small fraction of first-overtone stars. Several rich clusters with suitable ages have no Cepheids—a fact that is not understood and requires verification. Differences in the concentration of Cepheids toward their cluster centers probably reflect the fact that the clusters are at different stages of their dynamical evolution, with the Cepheids in cluster coronas being ejected from the cluster cores during dynamical interactions between stars. 相似文献