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1.
Two-time and two-point (two-particle) correlations are calculated for several parameters of the stellar motions, the densities, and the phase densities of model open clusters, in the vicinities of the cluster stars. The correlation times and correlation radii are determined in spaces of the parameters considered. The distributions of the two-point correlations for the distances between stars in the coordinate and velocity spaces of the stars are calculated. The local maxima of these distributions are used to determine the parameters of density waves, the potential, and the phase density in the model clusters. Analysis of the fine structure of regions of concentration in the two-point correlations in space of mutual distances between stars suggests the formation of polarization clouds near a number of such distances between stars. The distributions of the phase-density correlations are calculated, and the dynamics of these distributions analyzed. The dispersions of these distributions depend strongly on the presence of broad ??wings?? in the distributions (i.e., of strong correlations in the system). These dispersions are considered as a measure of the degree of correlation of phase-density fluctuations in model clusters. A growth in the correlations with time is observed for 50% of the cluster models considered. Flows of the phase-density correlations are investigated. A dominant correlation flow from the region of strong to the region of weak correlations is identified, leading to a flow of kinetic energy toward the cluster center. The rate at which this flow heats the model cluster core is estimated. Signs of weak turbulence are detected in the stellar motions in the model cluster cores with the highest degree of non-stationary in the regular field. 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》2008,52(11):888-899
The frequencies for homologous and non-homologous density flucutations in unisolated, ellipsoidal models of open clusters are determined. These fluctuations are stable. In the cases considered, the frequencies of non-homologous fluctuations are higher than those for homologous fluctuations of the ellipsoidal models. The instability of the proper fluctuations of the phase density at the center of a homogeneous ellipsoid whose central characteristics correspond to the core parameters of a dynamical, numericalmodel cluster is demonstrated. The development of instability of the phase-density flucutations at the center of such an ellipsoid leads to the domination of fluctuations with the frequency γ 1, corresponding to the growth increment γ 2 for this instability. Estimates of γ 1 and γ 2 agree with estimates obtained for the core of the corresponding dynamical, numerical model cluster. Astrophysical applications of the results are discussed. 相似文献
4.
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
5.
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
6.
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
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.
E. S. Safonova 《Astronomy Reports》2011,55(11):1016-1024
We propose a new method for estimating the HI deficiency in galaxies. The method is based on a semi-empirical relationship
between the total mass of HI and specific angular momentum of isolated galaxies. The atomic-hydrogen deficiency is estimated
for nearby spiral galaxies and for spiral galaxies in the Virgo and Coma clusters. The mean HI deficiencies determined for
these samples using our method are similar to those obtained with conventional methods, although there are considerable differences
in some cases. The HI deficiency in nearby galaxies does not depend on their degree of isolation, and there is no systematic
discrepancy between their HI and “normal” masses. Significant HI deficiencies are observed in the Virgo and Coma clusters,
out to distances of 1.5 and 3–4 Mpc from the cluster centers, respectively. At such distances, the ram pressure is too small
to sweep a considerable amount of gas from the galactic disks. Either these galaxies have passed through the dense cluster
center, or their gas deficiency is due to the fact that the halo had stopped accreting onto the disk when the galaxy entered
the cluster. 相似文献
9.
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
10.
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
11.
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
12.
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
13.
L. E. Pirogov 《Astronomy Reports》2009,53(12):1127-1135
Radial density profiles for the sample of dense cores associated with high-mass star-forming regions from southern hemisphere
have been derived using the data of observations in continuum at 250 GHz. Radial density profiles for the inner regions of
16 cores (at distances ≲0.2−0.8 pc from the center) are close on average to the ρ ∝ r
−α dependence, where α = 1.6 ± 0.3. In the outer regions density drops steeper. An analysis with various hydrostatic models showed that the modified
Bonnor-Ebertmodel, which describes turbulent sphere confined by external pressure, is preferable compared with the logotrope
and polytrope models practically in all cases. With a help of the Bonnor-Ebert model, estimates of central density in a core,
non-thermal velocity dispersion and core size are obtained. The comparison of central densities with the densities derived
earlier from the CS modeling reveals differences in several cases. The reasons of such differences are probably connected
with the presence of density inhomogenities on the scales smaller than the telescope beam. In most cases non-thermal velocity
dispersions are in agreement with the values obtained from molecular line observations. 相似文献
14.
The roles of neutrinos and convective instability in collapsing supernovae are considered. Spherically symmetrical computations of the collapse using the Boltzmann equation for the neutrinos lead to the formation of the condition of convective instability, \({\left( {\frac{{\partial P}}{{\partial s}}} \right)_{\rho {Y_l}}}\frac{{ds}}{{dr}} + {\left( {\frac{{\partial P}}{{\partial {Y_L}}}} \right)_{\rho s}}\frac{{d{Y_L}}}{{dr}}\) < 0, in a narrow region of matter accretion above the neutrinosphere. If instability arises in this region, the three-dimensional solution will represent a correction to the spherically symmetrical solution for the gravitational collapse. The mean neutrino energies change only negligibly in the narrow region of accretion. Nuclear statistical equilibrium is usually assumed in the hot proto-neutron stellar core, to simplify the computations of the collapse. Neutronization with the participation of free neutrons is most efficient. However, the decay of nuclei into nucleons is hindered during the collapse, because the density grows too rapidly compared to the growth in the temperature, and an appreciable fraction of the energy is carried away by neutrinos. The entropy of the matter per nucleon is modest at the stellar center. All the energy is in degenerate electrons during the collapse. If the large energy of these degenerate electrons is taken into account, neutrons are efficiently formed, even in cool matter with reduced Ye (the difference between the numbers of electrons and positrons per nucleon). This process brings about an increase in the optical depth to neutrinos, the appearance of free neutrons, and an increase in the entropy per nucleon at the center. The convectively unstable region at the center increases. The development of large-scale convection is illustrated using a multi-dimensional gas-dynamical model for the evolution of a stationary, unstable state (without taking into account neutrino transport). The time for the development of convective instability (several milliseconds) does not exceed the time for the existence of the unstable region at the center (10ms). The realization of this type of instability is fundamentally different from a spherically symmetrical model. The flux of neutrinos changes and the mean energy of the neutrinos is increased, which has important implications for the detection of neutrinos from supernovae. For these same reasons, the energy absorped in the supernova envelope also changes in the transition to such a multi-dimensional model. 相似文献
15.
The stability of multiple systems with known orbital elements and with subsystems occupying adjacent hierarchy levels is analyzed using six stability criteria and numerical simulations of their dynamical evolution. All the stability criteria considered are in qualitative agreement with the numerical computations. Of the 16 systems studied, 11 are confirmed to be stable and five (HD 40887, HD 136176, HD 150680, HD 217675, and HD 222326) may be unstable on time scales of ～106 yr or less. The small dynamical ages of the unstable systems may indicate that they have captured components during encounters between close binaries and field or moving cluster stars. The instability could also result from the perturbation of a stable system when it approaches a massive object (star, black hole, or molecular cloud). It is possible that some of the unstable systems are remnants of small clusters or stellar groups. 相似文献
16.
The physical properties of young stellar objects are studied as functions of the initial spatial distributions of the gas
surface density Σ and angular velocity Ω in pre-stellar cores using numerical hydrodynamic simulations. Two limiting cases
are considered: spatially homogeneous cores with Σ = const and Ω = const and centrally concentrated cores with radius-dependent
densities Σ ∝ r
−1 and Ω ∝ r
−1. The degree of gravitational instability and protostellar disk fragmentation is mostly determined by the initial core mass
and the ratio of the rotational to the gravitational energy, and depends only weakly on the initial spatial configuration
of pre-stellar cores, except for the earliest stages of evolution, when models with spatially homogeneous cores can be more
gravitationally unstable. The accretion of disk matter onto a protostar also depends weakly on the initial distributions of
Σ and Ω, with matter from the collapsing core falling onto the disk at a rate that is slightly higher in models with spatially
homogeneous cores. An appreciable dependence of the disk mass, disk radius, and the disk-to-protostar mass ratio on the initial
density and angular velocity profiles of the parent core is found only for class 0 young objects; this relationship is not
systematic in the later I and II stages of stellar evolution. The mass of the central protostar depends weakly on the initial
core configuration in all three evolutionary stages. 相似文献
17.
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
18.
Gholamreza Roshan Abdolazim Ghanghermeh José Antonio Orosa 《Arabian Journal of Geosciences》2014,7(9):3657-3674
Assessing the climatic characteristics and identifying the climatic parameters of a specific region can play a major role in human welfare. Thermal comfort conditions are among the most significant factors of climatic variables in the northwestern regions of Iran due to the considerable spatial and temporal variations and are vital for environmental, energy and economic management. It is therefore necessary to advance our knowledge of the climatic conditions in order to provide an appropriate tool for managing climatic extremes. This requires charting of the range of clusters of the thermal comfort conditions in this region. In this study, the general atmosphere circulation model HADCM3 and the A1 scenario, downscaled by the LARS-WG model, were employed to simulate the climatic conditions in Iran during the period 2011–2040. The data obtained were compared with sampled data from six Iranian climatic stations for the 30-year period (1961–1990). In order to tabulate this comparison, six clusters per climatic station were defined based on intrinsic similarity of data. Results show an increase in the annual average temperature of these six stations by 1.69 °C for the predicted years, projected from the base years 1961–1990. This factor has resulted in an increment of the annual average thermal comfort temperature inside buildings by a magnitude of 0.52 °C in future decades. When the thermal requirements of the studied region were evaluated based on the real temperature difference and the degree of thermal comfort, it becomes clear that apart from cluster 1, the energy required to reach thermal comfort inside buildings will increase in the future. As a result of this temperature increase, an increase of the energy required to reach the thermal comfort is expected. This new methodology is an interesting tool and needs to be seriously considered by engineers and architects in designing buildings of the future. 相似文献
19.
Possible paths for the formation of Ap/Bp stars—massive main-sequence stars with strong magnetic fields—are analyzed based
on modern theories for the evolution of single and binary stars. Assuming that the strong magnetic fields of these stars are
the main reason for their comparatively slow axial rotation and the observed anomalies in the chemical compositions of their
atmospheres, possible origins for these high magnetic fields are considered. Analysis of several possible scenarios for the
formation of these stars leads to the conclusion that their surface magnetic fields are probably generated in the convective
envelopes of the precursor stars. These precursors may be young, single stars with masses 1.5–3 M
⊙ that formed at the peripheries of forming star clusters and ended their accretion at the Hayashi boundary, or alternatively
close binaries whose components have convective envelopes, whose merger leads to the formation of an Ap/Bp star. Arguments
are presented supporting the view that the merger of close binaries is the main channel for the formation of Ap/Bp stars,
and a detailed analysis of this scenario is presented. The initial major axes of the merging binary systems must be in the
range 6–12 R
⊙, and the masses of their components in the range 0.7–1.5 M
⊙. When the merging components possess developed convective envelopes and fairly strong initial magnetic fields, these can
generate powerful magnetic fields “inherited” by the products of the merger—Ap/Bp stars. The reason the components of the
close binaries merge is a loss of angular momentum via the magnetic stellar winds of the components. 相似文献
20.
HST trigonometric distances, photometric metallicities, isochronic ages from the second revised version of the Geneva-Copenhagen
survey, and uniform spectroscopic Fe and Mg abundances from our master catalog are used to construct and analyze the age-metallicity
and age-relative Mg abundance relations for stars of the thin disk. The influences of selection effects are discussed in detail.
It is demonstrated that the radial migration of stars does not lead to appreciable distortions in the age dependence of the
metallicity. During the first several billion years of the formation of the thin disk, the interstellarmaterial in this disk
was, on average, fairly rich in heavy elements (〈[Fe/H]〉 ≈ −0.2) and poorly mixed. However, the metallicity dispersion continuously
decreased with age, from σ
[Fe/H] ≈ 0.22 to ≈0.13. All this time, the mean relative abundance of Mg was somewhat higher than the solar value (〈[Mg/Fe]〉 ≈ 0.1).
Roughly four to five billion years ago, the mean metallicity began to systematically increase, while retaining the same dispersion;
the mean relative Mg abundance began to decrease immediately following this. The number of stars in this subsystem increased
sharply at the same time. These properties suggest that the star-formation rate was low in the initial stage of formation
of the thin disk, but abruptly increased about four to five billion years ago. 相似文献