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1.
We explore the hypothesis that the outer boundaries (“cutoffs”) of the stellar disks observed in many galaxies are determined by the condition of local gravitational (Jeans) stability for the gaseous protodisks at large galactocentric distances. The ratio of the surface density of the disk Σdisk to the critical value for Jeans instability Σcrit is computed for a number of galaxies, assuming that the gas velocity dispersion in the forming disk corresponded to its current thickness and that the disk itself is in a quasi-equilibrium state. The mean estimated stellar velocity dispersion in the vicinity of the cutoff (12 km/s) is close to the typical velocity dispersions of gaseous clouds in disk galaxies. At greater distances, such velocity dispersions should ensure gravitational stability of the disk both at the present epoch and in the past. The cutoff radius of the disk R cut is correlated with other disk parameters, and the ratio Σdisk/Σcrit at R cut is close to unity in most cases. We conclude that the available observational data agree well with the hypothesis that stellar disk cutoffs are due to a rapid decrease in the star-formation rate beyond R cut, where the gaseous disk has always been stable. 相似文献
2.
Spectroscopic observations of three lenticular (S0) galaxies (NGC 1167, NGC 4150, and NGC 6340) and one SBa galaxy (NGC 2273) have been taken with the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences aimed to study the structure and kinematic properties of early-type disk galaxies. The radial profiles of the stellar radial velocities and the velocity dispersion are measured. N-body simulations are used to construct dynamical models of galaxies containing a stellar disk, bulge, and halo. The masses of individual components are estimated formaximum-mass disk models. A comparison of models with estimated rotational velocities and the stellar velocity dispersion suggests that the stellar disks in lenticular galaxies are “overheated”; i.e., there is a significant excess velocity dispersion over the minimum level required to maintain the stability of the disk. This supports the hypothesis that the stellar disks of S0 galaxies were subject to strong gravitational perturbations. The relative thickness of the stellar disks in the S0 galaxies considered substantially exceed the typical disk thickness of spiral galaxies. 相似文献
3.
Several scenarios for the formation of accretion and decretion disks in single and binary Ae and Be stars are proposed. It is shown that, in order for a rapidly rotating main-sequence Be star to lose mass via a disk, the star’s rotation must be quasi-rigid-body. Estimates show that such rotation can be maintained by the star’s magnetic field, which is probably a relict field. The evolution of single Be main-sequence stars is numerically simulated allowing for mass loss via the stellar wind and rotational mass loss assuming rigid-body rotation. The stellar wind is the factor that determines the maximum mass of Be stars, which is close to 30M ⊙. The evolution of Be stars in close binaries is analyzed in the approximation adopted in our scenario. Long gamma-ray bursts can be obtained as a result of the collapse of rapidly rotating oxygen—neon degenerate dwarfs—the accreting companions of Be stars—into neutron stars. 相似文献
4.
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
5.
A series of numerical N-body simulations is performed in order to dynamically model the properties of four galaxies (NGC 5603, NGC 3198, NGC 891, and NGC 1566) with known rotation curves, radial disk scales L, and velocity dispersions of old disk stars at various galactocentric distances r. Each model includes a three-dimensional collisionless disk and rigid spherical components, whose relative mass μ was treated as a free parameter that differed from simulation to simulation. The observed disk stellar velocity dispersions were assumed to be equal to or (in the general case) greater than the corresponding line-of-sight projections of the simulated values for the adopted μ after the initially unstable disk is heated and arrives at a steady state. A comparison of the simulated and observed rotational velocities and velocity dispersions provides evidence for “light” disks with μ≥2 in the disk (r<4L). 相似文献
6.
Published data on rotation curves and the radial distribution of the surface density of neutral hydrogen (HI) in galaxies
with a low gas content are used to calculate radial profiles of the volume density of HI in the planes of the galactic disks.
A self-consistent model for the disks is used, taking into account the self-gravitation of the gas and the presence of a pseudo-isothermal,
massive halo. Eleven low-surface-brightness (LSB) galaxies and three S0 galaxies in which HI is detected are considered. The
gaseous and stellar disks are taken to be in equilibrium and axially symmetric, and the velocity dispersion in the stellar
disk to be equal to the marginal value for gravitational perturbations; in general, this gives an upper limit for the gas
density. It is shown that, on average, the gas volume densities are two orders of magnitude lower in LSB galaxies than in
galaxies with normal brightnesses at the same R values, while the three S0 galaxies occupy an intermediate position. The volume density of gas observed at the galaxy peripheries
are less than 10−27 g/cm3, even in the plane of the disk. The role of the UV background in ionizing outer regions is discussed. The obtained gas densities
can be used to estimate the star-forming efficiency in regions of low density. 相似文献
7.
Models for 12 elliptical galaxies are constructed based on the second-order tensor virial theorem. This method takes into account the spatial shape and structure of the internal density layers for each galaxy. New data for the surface brightness and isophote structure of the E galaxies derived from deep photometric observations are used. A system of equations is used to find the components of the gravitational-energy, rotational-energy, and velocity-dispersion tensors. This makes it possible to calculate the anisotropy index β of the velocity dispersion of each galaxy. The values for the entire sample lie in the range 0.012 ? β ? 0.246. It is shown that taking into account variations in the mass-to-light ratio M/L (for the stellar component) can reduce the value of β by roughly 20%. The results are compared with those from other studies. Our values of β for small E galaxies (rapid rotators) agree with those found in the ATLAS3D project based on Jeans anisotropy models. However, our models for giant E galaxies (slow rotators) taking into account the triaxial shape and realistic structure of the density layers provide a much better agreement with observations than the simple axially symmetrical models from the SAURON project.
相似文献8.
We have obtained the stellar velocity dispersion in three mutually perpendicular directions in the halos and cores of clusters as a function of time for several non-stationary open-cluster models. During the dynamical evolution of the open-cluster models, the velocity dispersions undergo oscillations that do not decay during 5–10 violent-relaxation timescales, τ vr . We estimated the time for synchronization of the rotation of the open-cluster models and their motion around the center of the Galaxy, t s , which, depending on the model parameters, is t s ? (5–27)τ vr . Synchronization mechanisms for the models are discussed. The disruption of the systems in the force field of the Galaxy is strongly affected by tidal friction. We have also estimated the time for the formation of a spherical stellar-velocity distribution in the cluster models, t σ ? (6 ? 25)τ vr . The impact of instability in the stellar motions in a cluster on the formation of a spherical velocity distribution in the open-cluster models is discussed. We have noted a tendency for a weakening of the dependence of the coarse phase density of the cluster on small initial perturbations of the stellar phase coordinates in the model cluster cores for times about five times longer than the violent-relaxation time. 相似文献
9.
An analysis of the λ4244 Å continuum light curve of the eclipsing variable V444 Cyg is used to reconstruct the velocity law v(r) of the stellar wind of the WR star in terms of Lamers parametric and powerlaw models. Both models are inconsistent with the observed light curve, and can be rejected at the <2% significance level. Departures of the Lamers parametric relation from the empirical v(r) law reconstructed in previous papers from the same λ 4244 Å light curve on sets of concave and convexo-concave functions are statistically significant. The stellar wind of the WN5 star continues to accelerate at a considerable distance from the star’s center. This corresponds to an acceleration parameter β>1 in terms of a coarse Lamers-law approximation for the empirical v(r) field. 相似文献
10.
N-body dynamical simulations are used to analyze the conditions for the gravitational stability of a three-dimensional stellar disk in the gravitational field of two rigid spherical components—a bulge and halo whose central concentrations and relative masses vary over wide ranges. The number of point masses N in the simulations varies from 40 to 500 000 and the evolution of the simulated systems is followed over 10–20 rotation periods of the outer edge of the disk. The initially unstable disks are heated and, as a rule, reach a quasi-stationary equilibrium with a steady-state radial-velocity dispersion cr over five to eight turns. The radial behavior of the Toomre stability parameter QT(r) for the final state of the disk is estimated. Simple models are used to analyze the dependence of the gravitational stability of the disk on the relative masses of the spherical components, disk thickness, degree of differential rotation, and initial state of the disk. Formal application of existing, analytical, local criteria for marginal stability of the disk can lead to errors in cr of more than a factor of 1.5. It is suggested that the approximate constancy of QT?1.2–1.5 for r?(1–2)×L (where L is the radial scale of disk surface density), valid for a wide range of models, can be used to estimate upper limits for the mass and density of a disk based on the observed distributions of the rotational velocity of the gaseous component and of the stellar velocity dispersion. 相似文献
11.
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
12.
The influence of close passages of galaxies on the shapes of disk galaxies and the distribution of stars in them is studied for several types of interactions in the framework of the restricted N-body problem. Depending on the conditions adopted, either two spiral density waves or ring structures are formed in the stellar disk of the galaxy. These structures can generate star formation fronts with the corresponding shape, as are observed in disk galaxies. Our calculations can also be applied to study the influence of the passage of a nearby star on a protoplanetary disk. The formation of ring structures there could specify the type of planet formation in the outer regions of the planetary system and the distribution of semimajor axes for the planetary orbits. We use the same model to study the generation and evolution of spiral density waves in the stellar disks of galaxies as a result of the recently found asymmetry of the gravitational potential in the massive dark haloes in disk galaxies. The dipole component of the gravitational field of the halo can continuously permanently generate the spiral structure in disk galaxies. 相似文献
13.
14.
A series of numerical dynamical models for the LMC are constructed in order to fit the observed rotational velocities and stellar velocity dispersions at various galactocentric distances. The models include a three-dimensional spherical disk and nonevolving spherical components with various relative masses. The two LMC rotation curves presented by Kim et al. (1998) and Sofue (2000), which differ strongly in the inner region of the galaxy, are compared. The latter curve requires the presence of a massive dark bulge. Models based on the rotation curve of Sofue (2000) cannot account for the observed velocity dispersion or the presence of a long-lived bar in the galaxy. A model with no dark bulge is in good agreement with the observations if we assume that the disk dominates over the halo in terms of the mass within the optical radius (about 7 kpc). 相似文献
15.
Yu. A. Fadeev 《Astronomy Reports》2008,52(8):645-655
The evolution of Population I stars with initial masses 60 M ⊙ ≤ M ZAMS ≤ 120 M ⊙ is computed up to the Wolf-Rayet stage, when the central helium abundance decreases to Y c ≈ 0.05. Several models from evolutionary sequences in the core helium-burning stage were used as initial conditions when solving the equations of radiative hydrodynamics for self-exciting stellar radial pulsations. The low-density envelope surrounding the compact core during the core helium burning is unstable against radial oscillations in a wide range of effective temperatures extending to T eff ～ 105 K. The e-folding time of the amplitude growth is comparable to the dynamical time scale of the star, and, when the instability ceases growing, the radial displacement of the outer layers is comparable to the stellar radius. Evolutionary changes of the stellar radius and luminosity are accompanied by a decrease in the amplitude of radial pulsations, but, at the effective temperature T eff ≈ 105 K, the stellar oscillations are still nonlinear, with a maximum expansion velocity of the outer layers of about one-third the local escape velocity. The period of the radial oscillations decreases from 9 hr to 4 min as stellar mass decreases from M = 28 M ⊙ to M = 6 M ⊙ in the course of evolution. The nonlinear oscillations lead to a substantial increase of the radii of the Lagrangian mass zones compared to their equilibrium radii throughout the instability region. The instability of Wolf-Rayet stars against radial oscillations is due to the action of the κ mechanism in the iron-group ionization zone, which has a temperature of T ～ 2 × 105 K. 相似文献
16.
We analyze models for quasi-stationary, ultraluminous X-ray sources (ULXs) with luminosities 1038–1040 erg/s exceeding the Eddington limit for a ～1.4M⊙ neutron star. With the exception of relatively rare stationary ULXs that are associated with supernova remnants or background quasars, most ULXs are close binary systems containing a massive stellar black hole (BH) that accretes matter donated by a stellar companion. To explain the observed luminosities of ～1040 erg/s, the mass of the BH must be ～40M⊙ if the accreted matter is helium and ～60M⊙ if the accreted matter has the solar chemical composition. We consider donors in the form of main-sequence stars, red giants, red supergiants, degenerate helium dwarfs, heavy disks that are the remnants of disrupted degenerate dwarfs, helium nondegenerate stars, and Wolf-Rayet stars. The most common ULXs in galaxies with active star formation are BHs with Roche-lobe-filling main-sequence companions with masses ～7M⊙ or close Wolf-Rayet companions, which support the required mass-exchange rate via their strong stellar winds. The most probable candidate ULXs in old galaxies are BHs surrounded by massive disks and close binaries containing a BH and degenerate helium-dwarf, red-giant, or red-supergiant donor. 相似文献
17.
A new theory for the formation of the main structures of galaxies is proposed: these structures are viewed as low-frequency normal modes in disks consisting of precessing stellar orbits. Mathematically, the theory is based on an integral equation in the form of a classical eigenvalue problem, with the eigenvalues being equal to the angular velocities Ωp of the modes. Analysis of the general properties of the master integral equation (without finding concrete solutions) shows that it admits two types of solutions: barlike and spiral. The numerical algorithms are discussed and particular solutions of the integral equation are presented. If resonance interaction can be neglected, the bar mode represents a neutral perturbation of the disk. This mode can be amplified by the effect of the long-range gravitational field of the mode on stars located in the vicinity of the corotation and outer-Lindblad resonances. Spiral perturbations are waves with zero total angular momentum, and spiral modes are excited at the inner-Lindblad resonance. The approach proposed is compared to currently accepted mechanisms for the formation of galactic structures. In particular, Toomre's application of the swing amplification mechanism to explain the formation of global modes is critically discussed. In addition, we show that it is not correct to simulate the real stellar velocity dispersion in a galaxy using softened gravity. 相似文献
18.
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
19.
Soil–structure interaction problems are commonly encountered in engineering practice, and the resulting linear systems of equations are difficult to solve due to the significant material stiffness contrast. In this study, a novel partitioned block preconditioner in conjunction with the Krylov subspace iterative method symmetric quasiminimal residual is proposed to solve such linear equations. The performance of these investigated preconditioners is evaluated and compared on both the CPU architecture and the hybrid CPU–graphics processing units (GPU) computing environment. On the hybrid CPU–GPU computing platform, the capability of GPU in parallel implementation and high-intensity floating point operations is exploited to accelerate the iterative solutions, and particular attention is paid to the matrix–vector multiplications involved in the iterative process. Based on a pile-group foundation example and a tunneling example, numerical results show that the partitioned block preconditioners investigated are very efficient for the soil–structure interaction problems. However, their comparative performances may apparently depend on the computer architecture. When the CPU computer architecture is used, the novel partitioned block symmetric successive over-relaxation preconditioner appears to be the most efficient, but when the hybrid CPU–GPU computer architecture is adopted, it is shown that the inexact block diagonal preconditioners embedded with simple diagonal approximation to the soil block outperform the others. 相似文献
20.
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