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1.
We developed an efficient method for determining the surface-density distribution in a self-gravitating disk with an isolated central point mass from a specified angular-velocity distribution in the disk. An upper limit for the galactic-disk mass is shown to exist at a given black-hole mass. This limit significantly depends on the choice of rotation curves.  相似文献   

2.
We show that a fundamental choice between various models of an accretion disk around a black hole can be made based on the spectral wavelength distribution of the polarization. This conclusion is based on the possibility of comparing the observed spectral distribution of the polarization with its theoretical values obtained in various accretion disk models. The expected power-law wavelength (frequency) dependences of the polarization for various accretion disk models known in the literature are presented in the table.  相似文献   

3.
Recent data on the empirical metallicity distribution of G dwarfs in the disk solar neighbourhood are fitted in two different ways. We use an extended Poisson distribution in the limit where the probability of star formation is small, and a Gauss distribution in the limit where a large number of physical variables is required to determine stellar metal abundance. Both are found to reproduce the data at the same (acceptable) extent, with a slight preference for the former. The emprirical, differential metallicity distribution of G dwarfs in the disk solar neighbourhood is compared with its theoretical counterpart, in the picture of a closed, comoving model of chemical evolution. The limits of the currently used infall models are discussed and a scenario of galactic formation and evolution is presented. The Galactic history is thought as made of two main phases: contraction (which produces the extended component) and equilibrium (which gives the disk). In this view, the stars observed within the solar cylinder did not necessarily arise from the primordial gas which later collapsed into the disk solar neighbourhood. It is found that the G-dwarf problem is strongly alleviated, with the possible exception of the low-metallicity and high-metallicity tail of the distribution. The best choice of parameters implies: (i) a metal yield in the contraction phase which is larger by a factor of about 5 with respect to the equilibrium phase; (ii) a model halo mass fraction of about 0.3; (iii) a model disk mass fraction of about 0.6. It provides additional support to the idea of a generalized Schmidt star formation law, which is different in different phases of evolution. The model, cumulative, G-dwarf metallicity distribution in the disk solar neighbourhood is found to predict too may low-metallicity stars with respect to its empirical counterpart, related to a Poissonian or Gaussian fit. The main resons for the occurrence of a G-dwarf problem are discussed. Finally, a stochastic process of star formation, related to a Poisson distribution, is briefly outlined.  相似文献   

4.
A comparison of the observed distribution of absolute proper motions with a kinematical model of the Galaxy is presented. Proper motions with respect to galaxies were obtained for about 40 000 stars along the main Galactic meridian and in two fields near the North Galactic pole (programme MEGA). The Galaxy is considered as composed of the disk (main sequence and disk red giants), the thick disk and spheroid populations. For each subsystem, spatial velocity components and their dispersions were computed. The distribution of kinematical parameters were modelled for stars located in different directions of the Galaxy.  相似文献   

5.
The paper proposes a division of the local open clusters into two different disklike subpopulations on the base of the metallicity enhancement history, spatial distribution in the Galaxy and morphology of their C-M diagrams. The thin disk population objects exhibit higher metallicities and are more closely concentrated to the center and the disk of the Galaxy than the thick disk clusters. The former contain also frequently blue stragglers while the later exhibit the presence of Clump giant members, typical for the thick disk globular clusters. Kinematical properties of the two considered open cluster subpopulations remain unknown due to the lack of observational data.  相似文献   

6.
N-body simulations performed by us suggest a mechanism for the generation of spiral waves in galaxies in which a mutual quasi-ellipsoidal rotating equilibrium configuration increasing slowly by accretion from the surrounding disk influences the density distribution of stars in the disk such as to give rise to a trailing spiral density wave. Interaction of the spiral wave with the viscous interstellar gas and mutual gravitation between the stars in the disk are believed to influence the form of the spiral. Nevertheless the basic assumption of conventional density wave theory according to which the mutual interaction of stars in the disk is essential for the formation of spirals may not be true.  相似文献   

7.
Keiji Ohtsuki 《Icarus》2006,183(2):373-383
We derive an equation for the evolution of rotational energy of Keplerian particles in a dilute disk due to mutual collisions. Three-dimensional Keplerian motion of particles is taken into account precisely, on the basis of Hill's approximation. The Rayleigh distribution of particles' orbital eccentricities and inclinations, and the Gaussian distribution of their rotation rates are also taken into account. Performing appropriate variable transformation, we show that the equation can be expressed with two terms. The first term, which we call collisional stirring term, represents energy exchange between rotation and random motion via collisions. The second term, which we call rotational friction term, tends to equalize the mean rotational energy of particles with different sizes. The equation can describe the evolution of rotational energy of Keplerian particles with an arbitrary size distribution. We analytically evaluate the rates of stirring and friction for the random kinetic energy and rotational energy due to inelastic collisions, for non-gravitating particles in a dilute disk. Using these results, we discuss equilibrium states in a disk of spinning, non-gravitating Keplerian particles.  相似文献   

8.
The Fabry-Perot scanning interferometer mounted on the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences is used to study the distribution and kinematics of ionized gas in the peculiar galaxy Arp 212 (NGC 7625, IIIZw 102). Two kinematically distinct subsystems—the inner disk and outer emission filaments—are found within the optical radius of the galaxy. The first subsystem, at galactocentric distances r < 3.5 kpc, rotates in the plane of the stellar disk. The inner part of the ionized-gas disk (r<1.5–2 kpc) exactly coincides with the previously known disk consisting of molecular gas. The second subsystem of ionized gas is located at galactocentric distances 2–6 kpc. This subsystem rotates in a plane tilted by a significant angle to the stellar disk. The angle of orbital inclination in the outer disk increases with galactocentric distance and reaches 50° at r ≈ 6 kpc. The ionized fraction of the gaseous disk does not show up beyond this galactocentric distance, but we believe that the HI disk continues to warp and approaches the plane that is polar with respect to the inner disk of the galaxy. Hence Arp 212 can be classified as a galaxy with a polar ring (or a polar disk). The observed kinematics of the ionized and neutral gas can be explained assuming that the distribution of gravitational potential in the galaxy is not spherically symmetric. Most probably, the polar ring have formed via accretion of gas from the dwarf satellite galaxy UGC 12549.  相似文献   

9.
A suite of vast stellar surveys mapping the Milky Way, culminating in the Gaia mission, is revolutionizing the empirical information about the distribution and properties of stars in the Galactic stellar disk. We review and lay out what analysis and modeling machinery needs to be in place to test mechanism of disk galaxy evolution and to stringently constrain the Galactic gravitational potential, using such Galactic star-by-star measurements. We stress the crucial role of stellar survey selection functions in any such modeling; and we advocate the utility of viewing the Galactic stellar disk as made up of ‘mono-abundance populations’ (MAPs), both for dynamical modeling and for constraining the Milky Way’s evolutionary processes. We review recent work on the spatial and kinematical distribution of MAPs, and point out how further study of MAPs in the Gaia era should lead to a decisively clearer picture of the Milky Way’s dark-matter distribution and formation history.  相似文献   

10.
The formation of a disk galaxy within a slowly growing dark halo is simulated with a new chemo-dynamical model. The model describes the evolution of the stellar populations, the multi-phase ISM and all important interaction. I find, that the galaxy forms radially from inside-out and vertically from top-to-bottom. The derived stellar age distributions show that the inner halo is the oldest component, followed by the outer halo, the triaxial bulge, the halo-disk transition region and the disk. Despite the still idealized model, the final galaxy resembles present-day disk galaxies in many aspects. In particular, the stellar metallicity distribution in the halo of the model resembles the one of M31. The bulge in the model shows, at least two stellar subpopulations, an early collapse population and a population that formed later out of accreted disk mass. In the stellar metallicity distribution of the disk, I find a pronounced ‘G-dwarf problem’ which is the result of a pre-enrichment of the disk ISM with metal-rich gas from the bulge. This revised version was published online in September 2006 with corrections to the Cover Date.  相似文献   

11.
Exact radially-dependent particular solutions to the hydrodynamic equations for a disk galaxy are found. The mass density is calculated for a known Schwarzschild distribution function and also a distribution function is calculated from a known mass density.  相似文献   

12.
We numerically model the evolution of dust in a protoplanetary disk using a two-phase (gas+dust) Smoothed Particle Hydrodynamics (SPH) code, which is non-self-gravitating and locally isothermal. The code follows the three dimensional distribution of dust in a protoplanetary disk as it interacts with the gas via aerodynamic drag. In this work, we present the evolution of a disk comprising 1% dust by mass in the presence of an embedded planet for two different disk configurations: a small, minimum mass solar nebular (MMSN) disk and a larger, more massive Classical T Tauri star (CTTS) disk. We then vary the grain size and planetary mass to see how they effect the resulting disk structure. We find that gap formation is much more rapid and striking in the dust layer than in the gaseous disk and that a system with a given stellar, disk and planetary mass will have a different appearance depending on the grain size and that such differences will be detectable in the millimetre domain with ALMA. For low mass planets in our MMSN models, a gap can open in the dust disk while not in the gas disk. We also note that dust accumulates at the external edge of the planetary gap and speculate that the presence of a planet in the disk may facilitate the growth of planetesimals in this high density region.  相似文献   

13.
本文讨论了具有磁场的几何厚吸积盘结构和辐射。结果表明:磁场的存在明显地影响吸积盘的总光度和形状。这种模型可以解释类星体的辐射幂谱分布和高度偏振现象。它是类星体主要辐射区域的一种合理模型。  相似文献   

14.
Results of spectroscopic and photometric studies for the locally isolated lenticular galaxy NGC 4124 are presented. A model of the mass distribution consistent with photometric data has been constructed on the basis of a kinematic analysis. In this model, the halo mass within the optical radius is almost half the diskmass. The disk is shown to be in a dynamical state close to amarginally stable one. This rules out dynamical disk heating for the galaxy through a strong external action or a merger with a massive system. However, the presence of a gaseous disk inclined to the main plane of the galaxy in the central kiloparsec region suggests probable cannibalization of a small satellite that also produced a late starburst in the central region. This is confirmed by the younger mean age (～2 Gyr) of the stellar population in the galaxy’s central region than the disk age (5–7 Gyr).  相似文献   

15.
The evolution of initially balanced rotating disks of stars is investigated with a computer model for isolated disks of stars. An isolated, initially cold balanced disk is found to be violently unstable. Adding a sufficient amount of velocity dispersion will stabilize all small-scale disturbances. However, the disks are still unstable against slowly growing long wave-length modes and after about two rotations most disks tend to assume a bar-shaped structure. It is found that the final mass distribution over most of the disk can be closely approximated by an exponential variation, irrespective of the initial mass distribution. The gravitational two-stream instability is investigated by means of a modified computer model for infinite doubly periodic stellar systems.  相似文献   

16.
We compare the radial distributions of known localized gamma-ray bursts (GRBs) relative to the centers of their host galaxies with the distributions of known objects in nearby galaxies (supernovae of various types, X-ray binaries), the hypothetical dark-matter profiles, and the distribution of luminous matter in galaxies in the model of an exponential disk. By comparing the moments of empirical distributions, we show that the radial distribution of GRBs in galaxies differs significantly from that of other sources. We suggest a new statistical method for comparing empirical samples that is based on estimating the number of objects within a given radius. The exponential disk profile was found to be in best agreement with the radial distribution of GRBs. The distribution of GRBs relative to the centers of their host galaxies also agrees with the dark matter profile at certain model parameters.  相似文献   

17.
In accretion disk models of the solar nebula, turbulence is driven by convective instability. This mechanism requires high opacity, which must be provided by solid grains. Evolution of the grain size distribution in a turbulent disk is computed numerically, using realistic collisional outcomes and strengths of grain aggregates, rather than an arbitrary “sticking efficiency.” The presence of turbulence greatly increases the rate of grain collisions; the coagulation rate is initially much greater than in a nonturbulent disk. Aggregates quickly reach sizes ～0.1–1 cm, but erosion and breakup in collisions prevent growth of larger bodies for plausible aggregate impact strengths. These aggregates are too small to settle to the plane of the disk, and planetesimal formation is impossible as long as the turbulence persists. However, the opacity of the disk is reduced by aggregate formation; some combinations of opacity law and surface density produce an optically thin disk, cutting off turbulent convection. The disk may experience alternating periods of turbulence and quiescence, as grains are depleted by coagulation and replenished by infall from the presolar cloud. Planetesimals can form only during the quiescent intervals; it is argued that such episodes were rare during the lifetime of the accretion disk.  相似文献   

18.
The tidal force effects of a spherical galaxy passing head-on through a disk galaxy have been studied at various regions of the disk galaxy and for various orientations of the disk galaxy with respect to the direction of relative motion of the two galaxies. The density distribution of the disk galaxy is taken to be, (r)=ce–4r/R , where c is the central density andR is the radius of the disk. The density distribution of the spherical galaxy is taken to be that of a oolytrope of indexn=4. It is found that as a result of the collision, through the central parts and the outer parts of the disk galaxy remain intact, the region in between these two regions disrupts. Thus a ring galaxy with a nucleus embedded in the ring-i.e., a ring galaxy of the RN-type, is formed.  相似文献   

19.
The magnetic-field distribution outside a flat, infinitely conductive unbounded disk in the field of a point magnetic dipole is determined. A relationship is established between the problem of magnetic-field determination and the problem of the flow of an ideal incompressible fluid around an infinitely thin disk.  相似文献   

20.
The pumping up of orbital inclinations of asteroids caused by sweeping secular resonances associated with depletion of a protoplanetary disk is discussed, focusing on the dependence on the disk inclinations and surface density distribution. The asteroids have large mean inclinations that cannot be explained by present planetary perturbations alone. It has been suggested that the sweeping secular resonances caused by disk depletion are responsible for these high inclinations. Nagasawa et al. (2000, Astron. J.119, 1480-1497) showed that the inclinations of asteroids are pumped up if the disk is depleted in an inside-out manner on a time scale longer than 3×105 years. Their assumed disk midplane is not on the invariant plane. However, it should be affected by the inclination of the disk plane. Here we investigate the dependence on the disk inclinations. We assume a disk depletion model in which the disk inside the jovian orbit has been removed and the residual outer disk is uniformly depleted. We calculate the locations of the secular resonances and the excitation magnitude of the inclinations with analytical methods. We found that the inclinations are pumped up to the observational level for a depletion time scale longer than 106 years in the case of the disk plane that coincides with the invariant plane. The required time scale is longest (3×106 years) if the disk plane coincides with the jovian orbital plane. However, it is still within the observationally inferred depletion time scale. We also studied dependence on a disk surface density gradient and found that the results do not change significantly as long as the inner disk depletion is faster than the outer disk one.  相似文献   

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