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1.
%We study the evolution of galactic disks subject to tidal torques motivated by cosmological N-body simulations using analytic
and numerical techniques. We find that self-gravitating disks subject to these torques resemble observed warped galaxies.
The warps develop at a local surface density of 70 M
⊙ pc-2 and move out through the disk at a rate that depends on the surface density of the disk.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
2.
Planetary rings are common in the outer Solar System but their origin and long-term evolution is still a matter of debate. It is well known that viscous spreading is a major evolutionary process for rings, as it globally redistributes the disk’s mass and angular momentum, and can lead to the disk’s loosing mass by infall onto the planet or through the Roche limit. However, describing this process is highly dependent on the model used for the viscosity. In this paper we investigate the global and long-term viscous evolution of a circumplanetary disk. We have developed a simple 1D numerical code, but we use a physically realistic viscosity model derived from N-body simulations (Daisaka et al., 2001), and dependent on the disk’s local properties (surface mass density, particle size, distance to the planet). Particularly, we include the effects of gravitational instabilities (wakes) that importantly enhance the disk’s viscosity. This method allows to study the global evolution of the disk over the age of the Solar System.Common estimates of the disk’s spreading time-scales with constant viscosity significantly underestimate the rings’ lifetime. We show that, with a realistic viscosity model, an initially narrow ring undergoes two successive evolutionary stages: (1) a transient rapid spreading when the disk is self-gravitating, with the formation of a density peak inward and an outer region marginally gravitationally stable, and with an emptying time-scale proportional to (where M0 is the disk’s initial mass), (2) an asymptotic regime where the spreading rate continuously slows down as larger parts of the disk become non-self-gravitating due to the decrease of the surface density, until the disk becomes completely non-self-gravitating. At this point its evolution dramatically slows down, with an emptying time-scale proportional to 1/M0, which significantly increases the disk’s lifetime compared to the case with constant viscosity. We show also that the disk’s width scales like t1/4 with the realistic viscosity model, while it scales like t1/2 in the case of constant viscosity, resulting in much larger evolutionary time-scales in our model. We find however that the present shape of Saturn’s rings looks like a 100 million-years old disk in our simulations. Concerning Jupiter’s, Uranus’ and Neptune’s rings that are faint today, it is not likely that they were much more massive in the past and lost most of their mass due to viscous spreading alone. 相似文献
3.
M. Clutton-Brock 《Astrophysics and Space Science》1972,17(2):292-324
Computer simulations are performed which suggest that narrow intergalactic filaments can be produced by tidal forces. A fixed
potential perturber passes a self-gravitating disk of stars in a nearly parabolic orbit. Due to their large velocity dispersion,
the stars which are pulled out by the tide form only an amorphous cloud. The gas clouds are simulated by a disk of points
of zero mass, zero velocity dispersion superimposed on the stellar disk. The ‘gas clouds’ form remarkably long, narrow filaments;
but not so narrow as the narrowest filaments observed. 相似文献
4.
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
5.
Kathryn Z. Hadley Paul Fernandez James N. Imamura Erik Keever Rebecka Tumblin William Dumas 《Astrophysics and Space Science》2014,353(1):191-222
We studied global nonaxisymmetric hydrodynamic instabilities in an extensive collection of hot, self-gravitating polytropic disk systems, systems that covered a wide expanse of the parameter space relevant to protostellar and protoplanetary systems. We examined equilibrium disk models varying three parameters: the ratio of the inner to outer equatorial radii, the ratio of star mass to disk mass, and the rotation law exponent q. We took the polytropic index n=1.5 and examined the exponents q=1.5 and 2, and the transitional one q=1.75. For each of these sets of parameters, we examined models with inner to outer radius ratios from 0.1 to 0.75, and star mass to disk mass ratios from 0 to 103. We numerically calculated the growth rates and oscillation frequencies of low-order nonaxisymmetric disk modes, modes with azimuthal dependence ∝e im? . Low-m modes are found to dominate with the character and strength of instability strongly dependent on disk self-gravity. Representatives of each mode type are examined in detail, and torques and mass transport rates are calculated. 相似文献
6.
The bar formation is still an open problem in modern astrophysics. In this paper we present numerical simulations performed
with the aim of analyzing the growth of the bar instability inside stellar-gaseous disks, where the star formation is triggered,
and a central black hole is present. The aim of this paper is to point out the impact of such a central massive black hole
on the growth of the bar. We use N-body-SPH simulations of the same isolated disk-to-halo mass systems harboring black holes
with different initial masses and a different energy feedback on the surrounding gas. We compare the results of these simulations
with the one of the same disk without a black hole in its center. We make the same comparison (disk with and without black
hole) for a stellar disk in a fully cosmological scenario. A stellar bar, lasting 10 Gyrs, is present in all our simulations. 相似文献
7.
We compare high-resolution 2D and 3D numerical hydrocode simulations of asteroids striking the atmosphere of Venus. Our focus is on aerobraking and its effect on the size of impact craters. We consider impacts both by spheres and by the real asteroid 4769 Castalia, a severely nonspherical body in a Venus-crossing orbit. We compute mass and momentum fluxes as functions of altitude as global measures of the asteroid's progress. We find that, on average, the 2D and 3D simulations are in broad agreement over how quickly an asteroid slows down, but that the scatter about the average is much larger for the 2D models than for the 3D models. The 2D models appear to be rather strongly susceptible to the “butterfly effect,” in which tiny changes in initial conditions (e.g., 0.05% change in the impact velocity) produce quite different chaotic evolutions. By contrast, the global properties of the 3D models appear more reproducible despite seemingly large differences in initial conditions. We argue that this difference between 2D and 3D models has its root in the greater geometrical constraints present in any 2D model, and in particular in the global conservation of enstrophy in 2D that forces energy to pool in large-scale structures. It is the interaction of these artificial large-scale structures that causes slightly different 2D models to diverge so greatly. These constraints do not apply in 3D and large scale structures are not observed to form. A one-parameter modified pancake model reproduces the expected crater diameters of the 3D Castalias reasonably well. 相似文献
8.
We study the influence of an open cluster environment on the formation and current structure of the Oort cloud. To do this, we have run 19 different simulations of the formation of the Oort cloud for 4.5 Gyrs. In each simulation, the Solar System spends its first 100 Myrs in a different open cluster environment before transitioning to its current field environment. We find that, compared to forming in the field environment, the inner Oort cloud is preferentially loaded with comets while the Sun resides in the open cluster and that most of this material remains locked in the interior of the cloud for the next 4.4 Gyrs. In addition, the outer Oort cloud trapping efficiencies we observe in our simulations are lower than previous formation models by about a factor of 2, possibly implying an even more massive early planetesimal disk. Furthermore, some of our simulations reproduce the orbits of observed extended scattered disk objects, which may serve as an observational constraint on the Sun's early environment. Depending on the particular open cluster environment, the properties of the inner Oort cloud and extended scattered disk can vary widely. On the other hand, the outer portions of the Oort cloud in each of our simulations are all similar. 相似文献
9.
10.
The spiral arms of disk galaxies are very sensitive to various morphological properties, such as, the gas content, the disk-to-bulge
ratioetc. Here, the stability of self-gravitating annular disks surrounding the central rigid bulge component has been studied in
order to explain the transition from the tight spiral arms in Sa galaxies to rather open patterns in Sc galaxies as the central
amorphous component diminishes. Smooth spiral patterns are found associated with the dominant (or the fastest growing) modes
of the system. When the disk-to-bulge mass ratio is small, a tight pattern results restricted to the inner regions of the
disk. This pattern opens up and occupies larger disk areas as the disk component becomes comparable to the bulge. It is found
here that the ‘explosive’ instabilities of the global density waves do not occur in the presence of a massive bulge. The growth-rates
of the eigen-modes decrease as the disk-to-bulge mass ratio decreases. It is also found that unstable modes of the annular
disk can be suppressed by increasing the thermal pressure sufficiently. 相似文献
11.
D. Kilkenny L. A. Crause F. van Wyk 《Monthly notices of the Royal Astronomical Society》2005,361(2):559-564
A new statement of the eigenvalue problem of studying small perturbations in arbitrary integrable self-gravitating systems is presented. An example of such a system, a 2D stellar disc, is considered in detail. The theory, based on the general equation for disc eigenmodes, reveals mechanisms for the formation and growth of global galactic structures. This new point of view specifies the limits of the unified theory of bar-like and spiral modes that was based on the assumption that global galactic structures could be understood in terms of low-frequency disc modes. 相似文献
12.
We propose a novel mathematical method to construct an exact polytropic sphere in self-gravitating hydrostatic equilibrium, improving the non-linear Poisson equation. The central boundary condition for the present equation requires a ratio of gas pressure to total one at the centre, which is uniquely identified by the whole mass and molecular weight of the system. The special solution derived from the Lane–Emden equation can be reproduced. This scheme is now available for modelling the molecular cloud cores in interstellar media. The mass–radius relation of the first core is found to be consistent with the recent results of radiation hydrodynamic simulations. 相似文献
13.
A. M. Fateeva D. V. Bisikalo P. V. Kaygorodov A. Y. Sytov 《Astrophysics and Space Science》2011,335(1):125-129
We present results of 3D numerical simulations of the matter flow in the disk of a binary T Tauri star. It is shown that two
bow-shocks caused by the supersonic motion of the binary components in the gas of the disk are formed in the system having
parameters typical for T Tauri stars. These bow-shocks significantly change the flow pattern. In particular, for systems with
circular orbits they determine the size and shape of the inner gap. We also show that the redistribution of the angular momentum
due to the bow-shocks leads to occurrence of two matter flows propagating from the inner edge of the circumbinary disk to
the components. Further redistribution of this matter between the components is considered. 相似文献
14.
We carry out 2.5D MHD simulations to study the interaction between a dipolar magnetic field of a T Tauri Star, a circumstellar accretion disk, and the halo above the disk. The initial disk is the result of 1D radiation hydrodynamics computations with opacities appropriate for low temperatures. The gas is assumed resistive, and inside the disk accretion is driven by a Shakura–Sunyaev-type eddy viscosity. Magnetocentrifugal forces due to the rotational shear between the star and the Keplerian disk cause the magnetic field to be stretched outwards and part of the field lines are opened. For a solar-mass central star and an accretion rate of 10?8 solar masses per year a field strength of 100 G (measured on the surface of the star) launches a substantial outflow from the inner parts of the disk. For a field strength of 1 kG the inner parts of disk is disrupted. The truncation of the disk turns out to be temporary, but the magnetic field structure remains changed after the disk is rebuilt. 相似文献
15.
C. J. Clarke 《Monthly notices of the Royal Astronomical Society》2009,396(2):1066-1074
We present analytic models for the local structure of self-regulated self-gravitating accretion discs that are subject to realistic cooling. Such an approach can be used to predict the secular evolution of self-gravitating discs (which can usefully be compared with future radiation hydrodynamical simulations) and to define various physical regimes as a function of radius and equivalent steady state accretion rate. We show that fragmentation is inevitable, given realistic rates of infall into the disc, once the disc extends to radii >70 au (in the case of a solar mass central object). Owing to the outward redistribution of disc material by gravitational torques, we also predict fragmentation at >70 au even in the case of low angular momentum cores which initially collapse to a much smaller radius. We point out that 70 au is close to the median binary separation and propose that such delayed fragmentation, at the point that the disc expands to >70 au, ensures the creation of low mass ratio companions that can avoid substantial further growth and consequent evolution towards unit mass ratio. We thus propose this as a promising mechanism for producing low mass ratio binaries, which, while abundant observationally, are severely underproduced in hydrodynamical models. 相似文献
16.
Yoshiaki Kato 《Astrophysics and Space Science》2007,307(1-3):11-15
In spite of the large number of global three-dimensional (3-D) magnetohydrodynamic (MHD) simulations of accretion disks and
astrophysical jets, which have been developed since 2000, the launching mechanisms of jets is somewhat controversial. Previous
studies of jets have concentrated on the effect of the large-scale magnetic fields permeating accretion disks. However, the
existence of such global magnetic fields is not evident in various astrophysical objects, and their origin is not well understood.
Thus, we study the effect of small-scale magnetic fields confined within the accretion disk. We review our recent findings
on the formation of jets in dynamo-active accretion disks by using 3-D MHD simulations. In our simulations, we found the emergence
of accumulated azimuthal magnetic fields from the inner region of the disk (the so-called magnetic tower) and also the formation
of a jet accelerated by the magnetic pressure of the tower. Our results indicate that the magnetic tower jet is one of the
most promising mechanisms for launching jets from the magnetized accretion disk in various astrophysical objects. We will
discuss the formation of cosmic jets in the context of the magnetic tower model. 相似文献
17.
Numerical simulations of two-component (stars + gas) self-gravitating galactic disks show that the interstellar gas can significantly affect the dynamical evolution of the disk even if its mass fraction (relative to the total galaxy mass) is as low as several percent. Aided by efficient energy dissipation, the gas becomes gravitationally unstable onlocal scale and forms massive clumps. Gravitational scattering of stars by these clumps leads to suppression of bar instability usually seen in heavy stellar disks. In this case, gas inflow towards the galactic center is driven by dynamical friction which gas clumps suffer instead of bar forcing. 相似文献
18.
Solar System Research - Within the framework of Tsallis nonextensive statistics, the criteria for the Jeans gravitational instability are derived for a self-gravitating protoplanetary disk, whose... 相似文献
19.
We found the equilibrium conditions for a self-gravitating toroidal vortex by taking thermal pressure into account. These conditions are shown to significantly differ from those for a disk or a sphere. The evolution of a thin vortex turns it into a compact vortex that loses mechanical stability for low masses at a polytropic index γ<4/3 but retains stability for sufficiently high masses and densities determined by the velocity circulation in the vortex. 相似文献
20.
Ralph E. Pudritz 《Astrophysics and Space Science》2002,281(1-2):493-496
We present the results of very high resolution CDM simulations of galaxy formation designed to follow the formation and evolution
of self-gravitating, supergiant star-forming clouds. We find that the mass spectrum of these clouds is identical to that of
globular clusters and GMCs; dN/dM ∝ M
-1.7 ± 0.1.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献