Excitation of Turbulence in Protoplanetary Disks around Binary Stars     

E. P. Kurbatov  A. Yu. Sytov  D. V. Bisikalo 《Astronomy Reports》2017,61(12):1031-1037

Accretion disks around young binary stars are subject to strong forces exerted by the system components. Gas–dynamical interactions excite strong non-linear perturbations in the disk, which can give rise to turbulence. This study considers a mechanism for the excitation of turbulence based on the instability of non-linear waves in a rotating flow. It is shown that the spectrum of non-linear perturbations excited in the inner part of the disk leads to turbularization of the flow. Estimates of the Shakura–Syunyaev index, α ～ 0.01?0.05, are obtained for use in numerical models of accretion disks.  相似文献   

The development of large-scale instability in Keplerian stellar accretion disks     

A. Yu. Lugovskii  V. M. Chechetkin 《Astronomy Reports》2012,56(2):96-103

Our previous studies of large-scale vortical flows arising in shear flows of stellar accretion disks with Keplerian azimuthal velocity distributions as a result of the development of small perturbations are continued. The development of large-scale instability in an accretion disk is investigated via mathematical modeling. One result obtained is the change of the disk flow structure due to the formation of large vortices. In the limiting case, sufficiently long evolution leads to the formation of several asymmetric spiral structures of the flow of disk matter. The presence of large-scale structures leads to angular-momentum redistribution in the disk.  相似文献   

Early-type disk galaxies: Structure and kinematics     

A. V. Zasov  A. V. Moiseev  A. V. Khoperskov  E. A. Sidorova 《Astronomy Reports》2008,52(2):79-93

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   

The structure of cool accretion disks in semidetached binaries     

D. V. Bisikalo  A. A. Boyarchuk  P. V. Kaigorodov  O. A. Kuznetsov  T. Matsuda 《Astronomy Reports》2004,48(6):449-456

We present a qualitative analysis of possible changes in the structure of accretion disks that occur in the transition from hot to cool disks. We suggest that an additional spiral-density wave can exist in the inner parts of the disk, where gas-dynamical perturbations are negligible. We consider the formation of this wave and its parameters. The results of a three-dimensional gas-dynamical simulation of a cool accretion disk are presented; these results confirm the possibility of the formation of a new, “precessional,” spiral wave in the inner regions of a cool accretion disk. Possible observational manifestations of such a wave are discussed.  相似文献   

The structure of matter flows in semi-detached binaries after the termination of mass transfer     

D. V. Bisikalo  A. A. Boyarchuk  A. A. Kil’pio  O. A. Kuznetsov  V. M. Chechetkin 《Astronomy Reports》2001,45(8):611-619

We present the results of three-dimensional gas-dynamical simulations of matter flows in semi-detached binaries after termination of the mass transfer between the components of the system. The structure of the residual accretion disk is studied. When the mass transfer has ended, the quasi-elliptical disk becomes circular and its structure changes: tidal interactions result in the formation of a second arm in the spiral shock wave. In addition, a condensation (blob) moving through the disk with variable velocity is formed. The blob is maintained by interactions with the arms of the spiral shock and exists essentially over the entire lifetime of the disk. We also show that, for a viscosity corresponding to α～0.01 (typical for observed accretion disks), the lifetime of the residual disk is about 50 orbital periods.  相似文献   

The development of large-scale instability in stellar accretion disks and its influence on the redistribution of angular momentum     

A. Yu. Lugovskii  S. I. Mukhin  Yu. P. Popov  V. M. Chechetkin 《Astronomy Reports》2008,52(10):811-814

The paper continues our studies of large-scale instability arising during shearmotions in stellar accretion disks due to the development of small perturbations. The evolution of a local perturbation introduced into the outer part of a stationary accretion disk is modeled mathematically. The possible formation of large-scale structures that propagate throughout the disk, leading to an appreciable redistribution of angular momentum, is demonstrated.  相似文献   

Morphology of the interaction between the stream and cool accretion disk in a semidetached binary system     

D. V. Bisikalo  A. A. Boyarchuk  P. V. Kaigorodov  O. A. Kuznetsov 《Astronomy Reports》2003,47(10):809-820

We analyze heating and cooling processes in accretion disks in binaries. For realistic parameters of the accretion disks in close binaries (\(\dot M \simeq 10^{ - 12} - 10^7 M_ \odot /yr\) and α?10^?1–10^?2), the gas temperature in the outer parts of the disk is from ～10⁴ to ～10⁶ K. Our previous gas-dynamical studies of mass transfer in close binaries indicate that, for hot disks (with temperatures for the outer parts of the disk of several hundred thousand K), the interaction between the stream from the inner Lagrange point and the disk is shockless. To study the morphology of the interaction between the stream and a cool accretion disk, we carried out three-dimensional modeling of the flow structure in a binary for the case when the gas temperature in the outer parts of the forming disk does not exceed 13 600 K. The flow pattern indicates that the interaction is again shockless. The computations provide evidence that, as is the case for hot disks, the zone of enhanced energy release (the “hot line”) is located beyond the disk and originates due to the interaction between the circumdisk halo and the stream.  相似文献   

Periodic instabilities in protostellar disks with azimuthal magnetic fields     

V. V. Prudskikh 《Astronomy Reports》2013,57(6):440-447

The stability of magnetohydrodynamic oscillations in a protostellar disk with a toroidal magnetic field is analyzed. It is shown that, apart from the aperiodic magnetorotational instability, two other types of periodic instabilities of non-axisymmetric perturbations can exist. The simultaneous presence of azimuthal and vertical components of the wave vector are necessary for these to exist. One instability is due to the inductive winding-up of the azimuthal magnetic field of the wave, and the other arises when the field amplitude is increased by a comoving Hall wave, transferring magnetic field into a region of enhanced field intensity. The bandwidths of the unstable wave numbers are analyzed as a function of the Hall current, the β parameter of a plasma, and the angle between the direction of wave propagation and the plane of the disk. Regions in the accretion disks typical of T Tauri stars are indentified where these instabilities could be most active.  相似文献   

The volume density of gas in disk galaxies with low HI surface densities     

O. V. Abramova  A. V. Zasov 《Astronomy Reports》2011,55(3):202-213

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   

Models of astrophysical decretion-accretion diffusional disks     

A. V. Tutukov  Ya. N. Pavlyuchenkov 《Astronomy Reports》2004,48(10):800-812

The formation of gaseous diffusional accretion-decretion disks is an important stage in the evolution of numerous astronomical objects. Matter is accreted onto the object in the accretion part of these disks, while the angular momentum of the accreted matter is transported from the central region to the periphery in the decretion part. Here, we consider general questions connected with the formation and evolution of diffusive accretion-decretion disks in various astrophysical objects. Such disks can be described using nonstationary diffusion models. The phenomenological parameters of these models are the coefficients in the relations for the characteristic turbulent velocity and mean free path of diffusion elements in the disk. We have developed a numerical technique to compute the disk evolution for a number of models (a massive disk, a disk with continuous accretion, a purely decretion disk). Analytical expressions estimating the basic parameters of accretion-decretion disks are presented. We discuss the relationship between the models considered and the classical α model of an accretion disk.  相似文献   

Formation and evolution of inclined accretion disks in intermediate polars     

A. M. Fateeva  A. G. Zhilkin  D. V. Bisikalo 《Astronomy Reports》2016,60(1):87-98

The results of 3D modeling of the formation of the accretion disks of intermediate polars are presented. A model with misaligned rotation axes of accretor and the orbit is onsidered, in which it is assumed that the white dwarf has a dipolar magnetic field with its symmetry axis inclined to the whitedwarf rotation and orbital axes. The computations show that, in the early stages of formation of the disk, the action of magnetic field is able to create the initial (seed) inclination of the disk. This inclination is then supported mainly by the dynamical pressure of the flow from the inner Lagrangian point L₁. As themass of the disk increases, the inclination disappears. Under certain conditions, the disk inclination does not arise in systems with misaligned white-dwarf rotation and orbital axes. The influence of the magnetic field and asynchronous rotation of the accretor may result in the formation of spiral waves in the disk with amplitudes sufficient to be detected observationally.  相似文献   

Influence of luminosity bursts on properties of protostellar disks     

E. I. Vorobyov  Ya. N. Pavlyuchenkov  P. Trinkl 《Astronomy Reports》2014,58(8):522-536

A (2+ 1)-dimensional numerical model for the formation and evolution of young stellar objects with sub-solar masses is presented. The numerical hydrodynamicall code describing the formation and evolution of a pølar disk in a two-dimensional approximation is supplemented by one-dimensional code for the evolution of the star and an algorithm for establishing the vertical structure of the disk. This code is used to investigate the influence of luminosity bursts with intensities similar to those observed in FU Orionis objects (FUors) on the properties and thermal balance of pølar disks. A model with gravitational instability and fragmentation of the disk, with subsequent migration of the fragments onto the protostar, is used as a basic model for FUors. Typical FUor bursts (L ～ 100 L ^⊙) can appreciably influence the thermal balance of their disks and parent envelopes, leading to an increase in the disk temperature by more than a factor of two. On the other hand, massive fragments in the disk are only weakly perturbed by such bursts, partially due to screening by the disk and partially due to their high temperature brought about by adiabatic heating. Apart from massive fragments, the characteristic thermal time scales are appreciably shorter than the dynamical time scales throughout the radial extent of the disk and envelope; this enables the use of a stationary radiative-transfer equation when determining the vertical structure of the disk.  相似文献   

Stellar subsystems of the galaxy NGC 2366     

N. A. Tikhonov  O. A. Galazutdinova 《Astronomy Reports》2008,52(1):19-26

Hubble Space Telescope archive data are used to perform photometry of stars in seven fields at the center and periphery of the galaxy NGC 2366. The variation of the number density of stars of various ages with galactocentric radius and along the minor axis of the galaxy are determined. The boundaries of the thin and thick disks of the galaxy are found. The inferred sizes of the subsystems of NGC 2366 (Z _thin = 4 kpc and Z _thick = 8 kpc for the thin and thick disks, respectively) are more typical for spiral galaxies. Evidence for a stellar halo is found at the periphery of NGC 2366 beyond the thick disk of the galaxy.  相似文献   

Disk boundaries in spiral galaxies     

D. V. Bizyaev  A. V. Zasov 《Astronomy Reports》2002,46(9):721-730

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 Σ_disk to the critical value for Jeans instability Σ_crit 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 Σ_disk/Σ_crit 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   

A possible mechanism for the formation of tilted disks in intermediate polars     

D. V. Bisikalo  A. G. Zhilkin  P. V. Kaygorodov  V. A. Ustyugov  M. M. Montgomery 《Astronomy Reports》2013,57(5):327-337

Using 3D gas dynamics, we numerically simulate accretion-disk formation in typical cataclysmic variable intermediate polars with dipolar magnetic fields (B _a = 10⁵?5 × 10⁵ G) and misaligned white-dwarf magnetic and rotation axes. Our simulations confirm that a significant misalignment of the axes results in a significant misalignment of the disk to the orbital plane. However, over time, this disk tilt disappears: early in the simulation, the initial particle positions in the rarefied tilted disk are governed solely by the magnetic field of the white dwarf. Due to the increasing disk mass and hence increasing disk gas pressure, the tilted disk eventually becomes decoupled from the magnetic field. The tidal action of the donor leads to a retrograde (i.e., nodal) precession of the tilted disk’s streamlines, and the disk becomes twisted. When the disk tilt is greater than 4°, the incoming gas stream no longer strikes the disk rim (i.e., bright shocked region). Matter is now transported over and under the disk rim to the inner regions of the disk. Over time, the increased mass of inner parts of the disk due to the action of the colinear gas stream returns the inner-disk regions to a colinear configuration. Meanwhile, the outer regions of the tilted, twisted disk become warped. Our simulations suggest that the lifetime of an intermediate polar’s tilted disk could be several tens to thousands of orbital periods.  相似文献   

The possible nature of dips in the light curves of semidetached binaries with stationary disks     

D. V. Bisikalo  P. V. Kaigorodov  A. A. Boyarchuk  O. A. Kuznetsov 《Astronomy Reports》2005,49(9):701-708

  相似文献   

Ring structures in Orion KL     

L. I. Matveenko  P. J. Diamond  D. A. Graham 《Astronomy Reports》2000,44(9):592-610

We present the results of studies of the superfine structure of H₂O maser sources in the Orion Nebula. Powerful, low-velocity, compact maser sources are distributed in eight active zones. Highly organized structures in the form of chains of compact components were revealed in two of these, in the molecular cloud OMC-1. The component sizes are ～0.1 AU and their brightness temperatures are T _b =10¹²?10¹⁶ K. The structures correspond to tangential sections of concentric rings viewed edge-on. The ring emission is concentrated in the azimuthal plane, decreasing the probability of their discovery. The formation of protostars is accompanied by the development of accretion disks and bipolar flows, with associated H₂O maser emission. The accretion disks are in the stage of fragmentation into protoplanetary rings. In a Keplerian approximation, the protostars have low masses, possibly evidence for instability of the systems. Supermaser emission of the rings is probably triggered by precession of the accretion disk. The molecular cloud’s radial velocity is V _LSR=7.74 km/s and its optical depth is τ≈5. The emission from components with velocities within the maser window is additionally amplified. The components’ emission is linearly polarized via anisotropic pumping.  相似文献   

Two-dimensional hydrodynamical modeling of mass transfer in semidetached binaries with asynchronously rotating components     

V. V. Nazarenko  L. V. Glazunova 《Astronomy Reports》2003,47(12):1027-1037

We have modeled the mass transfer in the three semidetached binaries U Cep, RZ Sct, and V373 Cas taking into account radiative cooling both implicitly and explicitly. The systems have asynchronously rotating components and high mass-transfer rates of the order of 10^?6M_⊙/yr; they are undergoing various stages of their evolution. An accreting star rotates asynchronously if added angular momentum is redistributed over the entire star over a time that exceeds the synchronization time. Calculations have indicated that, in the model considered, mass transfer through the point L₁ is unable to desynchronize the donor star. The formation of an accretion disk and outer envelope depends on the component-mass ratio of the binary. If this ratio is of the order of unity, the flow makes a direct impact with the atmosphere of the accreting star, resulting in the formation of a small accretion disk and a relatively dense outer envelope. This is true of the disks in U Cep and V373 Cas. When the component-mass ratio substantially exceeds unity (the case in RZ Sct), the flow forms a large, dense accretion disk and less dense outer envelope. Taking into account radiative cooling both implicitly and explicitly, we show that a series of shocks forms in the envelopes of these systems.  相似文献   

Hall instability in protostellar disks     

V. V. Prudskikh 《Astronomy Reports》2012,56(7):489-495

Non-axial perturbations of a protostellar disk possessing vertical and azimuthal magnetic-field components are studied in the framework of Hall magnetohydrodynamics. The convective transport of the magnetic field by the Hall current leads to instability of fluctuations within a limited interval of wave numbers. The fundamental possibility of the existence of Hall instability in a medium that does not contain an inhomogeneous density distribution, which has not been discussed earlier, is demonstrated. The dependence of the instability increment on both the plasma parameter ?? and the degree of ionization of the protostellar material are analyzed. Possible consequences for weakly ionized astrophysical disks are discussed.  相似文献   

A unified theory for the formation of galactic structures     

V. L. Polyachenko  E. V. Polyachenko 《Astronomy Reports》2004,48(11):877-899

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 Ω_p 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