共查询到20条相似文献，搜索用时 31 毫秒
1.
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
2.
E. P. Velikhov K. R. Sychugov V. M. Chechetkin A. Yu. Lugovskii A. V. Koldoba 《Astronomy Reports》2012,56(2):84-95
We investigate the role of the magnetic field in the collapse of a gas-dust cloud into a massive gravitating object. Observations
of one such object (G31.41) indicate that the magnetic field has an hourglass shape oriented along the rotation axis of the
matter, due to the freezing-in of the magnetic-field lines in the accreting matter. It is believed that accretion in stellar
disks is associated with the transport of angular momentum from the center to the periphery, which could be initiated by large-scale
vortex structures arising in the presence of unstable rotational flows of matter. The numerical simulations have established
that the equilibrium configuration of a gas-dust disk rotating in a spherically symmetrical gravitational potential is subject
to the development of strong instability in the presence of a weak magnetic field. It is shown that the development of instability
leads to a transport of angular momentum to the disk periphery by large-scale vortex structures, together with the accretion
of matter onto the gravitating object. The magnetic-field lines near the equator take on a chaotic character, but an hourglass
configuration is observed near the rotation axis, in agreement with observations. 相似文献
3.
Ye. P. Velikhov A. Yu. Lugovsky S. I. Mukhin Yu. P. Popov V. M. Chechetkin 《Astronomy Reports》2007,51(2):154-160
We consider the origin and development of large-scale turbulence in a shear flow in a stellar accretion disk. The ratio of the kinetic energy of vortices originating in the turbulent flow and the total initial kinetic energy of the rotating disk is essentially constant. The large-scale structures that form are able to redistribute the angular momentum without any appreciable heating of the matter. 相似文献
4.
A. M. Fridman D. V. Bisikalo A. A. Boyarchuk Yu. M. Torgashin L. A. Pustil’nik 《Astronomy Reports》2009,53(8):750-768
The results of studies of the over-reflection mechanism for the development of hydrodynamical instability in the accretion disks of close binary stars are presented. The driving of this instability is shown to result in the generation of regular, large-scale, spiral-vortex structures and the development of turbulence in the disk. The derived estimates of the coefficient of turbulent viscosity are in good agreement with the observations, and are able to explain the high rate of angular-momentum transfer and the measured accretion rate. The developed theoretical model is used with the observational data to derive a power-law spectrum for the developed turbulence. 相似文献
5.
This paper continues a series of studies on three-dimensional hydrodynamical modeling of mass transfer in the binary system β Lyr. The model takes into account the stellar wind from the donor star, which outflows at a rate of , as demonstrated by radio observations. This stellar wind should appreciably influence the formation of the envelope in the binary. Computations have shown that the interaction of the matter flow from the Lagrangian point L1 and the accretor wind leads to the formation of an optically and geometrically thick gaseous envelope around the accretor. The matter flow meets the accretor wind, spreads out, accumulates over the outer edge of the wind, and forms a geometrically thick envelope (disk). The wind flows freely at the center of the disk, over the accretor poles. Jet-like structures arise beyond the wind-propagation region, above the thick accretion disk. The matter flowing from the outer edge of the disk interacts with the donor wind, leading to the formation of a standing shock between L1 and the outer edge of the disk, in the direction corresponding to orbital phase 0.25. This shock is able to explain the origin of the X-ray radiation from the binary β Lyr. 相似文献
6.
D. V. Bisikalo A. A. Boyarchuk E. Yu. Kilpio O. A. Kuznetsov 《Astronomy Reports》2002,46(12):1022-1029
A new mechanism is proposed to account for transitions between the quiescent and active states of symbiotic stars. A numerical study of the gas dynamics of the flows in the symbiotic star Z And shows that even small variations in the velocity of the wind from the cool giant can abruptly change the flow structure near the hot component. Such changes alter the accretion regime as the wind velocity increases: disk accretion makes a transition to accretion from the flow. Our calculations indicate that the accretion rate increases by a factor of several tens over a short time interval (～0.1 of the orbital period) during the rearrangement of the flow, when the accretion disk is destroyed. 相似文献
7.
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
8.
9.
M. Mitsumoto B. Jahanara T. Matsuda K. Oka D. V. Bisikalo E. Yu. Kilpio H. M. J. Boffin A. A. Boyarchuk O. A. Kuznetsov 《Astronomy Reports》2005,49(11):884-891
The results of three-dimensional modeling of the flow structure in the classical symbiotic system Z Andromedae are presented. Outbursts in systems of this type occur when the accretion rate exceeds the upper limit of the steady-burning range. Therefore, in order to realize the transition from a quiescent to an active state, it is necessary to find a mechanism capable of sufficiently increasing the accretion rate on the time scales typical for outburst development. Our calculations provide support for a mechanism for the transition from quiescence to outburst in classical symbiotic systems suggested earlier based on two-dimensional calculations (Bisikalo et al., 2002). Our results show that an accretion disk forms in the system for a wind velocity of 20 km s?1. The accretion rate for the solution with the disk is ～22.5–25% of the mass-loss rate of the donor, which is ～4.5?5 × 10?8M⊙ yr?1 for Z And. This value is in agreement with the steady-burning range for the white-dwarf masses usually accepted for this system. When the wind velocity increases from 20 to 30 km s?1, the accretion disk is destroyed and the disk material falls onto the accretor surface. This process is followed by an approximately twofold jump in the accretion rate. The resulting growth in the accretion rate is sufficient so as to exceed the upper limit of the steady-burning range, thus bringing the system into an active state. The time during which the accretion rate is above the steady-burning value is in very good agreement with observations. Our analysis leads us to conclude that small variations in the donor wind velocity can lead to the transition from disk accretion to wind accretion and, as a consequence, to the transition from a quiescent to an active state in classical symbiotic stars. 相似文献
10.
A. Yu. Sytov P. V. Kaigorodov D. V. Bisikalo O. A. Kuznetsov A. A. Boyarchuk 《Astronomy Reports》2007,51(10):836-846
We consider the structure and formation of the circumbinary envelopes in semi-detached binary systems. Three-dimensional numerical simulations of the gas dynamics are used to study the flow pattern in a binary system after it has reached the steady-state accretion regime. The outer parts of the circumbinary envelope are replenished by periodic ejections from the accretion disk and circum-disk halo through the vicinity of the Lagrange point L3. In this mechanism, the shape and position of a substantial part of the disk is specified by a precessional density wave. On timescales comparable to the orbital period, the precessional wave (and hence an appreciable fraction of the disk) will be virtually stationary in the observer’s frame, whereas the positions of other elements of the flow will vary due to the orbital rotation. The periodic variations of the positions of the disk and the bow shock formed when the inner parts of the circumbinary envelope flow around the disk result in variations in both the rate of angular-momentum transfer to the disk and the flow structure near L3. All these factors lead to a periodic increase of the matter flow into the outer layers of the circumbinary envelope through the vicinity of L3. The total duration of the ejection is approximately half the orbital period. 相似文献
11.
We consider disk accretion resulting purely from the loss of angular momentum due to the outflow of plasma from a magnetized disk. In this limiting case, the dissipation due to the viscosity and finite electrical conductivity of the plasma can be neglected. We have obtained self-consistent, self-similar solutions for dissipationless disk accretion. Such accretion may result in the formation of objects whose bolometric luminosities are lower than the flux of kinetic energy in the ejected material. 相似文献
12.
P. B. Isakova A. G. Zhilkin D. V. Bisikalo A. N. Semena M. G. Revnivtsev 《Astronomy Reports》2017,61(7):560-572
A two-dimensional numerical model in the axisymmetric approximation that describes the flow structure in the magnetosphere of the white dwarf in the EX Hya system has been developed. Results of simulations show that the accretion in EX Hya proceeds via accretion columns, which are not closed and have curtain-like shapes. The thickness of the accretion curtains depends only weakly on the thickness of the accretion disk. This thickness developed in the simulations does not agree with observations. It is concluded that the main reason for the formation of thick accretion curtains in the model is the assumption that the magnetic field penetrates fully into the plasma of the disk. An analysis based on simple estimates shows that a diamagnetic disk that fully or partially shields the magnetic field of the star may be a more attractive explanation for the observed features of the accretion in EX Hya. 相似文献
13.
We discuss characteristic features of the magnetic gas-dynamical structure of the flows in a semi-detached binary system obtained from three-dimensional simulations, assuming that the intrinsic magnetic field of the accreting star is dipolar. The turbulent diffusion of the magnetic field is taken into account. The SS Cyg system is considered as an example. Including the magnetic field can alter the basic parameters of the accretion disk, such as the accretion rate and the characteristic density. The magnetic field in the disk is primarily toroidal. 相似文献
14.
Typical changes of the accretion-disk structures in intermediate polars are studied as a function of the inclination of the accretor’s magnetic field. Thre-dimensional numerical modeling was performed for seven differentmagnetic-axis inclinations. The results showthat the system forms a magnetosphere region, and that column accretion occurs. The action of the magnetic field tilts the inner parts of the disk along the magnetic axis of the accretor. The magnetic-field inclination appreciably influences matter transfer in the disk and accretion processes. Generation of toroidal magnetic field, magnetic braking, and alignment of the dipole magnetic field result in oscillations of the accretion rate. A direct relationship between the field inclination and the oscillation amplitude is found, as well as an inverse relationship between the field inclination and the oscillation period. 相似文献
15.
We present the results of three-dimensional simulations of matter flows in semidetached binary systems with various viscosities. In low-viscosity systems, the flow structure displays the same qualitative features as in high-viscosity computations. A self-consistent solution shows the absence of a shock interaction between the stream flowing from the inner Lagrange point and the forming accretion disk (or hot spot) for any viscosity. 相似文献
16.
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
17.
The structures of plasma flows in close binary systems whose accretors have strong intrinsic magnetic fields are studied. A close binary system with the parameters of a typical polar is considered. The results of three-dimensional numerical simulations of the matter flow from the donor into the accretor Roche lobe are presented. Special attention is given to the flow structure in the vicinity of the inner Lagrangian point, where the accretion flow is formed. The interaction of the accretion-flow material from the donor’s envelope with the magnetic field of the accretor results in the formation of a hierarchical structure of the magnetosphere, because less dense areas of the accretion flow are stopped by the magnetic field of the white dwarf earlier than more dense regions. Taking into account this kind of magnetosphere structure can affect analysis results and interpretation of the observations. 相似文献
18.
Turbulence in the Accretion Disks in Semi-Detached Binary Systems with Various Component-Mass Ratios
Results of three-dimensional gas-dynamical numerical simulations of the structure of matter flows in semi-detached binary systems with various component-mass ratios are considered. The main elements of the flows in the models studied are described. The characteristics of density waves inside the accretion disk for various component-mass ratios are considered. The influence of the precessional density wave on the development of instability in the accretion disks and the characteristics of developing turbulence are analyzed. Values of the Shakura–Syunyaev coefficient α for the simulated systems are calculated. 相似文献
19.
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
20.
V. P. Grinin 《Astronomy Reports》2002,46(5):380-389
We consider the effect of binarity of young stars on the spectral energy distribution of the IR radiation from circumstellar dust. The formation of a common dust envelope in a binary system with a low-mass secondary component is strongly affected by the disk wind from the secondary. The small velocities in peripheral areas of the wind are such that it can be partially or entirely captured by the primary, even when the distance between the components is up to several astronomical units. As a result, an envelope with a rather complex spatial and kinematic structure is formed. Its mass is many orders of magnitude smaller than that of the accretion disk around the binary. However, the thermal radiation emitted by dust particles of the envelope can be comparable to the total radiation of the accretion disk. This result is discussed in the context of the deficit at near-IR wavelengths (2–10µm) in current models for accretion disks around young stars. 相似文献