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

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.     

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

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.     

A model for superoutbursts in SU UMa-type binaries

We suggest a new mechanism for the superoutbursts in SU UMa binaries, in which the increase in the accretion rate resulting in a superoutburst is associated with the formation of a spiral “precessional” wave in the inner parts of the disk, where gas-dynamical perturbations are negligible. The existence of such waves was suggested by us previously. The results of three-dimensional gas-dynamical simulations have shown that a considerable increase in the accretion rate (by up to an order of magnitude) is associated with the formation of the precessional wave. The features of the precessional spiral wave can explain both the energy release in the superoutburst and all its observational manifestations. One distinguishing feature of superoutbursts in SU UMa-type stars is the formation of a “superhump” in the light curve. Our model reproduces well both the formation of a superhump and its observational features, including its period, which is up to 3–7% longer than the orbital period, and the detectability of the superhump independent of the orbital inclination of the binary.     

Observational Manifestations of the Precessional Wave in the Accretion Disk of a Cataclysmic Variable Star

Effects due to the interaction of the steam from the inner Lagrangian point with the accretion disk in a cataclysmic variable star are considered. The results of three-dimensional gas-dynamical numerical simulations confirm that the disk thickness in the vicinity of the interaction with the stream is minimum when the component-mass ratio is 0.6. As a consequence, some of the matter from the stream does not collide with the outer edge of the accretion disk, and continues its motion unperturbed toward the accretor. This part of the stream subsequent interacts (collides) with a thickening of the accretion disk due to the presence of a precessional wave in the disk, leading to the appearance of an additional zone of heating at the disk surface. This additional zone of enhanced luminosity (hot spot) is a direct observational manifestation of the precessional wave in the accretion disk.     

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.     

Models of astrophysical decretion-accretion diffusional disks

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.     

Comparison between two-and three-dimensional modeling of the structure of an accretion disk in a binary system

We compare two-and three-dimensional modeling of gas-dynamical processes in the accretion disk of a binary system. The origin of spiral waves and the loss of the angular momentum related to them are considered. It is concluded that a steady state of the disk cannot exist without taking into account t he gas inflow from the donor star.     

Impact of the magnetic field on the structure of accretion disks in semi-detached binaries

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.     

Doppler mapping of the SS Cyg system during outburst

Spectral observations of the SS Cyg system in its active state are used to construct Hβ and Hγ Doppler tomograms. These are compared with analogous tomograms for the quiescent state and synthetic tomograms derived from the results of three-dimensional gas-dynamical modeling. The parameters of the accretion disk during the outburst are estimated. An explanation for the observed flow pattern is proposed, based on a numerical model with an elliptical accretion disk.     

The thickness of accretion α-disks: Theory and observations

Observations of X-ray binaries indicate substantial half-thicknesses for the accretion disks in these systems (up to h/R ≈ 0.25, where h is the disk half-thickness and R its radius), while standard α accretion disks predict appreciably smaller half-thicknesses. We study the theoretical vertical structure of such disks using two independent numerical methods, and show that their maximum half-thicknesses in the subcritical regime cannot exceed h/R ≈ 0.1. We consider various reasons for the apparent increase in the disk thickness, the most probable of which is the presence of matter above the disk in the form of a hot corona that scatters hard radiation from the central source and inner parts of the disk. As a result, the observed thickness of the disk and the illumination of its outer parts effectively increase. This mechanism can also explain both the optical-to-X-ray flux ratio in these systems and the observed parameters of eclipsing X-ray binaries.     

Magnetic-field structure in the accretion disks of semi-detached binary systems

The results of three-dimensional MHD numerical simulations are used to investigate the characteristic properties of the magnetic-field structures in the accretion disks of semi-detached binary systems. It is assumed that the intrinsic magnetic field of the accretor star is dipolar. Turbulent diffusion of the magnetic field in the disk is taken into account. The SS Cyg system is considered as an example. The results of the numerical simulations show the intense generation of a predominantly toroidal magnetic field in the accretion disk. Magnetic zones with well defined structures for the toroidal magnetic field form in the disk, which are separated by current sheets in which there ismagnetic reconnection and current dissipation. Possible observational manifestations of such structures are discussed. It is shown that the interaction of a spiral precessional wave with the accretor’s magnetosphere could lead to quasi-periodic oscillations of the accretion rate.     

Mathematical modeling of the structure of an accretion disk in a stellar binary system

Numerical simulations of gas-dynamical processes taking place in the accretion disk of a stellar binary system are presented. The initial state of the disk is an equilibrium gaseous configuration. Mechanisms for the development of spiral waves and associated variations in the angular momentum of the gas are considered. The influence of the ratio of the binary-component masses and the initial disk configuration are investigated. It is concluded that the existence of a steady-state disk is impossible without a flow of gas from the donor star.     

Formation and evolution of inclined accretion disks in intermediate polars

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.     

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

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.     

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

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.     

Evolution of a Viscous Protoplanetary Disk with Convectively Unstable Regions

The role of convection in the gas-dust accretion disk around a young star is studied. The evolution of a Keplerian disk is modeled using the Pringle equation, which describes the time variations of the surface density under the action of turbulent viscosity. The distributions of the density and temperature in the polar directions are computed simultaneously in the approximation that the disk is hydrostatically stable. The computations of the vertical structure of the disk take into account heating by stellar radiation, interstellar radiation, and viscous heating. The main factor governing evolution of the disk in this model is the dependence of the viscosity coefficient on the radius of the disk. The computations of this coefficient take into account the background viscosity providing the continuous accretion of the gas and the convective viscosity, which depends on the parameters of the convection at a given radius. The results of computations of the global evolution and morphology of the disk obtained in this approach are presented. It is shown that, in the adopted model, the accretion has burst-like character: after the inner part of the disk ($$R < 3$$ AU) is filled with matter, this material is transferred relatively rapidly onto the star, after which the process is repeated. Our results may be useful for explaining the activity of young FU Ori and EX Lup objects. It is concluded that convection may be one of the mechanisms responsible for the non-steady pattern of accretion in protostellar disks.     

Excitation of Turbulence in Protoplanetary Disks around Binary Stars

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.     

Disk wind in young binaries and its impact on the infrared excesses of young stars

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.     

Synthetic Doppler tomograms of gas flows in the binary system IP Peg

We have synthesized Doppler tomograms of gas flows in the binary system IP Peg using the results of three-dimensional gas-dynamical computations. Gas-dynamical modeling in combination with Doppler tomography enables identification of the key elements of flows in Doppler maps without solution of an ill-posed inverse problem. A comparison of the synthetic tomograms with observations shows that, in the quiescent state of the system, the most luminous components are (1) the shock wave induced by interaction between the circumbinary envelope and the stream from the Lagrange point L ₁ (the “hot line”) and (2) the gas condensation at the apogee of the quasi-elliptical disk. Both the single spiral shock wave arm in the gas-dynamical solution and the stream from L ₁ contribute little to the luminosity. In the active state of the system, when the stream from L ₁ does not play an appreciable role and the disk dominates, both areas of enhanced luminosity in the observational tomograms are associated with the two arms of the spiral shock wave in the disk.