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.     

The structure of cool accretion disks in semidetached binaries

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.     

Features of the accretion in the EX Hydrae system: Results of numerical simulation

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.     

Three-dimensional hydrodynamical modeling of accretion-disk formation in microquasars. Cen X-3

We have carried out three-dimensional hydrodynamical modeling of the formation of an accretion disk around a compact object due to radiative wind of a massive donor in a close binary system. The massive X-ray binary Cen X-3, which has a precessing accretion disk and may possess relativistic jets, is considered as an example. The computations show that, when the action of the central compact object on the formation of the wind is taken into account, the radiative wind forms an accretion disk with a radius of 0.16 (in units of the orbital separation), which accretes at a rate close to 1 × 10^?8 M _⊙/yr. In this model, the disk is spherically symmetrical and geometrically thick, with a tunnel going from the accretor to the upper layers of the disk along the accretor’s rotational axis at the disk center. The number density of the gas in the tunnel is five orders of magnitude lower than in the disk. The wind-disk interaction at the outer boundary of the disk produces a strong shock (wind-disk shock) directed toward the donor. The black-body emission of the disk and tunnel is nonstationary, and resembles the outbursts observed in Cen X-3. An analysis of the location of the region of nonstationary emission suggests that the outbursts occur in the wind-disk shock.     

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.     

Formation of a radiative wind and accretion disk in microquasars. Generation of flare activity in the disk due to an increase in the supply of material to the Lagrange point L<Subscript>1</Subscript>. The system LMC X-3

Three-dimensional numerical hydrodynamical modeling of a radiative wind and accretion disk in a close binary system with a compact object is carried out, using the massive X-ray binary LMC X-3 as an example. This system contains a precessing disk, and may have relativistic jets. These computations show that an accretion disk with a radius of about 0.20 (in units of the component separation) forms from the radiative wind from the donor when the action of the wind on the central source is taken into account, when the accretion rate is equal to the observed value (about 3.0 × 10^?8 M _⊙/year, which corresponds to the case when the donor overflows its Roche lobe by nearly 1%). It is assumed that the speed of the donor wind at infinity is about 2200 km/s. The disk that forms is geometrically thick and nearly cylindrical in shape, with a low-density tunnel at its center extending from the accretor through the disk along the rotational axis. We have also modeled a flare in the disk due to short-term variations in the supply of material through the Lagrange point L₁, whose brightnesses and durations are able to explain flares in cataclysmic variables and X-ray binaries. The accretion disk is not formed when the donor underfills its Roche lobe by 0.5%, which corresponds to an accretion rate onto the compact object of 2.0 × 10^?9 M _⊙/year. In place of a disk, an accretion envelope with a radius of about 0.03 forms, within which gas moves along very steep spiral trajectories before falling onto the compact object. As in the accretion-disk case, a tunnel forms along the rotational axis of the accretion envelope; a shock forms behind the accretor, where flares occur in a compact region a small distance from the accretor at a rate of about six flares per orbital period, with amplitudes of about 10 ^m or more. The flare durations are two to four minutes, and the energies of individual particles at the flare maximum are about 100–150 keV. These flares appear to be analogous to the flares observed in gamma-ray and X-ray burst sources. We accordingly propose a model in which these phenomena are associated with massive, close X-ray binary systems with component-mass ratios exceeding unity, in which the donor does not fill its Roche lobe. Although no accretion disk forms around the compact object, an accretion region develops near the accretor, where the gamma-ray and X-ray flares occur.     

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.     

An induction accelerator of cosmic rays on the axis of an accretion disk

The structure and magnitude of the electric field created by a rotating accretion disk with a poloidal magnetic field is found for the case of a vacuum approximation along the axis. The accretion disk is modeled as a torus filled with plasma and a frozen-in magnetic field. The dimensions and location of the maximum electric field as well as the energy of the accelerated particles are found. The gravitational field is assumed to be weak.     

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.     

Three-dimensional hydrodynamical modeling of the formation of the accretion disk in the SS 433 binary system

Three-dimensional hydrodynamical modeling of the formation of the accretion disk in the SS 433 binary system is carried out with various types of cooling and numerical grids. These computations show that a thick accretion disk with a height of 0.25–0.30 (in units of the component separation) is formed around the compact object, from a flow with a large radius (0.2–0.3 in the same units) that forms in the vicinity of the inner Lagrangian point. This disk has the form of a flattened torus. The number of orbits of a particle of gas in the disk is 100–150, testifying to a minimal influence of numerical viscosity in these computations. The computations also show that the stream flowing from L₁ is nearly conservative, and spirals in the disk are not formed due to the influence of the donor gravitation.     

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.     

The origin of the photometric and spectral variability of RW Aur

We have studied the relationships between the brightness, color, and emission-line profiles for the classical T Tauri star RW Aur, which displays an intense emission spectrum and signs of accretion and outflow. Spectral and photometric observations carried out from 1995 to 1999 have been analyzed in the framework of magnetospheric accretion concept, where the brightness of the star depends on the level of the non-photospheric continuum (veiling), which, in turn, depends on the accretion rate. We found that (1) the equivalent widths and profiles of broad emission lines vary independently of the brightness of the star, the brightness and color variations are due primarily to absorption in dust clouds formed by the disk wind; (2) at times when the accretion in the line of sight becomes stronger, all broad emission lines are weakened substantially due to the asymmetry of the magnetosphere and screening of the radiating region; (3) the periodic variations of the U-B and B-V color indices are due to the variable contribution of broad emission lines in the photometric bands; (4) the veiling of the photospheric spectrum is not correlated with either the brightness of the star or the intensity of the accretion components.     

Mathematical modeling of a jet in the vicinity of a compact object

An MHD model for the formation of a jet in the vicinity of a compact object with a supersonic accretion regime is constructed, assuming that the plasma is perfectly electrically conducting. A stable, collimated jet of plasma along the z axis symmetric about the plane of the accretion disk is obtained. The parameters of the jet are in good agreement-with available observational data on the structure and properties of jets.     

The mechanism of circumbinary envelope formation in close binaries

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.     

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.     

Periodic Accretion in the T Tauri Binary UZ Tau E

Results of three-dimensional numerical simulations of the gas dynamics of the envelope of the young T Tauri binary star UZ Tau E are considered. The flow structure in the circumstellar envelope of the system is analyzed. It is shown that a regime with the impulsive accretion of matter from the circumstellar disk is realized in the binary system, in which there is a periodic transfer of matter to the accretion disk of the primary component through the accretion disk of the secondary.

     

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.     

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.     

Non-steady-state accretion disks in X-ray novae: Outburst models for Nova Monocerotis 1975 and Nova Muscae 1991

We have fit outbursts of two X-ray novae (Nova Monocerotis 1975=A0620-00 and Nova Muscae GS 1991=1124-683) using a non-steady-state accretion-disk model. The model is based on a new solution for a diffusion-type equation for non-steady-state accretion and describes the evolution of a viscous α disk in a binary system after the peak of the outburst, when the matter in the disk is totally ionized. The accretion rate in the disk decreases according to a power law. We derive formulas for the accretion rate and effective temperature of the disk. The model has three free input parameters: the mass of the central object M, the turbulence parameter α, and the normalization parameter δt. The results of the modeling are compared with the observed X-ray and optical B and V light curves. The estimates for the turbulence parameter α are similar: 0.2–0.4 for A 0620-00 and 0.45–0.65 for GS 1124-683, suggesting a similar nature for the viscosity in the accretion disks around the compact objects in these sources. We have also derived the distances to these systems as functions of the masses of their compact objects.     

Three-dimensional gas-dynamical modeling of changes in the flow structure during the transition from the quiescent to the active state in symbiotic stars

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.