Oleg A. Kuznetsov

Three-dimensional numerical simulation of gaseous flow structure in semidetached binaries

The results of numerical simulation of mass transfer in semidetached non-magnetic binaries are presented. We investigate the morphology of gaseous flows on the basis of three-dimensional gas-dynamical calculations of interacting binaries of different types (cataclysmic variables and low-mass X-ray binaries). We find that taking into account a circumbinary envelope leads to significant changes in the stream–disc morphology. In particular, the obtained steady-state self-consistent solutions show an absence of impact between the gas stream from the inner Lagrangian point L1 and the forming accretion disc. The stream deviates under the action of the gas of the circumbinary envelope, and does not cause the shock perturbation of the disc boundary (traditional hotspot). At the same time, the gas of the circumbinary envelope interacts with the stream and causes the formation of an extended shock wave, located on the stream edge. We discuss the implication of this model without hotspot (but with a shock wave located outside the disc) for interpretation of the observations. The comparison of synthetic light curves with observations proves the validity of the discussed gas-dynamical model without hotspot. We have also considered the influence of the circumbinary envelope on the mass transfer rate in semidetached binaries. The obtained features of flow structure in the vicinity of L1 show that the gas of the circumbinary envelope plays an important role in the flow dynamics, and that it leads to significant (in order of magnitude) increase of the mass transfer rate. The most important contribution to this increase is from the stripping of the mass-losing star atmosphere by interstellar gas flows. The parameters of the formed accretion disc are also given in the paper. We discuss the details of the obtained gaseous flow structure for different boundary conditions on the surface of mass-losing star, and show that the main features of this structure in semidetached binaries are the same for different cases. The comparison of gaseous flow structure obtained in two- and three-dimensional approaches is presented. We discuss the common features of the flow structures and the possible reasons for revealed differences.

On the Azimuthal Stability of Shock Waves around Black Holes

Analytical studies and numerical simulations of time-dependent axially symmetric flows onto black holes have shown that it is possible to produce stationary shock waves with a stable position for both ideal inviscid and moderately viscous accretion disks. We perform several two-dimensional numerical simulations of accretion flows in the equatorial plane to study shock stability against nonaxisymmetric azimuthal perturbations. We find a peculiar new result. A very small perturbation seems to produce an instability as it crosses the shock, but after some small oscillations, the shock wave suddenly transforms into an asymmetric closed pattern and stabilizes with a finite radial extent, despite the fact that the inflow and outflow boundary conditions are perfectly symmetric. The main characteristics of the final flow are: (1) The deformed shock rotates steadily without any damping. It is a permanent feature, and the thermal energy content and the emitted energy vary periodically with time. (2) This behavior is also stable against further perturbations. (3) The average shock is still very strong and well defined, and its average radial distance is somewhat larger than that of the original axially symmetric circular shock. (4) Shocks obtained with larger angular momentum exhibit more frequencies and beating phenomena. (5) The oscillations occur in a wide range of parameters, so this new effect may have relevant observational consequences, such as (quasi-) periodic oscillations, for the accretion of matter onto black holes. Typical timescales for the periods are 0.01 and 1000 s for black holes with 10 and 106 M, respectively.

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.

Bending Instability of an Accretion Disk around a Black Hole

We demonstrate that generically an accretion disk around a compact object could have a new type of instability in that the accretion flow need not be symmetric with respect to the equatorial plane even when matter is supplied symmetrically farther out. We find that this behavior is mainly due to the interaction of outgoing matter bounced off the centrifugal barrier and the incoming accretion. We believe that X-ray variability could be developed by this instability.

HYDRODYNAMIC SIMULATIONS OF COUNTERROTATING ACCRETION DISKS

Time-dependent, axisymmetric hydrodynamic simulations have been used to study accretion disks consisting of counterrotating components with an intervening shear layer(s). Con—gurations of this type can arise from the accretion of newly supplied counterrotating matter onto an existing corotating disk. The grid-dependent numerical viscosity of our hydrocode is used to simulate the in—uence of a turbulent viscosity of the disk. First, we consider the case where the gas well above the disk midplane (z ( 0) rotates with angular rate ) )(r) and that well below (z 0) has the same properties but rotates with rate ( )(r). We —nd that there is angular momentum annihilation in a narrow equatorial boundary layer in which matter accretes supersonically with a velocity that approaches the free-fall velocity. This is in accord with the recent analytic model of Lovelace & Chou. The average accretion speed of the disk can be enormously larger than that for a conventional a-disk rotating in one direction. Under some condi- tions the interface between the corotating and counterrotating components shows signi—cant warping. Second, we consider the case of a corotating accretion disk for and a counterrotating disk for r r t In this case we observed, that matter from the annihilation layer lost its stability and propagated r ( r t . inward, pushing matter of inner regions of the disk to accrete. Third, we investigated the case where counterrotating matter in—owing from large radial distances encounters an existing corotating disk. Fric- tion between the in—owing matter and the existing disk is found to lead to fast boundary layer accretion along the disk surfaces and to enhanced accretion in the main disk. These models are pertinent to the formation of counterrotating disks in galaxies and possibly in active galactic nuclei and in X-ray pulsars in binary systems. For galaxies, the high accretion speed allows counterrotating gas to be transported into the central regions of a galaxy in a time much less than the Hubble time. Subject headings: accretion, accretion disksgalaxies: evolutiongalaxies: formation ¨ galaxies: nucleigalaxies: spiralhydrodynamicsHydrodynamic simulations have been used to study accretion disks consisting of counterrotating components with an intervening shear layer(s). Configurations of this type can arise from the accretion of newly supplied counterrotating matter onto an existing corotating disk. The grid-dependent numerical viscosity of our hydro code is used to simulate the influence of a turbulent viscosity of the disk. Firstly, we consider the case where the gas well above the disk midplane rotates with angular rate +Omega(r) and that well below has the same properties but rotates with rate -Omega(r). We find that there is angular momentum annihilation in a narrow equatorial boundary layer in which matter accretes supersonically with a velocity which approaches the free-fall velocity and the average accretion speed of the disk can be enormously larger than that for a conventional alpha-disk rotating in one direction. Secondly, we consider the case of a corotating accretion disk for rr_t. In this case we observed, that matter from the annihilation layer lost its stability and propagated inward pushing matter of inner regions of the disk to accrete. Thirdly, we investigated the case where counterrotating matter inflowing from large radial distances encounters an existing corotating disk. Friction between the inflowing matter and the existing disk is found to lead to fast boundary layer accretion along the disk surfaces and to enhanced accretion in the main disk. These models are pertinent to the formation of counterrotating disks in galaxies and possibly in Active Galactic Nuclei and in X-ray pulsars in binary systems.

Two-dimensional adaptive code for simulation of astrophysical magnetohydrodynamic flows

Integrated adaptive mesh for simulations of two-dimensional (2D) self-gravitating, axially symmetric magnetohydrodynamic (MHD) flows in spherical coordinates is constructed and a new adaptive numer...

Numerical simulation of protostar formation in magnetized molecular cloud cores

The two-dimensional numerical simulations of magnetized molecular cloud collapse has been done using integrated adaptive mesh in spherical coordinates. To take into account non-isothermality of late stages of collapse, we use a kind of parametrical approach to the radiative transfer through the cloud. Several numerical models are presented.

Numerical Simulations of the Astrophysical MHD Flows

Magnetic fields can play an important role in many astrophysical problems. At the present time, sufficient observational data have been gathered about the magnetic field of the interstellar medium, molecular clouds, star formation regions, Bok globules, and young stellar objects (Heiles et all, 1993; Dudorov, 1995; Vallee, 1997). This shows that star formation takes place in rotating magnetized interstellar clouds.

Modeling of Gas Flow Structure in Symbiotic Star Z And in Quiescent and Active States

2D modeling of gas flow structure in a binary system using the modified model on a fine grid has been carried out for the parameters of the classical symbiotic star Z And. The calculations have confirmed the mechanism of quiescent to active state change as a consequence of the transition from the disk accretion to the accretion from the flow proposed in our previous works. The simulations of the flow structure for the active state of the system have been also carried out. The thermonuclear runaway on the accretor has been modeled by the pressure jump on the accretor’s surface. It has been shown that the introduction of the pressure jump leads to the formation of the structure of two shocks and the contact discontinuity in the space between components of the system. The modeled changes of parameters caused by the formation of these shocks are found to be in agreement with the observed brightness changes.

Angular momentum evolution of protostellar clouds

The evolution of angular momentum during the collapse of protostellar clouds is investigated. The criteria for the efficiency of magnetic braking of rotation and angular momentum transfer are obtained analytically. These estimations are confirmed by the results of two-dimensional numerical simulations in the case of a frozen-in magnetic field.