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.     

Over-reflection instability and the developed turbulence in accretion disks in close binaries

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.     

The impact of large-scale turbulence on the redistribution of angular momentum in stellar accretion disks

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.     

Diagnostics for accretion disks around UX Ori stars based on Balmer, Paschen, and Brackett Lines

We have modeled the H_α, H_β, and H_γ (Balmer series), P₁₄ (Paschen series), and Br_γ (Brackett series) hydrogen lines formed in the inner regions of the accretion disk around the Herbig Ae star UX Ori. Our calculations are based on spectra obtained with the Nordic Optical Telescope (NOT) and the IRTF. We computed a grid of non-LTE models for a radiating area in the accretion disk and determined the basic parameters of the lines using the method of Sobolev. Analyzing the theoretical and observed line profiles, equivalent widths, and luminosities, we have estimated the accretion rate and electron-temperature distribution in the inner parts of the accretion disk. The accretion rate of UX Ori is about $\dot M_a = (3 - 10) \times 10^{ - 9} M_ \odot /yr$ , and the temperature distribution is consistent with the power law T(r)=T(r _*)(r/r _*)^?1/n , where the electron temperature near the stellar surface T(r _*) is 15000–20000 K and the power-law index n≈2–3 is about two to three. The resulting value for $\dot M_a $ eliminates problems connected with the application of magnetospheric accretion models to Herbig Ae/Be stars. Another important conclusion is that, at the estimated accretion rate, the energy release is substantially (about two orders of magnitude) lower than the stellar luminosity. Therefore, the optical radiation of UX Ori accretion disks cannot appreciably contribute to the observed variability of these stars, which must be determined mainly by variability in the circumstellar extinction.     

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.     

Formation of accretion disks in close-binary systems with magnetic fields

We have developed a three-dimensional numerical model and applied it to simulate plasma flows in semi-detached binary systems whose accretor possesses a strong intrinsic magnetic field. The model is based on the assumption that the plasma dynamics are determined by the slow mean flow, which forms a backdrop for the rapid propagation of MHD waves. The equations describing the slow motion of matter were obtained by averaging over rapidly propagating pulsations. The numerical model includes the diffusion of magnetic field by current dissipation in turbulent vortices, magnetic buoyancy, and wave MHD turbulence. A modified three-dimensional, parallel, numerical code was used to simulate the flow structure in close binary systems with various accretor magnetic fields, from 10⁵ to 10⁸ G. The conditions for the formation of the accretion disk and the criteria distinguishing the two types of flow corresponding to intermediate polars and polars are discussed.     

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 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 evolution of close binary stars

A review of our current understanding of the physics and evolution of close binary stars with various masses under the influence of the nuclear evolution of their components and their magnetic stellar winds is presented. The role of gravitational-wave radiation by close binaries on their evolution and the loss of their orbital angular momentum is also considered. The final stages in the evolution of close binary systems are described. The review also notes the main remaining tasks related to studies of the physics and evolution of various classes of close binaries, including analyses of collisions of close binaries and supermassive black holes in galactic nuclei. Such a collision could lead to the capture of one of the components by the black hole and the acceleration of the remaining component to relativistic speeds.     

Variations in the accretion rate and luminosity in gravitationally unstable protostellar disks

Self-consistent modeling of a protostar and protostellar disk is carried out for early stages of their evolution. The accretion rate at distances of sevral astronomical units from the protostar is appreciably variable, which is reflected in the protostar’s luminosity. The amplitude of the variations in the accretion rate and luminosity grows together with the sampling period, as a consequence of the nature of gravitationally unstable protostellar disks. A comparison of model luminosity variations with those derived from observations of nearby sites of star formation shows that the model variations are appreciably lower than the observed values for sampling periods of less than 10 years, indicating the presence of additional sources of variability on small dynamical distances from the protostar.     

Elliptical motions of stars in close binary systems

The motion of stars in close binary systems with conservative mass transfer is considered. It is shown that the Paczynski-Huang model that is currently used to determine the variations of the semimajor axis of the relative orbit of the stars is not correct, and leads to large errors in the derived semi-major axis. A new model is proposed, suitable for elliptical stellar orbits. The reaction forces and gravitational forces between the stars and the stream of overflowing matter are taken into account. The possibility of mass transfer in the presence of an accretion disk is considered.     

A possible manifestation of spiral shock waves in the accretion disks of cataclysmic variables

We present the results of three-dimensional numerical simulations of flow structures in binary systems with spiral shock waves. Variations of the mass-transfer rate perturb the equilibrium state of the accretion disk; consequently, a condensation (blob) behind the shock breaks away from the shock front and moves through the disk with variable speed. Our computations indicate that the blob is a long-lived formation, whose mean parameters do not vary substantially on timescales of several tens of orbital periods of the system. The presence of the spiral shocks maintains the compact blob in the disk: it prevents the blob from spreading due to the differential motion of matter in the disk, and dissipative spreading on this timescale is negligible. A number of cataclysmic variables display periodic or quasi-periodic photometric variations in their light curves with characteristic periods ～0.1–0.2P _orb, where P _orb is the orbital period. The blobs formed in systems with spiral shock waves are examined as a possible origin for these variations. The qualitative (and, in part, quantitative) agreement between our computations and observations of IP Peg and EX Dra provides evidence for the efficacy of the proposed model.     

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.     

Photoelectric lunar-occultation observations of several close binary stars

The results of photoelectric lunar-occultation observations for several stars are presented. Some of these are obvious close binary systems, while others are suspected close binaries or multiple systems, or show evidence for the presence of complex structure of some other kind. It is expected that new, efficient interferometric systems will enable detailed studies of the structure of many “ordinary” stars with an angular resolutions of 10^?4–10^?5 arcsec.     

Evolution of the masses of neutron stars in binary systems

We study the growth of the masses of neutron stars in binary systems due to the accumulation of mass from the optical donors accreted onto the neutron-star surface. Possible scenarios for this accretion are considered. The masses and magnetic-field strengths of radio pulsars derived using population-synthesis methods are compared to the observational data. The population-synthesis analysis indicates that a neutron star can increase its mass from the standard value of m _x ? 1.35M_⊙ to the Oppenheimer-Volkoff limit, m _x ? 2.5M_⊙, via accretion from a companion.     

High-velocity stars as a result of encounters between stars and massive binary black holes in galactic nuclei

Numerical simulations of the motions of stars in the gravitational fields of binary black holes with various component mass ratios have been carried out. Two models are considered: (1) the two-body problem with two fixed centers; (2) the general three-body problem. The first model is applicable only over short times Δt ? T, where T is the period of the binary system. The second model is applicable at all times except for during close encounters of stars with one of the binary components, r ≤ 0.00002 pc, where r is the distance from the star to the nearer black hole. In very close passages, relativistic corrections must be taken into account. Estimates of the probability of formation of high-velocity stars as a result of such interactions are obtained. It is shown that this mechanism is not suitable for the nucleus of our Galaxy due to the probable absence of a second massive black hole in the central region of the Galaxy.     

A model for the population of binary stars in the Galaxy

A comparative investigation of the population of Galactic binary stars is performed for two modes of star formation: star formation at a constant rate over 10¹⁰ yrs, and a burst of star formation that reprocesses the same mass of gas into stars over 10⁹ yrs. Estimates are presented for the star-formation rates and populations of about 100 types of binaries and the products of their evolution. For most close binary systems, the models depend only weakly on the common-envelope parameter α_ce.     

Accurate relative positions and motions of poorly studied binary stars

We have studied the relative motions of the components of 25 visual binary and multiple stars for which only a modest number of observations of various types had accumulated due to their small relative component motions. A uniform series of photographic observations obtained over many years on the 26-inch refractor of the Pulkovo Observatory are used to calculate the parameters of the apparent relative positions and motions, which can be used for further kinematic and dynamical studies of these stars. A comparison of the relative and proper motions of the components indicates that they are physically bound. Optical components are identified in six systems??for the first time in the three systemsWDS00082+6217, ADS 830, and ADS 7361. The relative motions in ADS 861 and ADS 12925 suggest the presence of perturbing invisible companions, but this requires confirmation based on more accurate observations. The apparent motion parameters method yields a family of orbits with a minimum period of 26 000 yrs for the nearby wide visual binary Gliese 745, for which the parallax and relative radial velocity of the components are known.     

Spectral distribution of the polarized radiation from standard accretion disks in Active Galactic Nuclei: Observational analysis

Spectropolarimetric observations of a number of Active Galactic Nuclei obtained using the SCORPIO-2 aperture focal reducer installed on the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences are used to estimate physical parameters of these objects. The measured polarization and its wavelength dependence are consistent with the expectations of a standard accretion-disk model taking into account the effect of Faraday depolarization over the mean free path of the emitted photons. Estimates of the magnetic field in the accretion disk near the innermost stable orbit and the spin of the accreting central black hole are obtained. It is concluded that supermassive black holes with standard accretion disks and equal magnetic and radiative pressures are primarily Kerr black holes.     

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.