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Most hot Jupiters have extended envelopes of gas that spread beyond their Roche lobes. A typical envelope is weakly gravitationally bound to the planet and, therefore, its structure and properties are strongly affected by stellar wind disturbances, for example, coronal mass ejections. Earlier, we performed gas-dynamic modeling of the interaction of a narrow coronal mass ejection (CME) with the envelope of the hot Jupiter HD 209458b. In this study, we investigate the influence of the planet’s magnetic field and stellar wind on the structure and dynamics of the HD 209458b envelope exposed to a similar CME. For this, an MHD model of the CME interaction with the envelope was developed. It was assumed that the field of the planet had a generally accepted value and corresponded to 1/10 of the magnetic moment of Jupiter. The field of the star was assumed to be weak (10–3 G), which ensured the super-Alfven flow of the stellar wind around the planet. The comparison of the MHD simulation with gas-dynamic calculations shows that, for the adopted values of the fields of the planet and the star, the effect of the magnetic field on the envelope is not decisive and the qualitative picture of the flow does not change. At the same time, taking magnetic fields into account leads to a change in the quantitative characteristics of the envelope and the mass loss rate, which can be important in determining the evolutionary status of the exoplanet.

We performed a three-dimensional numerical MHD simulation of the flow structure in the asynchronous polar CD Ind during the switching phases between the magnetic poles of the accretor—a white dwarf. To account for fast processes, a numerical non-stationary model has been developed. The calculations were performed under the assumption that the magnetic field of the accretor has a shifted dipole configuration. Based on the results of calculations, maps of hot spots on the surface of the accretor and light curves during the magnetic pole switching phases were constructed. We showed that in the adopted configuration of the magnetic field, when the field parameters at the south and north poles are different, the flow structure changes in different ways, depending on which pole the switching takes place. As a consequence, significant differences are also observed in the nature of the change in the light curves at different switching times. This circumstance allows us to hope that a comparison of the observed and synthetic light curves can provide information on the actual configuration of the magnetic field in the system.

This paper presents the results of calculating the efficiency of solar wind proton charge exchange as a function of variations in the column density of hydrogen atoms in the extended corona of Mars. The presence of regular density variations has been known for a long time and has been associated with the change of seasons on Mars. However, sporadic density variations in the upper Martian atmosphere were detected recently as well. They were caused by various processes in the underlying atmosphere, such as ejection of water vapor and ice particles at altitudes up to 100 km due to global dust storms, which was discovered in the Mars Express and MAVEN observations. Taking into account the variations in the column density with the observed amplitude up to one order of magnitude is absolutely necessary to correctly consider the interaction of solar wind protons with the Martian atmosphere. The calculations using the kinetic Monte Carlo model revealed that with a 2- and 5-fold increase in the column density of H atoms in the corona of Mars, the charge exchange efficiency also increases and reaches 6% and 8%, respectively, as compared to the base value of 4%. The energy spectrum of hydrogen atoms penetrating into the Martian atmosphere does not change and remains identical in structure to the spectrum of undisturbed solar wind protons. These estimates, combined with the previously developed kinetic model of the proton and hydrogen atom precipitation into the planetary atmosphere, make it possible to do the following: to trace all the stages of the penetration of the undisturbed solar wind protons into the dense layers of the atmosphere, as well as interpret the observed characteristics of proton auroras depending on the variations of atomic hydrogen content in the Martian corona.

—For the case of the sub-Alfven regime of the stellar wind flyby around the planet, the flow structure in the vicinity of a hot Jupiter have been studied by three-dimensional numerical simulation. As an example, the hot Jupiter HD 209458b was considered. The planet’s magnetic moment was set equal to 10% of the magnetic moment of Jupiter. The average magnetic field at the parent star’s surface was taken to be 0.5 G, corresponding to a relatively strong wind field. Under such conditions, calculations have shown that a quasi-closed ionospheric (gas) envelope with an induced shockless magnetosphere forms around a hot Jupiter. Under the influence of the strong magnetic field of stellar wind, envelope matter outflowing from the inner Lagrange point does not move along the ballistic trajectory, but rather moves along magnetic field lines of the wind toward the star. Therefore, for the sub-Alfven flow regime, a new ionospheric envelope type of hot Jupiters is considered to be discovered, supplementing the classification suggested earlier based on results of purely gas-dynamic modeling. Possible distinctive features of the observational manifestations of such envelopes are discussed.

The flow structure in the asynchronous polar CD Ind was investigated in the approximation of an offset dipole. Computation results made it possible to identify such system features as the drift of hot spots over the white dwarf surface, the flow structure dependence on the phase of the beat period, and the magnetic pole switching. To study the flow structure, a three-dimensional numerical MHD model based on the approximation of modified magnetic hydrodynamics was used. Numerical calculations were performed for ten phases of the beat period at a constant rate of mass transfer. In addition, for a more detailed study of the poles switching, additional calculation series were carried out in the corresponding phase ranges of the beat period. The energy release zones during the spin-orbit period shift in longitude on average by 20°, which corresponds to 0.05 of the orbital period phase. The magnetic pole switching occurs in a time that does not exceed 0.1 of the beat period. In the middle of this process, accretion occurs on both poles with the same intensity, and the flow resembles an arch. Based on the calculation results, synthetic light curves were constructed for the optical range of the spectrum.

In this paper, the effect of hot spots movement by accretor surface on the appearance of bolometric light curves for two types of polars - synchronous V808 Aur and asynchronous CD Ind is studied. The analysis was carried out under the assumption of a dipole configuration of the magnetic field, in which the axis of the dipole passes through the accretor center. It is shown that a noticeable shift of the flow maximum at the light curve corresponding to the position of the spots in synchronous polars is determined by a change in the magnitude of mass transfer rate. At the same time, the maximum deviation of the spots from the magnetic poles was 30°. In asynchronous polars, assuming a constant of the mass transfer rate, the spots movement caused by a change in the orientation of the dipole axis relative to the donor has a significant effect on the appearance of light curve. The greatest displacement of the spots from the magnetic poles, which equals to 20°, was observed at the moments when the accretion jet switched from one pole to the other. It is concluded that the comparison of synthetic and observational light curves provides an opportunity to study the physical properties of polars.

We numerically investigate the gravitational collapse of rotating magnetic protostellar clouds. The simulations are performed using 2D MHD code ‘Enlil’. The code is based on TVD scheme of increased order of accuracy. We developed a model of the initially non-uniform cloud, which self-consistently treats gas density and large-scale magnetic field distribution. Simulation results for the typical parameters of a solar mass cloud are presented. In agreement with our previous results for the uniform cloud, the isothermal collapse of the non-uniform cloud results in formation of hierarchical structure of the cloud, consisting of flattened envelope and thin quasi-magnetostatic primary disk near its equatorial plane. The non-uniform cloud collapses longer than the uniform one, since the magnetic field is dynamically stronger at the periphery of the cloud in the former case.

We investigate collapse of magnetic protostellar clouds of mass $10$ and $1 M_{\odot}$. The collapse is simulated numerically using the two-dimensional magnetohydrodynamic (MHD) code `Enlil'. The simulations show that protostellar clouds acquire a hierarchical structure by the end of the isothermal stage of collapse. Under the action of the electromagnetic force, the protostellar cloud takes the form of an oblate envelope with the half-thickness to radius ratio $Z/R \sim (0.20-0.95)$. A geometrically and optically thin primary disk with radius $0.2-0.7 R_0$ and $Z/R \sim (10^{-2}-10^{-1})$ forms inside the envelope, where $R_0$ is the initial radius of the cloud. Primary disks are the structures in magnetostatic equilibrium. They form when the initial magnetic energy of the cloud exceeds $20 \%$ of its gravitational energy. The mass of the primary disk is $30-80 \%$ of the initial mass of the cloud. The first hydrostatic core subsequently forms in the center of the primary disc. We discuss the role of primary disks in the further evolution of clouds, as well as possible observational appearance of the internal hierarchy of the collapsing cloud from the point of view of the features of the magnetic field geometry and the distribution of angular momentum at different levels of the hierarchy, Comment: Published in Astronomy Reports, 9 pages, 4 figures, 3 tables

In the paper a scenario of an electromagnetic response formation from the merging of two black holes is considered. In this scenario it’s assumed that the binary black hole is surrounded by an accretion disk. As a result of the black holes merging and mass loss, the accretion disk experiences a disturbance, which is accompanied by shock waves propagation of sufficiently high intensity. Heating of matter by shock waves leads to a sharp increase in the flux of electromagnetic radiation from the disk. The paper includes a calculated light curve, radiation spectrum, and the estimation of characteristic duration of the flare. This method can be used to discover of electromagnetic responses from gravitational-wave events, which registered by the LIGO (Laser Interferometer Gravitational-Wave Observatory) and the Virgo detectors. Supporting the registration of gravitational-wave events by observations in the electromagnetic channel has far-reaching prospects, since it corresponds to the multi-messenger approach to the study of astrophysical objects.

The structure of the flows in the eclipsing polar V808 Aur is studied. Comparison of computations with observed light curves is used to identify and clarify a number of features in the system, such as the drift of the hot spot—the place where the stream from the inner Lagrange point approaches the surface of the white dwarf, changes in the brightness in the secondary minimum, a dip in the light curve in the high state before entering the eclipse, the asymmetrical eclipse profile in high state, and others. A three-dimensional numerical MHD model based on the approximation of modified magnetic hydrodynamics is used in these studies. Numerical computations were carried out for several mass-exchange rates corresponding to different states of activity of the V808 Aur system. The computations show that, as the mass-exchange rate increases, the length of the ballistic part of the accretion stream increases, leading to changes in the spatial configuration of the flow and an appreciable drift of the region of energy release on the surface of the white dwarf (by up to 30° in longitude). These changes in the flow structure lead to effects in the light curve that are in good agreement with the available observations.

We show that the orbits of exoplanets of the "hot Jupiter" type, as a rule, are located close to the Alf\'{v}en point of the stellar wind of the parent star. At this, many hot Jupiters can be located in the sub-Alf\'{v}en zone in which the magnetic pressure of the stellar wind exceeds its dynamic pressure. Therefore, magnetic field of the wind must play an extremely important role for the flow of the stellar wind around the atmospheres of the hot Jupiters. This factor must be considered both in theoretical models and in the interpretation of observational data. The analysis shows that many typical hot Jupiters should have shock-less intrinsic magnetospheres, which, apparently, do not have counterparts in the Solar System. Such magnetospheres are characterized, primarily, by the absence of the bow shock, while the magnetic barrier (ionopause) is formed by the induced currents in the upper layers of the ionosphere. We confirmed this inference by the three-dimensional numerical simulation of the flow of the parent star stellar wind around the hot Jupiter HD 209458b in which we took into account both proper magnetic field of the planet and magnetic field of the wind., Comment: Accepted in Astronomy Reports, 23 pages, 10 figures

Hot Jupiters have extended gaseous (ionospheric) envelopes, which extend far beyond the Roche lobe. The envelopes are loosely bound to the planet and, therefore, are strongly influenced by fluctuations of the stellar wind. We show that, since hot Jupiters are close to the parent stars, magnetic field of the stellar wind is an important factor defining the structure of their magnetospheres. For a typical hot Jupiter, velocity of the stellar wind plasma flow around the atmosphere is close to the Alfvén velocity. As a result stellar wind fluctuations, such as coronal mass ejections, can affect the conditions for the formation of a bow shock around a hot Jupiter. This effect can affect observational manifestations of hot Jupiters.

A means of generating the radio emission of polars is proposed, based on cyclotron radiation of thermal electrons in a fluctuating magnetic field. The source of these fluctuations is Alfven turbulence. Expressions for the radiation spectrum and degree of polarization are obtained. The radio flux from the accretion flow is computed, using the polar AM Her as an example. The proposed model for the emission can reproduce the observed fluxes in the VLA frequency range with realistic plasma characteristics.

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, that 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 used 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., 21 pages, 11 figures

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 material 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 envelope of the donor 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 captured 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., 16 pages, 6 figures

In this paper, the characteristics of hot spots on an accretor surface are investigated for two types of polars: the eclipsing synchronous polar V808 Aur and the non-eclipsing asynchronous polar CD Ind in configuration of an offset and non-offset magnetic dipole. The drift of hot spots is analyzed based on the results of numerical calculations and maps of the temperature distribution over the accretor surface. It is shown that a noticeable displacement of the spots is determined by the ratio of ballistic and magnetic parts of the jet trajectory. In the synchronous polar, the dominant influence on the drift of hot spots is exerted by variations in the mass transfer rate, which entail a change in the ballistic part of the trajectory. It was found that when the mass transfer rate changes within the range of 10−10M⊙/year to 10−7M⊙/year, the displacement of the hot spot in latitude and longitude can reach 30∘. In the asynchronous polar, a change in the position of hot spots is mainly defined by the properties of the white dwarf magnetosphere, and the displacement of hot spots in latitude and longitude can reach 20∘.

The influence of the dipolar magnetic field of a “hot Jupiter” with the parameters of the object WASP-12b on the mass-loss rate from its atmosphere is investigated. The results of three-dimensional gas-dynamical and magnetohydrodynamical computations show that the presence of a magnetic moment with a strength of ~0.1 the magnetic moment of Jupiter leads to appreciable variations of the matter flow structure. For example, in the case of the exoplanet WASP-12b with its specified set of atmospheric parameters, the stream from the vicinity of the Lagrange point L1 is not stopped by the dynamical pressure of the stellar wind, and the envelope remains open. Including the effect of the magnetic field leads to a variation in this picture—the atmosphere becomes quasi-closed, with a characteristic size of order 14 planetary radii, which, in turn, substantially decreases the mass-loss rate by the exoplanet atmosphere (by~70%). This reduction of the mass-loss rate due to the influence of the magnetic fieldmakes it possible for exoplanets to form closed and quasi-closed envelopes in the presence of more strongly overflowing Roche lobes than is possible without a magnetic field.

The article presents a review of scientific problems and methods of ultraviolet astronomy, focusing on perspective scientific problems (directions) whose solution requires UV space observatories. These include reionization and the history of star formation in the Universe, searches for dark baryonic matter, physical and chemical processes in the interstellar medium and protoplanetary disks, the physics of accretion and outflows in astrophysical objects, from Active Galactic Nuclei to close binary stars, stellar activity (for both low-mass and high-mass stars), and processes occurring in the atmospheres of both planets in the solar system and exoplanets. Technological progress in UV astronomy achieved in recent years is also considered. The well advanced, international, Russian-led Spektr-UV (World Space Observatory—Ultraviolet) project is described in more detail. This project is directed at creating a major space observatory operational in the ultraviolet (115–310 nm). This observatory will provide an effective, and possibly the only, powerful means of observing in this spectral range over the next ten years, and will be an powerful tool for resolving many topical scientific problems.

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.

A mechanism for the formation of humps in the light curves of WZ Sge cataclysmic variables is proposed, based on the results of three-dimensional simulations of the gas dynamics in the V455 And system. Analysis of the computed flow structure shows the formation of an accretion disk and four shocks in the system, located at the outer edge of the disk. Moreover, a precessional spiral density wave forms in the accretion disk, which is essentially at rest in the observer’s frame. As a consequence, all four shocks (two arms of a tidal shock, a “hot line,” and an outgoing shock) pass through the location of the precessional density wave during each rotation of the disk. The interaction of the precessional spiral density wave with the shocks leads to an additional local density enhancement in the vicinity of the shocks. This strengthens the shocks and enhances the energy release at the shock fronts; this, in turn, should increase the brightness of the system, which could be observed as a hump in the light curve. The shift of the humps observed in the V455 And light curve in orbital phase as a function of the observing epoch is easy to explain in this picture, as a consequence of the difference in the velocities of the retrograde precession of the density wave and the orbital rotation of the binary. The variation in the observed number of humps can be explained by the presence of four, rather than two, strong shocks in the flow pattern and the conditions for their visibility at a given observing time.

We present new broad band optical photometry of two magnetic cataclysmic variable stars, the asynchronous polar BY Camelopardalis and the short period polar FL Ceti. Observations were obtained at the 2.1-m Otto Struve Telescope of McDonald Observatory with 3s and 1s integration times respectively. In an attempt to understand the observed complex changes in accretion flow geometry observed in BY Cam, we performed full 3D MHD simulations assuming a variety of white dwarf magnetic field structures. We investigate fields with increasing complexity including both aligned and non-aligned dipole plus quadrupole field components. We compare model predictions with photometry at various phases of the beat cycle and find that synthetic light curves derived from a multipolar field structure are broadly consistent with optical photometry. FL Ceti is observed to have two very small accretion regions at the foot-points of the white dwarf’s magnetic field. Both accretion regions are visible at the same time in the high state and are about 100 degrees apart. MHD modeling using a dipole plus quadrupole field structure yields quite similar accretion regions as those observed in FL Ceti. We conclude that accretion flows calculated from MHD modeling of multi-polar magnetic fields produce synthetic light curves consistent with photometry of these magnetic cataclysmic variables.

A numerical hydrodynamical model for the evolution of spherically symmetric collapsing clouds, designed for the calculation of the thermal structure of these objects in both the prestellar and protostellar stages of their evolution, is presented. Distinctive features of the model include the possibility of independently describing the temperatures of the gas and dust, which is extremely important when calculating the thermal structure of prestellar and protostellar clouds, and the account of the radiation flux from the central protostar. This model is used to compare the theoretical density and temperature distributions with observations for nearby sites of star formation obtained with the Herschel Space Observatory. Application of the diffusion approximation with a flux limiter describes well the radial density and temperature distributions in protostellar clouds. However, significant differences between the model and observational density profiles were found for prestellar stages, suggesting the presence of appreciable deviations from equilibrium in the prestellar clouds. An approximate method for calculating the thermal structure of a cloud based on the adaptive $\tau$-approximation is presented. Application of the $\tau$-approximation yields good agreement with the diffusion approximation for the prestellar phase, but produces appreciable discrepancies for the protostellar phase, when the thermal structure of the accreting envelope is determined by the radiation of the protostar.

The applicability of 3D Doppler tomography to mapping gas flows in polars is considered. Synthetic profiles of emission lines are calculated using solutions (for the densities, temperatures, and velocity components) obtained from 3D MHD modeling, which are then used to construct 3D Doppler tomograms in the velocity space (Vx, Vy, Vz). Subsequent analysis of these tomograms applying observational constraints (the signal-to-noise ratio, number of input profiles, etc.) enables evaluation of limits to the method’s applicability.

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.

A model for simulating the thermal and dynamical evolution of protostellar clouds is presented. In the model, the dust and gas temperatures are treated separately, making it possible to more precisely describe the initial stages of the cloud’s gravitational contraction and collapse. The model is fast enough to be applied in hydrodynamical computations, and has a high enough accuracy for the results to be used to compute emission spectra and comparing them with observational data. Two problems are considered as test examples and simple applications: calculation of the structure of clouds in thermal and hydrostatic equilibrium, and modeling the evolution of a protostellar cloud in a spherically symmetric approximation, including the formation of the first hydrostatic core.

The structure of plasma flows in close binary systems in which one of the components is a rapidly rotating magnetic white dwarf is studied. The main example considered is the AE Aquarii system; the spin period of the white dwarf is about a factor of 1000 shorter than the orbital period, and the magnetic field on the white dwarf surface is of order of 50 MG. The mass transfer in this system was analyzed via numerical solution of the system of MHD equations. These computations show that the magnetic field of the white dwarf does not significantly influence the velocity field of the material in its Roche lobe in the case of laminar flow regime, so that the field does not hinder the formation of a transient disk (ring) surrounding the magnetosphere. However, the efficiency of the energy and angular momentum exchange between the white dwarf and the surrounding material increases considerably with the development of turbulent motions in the matter, resulting in its acceleration at the magnetospheric boundary and further escape from the system at a high rate. The time scales of the transition of the system between the laminar and turbulent modes are close to those of the AE\,Aqr transition between its quiet and active phases., Comment: 16 pages, 4 figures

We present a physical and numerical model for studying the formation and evolution of protostellar objects with solar and sub-solar masses. The model covers several evolutionary phases of these objects starting from the gravitational collapse of an initially unstable pre-stellar cloud, proceeding through the formation and collapse of the first hydrostatic core, and ending with the complete dissipation of the initial cloud and formation of a protostar and protostellar disk. The model is described by a system of MHD equations that includes Ohmic dissipation and ambipolar diffusion, and also a scheme for calculating the thermal and ionization structure of the cloud. We employ the multicomponent approach for computing the thermal structure of collapsing protostellar cloud, in which the dust and gas temperatures are treated separately, allowing us to accurately describe the initial stages of the cloud’s gravitational contraction and collapse. We present the first results showing the structure of an initially q...

A three-dimensional numerical model for an accretion process investigation in the magnetosphere of a white dwarf in magnetic cataclysmic variables is developed. The model assumes that the white dwarf has a dipole magnetic field with its symmetry axis inclined to the rotation axis. The model is based on the equations of modified MHD, that describe the mean flow parameters in the wave MHD turbulence. Diffusion of the magnetic field and radiative heating and cooling are taken into account. The suitability of the model is confirmed by modeling the accretion in a typical intermediate polar. The computations show that a magnetosphere forms around the accretor, with the accretion occurring via columns. The accretion columns have a curtain-like shape, and arc-shaped zones of energy release form on the surface of the white dwarf in the magnetic poles area as a result of the matter infall., Comment: 10 pages, 4 figures

The flow structure in magnetic cataclysmic variables with strong magnetic field and fast rotation of the white dwarf was investigated. We model the AE Aqr system as a representative observational object whose spin period of the white dwarf is about 1000 times shorter than the orbital period of the system. Observations show that in spite of the fast rotation some part of the material from the inner Lagrangian point is present in the Roche lobe of the white dwarf. We analyze the possible physical mechanisms that can lead to the less effective angular momentum transfer from the rotating magnetosphere to the inflowing material. Among these mechanisms are the ambipolar diffusion, the pressure of the magneto-dipole radiation, the relativistic retard of magnetic field lines and the incomplete penetration of the magnetic field to plasma. Results of our analysis show that the last mechanism is the most effective. Taking this effect into account we can find the flow structure that is in an agreement with observatio...

Using results of 3D MHD numerical simulations we investigate the generation of the magnetic field in an accretion disk of a close binary star. Analysis of the numerical simulations shows that the magnetic field, which is mainly toroidal, is intensively generated in the accretion disk. In the disk, we can distinguish three regions: inner region of the intensive toroidal field generation due to the differential rotation; region of the current sheets; and outer region of the magnetic field dissipation. In the outer regions of the accretion disk of a magnetic close binary system two types of the dynamo may exist. One of them is the laminar dynamo that occurs as a result of the nonaxisymmetric motions. The other type is the turbulent αω-dynamo. Numerical simulations show that during the disk lifetime the dynamo effect in the outer region of the disk can be significant.

We present a review of the modern concept of physical processes which go on in magnetic CVs with the mass transfer between the components. Using results of 3D MHD simulations, we investigated variations of the main characteristics of accretion disks depending on the value of the magnetic induction on the surface of the accreting star. In the frame of a self-consistent description of the MHD flow structure in close binaries, we formulate conditions of the disk formation and find a criterion that separates two types of flows corresponding to intermediate polars (intermediate magnetic field) and polars (strong field).The influence of asynchronous rotation of the accretor on the flow structure in magnetic close binaries is also discussed. Simulations show that the accretion instability arising in binaries with rapid rotation of accretor (“propeller” regime) can explain the mechanism of quasi-periodic dwarf nova outbursts observed in DQ Her systems.

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.

The development of an inhomogeneous collapse of an initially homogeneous cloud in pressure equilibrium with an external medium is considered in a general-relativistic treatment. It is shown that a rarefaction wave propagating from the outer boundary toward the center of the cloud forms in the initial stage of the collapse. The rarefaction front moves through the collapsing gas at the sound speed, and separates the cloud material into an inner, uniform core and an outer, inhomogeneous envelope. Our analysis distinguishes two possible collapse regimes, depending on the ratio of the focusing time for the rarefaction wave and the free-fall time for the cloud. In massive clouds, the focusing time for the rarefaction wave exceeds the free-fall time, and the collapse of such clouds inevitably leads to the formation of an event horizon and black hole. In low-mass clouds, the focusing time is less than the free-fall time. After the focusing, the collapse of such clouds is fully inhomogeneous, and can be appreciably slowed by the pressure gradient. The compression of a cloud in a transitional regime separating these two scenarios is studied. In this case, a self-similar collapse regime is realized in the central part of the cloud near the focusing time for the rarefaction wave. The constructed self-similar solution describes both early stages in the compression up to the focusing of the rarefaction wave and later stages with the accretion of gas onto the black hole that is formed. Asymptotic distributions of quantities in the inhomogeneous region at large distances from the rarefaction front and in the accreting envelope are found. The structure of space-time in the vicinity of the black hole that is formed at the center of the cloud as a result of the focusing of the rarefaction wave is discussed.

Self-similar solutions describing the homogeneous, free, isothermal collapse of protostellar clouds are considered. One such solution correponds to the critical case of the propagation of a rarefaction wave near the time when it is focused in the central region of the cloud. The speed of the rarefaction front is finite and equal to three times the isothermal sound speed. The asymptotic distributions of gas-dynamical quantities in the central part of the collapsing cloud and the surrounding envelopes are considered at both early and late stages of compression, after the formation of an opaque core (protostar). These solutions are used in a magneto-kinematic approximation to study the geometry and evolution of the large-scale magnetic fields of collapsing protostellar clouds. All the solutions are verified using direct numerical simulations. It is shown that an initially uniform magnetic field acquires an “hourglass” geometry with time. The characteristic opening angles in the self-similar solutions are in satisfactory agreement with observations.

Context. In binary star systems, the winds from the two components impact each other, leading to strong shocks and regions of enhanced density and temperature. Potentially habitable circumbinary planets must continually be exposed to these interactions regions. Aims. We study, for the first time, the interactions between winds from low-mass stars in a binary system, to show the wind conditions seen by potentially habitable circumbinary planets. Methods. We use the advanced 3D numerical hydrodynamic code Nurgush to model the wind interactions of two identical winds from two solar mass stars with circular orbits and a binary separation of 0.5 AU. As input into this model, we use a 1D hydrodynamic simulation of the solar wind, run using the Versatile Advection Code. We derive the locations of stable and habitable orbits in this system to explore what wind conditions potentially habitable planets will be exposed to during their orbits. Results. Our wind interaction simulations result in the formation of two strong shock waves separated by a region of enhanced density and temperature. The wind-wind interaction region has a spiral shape due to Coriolis forces generated by the orbital motions of the two stars. The stable and habitable zone in this system extends from approximately 1.4 AU to 2.4 AU. (TRUNCATED), 15 pages, 11 figures, to be published in A&A

We present the results of spectroscopic observations and Doppler tomography of the short-period cataclysmic variable star V455 And. The investigated spectra indicate that the disk in the system periodically changes its size. Besides, we suppose that there is a dense region in the disk, moving in the retrograde precession. This region can periodically amplify shock waves, which may be seen in the light curves of the system. The Doppler tomograms, computed, using the spectra, obtained at different orbital phases, demonstrate different bright regions. This may be an additional argument, supporting the idea of the moving dense region.

The flow structure in magnetic cataclysmic variables was investigated taking into account the effects of strong magnetic field and fast rotation of the white dwarf. We modeled the AE Aqr system as a unique object that has the rotation period of the white dwarf is about 1000 times shorter than the orbital period of the binary system. Observations show that in spite of fast rotation of the white dwarf some part of the stream from the inner Lagrange point comes into the Roche lobe region. We analyzed possible mechanisms preventing material to outflow from the system., Comment: 5 pages, 6 figures. Proceedings of the Conference "Physics at the Magnetospheric Boundary" (Geneva, Switzerland, 25 - 26 June, 2013), to be published in "EPJ Web of Conferences"

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 $10^5$ to $10^8$ 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., 20 pages, 12 figures, Submitted in Astronomy Reports, 2010, 54, No. 12

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 is magnetic 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., Comment: 17 pages, 10 figures