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The Earth's N2-dominated atmosphere is a very special feature. Firstly, N2 as main gas is unique on terrestrial planets in the inner Solar System and gives a hint for tectonic activity. Studying the origins of atmospheric nitrogen and its stability provides insights into the uniqueness of the Earth's habitat. Secondly, the coexistence of N2 and O2 within an atmosphere is unequaled in the entire Solar System. Such a combination is strongly linked to the existence of aerobic lifeforms. The availability of nitrogen on the surface, in the ocean, and within the atmosphere can enable or prevent the habitability of a terrestrial planet, since nitrogen is vitally required by all known lifeforms. In the present work, the different origins of atmospheric nitrogen, the stability of nitrogen dominated atmospheres, and the development of early Earth's atmospheric N2 are discussed. We show why N2–O2 atmospheres constitute a biomarker not only for any lifeforms but for a-erobic lifeforms, which were the first major step that led to highly developed life on Earth. [ABSTRACT FROM AUTHOR]

The orientation parameter effect of a hot Jupiter's intrinsic dipole magnetic field on the flow structure in an extended gaseous (ionospheric) envelope of the planet is studied with a three-dimensional numerical simulation. For example, the hot Jupiter HD 209458b is considered. The magnitude of the planet's magnetic momentum was set equal to 0.1 of the magnetic momentum of Jupiter. The parameters of the stellar wind's magnetic field corresponding to the case of a super-Alfvén flow regime around the planet's atmosphere was taken into account. Under such conditions, calculations have shown that a quasi-closed gaseous (ionospheric) envelope with an induced shock magnetosphere is formed around a hot Jupiter, with a detailed structure determined by the magnetic dipole's slope angle. In this case, the mass-loss rate depends on the orientation of the planet's dipole magnetic field and increases with an increasing angle between the direction to the star and the direction closest to the inner Lagrange point's magnetic pole. This is due to the electromagnetic force increase that impedes the free movement of the matter in the emerging outflow when the magnetic pole approaches the inner Lagrange point. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

As a rule, the orbits of "hot Jupiter" exoplanets are located close to the Alfven point of the stellar wind of the host star. Many hot Jupiters could be in the sub-Alfven zone, where the magnetic pressure of the stellar wind exceeds the dynamical pressure. Therefore, the magnetic field in the wind should play an extremely important role in the process of stellar wind flowing around the atmosphere of a hot Jupiter. This must be taken into account when constructing theoretical models and interpreting observational data. Analyses show that many typical hot Jupiters should have shockless induced magnetospheres, which have no analogs in the solar system. Such magnetospheres are characterized first and foremost by the fact that there is no bow shock, and the magnetic barrier (ionopause) is formed by induced currents in upper layers of the ionosphere. This conclusion is confirmed here using three-dimensional numerical simulations of the flow of the stellar wind from the host star around the hot Jupiter HD 209458b, taking into account both the intrinsic magnetic field of the planet and the magnetic field in the wind. [ABSTRACT FROM AUTHOR]

The passage of a hot Jupiter with a quasi-closed extended envelope through a short coronal mass ejection (CME) with a small opening angle is considered. The results of three-dimensional gasdynamical simulations are used to determine the characteristics of the flow in the planet's envelope as it intersects the CME, and to infer possible observational manifestations of such an event. Two options are considered—entry of the planet into the CME at the beginning of the CME and a tangential interaction with the leading edge of the CME. The mass lost by the planet as a result of its interaction with the CME is estimated. [ABSTRACT FROM AUTHOR]

Results of a study of the influence of solar-type host stars superflares on the gas dynamics of the extended envelopes of giant exoplanets are presented. During flare events, the radiation intensity of the host star in the extreme ultraviolet and soft X-ray can increase by several orders of magnitude for a short time, leading to strong local heating of the exoplanet atmosphere on the side facing the star, with the formation of shocks in the atmosphere. Computations of the gas-dynamical response of the atmosphere of the hot Jupiter HD 209458b to characteristic superflares of solar-like stars were carried out earlier in [1] using a one-dimensional aeronomical model correctly taking into account heating and chemical processes in the atmosphere. To investigate the outflow of atmospheric gas, the results obtained with this onedimensional model were used as simple boundary conditions for computations of the three-dimensional flow structure after a flare. The results of these three-dimensional gas-dynamical computations show that the mass ejection of the flare increases the size of the envelope over several hours, which could be detected with existing observing facilities. It is shown that the mass-loss rates for the most powerful superflare considered could be enhanced by an order of magnitude over several tens of hours after the flare. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A one-dimensional aeronomic model of the upper atmosphere of a giant planet is used to study the reaction of the atmosphere of the hot Jupiter HD 209458b to additional heating by a stellar flare. It is shown that the absorption of additional energy from the stellar flare in the extreme ultraviolet leads to local heating of the atmosphere, accompanied by the formation of two shocks propagating in the atmosphere. Possible observational manifestations of these shocks and the feasibility of their detection are discussed. [ABSTRACT FROM AUTHOR]

The results of three-dimensional numerical simulations of the gaseous envelope of a contact binary star with parameters similar to those of SV Cen are presented. The outflow of matter from the vicinity of the Lagrange point L leads to the formation of a disk-like common envelope with a radius of order three times the component separation. The characteristics of this envelope and its structure and dynamics are discussed, as well as possible observational manifestations of such an envelope. [ABSTRACT FROM AUTHOR]

The discovery of the possible existence of huge quasi-stationary envelopes around a number of hot Jupiters (i.e., with sizes appreciably exceeding their Roche lobes) and the need to correctly take into account their properties when interpreting observational data require a careful analysis of the main physical processes influencing their atmospheres. One important factor is the possibility that the planet has a magnetic field. It was shown earlier that the presence of even a modest dipolar magnetic field of a hot Jupiter (with a magnetic moment approximately 1/10 the magnetic moment of Jupiter) influences the properties of the planetary atmosphere, in particular, leading to expansion of the range of parameters for which a giant, quasi-closed envelope can form around the planet. It was also established that the presence of a planetary magnetic field reduced the mass-loss rate from the envelope, since matter flowing out from the inner Lagrange point moves perpendicular to the field lines. Three-dimensional magnetohydrodynamical (MHD) modeling on time scales appreciably exceeding the time for the formation of the envelope show that pulsations arise in the atmospheres of hot Jupiters possessing dipolar magnetic fields, with characteristic periods ~0.27 P . This behavior is easy to understand physically, since even in the case of a spherical atmosphere, the continuous expansion of the ionized atmsphere of a hot Jupiter can lead to the accumulation of matter in regions bounded by closed field lines, and to the periodic rupture of the atmosphere beyond the magnetic field. In the case considered, when the system contains a giant envelope fed by a stream of matter from the inner Lagrange point, the presence of such pulsations gives rise to appreciable variations in the gas-dynamical structure of the flow. In particular, pulsations of the atmosphere lead to tearing off of part of the flow and sharp fluctuations in the size of the envelope, leading to variations in the envelope's observational properties. [ABSTRACT FROM AUTHOR]

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 L 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. [ABSTRACT FROM AUTHOR]

Vertical oscillations of the gas at the outer edge of the accretion disk in a semi-detached binary due to interaction with the stream of matter from the inner Lagrangian point L are considered. Mixing of the matter from the stream from L with matter of the disk halo results in the formation of a system of two diverging shocks and a contact discontinuity, or so-called 'hot line'. The passage of matter through the region of the hot line leads to an increase in its vertical velocity and a thickening of the disk at phases 0.7−0.8. Subsequently, the matter moving along the outer edge of the disk also experiences vertical oscillations, forming secondary maxima at phases 0.2−0.4. It is shown that, for systems with component mass ratios of 0.6, these oscillations will be amplified with each passage of the matter through the hotline zone, while the observations will be quenched in systems with component mass ratios ~0.07 and ~7. The most favorable conditions for the flow of matter from the stream through the edge of the disk arise for component mass ratios ~0.62. A theoretical relation between the phases of disk thickenings and the component mass ratio of the system is derived. [ABSTRACT FROM AUTHOR]

Accretion disks in binary systems can experience hydrodynamical influences at both their inner and outer edges. The former is typical for protoplanetary disks around young T Tauri stars, while the latter is typical for circumstellar disks in close binaries. This influence excites perturbations with various scales and amplitudes in the disk. The nonlinear evolution of perturbations with a finite, but small amplitude against the background of a sub-Keplerian flow is investigated. Nonlinear effects at the fronts of perturbation waves lead to the formation of discontinuities in the density and radial velocity; i.e., to formation of shocks. The tangential flow in the neighborhood of the shock becomes equivalent to a flow in a boundary layer. Due to an instability of the tangential flow, the disk becomes turbulent. The characteristics of the turbulence depend on the parameters of the perturbations, but the Shakura-Syunyaev α parameter does not exceed ~0.1. [ABSTRACT FROM AUTHOR]

A self-consistent, aeronomic model of the upper atmosphere of a 'hot Jupiter' including reactions involving suprathermal photoelectrons is presented. This model is used to compute the height profiles of the gas density, velocity, and temperature in the atmosphere of the exoplanet HD 209458b. It is shown that including suprathermal electrons when computing the heating and cooling functions reduces the mass loss rate of the atmosphere by a factor of five. [ABSTRACT FROM AUTHOR]

The structure ofmatter flows in close binary systems in which one of the components is a rapidly rotating magnetic white dwarf is studied. Themain example considered is the AEAquarii system; the period of the white dwarf's rotation is about a factor of 1000 shorter than the orbital period, and the magnetic field on the white-dwarf surface is of order 50MG. The matter flows in this system were analyzed via numerical solution of a systemofmagnetohydrodynamical equatons. These computations show that the white dwarf's magnetic field does not significantly influence the velocity field of the matter in its Roche lobe in the case of a 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 matter increases considerably with the development of turbulent motions in the matter, accelerating the matter at the magnetosphere boundary and leading to a high escape rate from the system. The time scales for the system's transition between the laminar and turbulent modes are close to those for the transition of AE Aquarii between its quiet and active phases. [ABSTRACT FROM AUTHOR]

The results of three-dimensional numerical simulations of the gas dynamics of the atmosphere of a 'hot Jupiter' exoplanet during the passage of a coronal mass ejection (CME) from the central star are presented. These computations assumed the parameters for the stellar wind and the CME to be typical of the solar values. The characteristic variations of the flow pattern are considered for quasi-closed and closed (but appreciably distorted by the gravitational influence of the star) gaseous envelopes of the exoplanet. It is shown that a typical CME is sufficient to tear off the outer part of an asymmetric envelope that is located beyond the Roche lobe and carry it away from the exoplanet. This leads to a substantial increase in the mass-loss rate from the exoplanet envelope during the passage of CMEs. The mass-loss rate grows by about a factor of 11 for a closed envelope, and by about a factor of 14 for a quasi-closed envelope. Possible evolutionary consequences of the loss of part of the atmosphere during the passage of CMEs are discussed. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The possibility of explaining the collected data for the hot Jupiter WASP-12b using a model with a stationary, quasi-closed, aspherical envelope around the exoplanet is considered. The influence of variations of the parameters of the stellar wind on the size and shape of the quasi-closed envelope is considered, and the maximum size of the envelope is determined to be Δ φ ≃ 0.17 ( φ is the orbital phase of the planet measured from the position of the main minimum). In order for the UV eclipse to begin at the phase indicated by observations with the Hubble Space Telescope, φ ≃ 0.83, the size of the envelope must be Δ φ ≃ 0.12, or 21 R ( R is the radius of the planet). This size is in very good agreement with theoretical estimates obtained assuming solar parameters for the stellar wind. Unfortunately, this conclusion is not firm, since the available observations begin at phase φ ≃ 0.83, hindering any conclusions about the character of the light curve at earlier phases. In the case of a quasi-closed or an open envelope, the UV eclipse could begin at earlier phases, φ ≃ 0.78 and φ ≃ 0.7, respectively. Determining the true nature of the envelope of WASP-12b will require additional UV observations beginning from a phase earlier than the phase corresponding to the eclipse of a quasi-closed envelope of the maximum size, φ < 0.78. [ABSTRACT FROM AUTHOR]

A kinetic model enabling investigation of the penetration and degradation of a flux of electrons into high-latitude regions of the hydrogen-dominant upper atmosphere of an exoplanet by means of numerical solution of the Boltzmann equation has been developed. It is shown for the case of a dipolar magnetic field that a one-dimensional model makes it possible to obtain a correct solution to this problem. Computations of the precipitation of a flux of electrons from the magnetosphere into the atmosphere of a typical hot Jupiter and the atmosphere of the planet Jupiter in our Solar system have been carried out. The computations assume a Maxwellian velocity distribution for electrons with three characteristic energies, E = 1, 10, and 100 keV. The efficiency of heating the atmosphere of a typical hot Jupiter and the planet Jupiter are considered. The heating efficiency displays only a weak dependence on the characteristic energy of the precipitating electrons. The heating efficiency for the upper atmosphere of Jupiter is also independent of the height, and lies in the range 7-9%. The heating efficiency for the atmosphere of a hot Jupiter depends appreciably on height, and varies from 7 to 18%. In the case of a hot Jupiter, the energy-absorption peaks for electrons with low kinetic energies lie in the region of higher heating efficiencies, substantially strengthening the contribution from precipitating electrons to the total heating of the atmosphere. [ABSTRACT FROM AUTHOR]

The possibility of explaining discrepancies between observed and theoretical limb-darkening coefficients as being a manifestation of the extended, non-spherical atmosphere of an eclipsing exoplanet is analyzed, assuming the model stellar atmospheres used are correct. [ABSTRACT FROM AUTHOR]

According to the compuations results obtained by Bisikalo et al. (2013) for the gas-dynamical effect of stellar winds on exoplanet atmospheres, three types of gaseous envelopes can form around hot Jupiters: closed, quasi-closed, and open. The type of envelope that forms depends on the position of the frontal collision point (where the dynamical pressure of the wind is equal to the pressure of the surrounding atmosphere) relative to the Roche-lobe boundaries. Closed envelopes are formed around planets whose atmospheres lie completely within their Roche lobes. If the frontal collision point is located outside the Roche lobe, the atmospheric material begins to flow out through the Lagrangian points L and L, which can result in the formation of quasi-closed (if the dynamical pressure of the stellar wind stops the outflow through L) or open gaseous envelopes. The example of the typical hot Jupiter HD 209458b is considered for four sets of atmospheric parameters, to determine the mass-loss rates for the different types of envelopes arising with these parameters. The mass-loss rates based on the modeling results were estimated to be Ṁ ≤ 10 g/s for a closed atmosphere, Ṁ ≃ 3 × 10 g/s for a quasi-closed atmosphere, and Ṁ ≃ 3 × 10 g/s for an open atmosphere. The matter in the closed and quasi-closed atmospheres flows out mainly through L, and the matter in open envelopes primarily through L. [ABSTRACT FROM AUTHOR]

As a rule, the orbital velocities of 'hot Jupiters,' i.e., exoplanets with masses comparable to the mass of Jupiter and orbital semi-major axes less than 0.1 AU, are supersonic relative to the stellar wind, resulting in the formation of a bow shock. Gas-dynamical modeling shows that the gaseous envelopes around hot Jupiters can belong to two classes, depending on the position of the collision point. if the collision point is inside the Roche lobe of the planet, the envelopes have the almost spherical shapes of classical atmospheres, slightly distorted by the influence of the star and interactions with the stellar-wind gas; if the collision point is located outside the Roche lobe, outflows from the vicinity of the Lagrangian points L and L arise, and the envelope becomes substantially asymmetrical. The latter class of objects can also be divided into two types. If the dynamical pressure of the stellar-wind gas is high enough to stop the most powerful outflow from the vicinity of the inner Lagrangian point L, a closed quasi-spherical envelope with a complex shape forms in the system. If the wind is unable to stop the outflow from L, an open aspherical envelope forms. The possible existence of atmospheres of these three types is confirmed by 3D numerical modeling. Using the typical hot Jupiter HD 209458b as an example, it is shown that all three types of atmospheres could exist within the range of estimated parameters of this planet. Since different types of envelopes have different observational manifestations, determining the type of envelope in HD 209458b could apply additional constrains on the parameters of this exoplanet. [ABSTRACT FROM AUTHOR]

Using 3D gas dynamics, we numerically simulate accretion-disk formation in typical cataclysmic variable intermediate polars with dipolar magnetic fields ( B = 10−5 × 10 G) and misaligned white-dwarf magnetic and rotation axes. Our simulations confirm that a significant misalignment of the axes results in a significant misalignment of the disk to the orbital plane. However, over time, this disk tilt disappears: early in the simulation, the initial particle positions in the rarefied tilted disk are governed solely by the magnetic field of the white dwarf. Due to the increasing disk mass and hence increasing disk gas pressure, the tilted disk eventually becomes decoupled from the magnetic field. The tidal action of the donor leads to a retrograde (i.e., nodal) precession of the tilted disk's streamlines, and the disk becomes twisted. When the disk tilt is greater than 4°, the incoming gas stream no longer strikes the disk rim (i.e., bright shocked region). Matter is now transported over and under the disk rim to the inner regions of the disk. Over time, the increased mass of inner parts of the disk due to the action of the colinear gas stream returns the inner-disk regions to a colinear configuration. Meanwhile, the outer regions of the tilted, twisted disk become warped. Our simulations suggest that the lifetime of an intermediate polar's tilted disk could be several tens to thousands of orbital periods. [ABSTRACT FROM AUTHOR]

Several scenarios for the formation of accretion and decretion disks in single and binary Ae and Be stars are proposed. It is shown that, in order for a rapidly rotating main-sequence Be star to lose mass via a disk, the star’s rotation must be quasi-rigid-body. Estimates show that such rotation can be maintained by the star’s magnetic field, which is probably a relict field. The evolution of single Be main-sequence stars is numerically simulated allowing for mass loss via the stellar wind and rotational mass loss assuming rigid-body rotation. The stellar wind is the factor that determines the maximum mass of Be stars, which is close to 30 M ⊙. The evolution of Be stars in close binaries is analyzed in the approximation adopted in our scenario. Long gamma-ray bursts can be obtained as a result of the collapse of rapidly rotating oxygen—neon degenerate dwarfs—the accreting companions of Be stars—into neutron stars. [ABSTRACT FROM AUTHOR]

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.© 2004 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]

We analyze models for quasi-stationary, ultraluminous X-ray sources (ULXs) with luminosities 1038–1040 erg/s exceeding the Eddington limit for a ∼1.4 M⊙ neutron star. With the exception of relatively rare stationary ULXs that are associated with supernova remnants or background quasars, most ULXs are close binary systems containing a massive stellar black hole (BH) that accretes matter donated by a stellar companion. To explain the observed luminosities of ∼1040 erg/s, the mass of the BH must be ∼40 M⊙ if the accreted matter is helium and ∼60 M⊙ if the accreted matter has the solar chemical composition. We consider donors in the form of main-sequence stars, red giants, red supergiants, degenerate helium dwarfs, heavy disks that are the remnants of disrupted degenerate dwarfs, helium nondegenerate stars, and Wolf–Rayet stars. The most common ULXs in galaxies with active star formation are BHs with Roche-lobe-filling main-sequence companions with masses ∼7 M⊙ or close Wolf–Rayet companions, which support the required mass-exchange rate via their strong stellar winds. The most probable candidate ULXs in old galaxies are BHs surrounded by massive disks and close binaries containing a BH and degenerate helium-dwarf, red-giant, or red-supergiant donor.© 2004 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]

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.© 2004 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]

We analyze heating and cooling processes in accretion disks in binaries. For realistic parameters of the accretion disks in close binaries (M ≃ 10[sup –12]–10[sup –7] M[sub ⊙]/yr and α ≃ 10[sup –1]–10[sup –2]), the gas temperature in the outer parts of the disk is from ∼ 10[sup 4] to ∼ 10[sup 6] 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.© 2003 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]

We have carried out a numerical study of rotational mass loss by rapidly rotating Be stars assuming preservation of rigid-body rotation during their main-sequence evolution. Evolutionary models are computed for stars with solar chemical composition and initial masses of 3, 10 and 30 M[sub ⊙]. As a result of their rapid initial rotation, these stars can lose one to four percent of their initial mass during the main-sequence stage. The amount of mass lost increases with the initial mass of the star. The matter lost by Be stars can form gas–dust disks with masses comparable to the masses of planets, which, in principle, makes possible the formation of planetary systems around such stars.© 2003 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]