arXiv subjects

D. V. Bisikalo

Publications and source records attributed to D. V. Bisikalo.

At least 37 records · Page 2Linked to original sources

arXiv2015

Characterising exoplanets and their environment with UV transmission spectroscopy

Exoplanet science is now in its full expansion, particularly after the CoRoT and Kepler space missions that led us to the discovery of thousands of extra-solar planets. The last decade has taught us that UV observations play a major role in advancing our understanding of planets and of their host stars, but the necessary UV observations can be carried out only by HST, and this is going to be the case for many years to come. It is therefore crucial to build a treasury data archive of UV exoplanet observations formed by a dozen "golden systems" for which observations will be available from the UV to the infrared. Only in this way we will be able to fully exploit JWST observations for exoplanet science, one of the key JWST science case.

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arXiv2015

Review of scientific topics for Millimetron space observatory

This paper describes outstanding issues in astrophysics and cosmology that can be solved by astronomical observations in a broad spectral range from far infrared to millimeter wavelengths. The discussed problems related to the formation of stars and planets, galaxies and the interstellar medium, studies of black holes and the development of the cosmological model can be addressed by the planned space observatory Millimetron (the "Spectr-M" project) equipped with a cooled 10-m mirror. Millimetron can operate both as a single-dish telescope and as a part of a space-ground interferometer with very long baseline.

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arXiv2014

On the possible turbulence mechanism in accretion disks in non-magnetic binary stars

The arising of turbulence in gas-dynamic (non-magnetic) accretion disks is a major issue of modern astrophysics. Such accretion disks should be stable against the turbulence generation, in contradiction to observations. Searching for possible instabilities leading to the turbulization of gas-dynamic disks is one of the challenging astrophysical problems. In 2004, we showed that in accretion disks in binary stars the so-called precessional density wave forms and induces additional density and velocity gradients in the disk. Linear analysis of the fluid instability of an accretion disk in a binary system revealed that the presence of the precessional wave in the disk due to tidal interaction with the binary companion gives rise to instability of radial modes with the characteristic growth time of tenths and hundredths of the binary orbital period. The radial velocity gradient in the precessional wave is shown to be responsible for the instability. A perturbation becomes unstable if the velocity variation the perturbation wavelength scale is about or higher than the sound speed. Unstable perturbations arise in the inner part of the disk and, by propagating towards its outer edge, lead to the disk turbulence with parameters corresponding to observations (the Shakura-Sunyaev parameter $α\lesssim 0.01$).

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arXiv2013

Types of Gaseous Envelopes of "Hot Jupiter" Exoplanets

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 $\mathrm L_1$ and $\mathrm L_2$ 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 $\mathrm L_1$, a closed quasi-spherical envelope with a complex shape forms in the system. If the wind is unable to stop the outflow from $\mathrm L_1$, 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.

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arXiv2013

Symbiotic Stars with Similar Line Profiles during Activity

Line profiles containing indication of bipolar collimated outflow along with P Cyg absorption during phase of activity of several symbiotic systems are considered. The Hgamma profile of Z And during its 2006 outburst consisted of four groups of components. The profile of the HeI λ 5876 line of the Hen 3-1341 system during its 1998-2004 outburst had high-velocity satellite components on one hand and a broad P Cyg absorption on other hand. The Halpha and Hbeta profiles of the BF Cyg system during its 2006-2012 outburst had satellite components, observed for the first time, along with P Cyg absorptions. These profiles are interpreted in the light of a model related to the strong recurrent outbursts of Z And. The AG Dra system during its 2006 outburst did not contain indication of collimated outflow. Its profiles are interpreted in the light of the model related to the first outburst of 2000-2012 active phase of Z And.

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arXiv2012

Gas dynamic simulation of the star-planet interaction using a binary star model

We have performed numerical simulations of the interaction between "hot Jupiter" planet and gas of the stellar wind using numerical code developed for investigations of binary stars. With this code we have modeled the structure of the gaseous flow in the system HD 209458. The results have been used to explain observations of this system performed with the COS instrument on-board the HST.

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arXiv2012

Full 3-D MHD calculations of accretion flow Structure in magnetic cataclysmic variable stars with strong and complex magnetic fields

We performed 3D MHD calculations of stream accretion in cataclysmic variable stars for which the white dwarf primary star possesses a strong and complex magnetic field. These calculations are motivated by observations of polars; cataclysmic variables containing white dwarfs with magnetic fields sufficiently strong to prevent the formation of an accretion disk. So an accretion stream flows from the L1 point and impacts directly onto one or more spots on the surface of the white dwarf. Observations indicate that the white dwarf, in some binaries, possesses a complex (non-dipolar) magnetic field. We perform simulations of 10 polars or equivalently one asynchronous polar at 10 different beat phases. Our models have an aligned dipole plus quadrupole magnetic field centered on the white dwarf primary. We find that for a sufficiently strong quadrupole component an accretion spot occurs near the magnetic equator for slightly less than half of our simulations while a polar accretion zone is active for most of the rest of the simulations. For one or two configurations; accretion at the dominant polar region and at an equatorial zone occurs simultaneously. These are the first 3D MHD calculations to confirm the existence of complex magnetic fields in magnetic CVs. We conclude, that it might be difficult observationally determine if the field is a pure dipole or if it is complex for polars, but there will be indications for some systems. Specifically, a complex magnetic field should be considered if the there is an accretion zone near the white dwarf's spin equator (orbital plane) or if there are two or more accretion regions that cannot be fit by a dipole magnetic field. For asynchronous polars, magnetic field constraints are expected to be substantially stronger, with clearer indicators of complex field geometry due to changes in accretion flow structure as a function of spin-orbit beat phase.

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arXiv2012

Morphology of the Interaction Between the Stream and Cool Accretion Disk in a Semi-detached Binary Systems

We analyze heating and cooling processes in accretion disks in binaries. For realistic parameters of the accretion disks in close binaries (with accretion rates from 1e-12 to 1e-7 Msun/year and αfrom 0.1 to 0.01), the gas temperature in the outer parts of the disk is from 1e4 to 1e6 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 13600 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 circum-disk halo and the stream.

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arXiv2012

Reverse Rotation of the Accretion Disk in RW Aur A: Observations and a Physical Model

Speckle interferometry of the young binary system RW Aur was performed with the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences using filters with central wavelengths of 550 nm and 800 nm and pass-band halfwidths of 20 nm and 100 nm, respectively. The angular separation of the binary components was 1.448"{\pm}0.005 and the position angle of the system was 255.9°{\pm}0.3° at the observation epoch (JD 2 454 255.9). We find using published data that these values have been changing with mean rates of +0.002"/yr and +0.02°/yr, respectively, over the past 70 years. This implies that the direction of the orbital motion of the binary system is opposite to the direction of the disk rotation in RW Aur A. We propose a physical model to explain the formation of circumstellar accretion disks rotating in the reverse direction relative to young binary stars surrounded by protoplanetary disks. Our model can explain the characteristic features of the matter flow in RW Aur A: the high accretion rate, small size of the disk around the massive component, and reverse direction of rotation.

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arXiv2011

Structure of the Circumbinary Envelope Around a Young Binary System

The structure of a circumstellar envelope around a young binary T Tauri star is considered. The supersonic orbital motion of the system components in the envelope gas leads to the formation of bow shocks around the star. Two- and three-dimensional numerical modeling indicates an important role of these shocks in the formation of the structure of the circumbinary envelope. In particular, for systems with circular orbits, the size of the central region of the envelope that is not filled with matter (the gap) is essentially determined by the parameters of the bow shocks. These modeling results are supported by comparisons of the obtained estimates for the gap parameters with observations.

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arXiv2010

Interpretation of the line spectrum of classical symbiotic stars in the scenario for their prototype Z And

Results of the study of the symbiotic binary Z And during its recent active phase 2000 -- 2010 when it experienced a series of six optical outbursts are presented. High-resolution spectra obtained during the first and fourth outburst, which was the strongest one, have been analyzed. These data are compared with results of theoretical computations. The comparison provides information about the behaviour of the system during the entire active phase rather than during an individual outburst. In particular it was found fundamental difference between the first outburst, which opened the active phase, and the recurrent outbursts - namely, the presence of bipolar collimated optical outflow during some of the recurrent outbursts. A scenario that can explain all the spectroscopic phenomena observed during this active phase as well as previous active phases of Z And is proposed. The possibility to use this scenario for explanation of the line spectrum of other classical symbiotic stars during their active phases is motivated.

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arXiv2010

Accretion Discs with an Inner Spiral Density Wave

In Montgomery (2009a), we show that accretion discs in binary systems could retrogradely precess by tidal torques like the Moon and the Sun on a tilted, spinning, non-spherical Earth. In addition, we show that the state of matter and the geometrical shape of the celestial object could significantly affect the precessional value. For example, a Cataclysmic Variable (CV) Dwarf Novae (DN) non-magnetic system that shows negative superhumps in its light curve can be described by a retrogradely precessing, differentially rotating, tilted disc. Because the disc is a fluid and because the gas stream overflows the tilted disc and particles can migrate into inner disc annuli, coupled to the disc could be a retrogradely precessing inner ring that is located near the innermost annuli of the disc. However, numerical simulations by Bisikalo et al. (2003, 2004) and this work show that an inner spiral density wave can be generated instead of an inner ring. Therefore, we show that retrograde precession in non-magnetic, spinning, tilted CV DN systems can equally be described by a retrogradely precessing and differentially rotating disc with an attached retrogradely precessing inner spiral density wave so long as the wave appears at the same radius as the ring and within the plane of the tilted disc. We find that the theoretical results generated in this work agree well with the theoretical results presented in Montgomery (2009a) and thus with the numerical simulations and select CV DN systems in Montgomery (2009b) that may have a main sequence secondary. Therefore, pressure effects do need to be considered in CV DN systems that exhibit negative superhumps if the accretion discs are tilted and have an inner spiral density wave that is in the plane of the disc.

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arXiv2010

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 $10^5$ 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.

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arXiv2010

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 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.

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arXiv2010

The hydrogen and helium lines of the symbiotic binary Z And during its brightening at the end of 2002

High resolution observations in the region of the lines Halpha, He II 4686 and Hgamma of the spectrum of the symbiotic binary Z And were performed during its small-amplitude brightening at the end of 2002. The profiles of the hydrogen lines were double-peaked. These profiles give a reason to suppose that the lines can be emitted mainly by an optically thin accretion disc. The Halpha line is strongly contaminated by the emission of the envelope, therefore for consideration of accretion disc properties we use the Hgamma line. The Halpha line had broad wings which are supposed to be determined mostly from radiation damping but high velocity stellar wind from the compact object in the system can also contribute to their appearance. The Hgamma line had a broad emission component which is assumed to be emitted mainly from the inner part of the accretion disc. The line He II 4686 had a broad emission component too, but it is supposed to appear in a region of a high velocity stellar wind. The outer radius of the accretion disc can be calculated from the shift between the peaks. Assuming, that the orbit inclination can ranges from 47$^\circ$ to 76$^\circ$, we estimate the outer radius as 20 - 50 R_sun. The behaviour of the observed lines can be considered in the framework of the model proposed for interpretation of the line spectrum during the major 2000 - 2002 brightening of this binary.

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arXiv2006

Bipolar ejection by the symbiotic binary system Z And during its 2006 outburst

High resolution data in the region of the line Halpha have been obtained at the time of the light maximum and after it of the 2006 optical outburst of the symbiotic binary Z And. A blue-shifted absorption component indicating outflow velocity of about 1400 km/s as well as additional emission components with similar velocities, situated on the two sides of the main peak of the line were observed during that time. It is suggested that all of them are spectral signature of bipolar outflow, observed for the first time in the optical spectrum of this binary. The emission measure and the mass of the nebular part of the streams were estimated approximately.

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arXiv2006

A Study of an Outburst in the Classical Symbiotic Star Z And in a Colliding-Wind Model

Two-dimensional gas-dynamical modeling of the mass-flow structure is used to study the outburst development in the classical symbiotic star Z And. The stage-by-stage rise of the light during the outburst can be explained in the framework of the colliding winds model. We suggest a scenario for the development of the outburst and study the possible influence of the changes of the flow structure on the light of the system. The model variations of the luminosity due to the formation of a system of shocks are in good agreement with the observed light variations.

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arXiv2006

Superhumps in Binary Systems and Their Connection to Precessional Spiral Density Waves

We consider a mechanism for the formation of superhumps in the TV Col system, based on the possible existence of a precessional spiral wave in the accretion disk of the system. This mechanism can act in binaries with arbitrary component-mass ratios, and our precessional spiral wave model can be applied to explain observed superhumps of all types.

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