arXiv subjects

D. V. Bisikalo

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

At least 19 recordsLinked to original sources

arXiv2023

Mass outflow from the symbiotic binary RS Oph during its 2021 outburst

RS Oph is a symbiotic recurrent nova containing a massive white dwarf with heavy mass loss during activity. In August 2021, it underwent its seventh optical eruption since the end of the 19th century. The goal of this work is to analyse the structure of the outflows from the outbursting object. Based on broad-band $U$, $B$, $V$, $R_{\rm C}$, and $I_{\rm C}$ photometry and high-resolution H$α$ spectroscopy obtained at days 11--15 of the outburst, we derived some parameters of the system's components and outflows and their changes during our observation. The effective temperature of a warm shell (pseudophotosphere) produced by the ejected material and occulting the hot component of the system was $T_{eff}=15000\pm1000$ K and the electron temperature of the nebula was $T_{e}=17000\pm3000$ K throughout the observations. The effective radius of the pseudophotosphere was $R_{ eff}=13.3\pm2.0$ R$_{\odot}$ and the emission measure of the nebula $EM=(9.50\pm0.59) $10$^{61}$ cm$^{-3}$ for day 11 and $R_{eff}=10.3\pm1.6$ R$_{\odot}$ and $EM=(5.60\pm0.35)$10$^{61}$ cm$^{-3}$ for day 15. To provide this emission measure, the bolometric luminosity of the outbursting object must exceed its Eddington limit. The mass-loss rate of the outbursting object through its wind is much greater than through its streams. The total rate (from wind + streams) was less than $(4-5)$ 10$^{-5}$ (d/1.6kpc)$^{3/2}$ M$_{\odot}$yr$^{-1}$. The streams are not highly collimated. Their mean outflowing velocities are $\upsilon_{b}=-3680\pm60$ km s$^{-1}$ for the approaching stream and $\upsilon_{r}=3520\pm50$ km s$^{-1}$ for the receding one if the orbit inclination is 50$^\circ$.

astro-ph.SR↗

arXiv2019

Features of the Accretion in the EX Hydrae System: Results of Numerical Simulation

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.

astro-ph.SR↗

arXiv2019

Features of the Flow Structure in the Vicinity of the Inner Lagrangian Point in Polars

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.

astro-ph.SR↗

arXiv2019

On one effect of coronal mass ejections influence on the envelopes of hot Jupiters

It is now established that the hot Jupiters have extensive gaseous (ionospheric) envelopes, which expanding far beyond the Roche lobe. The envelopes are weakly bound to the planet and affected by strong influence of stellar wind fluctuations. Also, the hot Jupiters are located close to the parent star and therefore the magnetic field of stellar wind is an important factor, determining the structure their magnetosphere. At the same time, for a typical hot Jupiter, the velocity of stellar wind plasma, flowing around the atmosphere, is close to the Alfvén velocity. This should result in stellar wind parameters fluctuations (density, velocity, magnetic field), that can affect the conditions of formation of bow shock waves around a hot Jupiter, i. e. to switch flow from sub-Alfvén to super-Alfvén mode and back. In this paper, based on the results of three-dimensional numerical MHD modeling, it is confirmed that in the envelope of hot Jupiter, which is in Alfvén point vicinity of the stellar wind, both disappearance and appearance of the bow shock wave occures under the action of coronal mass ejection. The paper also shows that this effect can influence the observational manifestations of hot Jupiter, including luminosity in energetic part of the spectrum.

astro-ph.EP↗

arXiv2019

On possible types of magnetospheres of hot Jupiters

We show that the orbits of exoplanets of the "hot Jupiter" type, as a rule, are located close to the Alfven point of the stellar wind of the parent star. At this, many hot Jupiters can be located in the sub-Alfven 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.

astro-ph.EP↗

arXiv2018

Atmospheric mass loss from hot Jupiters irradiated by stellar superflares

Because of their activity, late-type stars are known to host powerful flares producing intense high-energy radiation on short time-scales that may significantly affect the atmosphere of nearby planets. We employ a one-dimensional aeronomic model to study the reaction of the upper atmosphere of the hot Jupiter HD 209458b to the additional high-energy irradiation caused by a stellar flare. Atmospheric absorption of the additional energy produced during a flare leads to local atmospheric heating, accompanied by the formation of two propagating shock waves. We present estimates of the additional atmospheric loss occurring in response to the flare. We find the mass loss rate at the exobase level to significantly increase (3.8*10^10, 8*10^10, and 3.5*10^11 g s-1 for 10, 100, and 1000 times the high-energy flux of the quiet star, respectively) in comparison to that found considering the inactive star (2*10^10 g s-1).

astro-ph.EP↗

arXiv2018

On the possible electromagnetic manifestations of merging black holes

We consider scenario of merger of two stellar mass black holes surrounded by an accretion disk. Due to emission of gravitational waves, the mass of the central object decreases and accretion disk experiences perturbation. Calculations show that the main consequence of this disturbance is formation of a shock wave propagating from the center of the disk to its periphery. Light curve is computed and duration of the flash is estimated under assumption that the flash terminates when the luminosity returns to the initial value. It is shown that, if the total mass of the merging binary is 55 M_sol (like in the event GW170814), the flash produced by the shock will increase bolometric luminosity of the disk by 4--6 orders of magnitude, up to 10^{45} erg/s (absolute stellar magnitude -23.8^m). With account of the distance to the source (540 Mpc) and for reasonable assumptions on the parameters of the accretion disk, it turns out that the apparent magnitude of the flash at the maximum of the spectral flux density should be 12.8^m -- 14.2^m, while duration of the flash --- few minutes. The main part of the shock radiation flux is emitted in the X-ray and gamma-ray ranges. In the spectral band of the EPIC instrument of the XMM-Newton observatory or the telescope eROSITA of the Spectrum-RG observatory (0.3--10 keV), luminosity will increase by 3--4 orders of magnitude, up to 10^{44} erg/s, corresponding to the apparent stellar magnitude about 17^m. Luminosity is at maximum in the observational band of the IBIS instrument of the INTEGRAL observatory (20 keV -- 10 MeV) and will be 10^{44} -- 10^{45} erg/s, corresponding to the apparent flux 10^{-4} photons per cm^2/s/keV at the wavelength ~100 keV. From the far UV to the longer wavelengths, the brightening is virtually absent.

astro-ph.HE↗

arXiv2018

The influence of stellar flare on dynamical state of the atmosphere of exoplanet HD 209458b

By applying an one-dimensional aeronomic model of the upper atmosphere of the close-in giant planet HD 209458b, we study the reaction of the planetary atmosphere to an additional heating caused by the influence of a stellar flare. It is shown that the absorption of additional energy of the stellar flare in the extreme ultraviolet leads to local atmospheric heating, accompanied by formation of two shock waves, propagating in the atmosphere. We discuss possible observational manifestations of the shocks and feasibility of their detection.

astro-ph.EP↗

arXiv2018

On the possible mechanism of radio emission of polars

We suggest a mechanism for generation of radio emission from polars. It is based on the cyclotron radiation of thermal electrons at the background of fluctuating magnetic field. The source of fluctuations is Alfvén wave turbulence. Expressions for the radiation spectrum and degree of polarization are obtained. By the example of the polar AM~Her, the fluxes of radio emission from the accretion stream are calculated. Within the framework of proposed emission model, with realistic plasma characteristics, it is possible to reproduce observed radiation fluxes in the VLA bands.

astro-ph.SR↗

arXiv2017

The Influence of Coronal Mass Ejections on the Mass-loss Rates of Hot-Jupiters

Hot-Jupiters are subject to extreme radiation and plasma flows coming from their host stars. Past ultraviolet Hubble Space Telescope observations, supported by hydrodynamic models, confirmed that these factors lead to the formation of an extended envelope, part of which lies beyond the Roche lobe. We use gas-dynamic simulations to study the impact of time variations in the parameters of the stellar wind, namely that of coronal mass ejections (CMEs), on the envelope of the typical hot-Jupiter HD 209458b. We consider three CMEs characterized by different velocities and densities, taking their parameters from typical CMEs observed for the Sun. The perturbations in the ram-pressure of the stellar wind during the passage of each CME tear off most of the envelope that is located beyond the Roche lobe. This leads to a substantial increase of the mass-loss rates during the interaction with the CME. We find that the mass lost by the planet during the whole crossing of a CME is of ${\approx}10^{15}$ g, regardless of the CME taken into consideration. We also find that over the course of 1 Gyr, the mass lost by the planet because of CME impacts is comparable to that lost because of high-energy stellar irradiation.

astro-ph.EP↗

arXiv2017

Excitation of turbulence in accretion disks of binary stars by non-linear perturbations

Accretion disks in binary systems can experience hydrodynamic impact at inner as well as outer edges. The first case is typical for protoplanetary disks around young T Tau stars. The second one is typical for circumstellar disks in close binaries. As a result of such an impact, perturbations with different scales and amplitudes are excited in the disk. We investigated the nonlinear evolution of perturbations of a finite, but small amplitude, at the background of sub-Keplerian flow. Nonlinear effects at the front of perturbations lead to the formation of a shock wave, namely the discontinuity of the density and radial velocity. At this, the tangential flow in the neighborhood of the shock becomes equivalent to the flow in in the boundary layer. Instability of the tangential flow further leads to turbulization of the disk. Characteristics of the turbulence depend on perturbation parameters, but alpha-parameter of Shakura-Sunyaev does not exceed ~0.1.

astro-ph.SR↗

arXiv2016

A collimated wind interpretation for the spectral variability of Z And during its major 2006 eruption

High-resolution observations in the region, centered at 4400 Å and those of the lines HeII 4686, Hbeta and HeI 6678 of the spectrum of the symbiotic binary Z And were performed during its outburst in 2006. The line Hbeta had additional satellite high-velocity emission components situated on either side of its central peak. The lines of neutral helium presented two components, consisting of a nebular emission situated close to the reference wavelength and a highly variable P Cyg absorption. Close to the optical maximum the line HeII 4686 was weak emission feature, but with the fading of the light it changed into an intensive emission consisting of a central narrow component and a broad component with a low intensity. The lines of NIII and CIII were very broadened. We demonstrate that all of these groups of lines with very different profiles can be interpreted in the light of the same model, where a disc-shaped material surrounding the compact object collimates its stellar wind and gives rise to bipolar outflow.

astro-ph.SR↗

arXiv2016

The Influence of Coronal Mass Ejections on the Gas Dynamics of the Atmosphere of a "Hot~Jupiter" Exoplanet

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.

astro-ph.EP↗

arXiv2016

Mass-Loss Rates of "Hot-Jupiter" Exoplanets with Various Types of Gaseous Envelopes

According to the computations results obtained by Bisikalo et al. (2013b) 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 $\mathrm{L_1}$ and $\mathrm{L_2}$, which can result in the formation of quasi-closed (if the dynamical pressure of the stellar wind stops the outflow through $\mathrm{L_1}$) or open gaseous envelopes. The example of the typical hot Jupiter HD 209458 b 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 $\dot{M} \leq 10^{9}$ g/s for a closed atmosphere, $\dot{M} \simeq 3 \times 10^{9}$ g/s for a quasi-closed atmosphere, and $\dot{M} \simeq 3 \times 10^{10}$ g/s for an open atmosphere. The matter in the closed and quasi-closed atmospheres flows out mainly through $\mathrm{L_2}$, and the matter in open envelopes primarily through $\mathrm{L_1}$.

astro-ph.EP↗

arXiv2016

Features of the Matter Flows in the Peculiar Cataclysmic Variable AE Aquarii

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.

astro-ph.SR↗

arXiv2016

On the technique for the recovery of the spectrum of turbulence in astrophysical disks

We present a method that can be used to recover the spectrum of turbulence from observations of optically thin emission lines formed in astrophysical disks. Within this method we analyze how line intensity fluctuations depend on the angular resolution of the instrument, used for the observations. The method allows us to restore the slope of the power spectrum of velocity turbulent pulsations and estimate the upper boundary of the turbulence scale.

astro-ph.IM↗

arXiv2016

A Numerical Model for Accretion in Intermediate Polars with Dipolar Magnetic Fields

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.

astro-ph.SR↗

arXiv2015

Transient accretion disc-like envelope in the symbiotic binary BF Cygni during its 2006 - 2015 optical outburst

The optical light of the symbiotic binary BF Cygni during its last eruption after 2006 shows orbital variations because of an eclipse of the outbursting compact object. The first orbital minimum is deeper than the following ones. Moreover, the Balmer profiles of this system acquired additional satellite components indicating bipolar collimated outflow at one time between the first and second orbital minima. This behaviour is interpreted in the framework of the model of collimated stellar wind from the outbursting object. It is supposed that one extended disc-like envelope covering the accretion disc of the compact object and collimating its stellar wind forms in the period between the first and second minima. The uneclipsed part of this envelope is responsible for the decrease of the depth of the orbital minimum. The calculated $UBVR_{C}I_{C}$ fluxes of this uneclipsed part are in agreement with the observed residual of the depths of the first and second orbital minima. The parameters of the envelope require that it is the main emitting region of the line H$α$ but the H$α$ profile is less determined from its rotation and mostly from other mechanisms. It is concluded that the envelope is a transient nebular region and its destruction determines the increase of the depth of the orbital minimum with fading of the optical light.

astro-ph.SR↗