arXiv subjects

P. B. Ivanov

Publications and source records attributed to P. B. Ivanov.

At least 19 recordsLinked to original sources

arXiv2024

On the non-dissipative orbital evolution of a binary system comprising non-compact components with misaligned spin and orbital angular momenta

In this Paper we determine the non-dissipative tidal evolution of a close binary system with an arbitrary eccentricity in which the spin angular momenta of both components are misaligned with the orbital angular momentum. We focus on the situation where the orbital angular momentum dominates the spin angular momenta and so remains at small inclination to the conserved total angular momentum. Torques arising from rotational distortion and tidal distortion taking account of Coriolis forces are included. This extends the previous work of Ivanov & Papaloizou relaxing the limitation resulting from the assumption that one of the components is compact and has zero spin angular momentum. Unlike the above study, the evolution of spin-orbit inclination angles is driven by both types of torque. We develop a simple analytic theory describing the evolution of orbital angles and compare it with direct numerical simulations. We find that the tidal torque prevails near 'critical curves' in parameter space where the time-averaged apsidal precession rate is close to zero. In the limit of small spin, these curves exist only for systems that have at least one component with retrograde rotation. As in our previous work, we find solutions close to these curves for which the apsidal angle librates. As noted there, this could result in oscillation between prograde and retrograde states. We consider the application of our approach to systems with parameters similar to those of the misaligned binary DI Her.

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arXiv2024

On the evolution of a twisted thin accretion disc in eccentric inclined supermassive binary black holes

We propose a model of a twisted accretion disc around a Kerr black hole interacting with a secondary black hole of a smaller mass on an inclined eccentric orbit. We use parameters of the system, which may be appropriate for the so-called 'precessing massive' model of OJ 287. We calculate expressions for torque exerted on the disc by the secondary and a contribution of the secondary to the apsidal precession of disc elements by a double averaging procedure over the periods of the secondary and the disc elements. These expressions are used at all scales of interest, including the ones inside the binary orbit. We calculate numerically the evolution of the disc tilt and twist assuming a flat initial configuration. We consider the disc aspect ratio $h/r=10^{-3}$, a rather large viscosity parameter $α=0.1$ and several values of the primary rotational parameter, $χ$. We find that, after a few periods of Lense-Thirring precession of the orbit, the disc relaxes to a quasi-stationary configuration in the precessing frame with a non-trivial distribution of the disc inclination angle, $β$, over the radial scale. We propose an analytic model for this configuration. We show that the presence of the twisted disc leads to multiple crossings of the disc by the secondary per one orbital period, with time periods between the crossings being different from the flat disc model. Our results should be taken into account in the modelling of OJ 287. They can also be applied to similar sources.

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arXiv2023

On the non-dissipative tidal evolution of the misalignment between spin and orbital angular momenta

We extend our previous work on the evolution of close binary systems with misaligned orbital and spin angular momenta resulting from non-dissipative tidal interaction to include all physical effects contributing to apsidal motion. In addition to tidal distortion of the primary by the compact secondary these include relativistic Einstein precession and the rotational distortion of the primary. The influence of the precession of the line of nodes is included. The dependence of the tidal torque on the apsidal angle $\hat\varpi$ couples the apsidal motion to the rate of evolution of the misalignment angle $β$ which is found to oscillate. We provide analytical estimates for the oscillation amplitude $Δβ$ over a wide range of parameter space confirmed by numerical integrations. This is found to be more significant near critical curves on which $d{\hat \varpi } /dt=0$ for a specified $β$. We find that to obtain $0.1 < Δβ< \sim 1,$ the mass ratio, $q > \sim1$ the initial eccentricity should be modest, $\cos β< 1/\sqrt{5},$ with $\cosβ<0 $ corresponding to retrograde rotation, initially, and the primary rotation rate should be sufficiently large. The extended discussion of apsidal motion and its coupled evolution to the misalignment angle given here has potential applications to close binaries with anomalous apsidal motion as well as transiting exoplanets such as warm Jupiters.

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arXiv2022

Nonlinear tidal excitation of super-harmonic gravity waves in main-sequence stars in binary and exoplanetary systems

We study the role of nonlinear effects on tidally-excited internal gravity waves in stellar radiation zones in exoplanetary or binary systems. We are partly motivated to study tides due to massive short-period hot Jupiters, which preferentially orbit stars with convective cores, for which wave breaking near the stellar centre cannot operate. We develop a theory (and test it with numerical calculations) for the nonlinear excitation of super-harmonic "secondary" waves (with frequencies $2ω_p$) by a "primary" tidal wave (with frequency $ω_p$) near the interface between the radiation zone and convective envelope. These waves have the same horizontal phase speeds to leading order, and this nonlinear effect could contribute importantly to tidal dissipation if the secondary waves can efficiently damp the primary. We derive criteria involving the orbital and stellar parameters required to excite these secondary waves to large amplitudes using a local model of the radiative/convective interface, which we convert to apply to tides in a spherical star. We numerically evaluate the critical amplitudes required for this new nonlinear effect to become important using stellar models, comparing them to the "conventional" criteria for wave breaking in radiative cores and the application of WKBJ theory near convective cores. The criteria for this new effect are easier to satisfy than the conventional measures of nonlinearity in $1.4$ and $2M_\odot$ stars on the main-sequence. We predict nonlinear effects to be important even for planetary-mass companions around the latter, but this effect is probably less important in stars with radiative cores.

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arXiv2020

On the evolution of a binary system with arbitrarily misaligned orbital and stellar angular momenta due to quasi-stationary tides

We consider the evolution of a binary system interacting due to tidal effects without restriction on the orientation of the orbital, and where significant, spin angular momenta, and orbital eccentricity. We work in the low tidal forcing frequency regime in the equilibrium tide approximation. Internal degrees of freedom are fully taken into account for one component, the primary. In the case of the companion the spin angular momentum is assumed small enough to be neglected but internal energy dissipation is allowed for as this can be significant for orbital circularisation in the case of planetary companions. We obtain a set of equations governing the evolution of the orbit resulting from tidal effects. These depend on the masses and radii of the binary components, the form and orientation of the orbit, and for each involved component, the spin rate, the Coriolis force, the normalised rate of energy dissipation associated with the equilibrium tide due to radiative processes and viscosity, and the classical apsidal motion constant. These depend on stellar parameters with no need of additional assumptions or a phenomenological approach as has been invoked in the past. They can be used to determine the evolution of systems with initial significant misalignment of spin and orbital angular momenta as hypothesised for systems containing Hot Jupiters. The inclusion of the Coriolis force may lead to evolution of the inclination between orbital and spin angular momenta and precession of the orbital plane which may have observational consequences.

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arXiv2020

The secular evolution of a uniform density star cluster immersed in a compressible galactic tidal field

Nuclear stellar clusters are common in the center of galaxies. We consider the possibility that their progenitors assumed to be globular clusters may have formed elsewhere, migrated to and assembled near their present location. The main challenge for this scenario is whether globular clusters can withstand the tidal field of their host galaxies. Our analysis suggests that provided the mass-density distribution of background potential is relatively shallow, as in some galaxies with relatively flat surface brightness profiles, the tidal field near the center of galaxies may be shown to be able to compress rather than disrupt a globular cluster at a distance from the center much smaller than the conventionally defined `tidal disruption radius', $r_t$. To do so, we adopt a previously constructed formalism and consider the secular evolution of star clusters with a homogeneous mass density distribution. We analytically solve the secular equations in the limit that the mass density of stars in the galactic center approaches a uniform distribution. Our model indicates that a star cluster could travel to distances much smaller than $r_t$ without disruption, thus potentially contributing to the formation of the nuclear cluster. However, appropriate numerical N-body simulations are needed to confirm our analytic findings.

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arXiv2019

Search for isolated Galactic Centre stellar mass black holes in the IR and sub-mm range

We investigate a possibility to find an accreting isolated black hole (IBH) with mass $1-100~\mathrm{M}_{\odot}$ within Central Galactic Molecular Zone (CMZ) in the submillimetre and IR spectral range with help of planned space observatories James Webb Space Telescope (JWST) and Millimetron (MM). We assume the spherical mode of accretion. We develop the simplest possible quantitative model of the formation of radiation spectrum in this range due to synchrotron emission and show that it fully agrees with the more complicated model of Ipser and Price 1982 for expected values of accretion rate. If a substantial fraction of LIGO events was caused by merger of primordial black holes, the JWST would be able to find them provided that there is a cusp in distribution of dark matter in the Galaxy and that the accretion efficiency parameter $λ$ defined as the ratio of accretion rate onto IBH to its Bondi-Hoyle-Lyttleton value is larger than $\sim 10^{-2}$. A comparable amount of IBHs is also predicted by recent models of their formation due to stellar evolution. MM capabilities are hampered by the effect of confusion due to distant submillimetre galaxies, but it can also be used for such purposes if the confusion effect is properly dealt with. In case of efficient accretion with $λ\sim 1$, both instruments could detect IBHs even when their number densities are as low as $10^{-6}~\mathrm{pc}^{-3}$.

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arXiv2018

The dynamics of twisted disc formed after the tidal disruption of a star by a rotating black hole

We consider misaligned accretion discs formed after tidal disruption events occurring when a star encounters a supermassive rotating black hole. We use the linear theory of warped accretion discs to find the disc shape when the stream produced by the disrupted star provides a source of mass and angular momentum that is misaligned with the black hole. The evolution of the surface density and aspect ratio is found from a one dimensional vertically averaged model. We extend previous work which assumed a quasi-stationary disc to allow unrestricted dynamical propagation of disc tilt and twist through time dependent backgrounds. We consider a smaller value of the viscosity parameter, $α=0.01,$ finding the dynamics varies significantly. At early times the disc inclination is found to be nearly uniform at small radii where the aspect ratio is large. However, since torques arise from the Lense-Thirring effect and the stream there is non uniform precession. We propose a simple model for this requiring only the background surface density and aspect ratio. At these times the $α\sim 0.01$ disc exhibits a new feature. An inclined hot inner region joins an outer low inclination cool region via a thin transition front propagating outwards with a speed exceeding that of bending waves in the cool region. These waves accumulate where the propagation speeds match producing an inclination spike separating inner and outer discs. At late times a sequence of quasi-stationary configurations approximates disc shapes at small radii. We discuss observational implications of our results.

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arXiv2017

Dynamical tides in exoplanetary systems containing Hot Jupiters: confronting theory and observations

We study the effect of dynamical tides associated with the excitation of gravity waves in an interior radiative region of the central star on orbital evolution in observed systems containing Hot Jupiters. We consider WASP-43, Ogle-tr-113, WASP-12, and WASP-18 which contain stars on the main sequence (MS). For these systems there are observational estimates regarding the rate of change of the orbital period. We also investigate Kepler-91 which contains an evolved giant star. We adopt the formalism of Ivanov et al. for calculating the orbital evolution. For the MS stars we determine expected rates of orbital evolution under different assumptions about the amount of dissipation acting on the tides, estimate the effect of stellar rotation for the two most rapidly rotating stars and compare results with observations. All cases apart from possibly WASP-43 are consistent with a regime in which gravity waves are damped during their propagation over the star. However, at present this is not definitive as observational errors are large. We find that although it is expected to apply to Kepler-91, linear radiative damping cannot explain this dis- sipation regime applying to MS stars. Thus, a nonlinear mechanism may be needed. Kepler-91 is found to be such that the time scale for evolution of the star is comparable to that for the orbit. This implies that significant orbital circularisation may have occurred through tides acting on the star. Quasi-static tides, stellar winds, hydrodynamic drag and tides acting on the planet have likely played a minor role.

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arXiv2016

On the formation of a quasi-stationary twisted disc after a tidal disruption event

We investigate misaligned accretion discs formed after tidal disruption events that occur when a star encounters a supermassive black hole. We employ the linear theory of warped accretion discs to find the shape of a disc for which the stream arising from the disrupted star provides a source of angular momentum that is misaligned with that of the black hole. For quasi-steady configurations we find that when the warp diffusion or propagation time is large compared to the local mass accretion time and/or the natural disc alignment radius is small, misalignment is favoured. These results have been verified using SPH simulations. We also simulated 1D model discs including gas and radiation pressure. As accretion rates initially exceed the Eddington limit the disc is initially advection dominated. Assuming the $α$ model for the disc, where it can be thermally unstable it subsequently undergoes cyclic transitions between high and low states. During these transitions the aspect ratio varies from $\sim 1$ to $\sim 10^{-3}$ which is reflected in changes in the degree of disc misalignment at the stream impact location. For maximal black hole rotation and sufficiently large values of viscosity parameter $α> \sim 0.01-0.1$ the ratio of the disc inclination to that of the initial stellar orbit is estimated to be $0.1-0.2$ in the advection dominated state, while reaching of order unity in the low state. Misalignment descreases with decrease of $α$, but increases as the black hole rotation parameter decreases. Thus, it is always significant when the latter is small.

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arXiv2016

The evolution of a supermassive retrograde binary embedded in an accretion disk

In this note we discuss the main results of a study of a massive binary with unequal mass ratio, q, embedded in an accretion disk, with its orbital rotation being opposed to that of the disk. When the mass ratio is sufficiently large, a gap opens in the disk, but the mechanism of gap formation is very different from the prograde case. Inward migration occurs on a timescale of t_ev ~ M_p/(dot M), where M_p is the mass of the less massive component (the perturber), and dot M is the accretion rate. When q<< 1, the accretion takes place mostly onto the more massive component, with the accretion rate onto the perturber being smaller than, or of order of, q^(1/3)M. However, this rate increases when supermassive binary black holes are considered and gravitational wave emission is important. We estimate a typical duration of time for which the accretion onto the perturber and gravitational waves could be detected.

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arXiv2015

A new general normal mode approach to dynamic tides in rotating stars with realistic structure and its applications

We review our recent results on a unified normal mode approach to dynamic tides proposed in Ivanov, Papaloizou $\&$ Chernov (2013) and Chernov, Papaloizou $\&$ Ivanov (2013). Our formalism can be used whenever the tidal interactions are mainly determined by normal modes of a star with identifiable regular spectrum of low frequency modes. We provide in the text basic expressions for tidal energy and angular momentum transfer valid both for periodic and parabolic orbits, and different assumptions about efficiency of normal mode damping due to viscosity and/or non-linear effects and discuss applications to binary stars and close orbiting extrasolar planets.

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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

The evolution of a binary in a retrograde circular orbit embedded in an accretion disk

Supermassive black hole binaries may form as a consequence of galaxy mergers. Both prograde and retrograde orbits have been proposed. We study a binary of a small mass ratio, q, in a retrograde orbit immersed in and interacting with a gaseous accretion disk in order to estimate time scales for inward migration leading to coalescence and the accretion rate to the secondary component. We employ both semi-analytic methods and two dimensional numerical simulations, focusing on the case where the binary mass ratio is small but large enough to significantly perturb the disk. We develop the theory of type I migration for this case and determine conditions for gap formation finding that then inward migration occurs on a time scale equal to the time required for one half of the secondary mass to be accreted through the unperturbed disk, with accretion onto the secondary playing only a minor role. The semi-analytic and fully numerical approaches are in good agreement, the former being applicable over long time scales. Inward migration induced by interaction with the disk alleviates the final parsec problem. Accretion onto the secondary does not significantly affect the orbital evolution, but may have observational consequences for high accretion efficiency. The binary may then appear as two sources of radiation rotating around each other. This study should be extended to consider orbits with significant eccentricity and the effects of gravitational radiation at small length scales. Note too that torques acting between a circumbinary disk and a retrograde binary orbit may cause the mutual inclination to increase on a timescale that can be similar to, or smaller than that for orbital evolution, depending on detailed parameters. This is also an aspect for future study (abridged).

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arXiv2013

Dynamical tides excited in rotating stars of different masses and ages and the formation of close in orbits

We study the tidal response of rotating solar mass stars, as well as more massive rotating stars, of different ages in the context of tidal captures leading to either giant exoplanets on close in orbits, or the formation of binary systems in star clusters. To do this, we adopt approaches based on normal mode and associated overlap integral evaluation, developed in a companion paper by Ivanov et al., and direct numerical simulation, to evaluate energy and angular momentum exchanges between the orbit and normal modes. The two approaches are found to be in essential agreement apart from when encounters occur near to pseudosynchronization, where the stellar angular velocity and the orbital angular velocity at periastron are approximately matched. We find that the strength of tidal interaction being expressed in dimensionless natural units is significantly weaker for the more massive stars, as compared to the solar mass stars, because of the lack of significant convective regions in the former case. On the other hand the interaction is found to be stronger for retrograde as opposed to prograde orbits in all cases. In addition, for a given pericentre distance, tidal interactions also strengthen for more evolved stars on account of their radial expansion. In agreement with previous work based on simplified polytropic models, we find that energy transferred to their central stars could play a significant role in the early stages of the circularisation of potential 'Hot Jupiters'.

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arXiv2013

A unified normal mode approach to dynamic tides and its application to rotating Sun-like stars

We determine the response of a uniformly rotating star to tidal perturbations due to a companion. General periodic orbits and parabolic flybys are considered. We evaluate energy and angular momentum exchange rates as a sum of contributions from normal modes allowing for dissipative processes. We consider the case when the response is dominated by the contribution of an identifiable regular spectrum of low frequency modes, such as gravity modes and evaluate it in the limit of very weak dissipation. Our formalism may be applied both to Sun-like stars with radiative cores and convective envelopes and to more massive stars with convective cores and radiative envelopes. We provide general expressions for transfer of energy and angular momentum valid for an orbit with any eccentricity. Detailed calculations are made for Sun-like stars in the slow rotation regime where centrifugal distortion is neglected in the equilibrium and the traditional approximation is made for the normal modes. We use both a WKBJ procedure and direct numerical evaluation which are found to be in good agreement for regimes of interest. Finally we use our formalism to determine the evolution time scales for an object, in an orbit of small eccentricity, around a Sun-like star in which the tidal response is assumed to occur. Systems with either no rotation or synchronous rotation are considered. Only rotationally modified gravity modes are taken into account under the assumption that wave dissipation occurs close to the stellar centre.

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arXiv2011

Close encounters of a rotating star with planets in parabolic orbits of varying inclination and the formation of Hot Jupiters

(abbreviated) We extend the theory of close encounters of a planet on a parabolic orbit with a star to include the effects of tides induced on the central rotating star. Orbits with arbitrary inclination to the stellar rotation axis are considered. We obtain results both from an analytic treatment and numerical one that are in satisfactory agreement. These results are applied to the initial phase of the tidal circularisation problem. We find that both tides induced in the star and planet can lead to a significant decrease of the orbital semi-major axis for orbits having periastron distances smaller than 5-6 stellar radii (corresponding to periods $\sim 4-5$ days after the circularisation has been completed) with tides in the star being much stronger for retrograde orbits compared to prograde orbits. We use the simple Skumanich law for the stellar rotation with its rotational period equal to one month at the age of 5Gyr. The strength of tidal interactions is characterised by circularisation time scale, $t_{ev}$ defined as a time scale of evolution of the planet's semi-major axis due to tides considered as a function of orbital period $P_{obs}$ after the process of tidal circularisation has been completed. We find that the ratio of the initial circularisation time scales corresponding to prograde and retrograde orbits is of order 1.5-2 for a planet of one Jupiter mass and $P_{obs}\sim $ four days. It grows with the mass of the planet, being of order five for a five Jupiter mass planet with the same $P_{orb}$. Thus, the effect of stellar rotation may provide a bias in the formation of planetary systems having planets on close orbits around their host stars, as a consequence of planet-planet scattering, favouring systems with retrograde orbits. The results may also be applied to the problem of tidal capture of stars in young stellar clusters.

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arXiv2010

Inertial waves in rotating bodies: a WKBJ formalism for inertial modes and a comparison with numerical results

(abbreviated) In this paper we develop a consistent WKBJ formalism, together with a formal first order perturbation theory for calculating the properties of the inertial modes of a uniformly rotating coreless body (modelled as a polytrope and referred hereafter to as a planet) under the assumption of a spherically symmetric structure. The eigenfrequencies, spatial form of the associated eigenfunctions and other properties we obtained analytically using the WKBJ eigenfunctions are in good agreement with corresponding results obtained by numerical means for a variety of planet models even for global modes with a large scale distribution of perturbed quantities. This indicates that even though they are embedded in a dense spectrum, such modes can be identified and followed as model parameters changed and that first order perturbation theory can be applied. This is used to estimate corrections to the eigenfrequencies as a consequence of the anelastic approximation, which we argue here to be small when the rotation frequency is small. These are compared with simulation results in an accompanying paper with a good agreement between theoretical and numerical results. The results reported here may provide a basis of theoretical investigations of inertial waves in many astrophysical and other applications, where a rotating body can be modelled as a uniformly rotating barotropic object, for which the density has, close to its surface, an approximately power law dependence on distance from the surface.

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