Accretion Disks In Binary Systems Research Articles

Planet migration in accretion discs in binary systems

AbstractWe model evolution of exoplanets of S-type in close binary systems at the stage when the companion starts to lose mass via a slow stellar wind. At this stage an accretion disc is formed around the planets’ host. Detailed structure of such discs is calculated in quasi-stationary and non-stationary approaches. We model migration of planets embedded in these discs.

Hydrodynamical Simulations of Misaligned Accretion Discs in Binary Systems: Companions tear discs

AbstractAccretion discs appear in many astrophysical systems. In most cases, these discs are probably not completely axisymmetric. Discs in binary systems are often found to be misaligned with respect to the binary orbit. In this case, the gravitational torque from a companion induces nodal precession in misaligned rings of gas. We first calculate whether this precession is strong enough to overcome the internal disc torques communicating angular momentum. For typical parameters, precession torque wins. To check this result, we perform numerical simulations using the Smoothed Particle Hydrodynamics code, PHANTOM, and confirm that sufficiently thin and sufficiently inclined discs can break into distinct planes that precess effectively independently. Disc tearing is widespread and severely changes the disc structure. It enhances dissipation and promotes stronger accretion onto the central object. We also perform a stability analysis on isolated warped discs to understand the physics of disc breaking and tearing observed in numerical simulations. The instability appears in the form of viscous anti-diffusion of the warp amplitude and the surface density. The discovery of disc breaking and tearing has revealed new physical processes that dramatically change the evolution of accretion discs, with obvious implications for observed systems.

Planet migration in wind-fed accretion discs in binaries

Planet migration originally refers to protoplanetary disks, which are more massive and dense than typical accretion disks in binary systems. We study planet migration in an accretion disk in a binary system consisting of a solar-like star hosting a planet and a red giant donor star. The accretion disk is fed by a stellar wind. %, disk self-gravity is neglected. We use the $\alpha$-disk model and consider that the stellar wind is time-dependent. Assuming the disk is quasi-stationary we calculate its temperature and surface density profiles. In addition to the standard disk model, when matter is captured by the disk at its outer edge, we study the situation when the stellar wind delivers matter on the whole disc surface inside the accretion radius with the rate depending on distance from the central star. Implying that a planet experiences classical type I/II migration we calculate migration time for a planet on a circular orbit coplanar with the disk. Potentially, rapid inward planet migration can result in a planet-star merger which can be accompanied by an optical or/and UV/X-ray transient. We calculate timescale of migration for different parameters of planets and binaries. Our results demonstrate that planets can fall on their host stars within the lifetime of the late-type donor for realistic sets of parameters.

Simulating the synchrotron emission of AGN with grid based relativistic hydrodynamics

Grid based numerical hydrodynamic codes have been used to simulate the evolution of a variety of astrophysical environments, ranging from stellar winds to accretion disks in binary systems. These simulations can be used as valuable tools to correlate the theoretical models and observational data. In this study a grid based relativistic hydrodynamic simulation is created, using the freely available numerical code PLUTO ver 4.2, to study possible causes of variability in AGN. The code uses shock-capturing Godunov-type methods to solve the fluid dynamic conservation equations on a structured mesh grid. To simulate the production of a jet similar to those observed in AGN a three dimensional static grid was created containing 256 × 256 × 512 computational cells. A uniform background medium was assigned to this grid, while a nozzle was defined on the z = 0 boundary to inject a pressure matched relativistic jet, with Γ = 10 (v = 0:995c), into the medium. The simulation showed the formation of a central collimated relativistic beam along with a surrounding cocoon region. In addition, a post-processing emission modelling code is being developed, in order to determine the emission produced by the relativistic jet. The emission model utilizes the data produced by the hydrodynamic code to calculate intensity maps due to synchrotron self absorption. The emission code takes into account the effects produced by the relativistic motion of the emitting regions (excluding light travel time) which can lead to effects such as the Doppler boosting observed in blazars. Synchrotron based emission models can be applied to other astrophysical objects, such as X-ray binary systems, which means that the application of the emission code can be adapted to model a variety of astrophysical sources.

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.

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.

New solution to viscous evolution of accretion disks in binary systems

The analytical investigation of time-dependent ac- cretion in disks is carried out. We consider a time-dependent disk in a binary system at outburst which has a fixed tidally-truncated outer radius. The standard model (Shakura-Sunyaev) of the disk is considered. The vertical structure of the disk is accurately de- scribed in two regimes of opacity: Thomson and free-free. The fully analytical solutions are obtained, characterized by power- law variations of accretion rate with time. The solutions supply asymptotic description of the disk evolution in flaring sources in the periods after outbursts while the disk is fully ionized. The X-ray flux of multicolor (black-body) -disk is obtained as varying quasi-exponentially. The application to X-ray novae is briefly discussed concerning the observed faster-than-power decays of X-ray light curves. The case of time-dependent ad- vective disk when the exponential variations of accretion rate can occur is discussed.

DYNAMICAL EVOLUTION OF ROTATING ACCRETION USING DIFFERENT BOUNDARY CONDITIONS: STATE AFTER STABLE ACCRETION DISK CREATED

The 2D time-dependent solution of the thin and stable accretion disk with two-armed spiral shock waves in a closed binary system have been presented on the equatorial plane around the Schwarzschild black hole in Donmez (2004).2 The subject of this paper is to study the influence of two different boundary conditions, far away from a black hole called the outer boundary, on an accretion disk around the black hole during the time evolution. We have started with a stable accretion disk after the point where two-armed spiral shock waves were created (Donmez, 2004).2 The initial data which is also called the freezing boundary is used as a first boundary condition. As a second one, we use the outflow boundary condition. In both cases, the accretion disk is created and gases on the disk made closed trajectories. As a stable tori close to the black hole is created by using the first boundary, freezing condition, which has a ~10M radius where M is the mass of black hole, and the other boundary, outflow, creates stable two-armed spiral shock waves. The last stable circular orbit around the Schwarzschild black hole for this type of accretion disk is located around 11M in the case of the freezing boundary condition. The results of these simulations show that the tori and spiral shock waves are created in each case using freezing and the outflow boundary, respectively, and it also suggests that spiral waves are a robust feature of accretion disks in binary systems, and that these spiral shocks can indeed transfer the gravitational energy to the radiation energy observed by different X-ray satellites.

Accretion disc dynamics in extreme mass ratio compact binaries

An analysis is presented of a numerical investigation of the dynamics and geometry of accretion discs in binary systems with mass ratios q = M 2 /M 1 < 0.1, applicable to ultracompact X-ray binaries, AM CVn stars and very short period cataclysmic variables. The steady-state geometry of the disc in the binary reference frame is found to be quite different from that expected at higher mass ratios. For q ∼ 0.1, the disc takes on the usual elliptical shape, with the major axis aligned perpendicular to the line of centres of the two stars. However, at smaller mass ratios the elliptical gaseous orbits in the outer regions of the disc are rotated in the binary plane. The angle of rotation increases with gas temperature, but is found to vary inversely with q. At q = 0.01, the major axis of these orbits is aligned almost parallel to the line of centres of the two stars. These effects may be responsible for the similar disc structure inferred from Doppler tomography of the AM CVn star GP Com, which has q = 0.02. The steady-state geometry at low mass ratios is not predicted by an inviscid, restricted three-body model of gaseous orbits; it is related to the effects of tidal-viscous truncation of the disc near the Roche lobe boundary. Since the disc geometry can be inferred observationally for some systems, it is proposed that this may offer a useful diagnostic for the determination of mass ratios in ultracompact binaries.

Testing and Improving the Dynamical Theory of Mass Exchange

AbstractThe study of the flow structure is of great importance, and the results can be used both for consideration of the evolutionary status of binary stars and for the interpretation of observational data. In this report we present the review of 3D gas dynamic models used for the description of the mass exchange in close binaries.Main features of the flow structure in steady-state close binaries are summarized. It is shown that in self-consistent considerations the interaction between the stream from the inner Lagrangian point and the forming accretion disk is shock-free, and, hence, a “hot spot” does not form at the outer edge of the disk. To explain the presence of the observed zones of high luminosity in close binaries a self-consistent “hot line” model was proposed according to which the excess energy is released in a shock wave formed due to interaction between the circumdisk halo and the stream. The “hot line” model was confronted with observations and confirmed by virtue of comparison of synthetic and observational light curves for cataclysmic variables and by the analysis of Doppler tomograms.The special attention is paid to the physics of accretion disks in binary systems and particularly to waves in disks. The possible observational manifestations of the “hot line” wave and two arms of the tidal shocks are discussed. We also suggest that an additional spiral density wave can exist in inner parts of the cold accretion disk. This spiral wave is due to the retrograde precession of flow lines in the binary system. The results of 3D gas dynamic simulation have shown that a considerable increase in the accretion rate (by an order of magnitude) is associated with the formation of the “precessional” spiral wave. Based on this fact we suggest a new mechanism for the superoutbursts and superhumps in close binaries.

The Interaction of Binary Systems with Accretion Disks

AbstractWe study the (time) fluctuations in the outgoing radiation of accretion disks in binary systems in order to obtain properties of those systems (via comparison with observations), such as the angular momentum of the compact star within the disk. The effects of the companion star are discussed, and a prediction is made on the frequency of the outgoing (modulated) X-ray radiation.

Precessing accretion discs in binary systems

AbstractSuperhumps – quasi-periodic oscillations at periods a few percent different than the orbital periods – have been observed in several dozen binary systems of various types. It is well-accepted now that they are caused by the precession of the accretion disc. New findings on TV Col, TX Col and V4742 Sgr are presented. These cataclysmic variables (CVs) have exceptional precessing periods that cast strong doubt on the predictions of the Tidal Disc Instability model.

Early humps in WZ Sge stars

Photometric humps in outburst that are locked with the binary orbital period have been observed exclusively in the early phase of outbursts of WZ Sge stars. It is suggested that this “early hump” phenomenon is the manifestation of the tidal 2:1 resonance in accretion disks of binary systems with extremely low mass ratios. The “early humps” can be understood by the two-armed spiral pattern of tidal dissipation generated by the 2:1 resonance, first discussed by Lin & Papaloizou ([CITE]). The tidal removal of angular momentum from the disk during outbursts of dwarf novae, an important feature, is discussed in the context of the disk instability model. The ordering of tidal truncation radius, the 3:1 and 2:1 resonance radius in systems of different mass ratio naturally leads to a classification of dwarf nova systems in three groups according to their mass ratio. The WZ Sge stars are those systems which have the lowest mass ratios and are therefore characterized by “early humps”.

The tidally induced warping, precession and truncation of accretion discs in binary systems: three-dimensional simulations

We present the results of non-linear, hydrodynamic simulations, in three dimensions, of the tidal perturbation of accretion discs in binary systems where the orbit is circular and not necessarily coplanar with the disc mid-plane. The accretion discs are assumed to be geometrically thin, and of low mass relative to the stellar mass so that they are governed by thermal pressure and viscosity, but not self-gravity. The parameters that we consider in our models are the ratio of the orbital distance to the disc radius, D/R, the binary mass ratio, Ms/Mp, the initial inclination angle between the orbit and disc planes, δ, and the Mach number in the outer parts of the unperturbed disc, ℳ. Since we consider non-self-gravitating discs, these calculations are relevant to protostellar binaries with separations below a few hundred au. For binary mass ratios of around unity and D/R in the range 3 to 4, we find that the global evolution of the discs is governed primarily by the value of ℳ. For relatively low Mach numbers (i.e. ℳ = 10 to 20) we find that the discs develop a mildly warped structure, are tidally truncated, and undergo a near rigid body precession at a rate which is in close agreement with analytical arguments. For higher Mach numbers (ℳ ≈ 30), the evolution is towards a considerably more warped structure, but the disc none the less maintains itself as a long-lived, coherent entity. A further increase in Mach number to ℳ = 50 leads to a dramatic disruption of the disc as a result of differential precession, since the sound speed is too low to allow efficient communication between constituent parts of the disc. Additionally, it is found that the inclination angle between the disc and the orbital angular momentum vectors evolves on a longer time-scale, which is probably the viscous evolution time-scale of the disc. The calculations are relevant to a number of observed astrophysical phenomena, including the precession of jets associated with young stars, the high spectral index of some T Tauri stars, and the light curves of X-ray binaries such as Hercules X-1 which suggest the presence of precessing accretion discs.

The formation and evolution of accretion disks in binary systems

In this paper, we consider a close binary system consisting of a compact star and a optical Main-Sequence star which fills its critical Roche lobe and transfers matter through the inner Lagrangian pointL 1 toward the compact object. We use the Hill's problem as the dynamical model of the binaries. The following binaries are calculated by Duncanet al.'s mapping approximation: RY Per, RZ Sct, RS Vul, and λ Tau. Figures 3–6 show that the trajectories of accretion disk particles in the binaries. The relation between the dimensionless semi-major axis ɛ and times of conjunctionN are presented in Figures 7–10.

On the effects of tidal interaction on thin accretion disks: an analytic study

We calculate tidal effects on two-dimensional thin accretion disks in binary systems. We apply a perturbation expansion to obtain an analytic solution of the tidally induced waves. We obtain spiral waves that are stronger at the inner parts of the disks, in addition to a local disturbance which scales like the strength of the local tidal force. Our results agree with recent calculations of the linear response of the disk to tidal interaction.

On neutron star structure and the millisecond pulsar

The recently discovered millisecond pulsar (PSR1937-214) is observed to be rotating close to the limit of dynamical instability for a neutron star. Despite its extremely rapid rotation, measurements of the period derivative put a stringent upper limit on the energy loss from gravitational radiation, thus requiring that the quadrupole moment be quite small. The pulsar must also be rotating below the critical frequency at which its equilibrium configuration would become non-axisymmetric, since the lifetime of this configuration against decay by gravitational radiation is very short. This critical frequency, given by the theory of rotating ellipsoids, imposes a restriction on the rotation rate more severe than the break-up frequency and may be used to set a lower limit, rho 2 x 10 to the 14th power g/cu cm, on the density of the star. If the mass is 0.5 - 1.5 solar mass, several of the stiffer neutron star equations of state may be ruled out, and the radius should be less than 16 km. The condition for axisymmetry also imposes an upper limit on the rotation rate to which neutron stars may be spun up by accretion disks in binary systems, a model recently proposed for the evolution of the millisecond pulsar.

On the orbital phase dependence of the turn-on times of Hercules X-1

The present investigation is concerned with the dynamics of tilted accretion disks in binary systems. It is shown that the expected motion of a ring of matter in an accretion disk is more complicated than simple precession at a uniform rate. The motion is found to be better described by a vector normal to the plane of the ring of matter whose tip periodically traces out a small ellipse centered on the tip of a uniformly precessing vector. It is also demonstrated that this 'wobble' can be utilized to explain why the turn-ons of Hercules X-1 preferentially occur around orbital phases 0.2 and 0.7. If an accretion disk in a binary system is tilted with respect to the binary orbital plane, then it must undergo forced precession with the corresponding 'wobble'. This 'wobble' may be, at least, partially responsible for the phase dependence of the distribution of X-ray turn-on times of Hercules X-1.

Tidal torques on accretion discs in binary systems with extreme mass ratios

This paper investigates the tidal transport of angular momentum in accretion disc flow in a close binary system with small mass ratio, q. We demonstrate that the disc can only be tidally truncated if q is greater than the inverse of the Reynolds number. We derive the existence of a strong 2:1 resonance. Near the resonance, a spiral dissipation pattern is expected. If the Reynolds number is greater than ∼q−2, a dissipation and angular momentum loss from the disc may be sufficient to truncate it at the resonance. Finally we discuss the application of these results to dwarf novae.