Angular Momentum Accretion Research Articles

Gas dynamic stripping and X-ray emission of cluster elliptical galaxies

Detailed three-dimensional numerical simulations of an elliptical galaxy orbiting in a gas-rich cluster of galaxies indicate that gas dynamic stripping is less efficient than the results from previous, simpler calculations by Takeda et al. and Gaetz et al. implied. This result is consistent with X-ray data for cluster elliptical galaxies. Hydrodynamic torques and direct accretion of orbital angular momentum can result in the formation of a cold gaseous disc, even in a non-rotating galaxy. The gas lost by cluster galaxies via the process of gas dynamic stripping tends to produce a colder, chemically enriched cluster gas core. A comparison of the models with the available X-ray data of cluster galaxies shows that the X-ray luminosity distribution of cluster galaxies may reflect hydrodynamic stripping, but also that a purely hydrodynamic treatment is inadequate for the cooler interstellar medium near the centre of the galaxy.

Advection‐dominated Accretion Flows in the Kerr Metric. II. Steady State Global Solutions

In a previous paper we have written down equations describing steady-state, optically thin, advection-dominated accretion onto a Kerr black hole (Gammie and Popham 1997, hereafter Paper I). In this paper we survey the numerical solutions to these equations. We find that the temperature and density of the gas in the inner part of the accretion flow depend strongly on the black hole spin parameter $a$. The rate of angular momentum accretion is also shown to depend on $a$; for $a$ greater than an equilibrium spin parameter $a_{eq}$ the black hole is de-spun by the accretion flow. We also investigate the dependence of the flow on the angular momentum transport efficiency $\alpha$, the advected fraction of the dissipated energy $f$, and the adiabatic index $\gamma$. We find solutions for $-1 < a < 1$, $10^{-4} \le \alpha \le 0.44$, $0.01 \le f \le 1$, and $4/3 < \gamma < 5/3$. For low values of $\alpha$ and $f$ the inner part of the flow exhibits a pressure maximum and appears similar to equilibrium thick disk solutions.

Tidal mass transfer in elliptical-orbit binary stars

High mass X-ray binary star systems with elliptical orbits, like GX 301-2, often exhibit a peak in X-ray luminosity associated with periastron passage. We use a two dimensional hydrodynamics code to examine the possibility that these X-ray flares result from tidal stripping of gas from the primary star, and subsequent accretion of this gas onto the compact companion. We find that if the primary star is rotating near corotation with the orbiting compact companion at periastron, tidally stripped gas can accrete, causing X-ray flares. Such a tidally induced flare will occur substantially after periastron, at a phase of ∼ 0.2 for the parameters used in our model. This flare is characterized by a brief disk accretion phase with a large angular momentum accretion rate. However, in our particular model this disk accretion phase was followed by an equally brief phase of accretion with the opposite sign of angular momentum, resulting in no long-term spin up of the X-ray pulsar.

7.14. Gaseous accretion flows in the inner parsec of the Galaxy

Many characteristics of galactic nuclei may be associated with the accretion of ambient gas by a central concentration of mass. Using a 3D hydrodynamical code, we have been simulating this accretion process for Sgr A∗, the compact nonthermal source at the center of the Milky Way, in order to realistically model the gaseous flows in the inner parsec of our Galaxy. In the most recent simulations, we have taken into account the multi-point-like distribution of wind sources and we find that the structure of the flow can be significantly different from that due to a uniform medium. We here present our results concerning the mass and angular momentum accretion rates and discuss how these may be used to set constraints on our Galaxy's central engine.

Accretion of Mass and Spin Angular Momentum by a Planet on an Eccentric Orbit

We have extended our previous calculations of planetary accretion rates (Y. Greenzweig and J. J. Lissauer 1990, Icarus87, 40–77, and 1992, Icarus100, 440–463) to the case of a planet on an eccentric orbit. We have found that the dependence of accretion rates on planetary and planetesimal eccentricities can be reduced to a single parameter and that the extent of a planet's single-pass feeding zones depends (in the limit of small planetary mass) only upon the sum of the eccentricities of the planet and the planetesimals and their mutual inclination. We have also addressed the problem of the origin of planetary rotation by calculating the spin angular momentum contributed by all particles which are accreted by a planet in a uniform particle disk. Previous authors (J. J. Lissauer and D. M. Kary 1991, Icarus94, 126–159; L. Dones and S. Tremaine 1993, Icarus103, 67–92) have obtained analytic solutions for nongravitating planets on circular orbits in two-dimensional disks. We have developed new techniques which allow us to extend these calculations to the cases in which the planet's orbit is eccentric and the disk is three-dimensional. Nongravitating planets on eccentric orbits accrete the same specific rotational angular momentum as planets on circular orbits in disks of planetesimals on eccentric orbits. Planetesimal inclinations reduce the specific angular momentum accreted by nongravitating planets by 25–65%. By performing a series of numerical three-body simulations, we have found that a gravitating planet on an orbit with eccentricity e 0⪡ in a disk of planetesimals on circular orbits obtains the same rotation rate as a planet on a circular orbit in a disk of planetesimals which have e= e 0. When the planet and the planetesimals are both on eccentric orbits with e⪡ 1, the spin angular momentum is determined by the distribution of relative eccentricity vectors. If the eccentricities of the planet and planetesimals differ significantly, inclination typically acts to reduce the magnitude of the rotation rate by a few tens of percent. However, in certain cases where the eccentricities of the planet and planetesimals are comparable, inclination can change slow retrograde rotation to slow prograde rotation. A planet which accretes material primarily from the extremities of its feeding zone during the latter stage of its growth attains rapid systematic prograde rotation only if the amplitude of its epicyclic motion is comparable to or smaller than the radius of its Hill sphere.

The Dark Mass Concentration at the Galactic Center

AbstractStellar kinematic studies indicate the presence of a concentrated central mass at just under 2 × 106M⊙, in close agreement with the mass deduced from gas velocities measured with the [Ne II] line. Although this mass is most likely a black hole, it may be dominated by a tightly concentrated cluster of stellar remnants. If Sgr A*, a point radio source coincident with this central mass, is a massive black hole embedded in a region with strong gaseous outflows, as suggested by the observation of He I, Brα and Brγ line emission, it is accreting from its environment via the Bondi-Hoyle process. We discuss the consequences of this activity, including the expected mass and angular momentum accretion rate onto the black hole, and the resulting observable characteristics. The latest infrared images of this region appear to rule out the possibility that this large scale flow settles down into a standard α-disk at small radii. We discuss some possible scenarios that might account for this, including strong advection in the disk or the presence of a massive, fossilized disk. Not all of the gas affected in this way by Sgr A*’s strong gravitational field becomes bound. Some of it is redirected into a focused flow that in turn interacts with other coherent gas structures near the black hole. We suggest that the mini-cavity (to the south-west of Sgr A*) may be formed as a result of this activity, and argue that the characteristics of the mini-cavity lend some observational support for the presence of a concentrated mass near Sgr A*. We show, however, that as far as the mini-cavity is concerned, this concentrated mass need not be in the form of a point mass, but may instead be a highly concentrated cluster of stellar remnants.

Spectra and Line Profiles of FU Orionis Objects: Comparisons between Boundary Layer Models and Observations

We present solutions for the accretion disks and boundary layers in pre-main-sequence stars undergoing FU Orionis outbursts. These solutions differ from earlier disk solutions in that they include a self-consistent treatment of the boundary layer region. In a previous paper (Popham 1996), we showed that these stars should stop accreting angular momentum once they spin up to modest rotation rates. Here we show that for reasonable values of $\alpha$, these low angular momentum accretion rate solutions fit the spectra and line profiles observed in FU Orionis objects better than solutions with high rates of angular momentum accretion. We find solutions which fit the observations of FU Orionis and V1057 Cygni. These solutions have mass accretion rates of 2 and $1 \times 10^{-4} \msyr$, stellar masses of 0.7 and $0.5 \msun$, and stellar radii of 5.75 and $5.03 \rsun$, respectively. They also have modest stellar rotation rates $8-9 \xss$, comparable to the observed rotation rates of T Tauri stars, and angular momentum accretion rates of zero. This supports our earlier suggestion that FU Orionis outbursts may regulate the rotation rates of T Tauri stars.

Can FU Orionis Outbursts Regulate the Rotation Rates of T Tauri Stars?

We propose that FU Orionis outbursts may play an important role in maintaining the slow rotation of classical disk-accreting T Tauri stars. Current estimates for the frequency and duration of FU Orionis outbursts and the mass accretion rates of T Tauri and FU Orionis stars suggest that more mass may be accreted during the outbursts than during the T Tauri phases. If this is the case, then the outbursts should also dominate the accretion of angular momentum. During the outbursts, the accretion rate is so high that the magnetic field of the star should not disrupt the disk, and the disk will extend all the way in to the stellar surface. Standard thin disk models then predict that the star should accrete large amounts of angular momentum, which will produce a secular spinup of the star. We present boundary layer solutions for FU Orionis parameters which show that the angular momentum accretion rate drops below zero for stellar rotation rates which are always substantially below breakup, but depend on the mass accretion rate and on the adopted definition of the stellar radius. When the angular momentum accretion rate drops close to zero, the star will stop spinning up. Faster stellar rotation rates will produce negative angular momentum accretion which will spin the star down. Therefore, FU Orionis outbursts can keep the stellar rotation rate close to some equilibrium value for which the angular momentum accretion rate is small. We show that this equilibrium rotation rate may be similar to the observed rotation rates of T Tauri stars; thus we propose that FU Orionis outbursts may be responsible for the observed slow rotation of T Tauri stars. This mechanism is independent of whether the disk is disrupted by the stellar magnetic field during the T Tauri phase.

Magnetic Interaction between Classic T Tauri Stars and Their Associated Disks

In earlier models of magnetic interaction between a central T Tauri star and its associated disk, a stellar corotating magnetosphere was assumed, and consequently a large vertical angular velocity shear exists between the disk surface and the magnetosphere. The evaluated magnetic torque on the star is then dependent on the macroresistivity of the disk, regardless of true nature of the accretion flow. However, we may argue that a corotating magnetosphere with an enormous rotational shearing placed on the disk surface does not exist in reality, and the stellar magnetic fields are anchored to the disk non-slippage assumption. Hence the disk-star system may undergo a dynamical coupling in which a field-aligned plasma motion, via either the accretion flow or a disk wind, removes a local winding of the magnetic fields. As a result, the accretion flow and the magnetic torque are closely related through MHD equations. We calculated the angular momentum accretion rate in our model with parameters typically for classic T Tauri stars (CTTSs) and found that it is about one order of magnitude smaller than that of a hydrodynamical accretion, indicating that most of the angular momentum of the accretion flow goes to the disk (but not to the star). The result may lead to the conclusion that the magnetized accretion of angular momentum onto the central CTTS can be quite inefficient. This may naturally explain why CTTSs rotate more slowly than T Tauri stars without disks. The model predicts that if the outer radius of the magnetosphere is large the central star is in a rotational equilibrium at r<SUB>in</SUB> ≃ 0.7r<SUB>co</SUB>, where r<SUB>in</SUB> and r<SUB>co</SUB> are the inner disk and the corotation radii.

Acceleration in astrophysics

The origin of cosmic rays and applicable laboratory experiments are discussed. The principle problems of shock acceleration for the production of cosmic rays in the context of astrophysical conditions are found to be: (1) The presumed unique explanation of the power law spectrum is shown instead to be expected as a universal property of all accelerators with high loss; (2) The extraordinary isotropy of cosmic rays and the limited diffusion distances implied by supernova induced shock acceleration requires a more frequent and space-filling source than supernova; (3) Near perfect adiabaticity of reflection by strong hydromagnetic turbulence is required in order to accelerate the particles. In each doubling in energy roughly 105–6 scatterings are required with negligible energy loss. This seems most unlikely by strong hydromagnetic waves; (4) The evidence for acceleration due to quasi-parallel heliosphere shocks is weak. There is small evidence for the expected strong hydromagnetic turbulence, and instead, only a small number of particles are observed to be accelerated after only a few shock traversals; (5) The acceleration of electrons in the same collisionless shock that accelerates ions is difficult to reconcile with the theoretical picture of strong hydromagnetic turbulence that reflects the ions. The hydromagnetic turbulence will appear adiabatic to the electrons at their much higher Larmor frequency and so the electrons should not be scattered incoherently as they must be for acceleration. Therefore the electrons must be accelerated by a different mechanism. This is unsatisfactory, because wherever electrons are accelerated these sites, observed in radio emission, may accelerate ions more favorably. Because of these difficulties, an alternate explanation is given where reconnection of twisted magnetic fields (from gravitational condensation, accretion, and conservation of angular momentum) accelerates particles along field lines. The acceleration is coherent provided the reconnection is coherent, in which case the total flux, as for example of collimated radio sources, predicts single charge accelerated energies significantly greater than observed. The same acceleration process should occur during the formation of stars associated with the T-Tauri phase and bi-polar outflows. Because of the ubiquitous nature of matter condensations in the universe, and the near universal excess of angular momentum, the acceleration is inherently isotropic and space-filling in nature. Four laboratory experiments are suggested that would form the basis of a substantiated science of plasma processes of acceleration. These include an α-ω dynamo using liquid sodium, a one-ended plasma pinch that simulates the formation of collimated radio sources or bi-polar out-flows, reconnection or current interruptions in a tokamak, which simulates acceleration by coherent reconnection, and finally a collisionless shock experiment to simulate the classical shock acceleration process.

Non linear stability of slender accretion disks by bifurcation method

Abstract The nonlinear evolution of a constant angular momentum accretion disk subject to Papaloizou and Pringle (1984; hereafter PP) instability is investigated. The analysis is performed on an inviscid incompressible two-dimensional model using a formalism suitable for Hamiltonian systems. Only the most unstable modes are taken into account. It is found that relevant nonlinear terms have a stabilizing influence on the system. This supports recent numerical experiments showing a transition towards a quasi-stable planet configuration. The extension of the method to fat disk instabilities, more relevant to AGN disks, is discussed.

On the Origin of Planetary Spins

We examine the rate of accretion of mass and spin angular momentum by a spherical solid body ("planet") immersed in a differentially rotating disk of particles ("planetesimals"). If the planet travels on a circular orbit, the accretion process is described by two dimensionless parameters r and s , which measure the ratio of the planet's radius to its Hill radius and the ratio of the rms radial velocity of the planetesimals to the shear across the Hill radius, respectively. Using a combination of analytic arguments and numerical simulations, we derive the mass and angular momentum accretion rates and their scaling with r and s. By introducing an additional parameter, the effective mass of the planetesimals relative to the planet, we can derive the obliquities and spins arising from the stochastic nature of the accretion process. Our results are consistent with those of previous calculations by J. J. Lissauer and D. M. Kary (Icarus 94, 126-159 (1991)) wherever there is overlap in parameter space. In particular, we conclude that ordered accretion from a uniform or slowly varying disk of small bodies cannot result in rotation as rapid as that of Earth or Mars for any value of the disk velocity dispersion s. The spin rates of these planets are most naturally explained as arising from one or a few "giant" impacts by planetesimals.

Numerical simulations of two-dimensional and three-dimensional wind accretion flows of an isothermal gas

We present the results of two- and three-dimensional (2D) and (3D) numerical hydrodynamical calculations of accretion flows of an isothermal gas past a gravitating compact object. The calculations were performed both for a homogeneous medium and for a medium containing a transverse density or velocity gradient. We find that 2D isothermal flows exhibit the flip-flop instability (previously seen in adiabatic calculations) both in the homogeneous and the inhomogeneous cases. In the 3D case, while some unsteadiness is observed, the instability is much less violent than in 2D. We calculate (for the first time with a 3D hydrocode) the rate of accretion of mass and angular momentum from an inhomogeneous medium

The Formation of Non-Equal Mass Binary and Multiple Systems

AbstractCalculations of the formation of non-equal mass binary and multiple systems are presented. Binary formation results from the elongated shape of the initial cloud, one fragment forming on each side of the equatorial plane. Slight linear density gradients along the cloud’s major axis are sufficient to form non-equal mass fragments. Resultant mass ratios range from 0.1 to 1.0, in agreement with observations.In the presence of rotation, the fragments form with surrounding disks. The primary forms with a larger disk than does the secondary. Due to it’s orbital motion around the primary, the secondary can directly accrete the matter that has gathered in the primary’s disk. The secondary is thus less likely to have an appreciable disk, accounting for a redder primary. The binary system’s initially high eccentricity decreases with the continued accretion of higher specific angular momentum at apastron and lower specific angular momentum matter at periastron. Non-coplanar multiple systems with unequal mass components are also formed. The systems are composed of an inner binary and a more distant companion. The mass ratio of the companion binary system is usually less than that of the binary.

Three-dimensional simulations of black hole tori

The global hydrodynamic nonaxisymmetric instabilities in thick constant angular momentum (l) accretion gas tori in orbit around a Schwarzschild black hole are investigated via 2D and 3D numerical simulations. A radially-wide torus is found to develop and m = 1 nonaxisymmetric density perturbation near the pressure maximum and a trailing spiral wave that extends through the outer part of the torus. This global wave transports angular momentum through the torus, causing the average angular momentum distribution to evolve slowly away from l = constant. The unstable mode drives an accretion flow from the torus into the black hole. The role of accretion is investigated by modeling a wide torus with an inner boundary at the cusp of the Schwarzschild effective potential. In such a torus, accretion is present throughout the evolution; only modest unstable mode growth is observed, and saturation occurs at low amplitude. Comparison simulations performed in the torus equatorial plane show qualitative similarities between the 2D and 3D system, although the 3D modes have a function dependence on height.

Numerical models of giant planet formation with rotation

We present models of giant planet formation including the accretion and transport of angular momentum. The calculations are carried out in a one-dimensional quasispherical approximation, assuming hydrostatic equilibrium. The approximation of quasisphericity limits the calculations to relatively slowly rotating models. As in previous calculations without rotation (Bodenheimer and Pollack 1986), the so-called “core-accretion” model is adopted. Angular momentum transport is included for both radiative and convective zones. Convective regions are not forced to rotate uniformly at all times, but the angular momentum distribution in those regions is driven to uniform rotation, with transport coefficients based on mixing-length theory. Radiative regions whose angular momentum distributions are unstable according to the Goldreich-Schubert-Fricke criterion are driven to states of marginal stability, on timescales derived from linear theory and previous hydrodynamical modeling. The calculations are carried up to masses approximately equal to that of Saturn. Angular momentum transport is highly effective, producing models with a strong concentration of angular momentum in the outer convective zones that develop at envelope masses above 30 M +. Near the end of the calculation, gas accretion was turned off, and the models allowed to contract. The ratio of centrifugal force to gravity at the outer radius then rises high enough to validate the quasispherical approximation. At that point, mass is assumed to be shed to form a disk. We find that ∼1 M +, and ∼80% of the angular momentum, are lost to a disk. Such values of disk mass and angular momentum are amply sufficient for satellite system formation. As a result of the loss of angular momentum, however, the final planetary models have only one-third to one-half the present-day value of angular momentum of Saturn, indicating that yet larger amounts of angular momentum may have been present during formation.

Enhanced winds and tidal streams in massive X-ray binaries

The tidal effects created by the presence of a compact companion are expected to induce a stream of enhanced wind from the early-type primary star in massive X-ray binary systems. In this paper, two-dimensional gasdynamical simulations of such streams are presented. It is found that the wind enhancement is a sensitive function of the binary separation, and develops into a tidal stream as the primary approaches its critical surface. For typical system parameters, the Coriolis force deflects the stream sufficiently that it does not impact directly on the compact companion but passes behind it. The density in the stream can reach values of 20-30 times the ambient wind density, leading to strong attenuation of the X-ray flux that passes through the tidal stream, providing a possible explanation of the enhanced absorption events seen at later phases in the X-ray observations of massive X-ray binary systems such as Vela X-1. In contrast to the time-variable accretion wake, the tidal stream is relatively stationary, producing absorption features that should remain fixed from orbit to orbit. For systems with a strong tidal stream, the large asymmetry in the accreting wind results in the accretion of angular momentum of constant sign, as opposed to systems without streams, where the sign of the accreted angular momentum can change.

Accretion from Stellar Winds

AbstractRecent calculations have demonstrated that accretion from a stellar wind is very probably unsteady. The average rate of accretion of angular momentum is lower by about a factor 5 than the rate at which angular momentum is deposited into the Bondi-Hoyle accretion cylinder. This makes disk formation from wind accretion very difficult, in particular in the case of massive x-ray binaries. A combination of x-ray, uv and optical observations of symbiotic and related systems, as well as spin-up information on x-ray binaries, can be used to determine whether an accretion disk does form. Such observations can provide us with valuable information on the process of accretion from an inhomogeneous medium.

Accretion from an inhomogeneous medium – III. General case and observational consequences

We study the problem of accretion by a compact object from an inhomogeneous medium, in the general |$\gamma \ne 1$| case. The mass accretion rate is found to decrease with increasing γ. The rate of accretion of angular momentum is found to be significantly lower than the rate at which angular momentum is deposited into the Bondi–Hoyle, symmetrical, accretion cylinder. We discuss the consequences of our results for the cases of neutron stars accreting from the winds of early-type companions and white dwarfs and main-sequence stars accreting from winds of cool giants.

Accretion of angular momentum from an inhomogeneous medium - II. Isothermal flow

Accretion of angular momentum from an inhomogeneous medium - II. Isothermal flow