arXiv subjects

R. V. E. Lovelace

Publications and source records attributed to R. V. E. Lovelace.

86 records · Page 5Linked to original sources

arXiv1999

Eccentric Behavior of Disk Galaxies

A theory is developed for the dynamics of eccentric perturbations $[\propto \exp(\pm iϕ)]$ of a disk galaxy residing in a spherical dark matter halo and including a spherical bulge component. The disk is represented as a large number $N$ of rings with shifted centers and with perturbed azimuthal matter distributions. Account is taken of the dynamics of the shift of the matter at the galaxy's center which may include a massive black hole. The gravitational interactions between the rings and between the rings and the center is fully accounted for, but the halo and bulge components are treated as passive gravitational field sources. Equations of motion and a Lagrangian are derived for the ring + center system, and these lead to total energy and total angular momentum constants of the motion. We study the eccentric dynamics of a disk with an exponential surface density distribution represented by a large number of rings. The inner part of the disk is found to be strongly unstable. Angular momentum of the rings is transferred outward and to the central mass if present, and a trailing one-armed spiral wave is formed in the disk. We also analyze a disk with a modified exponential density distribution where the density of the inner part of the disk is reduced. In this case we find much slower, linear growth of the eccentric motion. A trailing one-armed spiral wave forms in the disk and becomes more tightly wrapped as time increases. The motion of the central mass if present is small compared with that of the disk.

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arXiv1999

Magnetic Field Limitations on Advection Dominated Flows

Recent papers discussing advection dominated accretion flows (ADAF) as a solution for astrophysical accretion problems should be treated with some caution because of their uncertain physical basis. The suggestions underlying ADAF involve ignoring the magnetic field reconnection in heating of the plasma flow, assuming electron heating due only to binary Coulomb collisions with ions. Here, we analyze the physical processes in optically thin accretion flows at low accretion rates including the influence of an equipartition turbulent magnetic field. For these conditions there is continuous destruction of magnetic flux by reconnection. The reconnection is expected to significantly heat the electrons which can efficiently emit magnetobremstrahlung radiation. Because of this electron emission, the radiative efficiency of the ADAF is not small. We suggest that the small luminosities of nearby galactic black holes is due to outflows rather than ADAF accretion.

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arXiv1999

Magnetohydrodynamic Origin of Jets from Accretion Disks

A review is made of recent magnetohydrodynamic (MHD) theory and simulations of origin of jets from accretion disks. Many compact astrophysical objects emit powerful, highly-collimated, oppositely directed jets. Included are the extra galactic radio jets of active galaxies and quasars, and old compact stars in binaries, and emission line jets in young stellar objects. It is widely thought that these different jets arise from rotating, conducting accretion disks threaded by an ordered magnetic field. The twisting of the magnetic field by the rotation of the disk drives the jets by magnetically extracting matter, angular momentum, and energy from the accretion disk. Two main regimes have been discussed theoretically, hydromagnetic winds which have a significant mass flux, and Poynting flux jets where the mass flux is negligible. Over the past several years, exciting new developments on models of jets have come from progress in MHD simulations which now allow the study of the origin - the acceleration and collimation - of jets from accretion disks. Simulation studies in the hydromagnetic wind regime indicate that the outflows are accelerated close to their region of origin whereas the collimation occurs at much larger distances.

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arXiv1998

On the Particle Heating and Acceleration in Black Hole Accretion Systems

The lack of our knowledge on how angular momentum is transported in accretion disks around black holes has prevented us from fully understanding their high energy emissions. We briefly highlight some theoretical models, emphasizing the energy flow and electron energization processes. More questions and uncertainties are raised from a plasma physics point of view.

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arXiv1998

Magneto-centrifugally driven winds: comparison of MHD simulations with theory

Stationary magnetohydrodynamic (MHD) outflows from a rotating, conducting Keplerian accretion disk threaded by B-field are investigated numerically by time-dependent, axisymmetric (2.5D) simulations using a Godunov-type code. A large class of stationary magneto-centrifugally driven winds are found where matter is accelerated from a thermal speed at the disk to much larger velocity, greater than the fast magnetosonic speed and larger than the escape speed. The flows are approximately spherical outflows with only small collimation within the simulation region. Numerical results are shown to coincide with the theoretical predictions of ideal, axisymmetric MHD to high accuracy. Investigation of the influence of outer boundary conditions, particularly that on the toroidal component of magnetic field shows that the commonly used ``free'' boundary condition leads to artificial magnetic forces which can act to give spurious collimation. New boundary conditions are proposed which do not generate artificial forces. Artificial results may also arise for cases where the Mach cones on the outer boundaries are partially directed into the simulation region.

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arXiv1998

Magnetic propeller outflows

A model is developed for magnetic `propeller'-driven outflows which cause a rapidly rotating magnetized star accreting from a disk to spin-down. Energy and angular momentum lost by the star goes into expelling most of the accreting disk matter. The theory gives an expression for the effective Alfven radius $R_A$ (where the inflowing matter is effectively stopped) which depends on the mass accretion rate, the star's mass and magnetic moment, and the star's rotation rate. The model points to a mechanism for `jumps' between spin-down and spin-up evolution and for the reverse transition, which are changes between two possible equilibrium configurations of the system. In for example the transistion from spin-down to spin-up states the Alfven radius $R_A$ decreases from a value larger than the corotation radius to one which is smaller. In this transistion the `propeller' goes from being ``on'' to being ``off.'' The ratio of the spin-down to spin-up torque (or the ratio for the reverse change) in a jump is shown to be of order unity.

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arXiv1998

Spherical Bondi accretion onto a magnetic dipole

Quasi-spherical supersonic (Bondi-type) accretion to a star with a dipole magnetic field is investigated using resistive magnetohydrodynamic simulations. A systematic study is made of accretion to a non-rotating star, while sample results for a rotating star are also presented. A new stationary subsonic accretion flow is found with a steady rate of accretion to the magnetized star smaller than the Bondi accretion rate. Dependences of the accretion rate and the flow pattern on the magnetic momentum of the star and the magnetic diffusivity are presented. For slow star's rotation the accretion flow is similar to that in non-rotating case, but in the case of fast rotation the structure of the subsonic accretion flow is fundamentally different and includes a region of ``propeller'' outflow. The methods and results described here are of general interest and can be applied to systems where matter accretes with low angular momentum.

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arXiv1998

Hydrodynamic Simulations of Counterrotating Accretion Disks

Hydrodynamic simulations have been used to study accretion disks consisting of counterrotating components with an intervening shear layer(s). Configurations of this type can arise from the accretion of newly supplied counterrotating matter onto an existing corotating disk. The grid-dependent numerical viscosity of our hydro code is used to simulate the influence of a turbulent viscosity of the disk. Firstly, we consider the case where the gas well above the disk midplane rotates with angular rate +Ω(r) and that well below has the same properties but rotates with rate -Ω(r). We find that there is angular momentum annihilation in a narrow equatorial boundary layer in which matter accretes supersonically with a velocity which approaches the free-fall velocity and the average accretion speed of the disk can be enormously larger than that for a conventional α-disk rotating in one direction. Secondly, we consider the case of a corotating accretion disk for r r_t. In this case we observed, that matter from the annihilation layer lost its stability and propagated inward pushing matter of inner regions of the disk to accrete. Thirdly, we investigated the case where counterrotating matter inflowing from large radial distances encounters an existing corotating disk. Friction between the inflowing matter and the existing disk is found to lead to fast boundary layer accretion along the disk surfaces and to enhanced accretion in the main disk. These models are pertinent to the formation of counterrotating disks in galaxies and possibly in Active Galactic Nuclei and in X-ray pulsars in binary systems.

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arXiv1998

Rossby Wave Instability of Keplerian Accretion Disks

We find a linear instability of non-axisymmetric Rossby waves in a thin non-magnetized Keplerian disk when there is a local maximum in the radial profile of a key function ${\cal L}(r) \equiv {\cal F}(r) S^{2/Γ}(r)$, where ${\cal F}^{-1} = \hat {\bf z}\cdot ({\bf \nabla}\times {\bf v}) /Σ$ is the potential vorticity, $S = P/Σ^Γ$ is the entropy, $Σ$ is the surface mass density, $P$ is the vertically integrated pressure, and $Γ$ is the adiabatic index. We consider in detail the special case where there is a local maximum in the disk entropy profile $S(r)$. This maximum acts to trap the waves in its vicinity if its height to width ratio ${\rm max}(S)/Δr$ is larger than a threshold value. The pressure gradient derived from this entropy variation provides the restoring force for the wave growth. We show that the trapped waves act to transport angular momentum outward. A plausible way to produce an entropy variation is when an accretion disk is starting from negligible mass and temperature, therefore negligible entropy. As mass accumulates by either tidal torquing, magnetic torquing, or Roche-lobe overflow, confinement of heat will lead to an entropy maximum at the outer boundary of the disk. Possible nonlinear developments from this instability include the formation of Rossby vortices and the formation of spiral shocks. What remains to be determined from hydrodynamic simulations is whether or not Rossby wave packets (or vortices) ``hold together'' as they propagate radially inward.

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arXiv1997

Tilting of a Disk of Gravitating Rings

The present work represents an attempt to understand the `rules of behavior' of observed warps in the HI disks of spiral galaxies found by Briggs (1990). In contrast with most earlier theoretical work, the present study investigates different initial value problems of a warped disk in an oblate (or prolate) halo potential, and it represents the disk warp in terms of $N$ independently tilted, self-gravitating, concentric rings. This representation gives new insight into the disk warping. A new constant of the motion of $N$ tilted rings is identified (in addition to the energy). The phenomenon of phase-locking of the lines-of-nodes of nearby rings due to self-gravity is demonstrated. We consider the influence of dynamical friction due to ring motion through the halo matter as well as friction between gaseous rings with different vertical motions due to turbulent viscosity. We first consider the dynamics of one, two, and three tilted rings of different radii in a halo potential. We go on to develop dynamical equations for $N-$rings which are most simply expressed in terms of the complex tilt angles $Θ_j = θ_j exp(-iϕ_j)$, where $θ_j$ is the actual tilt angle and $ϕ_j$ the line-of-nodes angle for the $j^{th}$ ring ($j=1..N$). We numerically solve the equations for $Θ_j$ for four different types of initial conditions: (1) warp excitation by a passing satellite, (2) excitation by a sinking compact minor satellite, (3) warp evolution due toa tilted halo potential, and (4) warp evolution resulting from an initially tilted disk plane.

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arXiv1997

Influence of Ohmic Heating on Advection-Dominated Accretion Flows

Advection-dominated, high-temperature, quasi-spherical accretion flow onto a compact object, recently considered by a number of authors, assume that the dissipation of turbulent energy of the flow heats the ions and that the dissipated energy is advected inward. It is suggested that the efficiency of conversion of accretion energy to radiation can be very much smaller than unity. However, it is likely that the flows have an equipartition magnetic field with the result that dissipation of magnetic energy at a rate comparable to that for the turbulence must occur by Ohmic heating. We argue that this heating occurs as a result of plasma instabilities and that the relevant instabilities are current driven in response to the strong electric fields parallel to the magnetic field. We argue further that these instabilities heat predominantly the electrons. We conclude that the efficiency of conversion of accretion energy to radiation can be much smaller than unity only for the unlikely condition that the Ohmic heating of the electrons is negligible.

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arXiv1996

Two-Stream Instability of Counter-Rotating Galaxies

The present study of the two-stream instability in stellar disks with counter-rotating components of stars and/or gas is stimulated by recently discovered counter-rotating spiral and S0 galaxies. Strong linear two-stream instability of tightly-wrapped spiral waves is found for one and two-armed waves with the pattern angular speed of the unstable waves always intermediate between the angular speed of the co-rotating matter ($+Ω$) and that of the counter-rotating matter ($-Ω$). The instability arises from the interaction of positive and negative energy modes in the co- and counter-rotating components. The unstable waves are in general convective - they move in radius and radial wavenumber space - with the result that amplification of the advected wave is more important than the local growth rate. For a galaxy of co-rotating stars and counter-rotating stars of mass-fraction $ξ_* < {1\over 2}$, or of counter-rotating gas of mass-fraction $ξ_g < {1\over 2}$, the largest amplification is usually for the one-armed leading waves (with respect to the co-rotating stars). For the case of both counter-rotating stars and gas, the largest amplifications are for $ξ_*+ξ_g \approx {1\over 2}$, also for one-armed leading waves. The two-armed trailing waves usually have smaller amplifications. The growth rates and amplifications all decrease as the velocity spreads of the stars and/or gas increase. It is suggested that the spiral waves can provide an effective viscosity for the gas causing its accretion.

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arXiv1996

Counter-rotating Accretion Disks

We consider accretion disks consisting of counter-rotating gaseous components with an intervening shear layer. Configurations of this type may arise from the accretion of newly supplied counter-rotating gas onto an existing co-rotating gas disk. For simplicity we consider the case where the gas well above the disk midplane is rotating with angular rate $+Ω$ and that well below has the same properties but is rotating with rate $-Ω$. Using the Shakura-Sunyaev alpha turbulence model, we find self-similar solutions where a thin (relative to the full disk thickness) equatorial layer accretes very rapidly, essentially at free-fall speed. As a result the accretion speed is much larger than it would be for an alpha disk rotating in one direction. Counter-rotating accretion disks may be a transient stage in the formation of counter-rotating galaxies and in the accretion of matter onto compact objects.

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arXiv1994

Spin-Up/Spin-Down of Magnetized Stars with Accretion Disks and Outflows

An investigation is made of disk accretion of matter onto a rotating star with an aligned dipole magnetic field. A new aspect of this work is that when the angular velocity of the star and disk differ substantially we argue that the $\bf B$ field linking the star and disk rapidly inflates to give regions of open field lines extending from the polar caps of the star and from the disk. The open field line region of the disk leads to the possibility of magnetically driven outflows. An analysis is made of the outflows and their back affect on the disk structure assuming an ``$\ap$" turbulent viscosity model for the disk and a magnetic diffusivity comparable to this viscosity. The outflows are found to extend over a range of radial distances inward to a distance close to $r_{to}$, which is the distance of the maximum of the angular rotation rate of the disk. We find that $r_{to}$ depends on the star's magnetic moment, the accretion rate, and the disk's magnetic diffusivity. The outflow regime is accompanied in general by a spin-up of the rotation rate of the star. When $r_{to}$ exceeds the star's corotation radius $r_{cr} = (GM/\om_*^2)^{1\ov 3}$, we argue that outflow solutions do not occur, but instead that ``magnetic braking" of the star by the disk due to field-line twisting occurs in the vicinity of $r_{cr}$. The magnetic braking solutions can give spin-up or spin-down (or no spin change) of the star depending mainly on the star's magnetic moment and the mass accretion rate. For a system with $r_{to}$ comparable to $r_{cr}$, bimodal behavior is possible where extraneous perturbations cause the system to flip between spin-up and spin-down.

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