Structure Of Accretion Disk Research Articles

The effects of X-ray illumination on accretion disk interiors

The structure of accretion disks illuminated by X-ray from a central compact object. At radii larger than a certain value, X-ray heating can dominate the viscous heating and determine the disk structure in a standard alpha-disk model. The disk structure resulting from X-ray illumination is calculated both analytically and numerically for low-mass X-ray binaries as well as for active galactic nuclei. X-ray illumination does have important effects on the disk structures, under a variety of circumstances; instabilities may arise under significant X-ray heating.

Superhumps, resonances and accretion discs

The structure of accretion discs within binary systems is shown to be influenced by the excitation of resonances within the disc. Of particular importance is that near the 3:1 commensurability with the stars' orbit. This can be used to explain the superhump phenomenon of SU UMa dwarf novae in superoutburst. This resonance can only appear for mass ratios which satisfy M 1 /M 2 ≤ 0.25-0.33, for larger mass ratios the available resonances are weaker and of the wrong form to produce the superhump phenomenon. The mass-transfer stream is shown to be an important contributor to growth rate of the resonance.

Structure and evolution of irradiated accretion disks. I - Static thermal equilibrium structure. II - Dynamical evolution of a thermally unstable torus

The thermal equilibrum structure and dynamical behavior of externally irradiated accretion disks are investigated. For radiative disks only the surface layer is heated, while for convective disks the heat penetrates deeply into the disk. For sufficiently strong radiation and given irradiation flux F(irr), the disk is completely stabilized against thermal instabilities of the sort invoked to explain dwarf novae. For moderately strong irradiation there is still an unstable branch in the thermal equilibrium curve. In typical soft X-ray transients, the disk is unstable against the dwarf-nova type instability. Fixed F(irr) on accretion disk annuli reduces the amplitude and the quiescent times and increases the outburst duration of the resultant light curves. Varying F(irr) in proportion to the mass accretion rate at the disks's inner edge results in light curves with a plateau in the decay from outbursts. In the case when irradiation is suddenly switched on, a temperature inversion results which leads to the formation of an accretion corona.

Structure and Evolution of Irradiated Accretion Disks. II. Dynamical Evolution of a Thermally Unstable Torus

La reponse dynamique du tore une-zone d'un disque d'accretion aux instabilites thermiques et aux sources de rayonnement externes est analysee. L'evolution non-lineaire d'une instabilite thermique dans un disque d'accretion est simulee pour un disque non-irradie et pour un disque soumis a un rayonnement RX externe. Dans le cas d'un disque irradie, trois types d'irradiation sont consideres : une irradiation constante, une irradiation variable ou le flux d'irradiation varie proportionnellement a la vitesse d'accretion massique sur l'etoile compacte et une irradiation brusque appliquee sur le disque non-irradie.

Vertical structure of accretion disks - A simplified analytical model

A simplified model of the vertical structure of accretion disks is derived. Analytical expressions for the temperature and density structure, which represent a generalization of the gray model long known in the theory of classical stellar atmospheres, are presented. The formalism naturally explains similarities and differences between the structure of a disk and a stellar atmosphere. In particular, the influence of viscous dissipation and external irradiation of the disk by the central star, as well as of the finite optical thickness of the disk, may be easily accounted for and explained by the present model.

Structure of line-emitting accretion disks in active galactic nuclei - ARP 102B

The prime objects of the present self-consistent model of a line-emitting accretion disk able to account for the properties observed in a small class of AGNs are Arp 102B and 3C 390.3, whose double-peaked emission lines have been attributed to a Keplerian disk. Improved calculations of the line profile of a relativistic Keplerian disk, generalized to include a variety of emissivity laws as well as local broadening due to electron scattering or turbulence, are noted to fit Arp 102B; analytic and numerical calculations of the solid angle presented by the outer thin disk to an extended isotropic source of illumination demonstrate that the energy budget requirements for line emission from the disk are also satisfied.

Effects of electron-positron pairs on accretion flows

Consideration is given to the effects of thermal and nonthermal electron-positron pair production upon the structure of two-temperature accretion disks and upon the two-temperature criterion in quasi-spherical accretion flows, by balancing pair creation with annihilation and inflow. It is found that thermal pair creation is unimportant in both the disk and the quasi-spherical flows for reasonable values of parameters consistent with the observed spectra of active galactic nuclei. However, pairs produced nonthermally can overwhelm ions and significantly alter the flow structure for a wide range of parameter values. 32 refs.

How much boundary layer heating occurs in an accreting prenova white dwarf?

Understanding boundary layer heating is crucial in determining the thermal structure of the accreted envelope of a prenova white dwarf. The matched asymptotic expansion method was used to solve consistently for the structure of accretion disks transferring matter onto rotating white dwarfs. The fraction of accretion energy transported into prenova white dwarf envelopes was calculated. These results should be used by modelers of nova eruptions; they will produce significantly lower degeneracies and weaker explosions than expected until now. Detailed models of accretion disks and boundary layers can also be used to calculate the amount of white dwarf heating during a dwarf nova outburst. In general, such models can serve as input to model atmosphere codes to predict more realistic spectra of disk-accreting objects. 29 refs.

Thermal cycling and fluctuations in the protoplanetary nebula

The cooling of the protoplanetary nebula is controlled largely by the opacity of dust grains. Usually nebula structure models employ standard opacity tables calculated for ensembles of micron-sized dust grains, which are assumed to dominate the opacity. In a recent paper, Morfill (1988, Icarus, in press) calculated the radial structure of viscous accretion disks, assuming that “large” millimeter-sized dust particles dominate the opacities. Such disks have an inner “transition zone,” the structure of which is controlled by evaporation and condensation of refractory material. We examine this hot inner region (at temperatures around 1500° K, where refractory material can evaporate) and conclude that the transition zone may evolve through a number of heating and cooling phases. The physical process is the following: If excess energy is supplied, the nebula heats up, and small refractory dust grains evaporate. This lowers the opacity and the nebula cools again. As the gas becomes cooler, the refractory vapor recondenses on surviving (larger) dust particles. This increases the mean particle size, as well as the opacity, and the nebula can heat up once more. This starts the cycle of evaporation up again, etc. In this way, the transition zone may remain at a mean temperature of ∼1500°K, with rapid fluctuations around this mean value occurring. Simple model calculations give temperature fluctuation rates of several 10s of degrees per hour, about 3 to 4 orders of magnitude higher than the temperature cycling due to convective transport. The fluctuations are damped and need to be triggered by short-term low-power heating events. It is possible that, for instance, lightning discharges or rapid magnetic reconnection events might provide such trigger action. There is a great deal of direct evidence of rapid heating and cooling events in meteoritic material, and it is suggested that the above processes may help us to understand some aspects of these observations.

Protoplanetary accretion disks with coagulation and evaporation

Recently, three new developments have influenced the concept, purpose, and action of accretion disk theory in the context of star and planet formation. These are: (1) The strength of convectively driven turbulence may be as much as three orders of magnitude lower than was believed hitherto. (2) Coagulation of dust could significantly change the opacity of the nebula, in fact it probably has to, if planet formation is a general feature. It has been suggested that this could inhibit turbulence and possibly make it intermittent. (3) The role of grain evaporation in the inner disk region has been analyzed and it was concluded that the structure of accretion disks could be profoundly influenced by the nonlinear coupling introduced via associated opacity changes. In this paper we investigate steady disk models which incorporate these new features. In particular we show that accretion disks can adjust to remain optically thick even if substantial coagulation of dust gives very much smaller (than the canonical) opacities. Also, we demonstrate that evaporation of grains and the associated lowering of opacities leads to a protoplanetary disk which has a broad radial regime with central temperature ∼1500°K. For inefficient turbulence and high mass accretion rates this regime may extend beyond the orbit of Earth. Cosmochemical consequences are briefly discussed.

On convection-induced viscosity in accretion disks in cataclysmic variables

The vertical structure of accretion disks in cataclysmic variable systems is examined on the assumption that the convection is solely responsible for inducing the viscous heating. The viscous heating is distributed over the entire vertical structure so that purely radiative zones are able to carry a radial mass flux. The convective flux is computed by two different methods in separate calculations and the results from each method are compared. The values of the viscosity obtained are compared with those values inferred from the observed time scales of dwarf nova eruptions. Sequences of equilibrium solutions for constant radius and varying mass accretion rate are examined, the relations between viscosity and surface density are discussed in the context of recent models for the eruptions of dwarf novae.

Possible evidence for disk emission in SS 433

view Abstract Citations (37) References (24) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Possible Evidence for Disk Emission in SS 433 Filippenko, Alexei V. ; Romani, Roger W. ; Sargent, Wallace L. W. ; Blandford, Roger D. Abstract Optical and N/R spectra of SS 433 having moderate resolution (2-4 A) and exceptionally high signal-to-noise ratios (100/1 to 200/1), obtained a few days prior to the crossover time of the moving-line systems are presented. The strong Paschen lines of the stationary (rest wavelength) system exhibit a remarkable double-peaked structure (Delta v about 290 km/s), suggestive of emission from a rotating disklike region. Several unblended Fe II lines also consist of double peaks centered on the systemic velocity. Both the Paschen and the Balmer stationary-line profiles show substantial variation over the three nights spanning the observations. Interesting changes are also seen in the moving-line system; in particular, emission is detected from several members of the Paschen series. If the proposed identification of the disk emission is confirmed, detailed observations would probe the accretion disk structure and provide estimates of the mass of each component in the system. Publication: The Astronomical Journal Pub Date: July 1988 DOI: 10.1086/114806 Bibcode: 1988AJ.....96..242F Keywords: Accretion Disks; Binary Stars; Doppler Effect; Emission Spectra; Stellar Mass; Stellar Models; Infrared Spectra; Near Infrared Radiation; Paschen Series; Signal To Noise Ratios; Visible Spectrum; Astrophysics; STARS: WHITE DWARFS; STARS: CARBON; ULTRAVIOLET: GENERAL; X-RAYS: SOURCES full text sources ADS | data products SIMBAD (2)

The very low frequency power spectrum of Centaurus X-3

The long-term variability of Cen X-3 on time scales ranging from days to years has been examined by combining data obtained by the HEAO 1 A-4 instrument with data from Vela 5B. A simple interpretation of the data is made in terms of the standard alpha-disk model of accretion disk structure and dynamics. Assuming that the low-frequency variance represents the inherent variability of the mass transfer from the companion, the decline in power at higher frequencies results from the leveling of radial structure in the accretion disk through viscous mixing. The shape of the observed power spectrum is shown to be in excellent agreement with a calculation based on a simplified form of this model. The observed low-frequency power spectrum of Cen X-3 is consistent with a disk in which viscous mixing occurs about as rapidly as possible and on the largest scale possible.

X-ray irradiated accretion disks and bimodal states

X-ray irradiation plays an important role in the structure of an outer accretion disk. The authors examine the outer portion of an accretion disk, carefully taking into account X-ray irradiation within the context of α-viscosity. Bimodal states observed in the X-ray sources Cyg X-1 and GX 339-4 are explained naturally in terms of an accretion disk irradiated by X-rays originating at the inner portion of the disk.

A linear thermal stability analysis for the vertical structure of alpha model accretion disks

The linear equations that describe the thermal stability of the convective and radiative vertical structure of an alpha model accretion disk are derived. For a specific model chosen to be representative of a dwarf nova disk, it is found that the eigenfunction for the temperature shows large excursions in the zone of partial ionization, but that the eigenfunctions for the unstable fundamental mode perturbations of the vertical distance and flux are nearly constant over the vertical extent of the disk. The growth of any instability is thus nearly homologous and the stability criteria derived for vertically averaged structures represent an adequate approximation.

Accretion disk structure in SS Cygni

High-resolution Coude observations of non-axisymmetric line emission from the dwarf nova SS Cygni are presented. By subtracting the constant line component, the asymmetric line emission responsible for the observed phase shift between the absorption and emission line radial velocity curves can be isolated. The extra emission is a large fraction of the total line emission and extends to large velocities (∼1500 km sec−1). The phase stability of the emission demands a large-scale structure which is fixed in the frame of the binary. A magnetic origin of the excitation cannot be ruled out but is implausible. A simple explanation is that the accretion stream from the companion star is able to spill over the edge of the disk, introducing emission at non-circular velocities and most likely disturbing the upper layers of the accretion disk.

Accretion Disk Structure in SS Cygni

AbstractHigh-resolution Coude observations of non-axisymmetric line emission -from the dwarf nova SS Cygni are presented. By subtracting the constant line component, the asymmetric line emission responsible for the observed phase shift between the absorption and emission line radial velocity curves can be isolated. The extra emission is a large fraction of the total line emission and extends to large velocities (~1500 km sec−1). The phase stability of the emission demands a large-scale structure which is fixed in the frame of the binary. A magnetic origin of the excitation cannot De ruled out but is implausible. A simple explanation is that the accretion stream from the companion star is able to spill over the sags of the disk, introducing emission at non-circular velocities and most likely disturbing the upper layers of the accretion disk.

A critical examination of thick disks with equatorial accretion

view Abstract Citations (3) References (9) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Critical Examination of Thick Disks with Equatorial Accretion Hunter, J. H., Jr. ; Wilson, R. E. Abstract In order to deal with the problem of accretion disks around black holes, numerous models have been introduced during the past five years. The present paper is in particular concerned with the models discussed by Paczynski and Abramowicz (1982), taking into account a reproduction of the numerical integrations. Attention is given to details regarding the integration of the differential equations, the interior structure, model refinements, and cusp energetics. It is concluded that the structure of thick accretion disks of the type proposed by Paczynski and Abramowicz is extraordinary sensitive to the conditions at large radii. At some places in parameter space, only a minute change in the starting values separates two very different families of solutions. It is speculated that the solutions are globally unstable, even though they are stable to local axisymmetric perturbations by the Hoiland criterion. Publication: The Astrophysical Journal Pub Date: March 1986 DOI: 10.1086/163967 Bibcode: 1986ApJ...302...11H Keywords: Accretion Disks; Stellar Mass Accretion; Stellar Models; Thickness; Angular Momentum; Black Holes (Astronomy); Differential Equations; Radii; Stellar Orbits; Stellar Structure; Astrophysics; STARS: ACCRETION full text sources ADS | Related Materials (1) Erratum: 1986ApJ...308.1018H

Compact Binary X-Ray Sources

AbstractCompact binary X-ray sources include white dwarfs, neutron stars, and black holes that are accreting matter from a companion star. The X-ray emission from these systems is produced by the accreting matter as it flows through an accretion disk and strikes the surface of the compact object. The emitting regions have opacities dominated by electron scattering, and radiation pressure is likely to play an important role in the hydrodynamics. Strong magnetic fields greatly modify the hydrodynamics and radiation transfer in the pulsating neutron star sources. Accretion disks have complex structure, including an electron scattering corona, a cool outer region, and possibly a thick torus in their inner region. The structure and stability properties of accretion disks are only partially understood. Major problems exist with the interpretation of the spectra and luminosities of the X-ray burst sources. The pulsed X-ray emission from the pulsating binary X-ray sources probably comes from “mounds” of accreting gas at the magnetic poles of neutron stars, in which the accreting matter is decelerated by radiation pressure. The physics of these systems is reviewed, with an emphasis on problems for which hydrodynamical simulations may be especially useful.

The influence of nuclear burning on the stability of degenerate and nondegenerate accretion disks

view Abstract Citations (11) References (21) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The influence of nuclear burning on the stability of degenerate and nondegenerate accretion disks. Taam, R. E. ; Fryxell, B. A. Abstract The structure and stability of accretion disks composed of hydrogen-rich matter rotating about a central neutron star have been investigated for known sources of viscosity. Two general classes of solutions have been found. For one class the energy generated in the disk is provided by hydrogen burning, whereas for the other class the gravitational binding energy released by viscous dissipation dominates. The former solutions are thermally unstable (stable) whenever hydrogen burns via the normal CNO cycle (pp chain) in a partially or fully degenerate region of the disk. Solutions characterized by nuclear-burning via the Beta-limited CNO cycle or by viscous dissipation only are always stable. On the basis of a local analysis it is shown that modulations of the mass flow in the disk are possible for a range of mass inflow rates into the disk. In such circumstances the disk can undergo a phase transition from a cold, low-viscosity state to a hot, high-viscosity state as a result of the thermonuclear flash instability. Phase transitions from the hot state to the cold state also occur whenever the mass input rate into the disk is less than the equilibrium mass flow rate corresponding to the hot state. It is also shown that for sufficiently high mass flow rates all the hydrogen-rich matter can be processed to helium in the inner regions of the disk before it can be accreted by a neutron star. Publication: The Astrophysical Journal Pub Date: July 1985 DOI: 10.1086/163298 Bibcode: 1985ApJ...294..303T Keywords: Accretion Disks; Mass Flow; Neutron Stars; Nuclear Astrophysics; Nuclear Fusion; Stellar Models; Thermal Stability; Mass Flow Rate; Viscosity; X Ray Sources; Astrophysics full text sources ADS | data products SIMBAD (1)