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1.
Mass flow and accretion through gaps in accretion discs 总被引：1，自引：0，他引：1
W. Kley 《Monthly notices of the Royal Astronomical Society》1999,303(4):696-710
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Fractal concepts have been introduced in the accretion disc as a new feature. Due to the fractal nature of the flow, its continuity condition undergoes modifications. The conserved stationary fractal flow admits only saddle points and centre-type points in its phase portrait. Completely analytical solutions of the equilibrium point conditions indicate that the fractal properties enable the flow to behave like an effective continuum of lesser density, and facilitate the generation of transonicity. However, strongly fractal flows inhibit multitransonicity from developing. The mass accretion rate exhibits a fractal scaling behaviour, and the entire fractal accretion disc is stable under linearized dynamic perturbations. 相似文献
3.
Charles F. Gammie 《Monthly notices of the Royal Astronomical Society》1998,297(3):929-935
We show that radiation-dominated accretion discs are likely to suffer from a 'photon bubble' instability similar to that described by Arons in the context of accretion on to neutron star polar caps. The instability requires a magnetic field for its existence. In an asymptotic regime appropriate to accretion discs, we find that the overstable modes obey the remarkably simple dispersion relation
ο2 =−i gkF ( B , k ).
Here g is the vertical gravitational acceleration, B is the magnetic field, and F is a geometric factor of order unity that depends on the relative orientation of the magnetic field and the wavevector. In the non-linear outcome it seems likely that the instability will enhance vertical energy transport and thereby change the structure of the innermost parts of relativistic accretion discs. 相似文献
ο
Here g is the vertical gravitational acceleration, B is the magnetic field, and F is a geometric factor of order unity that depends on the relative orientation of the magnetic field and the wavevector. In the non-linear outcome it seems likely that the instability will enhance vertical energy transport and thereby change the structure of the innermost parts of relativistic accretion discs. 相似文献
4.
V. A. Urpin 《Astrophysics and Space Science》1983,90(1):79-107
A simple but sufficiently accurate method for calculating an accretion disk structure is developed. The detailed analysis of the accretion disk is fulfilled by using this method. The effect of turbulent heat transfer on the disk structure is taken into account along with the effect of radiative transfer. The turbulent heat transfer is shown to play an important role, and may be even predominant in the inner disk region. The dependence of temperatureT and density on thez-coordinate is found. Simple analytic expressions are proposed for the run of the density in all the disk zones. It is shown that the inner disk region is convectively stable. The main parameters of all the zones are derived. The geometry of regular motions is studied; the regular hydrodynamical flows are found to appear directed both toward and outside the central object. A detailed comparison is made with the results of other authors. 相似文献
5.
Renzo Sancisi Filippo Fraternali Tom Oosterloo Thijs van der Hulst 《Astronomy and Astrophysics Review》2008,15(3):189-223
Evidence for the accretion of cold gas in galaxies has been rapidly accumulating in the past years. HI observations of galaxies and their environment have brought to light new facts and phenomena which are evidence of ongoing
or recent accretion: (1) A large number of galaxies are accompanied by gas-rich dwarfs or are surrounded by HI cloud complexes, tails and filaments. This suggests ongoing minor mergers and recent arrival of external gas. It may be regarded,
therefore, as direct evidence of cold gas accretion in the local universe. It is probably the same kind of phenomenon of material
infall as the stellar streams observed in the halos of our galaxy and M 31. (2) Considerable amounts of extra-planar HI have been found in nearby spiral galaxies. While a large fraction of this gas is undoubtedly produced by galactic fountains,
it is likely that a part of it is of extragalactic origin. Also the Milky Way has extra-planar gas complexes: the Intermediate-
and High-Velocity Clouds (IVCs and HVCs). (3) Spirals are known to have extended and warped outer layers of HI. It is not clear how these have formed, and how and for how long the warps can be sustained. Gas infall has been proposed
as the origin. (4) The majority of galactic disks are lopsided in their morphology as well as in their kinematics. Also here
recent accretion has been advocated as a possible cause. In our view, accretion takes place both through the arrival and merging
of gas-rich satellites and through gas infall from the intergalactic medium (IGM). The new gas could be added to the halo
or be deposited in the outer parts of galaxies and form reservoirs for replenishing the inner parts and feeding star formation.
The infall may have observable effects on the disk such as bursts of star formation and lopsidedness. We infer a mean “visible”
accretion rate of cold gas in galaxies of at least . In order to reach the accretion rates needed to sustain the observed star formation (), additional infall of large amounts of gas from the IGM seems to be required. 相似文献
6.
The eccentric luminosity variations of quasars are still a mystery. Analytic results of this behavior ranged from multi-periodic behavior to a purely random process. Recently, we have used the non-linear time-series analysis techniques to analyze the optical light curve of 3C 273 and found that its eccentric behavior may be a low-dimensional chaos. This result induces us to seek some non-linear mechanism for the eccentric luminosity variation. In this paper, we propose a simple non-linear accretion disk model and find that under some circumstances, the chaos appears both in the disk and in the light curve. Then we compute the outburst energy ΔI, defined by the difference between the maximum luminosity and the minimum luminosity, and the mean luminosity 〈I〉 and find that when chaos appears, ΔI～〈I〉 α ～M 0.5α , where M is the mass of black hole and α≈1. These results are confirmed by and/or compatible with the observational data analysis. 相似文献
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I. G. Kovalenko & M. A. Eremin 《Monthly notices of the Royal Astronomical Society》1998,298(3):861-870
We examine the spatial stability of spherical adiabatic Bondi accretion on to a point gravitating mass against external perturbations. Both transonic critical and subsonic subcritical accretion are shown to be stable against purely radial acoustic, vortex or entropy perturbations. In the case of non-radial perturbations the amplitude of the perturbations grows without limit with smaller radii. Instability manifests itself only if the size of the accreting body is much less than the Bondi radius so that the inflow is highly supersonic or highly subsonic at the surface of the accretor in the case of critical or subcritical accretion respectively. These asymptotics hold and consequently the instability may develop for adiabatic index of accreting gas γ < 5/3. We suggest that this instability may lead to an essential thermalization of accreting flow thus, particularly, solving the problem of otherwise inefficient energy release in spherical accretion on to a black hole. 相似文献
10.
N.Z. Scoville 《Astrophysics and Space Science》1999,266(1-2):149-155
Recent high resolution near infrared (HST-NICMOS) and mm-interferometric imaging have revealed dense gas and dust accretion disks in nearby ultra-luminous galactic nuclei. In the best studied ultraluminousIR galaxy, Arp 220, the 2m imaging shows dust disks in both of the merging galactic nuclei and mm-CO line imaging indicates molecular gasmasses 109M for each disk. The two gas disks in Arp 220 are counterrotating and their dynamical masses are 2×109
M
, that is, only slightly largerthan the gas masses. These disks have radii 100 pc and thickness 10-50 pc. The high brightness temperatures of the CO lines indicatethat the gas in the disks has area filling factors 25-50% and mean densitiesof 104 cm-3. Within these nuclear disks, the rate of massive star formation is undoubtedly prodigious and, given the high viscosity of the gas, there will also be high radial accretion rates, perhaps 10 M yr -1. If this inflow persists to very small radii, it is enough to feed even the highest luminosity AGNs. 相似文献
11.
I. A. Bonnell M. R. Bate C. J. Clarke J. E. Pringle 《Monthly notices of the Royal Astronomical Society》2001,323(4):785-794
We investigate the physics of gas accretion in young stellar clusters. Accretion in clusters is a dynamic phenomenon as both the stars and the gas respond to the same gravitational potential. Accretion rates are highly non-uniform with stars nearer the centre of the cluster, where gas densities are higher, accreting more than others. This competitive accretion naturally results in both initial mass segregation and a spectrum of stellar masses. Accretion in gas-dominated clusters is well modelled using a tidal-lobe radius instead of the commonly used Bondi–Hoyle accretion radius. This works as both the stellar and gas velocities are under the influence of the same gravitational potential and are thus comparable. The low relative velocity which results means that R tidal < R BH in these systems. In contrast, when the stars dominate the potential and are virialized, R BH < R tidal and Bondi–Hoyle accretion is a better fit to the accretion rates. 相似文献
12.
Yasushi Kawai 《Astrophysics and Space Science》1989,151(1):81-101
We have studied the effect of the flow in the accretion disk. The specific angular momentum of the disk is assumed to be constant and the polytropic relation is used. We have solved the structure of the disk and the flow patterns of the irrotational perfect fluid.As far as the obtained results are concerned, the flow does not affect the shape of the configuration in the bulk of the disk, although the flow velocity reaches even a half of the sound velocity at the inner edge of the disk. Therefore, in order to study accretion disk models with the moderate mass accretion rate—i.e.,
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相似文献
13.
D. Steeghs 《Astronomische Nachrichten》2004,325(3):185-188
Since its conception, Doppler tomography has matured into a versatile and widely used tool. It exploits the information contained in the highly‐structured spectral line‐profiles typically observed in mass‐transferring binaries. Using inversion techniques akin to medical imaging, it permits the reconstruction of Doppler maps that image the accretion flow on micro‐arcsecond scales. I summarise the basic concepts behind the technique and highlight two recent results; the use of donor star emission as a means to system parameter determination, and the real‐time movies of the evolving accretion flow in the cataclysmic variable WZ Sge during its 2001 outburst. I conclude with future opportunities in Doppler tomography by exploiting the combination of superior data sets, second generation reconstruction codes and simulated theoretical tomograms to delve deeper into the physics of accretion flows. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
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E. I. Vorobyov Shantanu Basu 《Monthly notices of the Royal Astronomical Society》2007,381(3):1009-1017
We present a numerical model for the evolution of a protostellar disc that has formed self-consistently from the collapse of a molecular cloud core. The global evolution of the disc is followed for several million years after its formation. The capture of a wide range of spatial and temporal scales is made possible by use of the thin-disc approximation. We focus on the role of gravitational torques in transporting mass inward and angular momentum outward during different evolutionary phases of a protostellar disc with disc-to-star mass ratio of order 0.1. In the early phase, when the infall of matter from the surrounding envelope is substantial, mass is transported inward by the gravitational torques from spiral arms that are a manifestation of the envelope-induced gravitational instability in the disc. In the late phase, when the gas reservoir of the envelope is depleted, the distinct spiral structure is replaced by ongoing irregular non-axisymmetric density perturbations. The amplitude of these density perturbations decreases with time, though this process is moderated by swing amplification aided by the existence of the disc's sharp outer edge. Our global modelling of the protostellar disc reveals that there is typically a residual non-zero gravitational torque from these density perturbations, i.e. their effects do not exactly cancel out in each region. In particular, the net gravitational torque in the inner disc tends to be negative during first several million years of the evolution, while the outer disc has a net positive gravitational torque. Our global model of a self-consistently formed disc shows that it is also self-regulated in the late phase, so that it is near the Toomre stability limit, with a near-uniform Toomre parameter Q ≈ 1.5–2.0. Since the disc also has near-Keplerian rotation, and comparatively weak temperature variation, it maintains a near-power-law surface density profile proportional to r
16.
An asymptotic treatment of thin accretion disks, introduced by Kluźniak and Kita [Kluźniak, W., Kita, D., 2000. Three-dimensional structure of an alpha accretion disk. Available from: <arXiv:astro-ph/0006266v1> (KK)] for a steady-state disk flow, is extended to a time-dependent problem. Transient growth of axisymmetric disturbances is analytically shown to occur on the global disk scale. The implications of this result on the theory of hydrodynamical thin accretion disks, as well as future prospects, are discussed. 相似文献
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Feng Yuan † 《Monthly notices of the Royal Astronomical Society》2001,324(1):119-127
We find a new two-temperature hot branch of equilibrium solutions for stationary accretion discs around black holes. In units of Eddington accretion rate defined as 10 L
19.
Accretion disks around magnetized, compact stars are expected to be tilted near their inner edges, due to the stresses exerted by the corotating magnetosphere of the inclined central rotator. We reassess numerically the results obtained analytically by Lipunovet al. (1981). Four qualitatively different situations occur, depending on the relative orientations of the outer accretion disk, the spin of the central rotator, and its magnetic dipole axis. In at least two of them, the inner part of the disk is expected to be decomposed into massive, magnetically confined clumps. 相似文献
20.
A detailed understanding of how the infalling matter in accretion columns is decelerated is essential for the calculation of the emitted radiation. On neutron stars, the deceleration takes place mainly by the interaction of the plasma with radiation, at least for the high-luminosity sources. We report on our two-dimensional calculations of the hydrodynamic flow in such accretion columns. The radiation transport is treated in the diffusion approximation, and we are looking for a stationary solution for the velocity field. The dependence of the results on physical parameters, especially on the accretion rate is discussed. Due to the non-linearity of the problem it turns out that only in certain parameter ranges stationary solutions seem to exist. For accretion rates higher than a critical value there are no stationary accretion flows. This leads us to the conclusion that a time-dependent picture for the accretion is unavoidable.Paper presented at the IAU Colloquium No. 93 on Cataclysmic Variables. Recent Multi-Frequency Observations and Theoretical Developments, held at Dr. Remeis-Sternwarte Bamberg, F.R.G., 16–19 June, 1986. 相似文献
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