共查询到20条相似文献，搜索用时 31 毫秒
1.
We have studied the structure of hot accretion flow bathed in a general large-scale magnetic field. We have considered magnetic parameters , where are the Alfvén sound speeds in three direction of cylindrical coordinate (r,φ,z). The dominant mechanism of energy dissipation is assumed to be the magnetic diffusivity due to turbulence and viscosity in the accretion flow. Also, we adopt a more realistic model for kinematic viscosity (ν=αc s H), with both c s and H as a function of magnetic field. As a result in our model, the kinematic viscosity and magnetic diffusivity (η=η 0 c s H) are not constant. In order to solve the integrated equations that govern the behavior of the accretion flow, a self-similar method is used. It is found that the existence of magnetic resistivity will increase the radial infall velocity as well as sound speed and vertical thickness of the disk. However the rotational velocity of the disk decreases by the increase of magnetic resistivity. Moreover, we study the effect of three components of global magnetic field on the structure of the disk. We found out that the radial velocity and sound speed are Sub-Keplerian for all values of magnetic field parameters, but the rotational velocity can be Super-Keplerian by the increase of toroidal magnetic field. Also, Our numerical results show that all components of magnetic field can be important and have a considerable effect on velocities and vertical thickness of the disk. 相似文献
2.
As gas flowed from the solar accretion disk or Solar Nebula onto the proto-Sun, magnetic pressure gradients in the solar magnetosphere and the inner Solar Nebula provided an environment where some of this infalling flow was diverted to produce a low pressure, high temperature, gaseous, “infall” atmosphere around the inner Solar Nebula. The pressure in this inner disk atmosphere was mainly dependant on the accretion flow rate onto the star. High flow rates implied relatively high pressures, which decreased over time as the accretion rate decreased.In the first hundred thousand years after the formation of the Solar Nebula, accretional flow gas pressures were high enough to create submicron-sized Refractory Metal Nuggets (RMNs) – the precursors to Calcium Aluminum Inclusions (CAIs). Optimal temperatures and pressures for RMN formation may have occurred between 20,000 and 100,000 years after the formation of the Solar Nebula. It is possible that conditions were conducive to RMN/CAI formation over an 80,000 year timescale. The “infall” atmosphere and the condensation of refractory particles within this atmosphere may be observable around the inner disks of other protostellar systems.The interaction of forces from magnetic fields with the radiation pressure from the proto-Sun and the inner solar accretion disk potentially produced an optical-magnetic trap above and below the inner Solar Nebula, which provided a relatively stable environment in which the RMNs/proto-CAIs could form and grow. These RMN formation sites only existed during accretion events from the proto-solar disk onto the proto-Sun. As such, the formation and growth time of a particular RMN was dependent on the timescale of its nascent accretion event.Observational evidence suggests that RMNs were the nucleation particles for CAIs. As a consequence, the observed bimodal distribution of 26Al in CAIs, where some CAIs have 26Al while others do not, is probably due to the injection 26Al during the short CAI formation period, where 26Al was not present when the first CAIs were formed. 相似文献
3.
Christian Fendt 《Astrophysics and Space Science》2003,287(1-4):59-64
The evolution of a stellar, initially dipole type magnetosphere interacting with an accretion disk is investigated using numerical ideal MHD simulations. The simulations follow several 1000 Keplerian periods of the inner disk (for animated movies see http://www.aip.de～cfendt).Our model prescribes a Keplerian disk around a rotating star as a fixed boundary condition. The initial magnetic field distribution remains frozen into the star and the disk. The mass flow rate into the corona is fixed for both components. The initial dipole type magnetic field develops into a spherically radial outflow pattern with two main components – a disk wind and a stellar wind – both evolving into a quasi-stationary final state. A neutral field line divides both components, along which small plasmoids are ejected in irregular time intervals. The half opening angle of the stellar wind cone varies from 30° to55° depending on the ratio of the mass flow rates of disk wind and stellar wind. The maximum speed of the outflow is about the Keplerian speed at the inner disk radius. An axial jet forms during the first decades of rotations. However, this feature does not survive on the very long time scale and a pressure driven low velocity flow along the axis evolves. Within a cone of 15° along the axis the formation of knots may be observed if the stellar wind is weak. With the chosen mass flow rates and field strength we see almost no indication for a flow self-collimation. This is due to the weak net poloidal electric current in the magnetosphere which is in difference to typical jet models. 相似文献
4.
We calculate the polarization of the radiation from an optically thick accretion disk with a vertical averaged magnetic field.
The polarization arises from the scattering of light by free electrons in a magnetized disk plasma. The Faraday rotation of
the polarization plane during the propagation of a photon in a medium with a magnetic field is considered as the main effect.
We discuss various models of optically thick accretion disks with a vertical averaged magnetic field. Our main goal is to
derive simple asymptotic formulas for the polarization of radiation in the case where the Faraday rotation angle Ψ ≫ 1 at
the Thomson optical depth τ = 1. The results of our calculations allow the magnetic field strength in the region of the marginally
stable orbit near a black hole to be estimated from polarimetric observations, including X-ray observations expected in the
future. Since the polarization spectrum of the radiation strongly depends on the accretion disk model, a realistic physical
model of the accretion disk can be determined from data on the polarization of its radiation. 相似文献
5.
6.
We carry out 2.5D MHD simulations to study the interaction between a dipolar magnetic field of a T Tauri Star, a circumstellar accretion disk, and the halo above the disk. The initial disk is the result of 1D radiation hydrodynamics computations with opacities appropriate for low temperatures. The gas is assumed resistive, and inside the disk accretion is driven by a Shakura–Sunyaev-type eddy viscosity. Magnetocentrifugal forces due to the rotational shear between the star and the Keplerian disk cause the magnetic field to be stretched outwards and part of the field lines are opened. For a solar-mass central star and an accretion rate of 10?8 solar masses per year a field strength of 100 G (measured on the surface of the star) launches a substantial outflow from the inner parts of the disk. For a field strength of 1 kG the inner parts of disk is disrupted. The truncation of the disk turns out to be temporary, but the magnetic field structure remains changed after the disk is rebuilt. 相似文献
7.
We simulate the impact of a dipolar stellar magnetic field rooted in a classical T Tauri star on the accretion disk and the halo above using a 2.5D finite difference code. The gas is assumed resistive, and inside the disk accretion is driven by a Shakura-Sunyaev-type eddy viscosity. The rotational shear between the star and the Keplerian disk causes the magnetic field to be wound up and stretched outwards, away from the star. Part of the field lines open and an outflow is launched. Direct disk disruption by the Lorentz force only occurs for sufficient field strength. For our model system with a solar-mass central star, an accretion rate of 10-7M⊙/a, and a viscosity parameter αSS=0.01, a field strength of 1 kG, measured at the poles on the surface of the star, was found insufficient for disk disruption. 相似文献
8.
Most astrophysical accretion disks are likely to be warped.In X-ray binaries,the spin evolution of an accreting neutron star is critically dependent on the interaction between the neutron star magnetic field and the accretion disk.There have been extensive investigations on the accretion torque exerted by a coplanar disk that is magnetically threaded by the magnetic field lines from the neutron stars,but relevant works on warped/tilted accretion disks are still lacking.In this paper we develop a simplified twocomponent model,in which the disk is comprised of an inner coplanar part and an outer,tilted part.Based on standard assumption on the formation and evolution of the toroidal magnetic field component,we derive the dimensionless torque and show that a warped/titled disk is more likely to spin up the neutron star compared with a coplanar disk.We also discuss the possible influence of various initial parameters on the torque. 相似文献
9.
A study is made of axisymmetric, low sonic-Mach-number flows of a viscous fluid with angular momentum outside of a black-hole. The viscosity is an eddy viscosity due to turbulence in the sheared flows. Self-similar solutions arise naturally, reducing the Navier-Stokes equations to a set of nonlinear ordinary differential equations. These equations are solved analytically for flows of constant specific angular momentum and numerically for more general flows. For flows with non-constant specific angular momentum, the momentum flux density includes a planar discontinuity which is interpreted as an accretion disc. In general, two flow regions appear on each side of the disk, corresponding to accretion onto the disk and jet-like outflows along the ±z-axes. Physical interpretations of the solutions show that these flows arise in response to point sources of axial momentum at the origin directed in the ±z-directions. The power needed to maintain this momentum input is assumed to come from the mass accretion onto the black hole.The hydrodynamic flows are generalized to include a magnetic field. In the limit of infinite electrical conductivity, the possible types of flow patterns are the same as in hydrodynamic case. The magnetic field alters the relative amounts of reversible and irreversible momentum and angular momentum transport by the flow. For a flow with turbulent viscosity, the magnetic field acts to reduce the level of the turbulence and the effective value of the eddy viscosity. 相似文献
10.
Modern models of the formation of the regular satellites of giant planets, constructed with consideration for their structure and composition suggest that this process lasted for a considerable period of time (0.1–1 Myr) and developed in gas-dust circumplanetary disks at the final stage of giant planet formation. The parameters of protosatellite disks (e.g., the radial distribution of surface density and temperature) serve as important initial conditions for such models. Therefore, the development of protosatellite disk models that take into account currently known cosmochemical and physical restrictions remains a pressing problem. It is this problem that is solved in the paper. New models of the accretion disks of Jupiter and Saturn were constructed with consideration for the disk heating by viscous dissipation of turbulent motions, by accretion of material from the surrounding region of the solar nebula, and by radiation from the central planets. The influence of a set of input model parameters (the total rate of mass infall onto the disk, the turbulent viscosity and opacity of disk material, and the centrifugal radius of the disk) on thermal conditions in the accretion disks was studied. The dependence of opacity on temperature and the abundance and size of solid particles present in the disk was taken into account. Those constructed models that satisfy the existing constraints limit the probable values of input parameters (primarily rates of mass infall onto the disks of Jupiter and Saturn at the stage of regular satellite formation and, to a lesser extent, the disk opacities). Constraints on the location of the regions of formation of the major satellites of Jupiter and Saturn are suggested based on the constructed models and simple analytical estimates concerning the formation of satellites in the accretion disks. It is shown that Callisto and Titan could hardly be formed at significantly greater distances from their planets. 相似文献
11.
N. A. Silant’ev Yu. N. Gnedin S. D. Buliga M. Yu. Piotrovich T. M. Natsvlishvili 《Astrophysical Bulletin》2013,68(1):14-25
Estimates of magnetic fields for a number of active galactic nuclei are presented. These estimates are based on the observed polarization degrees and position angles of broad Hα lines and in the nearby continuum and on asymptotic analytical formulas for the Stokes parameters of the radiation emerging from a magnetized accretion disk (the Milne problem in a magnetized atmosphere). The characteristic observed feature of the wavelength dependence of the polarization degree inside the line—a minimum at the center and a fast increase of the position angle from one wing to another—can be explained by the superposition of resonance emission from two or more clouds located in the right (Keplerian velocity directed away from the observer) and left (Keplerian velocity directed toward the observer) parts of the orbit in the rotating magnetized accretion disk. The main component in our mechanism is the azimuthal magnetic field in the disk. The presence of a magnetic field perpendicular to the disk plane (which is usually weaker than the azimuthal field) results in the asymmetry of the distribution of the polarization degree and position angle inside the line. The inferred magnetic field strengths at the galactocentric distances where broad lines are emitted can be used to estimate the magnetic fields in the region of the centermost stable orbit and at the horizon of the central black hole, using the power-law dependence of the magnetic field strength corresponding to the standard model of the accretion disk. 相似文献
12.
13.
本文讨论了具有磁场的几何厚吸积盘结构和辐射。结果表明:磁场的存在明显地影响吸积盘的总光度和形状。这种模型可以解释类星体的辐射幂谱分布和高度偏振现象。它是类星体主要辐射区域的一种合理模型。 相似文献
14.
M. Yu. Piotrovich Yu. N. Gnedin N. A. Silant'ev T. M. Natsvlishvili S. D. Buliga 《Astronomische Nachrichten》2015,336(10):1013-1016
Magnetic fields in an accretion disk around the central black hole can modify the size of the innermost stable circular orbit (ISCO) and can produce a difference to the classical Novikov‐Thorne radius. We estimated the ISCO magnetic field strength from the polarimetric observations of the accretion‐disk radiation. This estimate is obtained taking into account the effect of the Faraday rotation of the polarization plane at the distance of the mean free path of photons between successive electron scattering events. We present the new method for estimating the ISCO radius in the accretion disk, i.e. in the nearest vicinity of a central black hole. Our estimates confirmed the Frolov, Shoom & Tzounis (2014) and Ranea‐Sandoval & Garcia (2015) conclusion that the magnetic field in the accretion disk decreases the size of the innermost stable circular orbit. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
15.
N. A. Silant’ev M. Yu. Piotrovich Yu. N. Gnedin T. M. Natsvlishvili 《Astronomy Letters》2010,36(8):549-553
We compare two methods for estimating the magnetic field near a black hole—based on a statistical formula that relates the
magnetic field on the horizon of the black hole to its mass and on a direct magnetic field determination from optical polarimetric
observations. An optically thick magnetized accretion disk is usually formed around the black hole in active galactic nuclei.
The linear polarization of the radiation emerging from the disk is related to the local magnetic field and the polarization
in Milne’s classical problem. The magnetic field in the region from where the radiation with the observed wavelength comes
can be estimated by measuring this polarization. On the other hand, this field can be estimated from the above statistical
formula if the field near the black hole is extrapolated into the accretion disk using a standard power law. Comparison of
the magnetic field obtained from the observed polarization with the extrapolated field can serve to estimate the accuracy
of the latter and the exponent in the adopted extrapolation law. As an example, we compare these two methods for the source
NGC 4258. 相似文献
16.
Equilibrium configuration of the magnetosphere of a star loaded by the gravitationally accreted plasma having its own magnetic
field is investigated. Axisymmetry around the star’s magnetic axis is assumed for simplicity. It is seen that two distinct
configurations appear for the cases of parallel and antiparallel magnetic field of the accreted plasma with respect to the
star’s magnetic moment. If the external field is antiparallel to the star’s magnetic moment, the stellar magnetosphere is
confined within a spherical region surrounded by the external field with a separatric surface between them. This is an extension
of the case of the spherical accretion of non-magnetic plasma dealt with thus far in connection with the mass accretion by
the degenerate stars in X-ray binaries. It is noticed that the mass slides down along the field lines to the point closest
to the star and is stratified hydrostatically in equilibrium to form a disk in the equatorial plane. The mass loading compresses
the sphere as a whole in this case. If, on the other hand, the external field is parallel to the star’s magnetic moment, there
appears a ring of magnetic neutral point in the equatorial plane. Polar field is open and extends to infinity while the low-latitude
field is closed and faces the external field of opposite polarity across the neutral point. The increase of the loaded mass
in this case causes a shrink of the closed field region, and the open polar flux is increased. Therefore, the transition between
equilibria with small and large amount of the loaded mass requires the reconnection of magnetic lines of force, and the reconnection
of the flux through the magnetic neutral ring is proposed as the mechanism of the steady or the intermittent mass leakage
like the ones postulated for some X-ray bursters.
Visiting Scientist supported by the Japan Society for the Promotion of Sciences. 相似文献
17.
Acounter-rotating gas disk has been detected in the SA0 galaxy IC 560 located at the periphery of a sparse group of six late-type galaxies. The pattern of gas excitation and mid-infrared colors are indicative of ongoing star formation within 1 kpc of the center. Outside the gas disk with star formation the large-scale stellar disk of the galaxy has an old age and a very low metallicity, [Z/H] ≈ ?1. The source of external gas accretion onto IC 560 is undetected; the only option is a single infall of a companion rich in high-metallicity gas. 相似文献
18.
By taking magnetic stress in place of viscosity as the mechanism for angular moaentum transfer, the effect of frozen magnetic field on the structure of a geometrically thin accretion disk is examined. It is shown that the disk is quasi-Keplerian and its total luminosity is twice the luminosity in the standard disk model. In the inner region, there exists a narrow cool region and the highly collimated jet is formed under the action of the azimuthal component of the magnetic field. Also, we discuss the possibility that a magnetized corona be formed near the surface of the accretion disk and a wide band radiation issuing therefrom. The model suggested here can easily and reasonably explain the major AGN properties such as the radiation variation, the “bumps” in the optical, ultraviolet and soft X-ray ranges, etc. 相似文献
19.
We present measurements of magnetic field strength and geometry on the surfaces of T Tauri stars (TTS) with and without circumstellar disks. We use these measurements to argue that magnetospheric accretion models should not assume that a fixed fraction of the stellar surface contains magnetic field lines that couple with the disk. We predict the fractional area of accretion footpoints, using magnetospheric accretion models and assuming field strength is roughly constant for all TTS. Analysis of Zeeman broadened infrared line profiles shows that individual TTS each have a distribution of surface magnetic field strengths extending up to 6 kG. Averaging over this distribution yields mean magnetic field strengths of 1-3 kG for all TTS, regardless of whether the star is surrounded by a disk. These strong magnetic fields suggest that magnetic pressure dominates gas pressure in TTS photospheres, indicating the need for new model atmospheres. The He I 5876 Å emission line in TTS can be strongly polarized, so that magnetic field lines at the footpoints of accretion have uniform polarity. The circular polarization signal appears to be rotationally modulated, implying that accretion and perhaps the magnetosphere are not axisymmetric. Time series spectropolarimetry is fitted reasonably well by a simple model with one magnetic spot on the surface of a rotating star. On the other hand, spectropolarimetry of photospheric absorption lines rules out a global dipolar field at the stellar surface for at least some TTS. 相似文献
20.
本文从磁流体力学基本方程组出发,讨论了具有类偶极磁场的厚吸积盘模型。其结果表明:引入这种磁场位形后,会影响盘的位形、光度和吸积率。对光度的影响是通过改变盘的位形实现的,它使盘变厚,会在厚盘的内区形成更窄更陡的漏斗状结构,这对解释类星体的高度准直性的喷流更为有利;并且还预言不对称喷流或单边喷流的存在。 相似文献