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1.
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
2.
K. Tsinganos N. Vlahakis S. Bogovalov C. Sauty E. Trussoni J.J.G. Lima 《Astrophysics and Space Science》2003,287(1-4):103-108
We explain in simple terms why a rotating and magnetized outflow forms a core with a jet and show numerical simulations which substantiate this argument. The outflow from a solar-type inefficient magnetic rotator is found to be very weakly collimated while the outflow from a ten times faster rotating YSO is shown to produce a tightly collimated jet. This gives rise to an evolutionary scenario for stellar outflows. We also propose a two-component model consisting of a wind outflow from a central object and a faster rotating outflow launched from a surrounding accretion disk which plays the role of the flow collimator. 相似文献
3.
We present the optical spectroscopic study of two classical Be stars, 59 Cyg and OT Gem obtained over a period of few months in 2009. We detected a rare triple-peak H α emission phase in 59 Cyg and a rapid decrease in the emission strength of H α in OT Gem, which are used to understand their circumstellar disks. We find that 59 Cyg is likely to be rapid rotator, rotating at a fractional critical rotation of ～0.80. The radius of the H α emission region for 59 Cyg is estimated to be R d/R ? ～ 10.0, assuming a Keplerian disk, suggesting that it has a large disk. We classify stars which have shown triple-peaks into two groups and find that the triple-peak emission in 59 Cyg is similar to ζ Tau. OT Gem is found to have a fractional critical rotation of ～0.30, suggesting that it is either a slow rotator or viewed in low inclination. In OT Gem, we observed a large reduction in the radius of the H α emission region from ～6.9 to ～1.7 in a period of three months, along with the reduction in the emission strength. Our observations suggest that the disk is lost from outside to inside during this disk loss phase in OT Gem. 相似文献
4.
P. P. Petrov 《Kinematics and Physics of Celestial Bodies》2010,26(4):192-199
Basic ideas concerning the nature of young T Tauri stars (TTS) are briefly outlined and some examples of spectral investigations
of those stars are considered. The photometric and spectral variability of TTS is believed to be due to circumstellar extinction,
magnetic activity, and accretion of matter from the circumstellar disk onto the stellar surface. In the 1990s, a series of
high resolution spectra of several TTS were obtained using the SOFIN echelle spectrograph with the Nordic Optical Telescope
(NOT). In particular, the emission lines in the spectra of the star RW Aur A were shown to be rotationally modulated with
a period of 2.7 days, which was interpreted in terms of the magnetospheric accretion model with an inclined magnetic rotator.
The spectra of TTS obtained using the UVES spectrograph with the VLT demonstrated that the effect of veiling the photospheric
spectrum, usually attributed to accretion, was largely due to chromospheric extinction. The accretion is suggested to be a
complementary heating source in chromospheres of TTS. 相似文献
5.
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
6.
Yoshiaki Kato 《Astrophysics and Space Science》2007,307(1-3):11-15
In spite of the large number of global three-dimensional (3-D) magnetohydrodynamic (MHD) simulations of accretion disks and
astrophysical jets, which have been developed since 2000, the launching mechanisms of jets is somewhat controversial. Previous
studies of jets have concentrated on the effect of the large-scale magnetic fields permeating accretion disks. However, the
existence of such global magnetic fields is not evident in various astrophysical objects, and their origin is not well understood.
Thus, we study the effect of small-scale magnetic fields confined within the accretion disk. We review our recent findings
on the formation of jets in dynamo-active accretion disks by using 3-D MHD simulations. In our simulations, we found the emergence
of accumulated azimuthal magnetic fields from the inner region of the disk (the so-called magnetic tower) and also the formation
of a jet accelerated by the magnetic pressure of the tower. Our results indicate that the magnetic tower jet is one of the
most promising mechanisms for launching jets from the magnetized accretion disk in various astrophysical objects. We will
discuss the formation of cosmic jets in the context of the magnetic tower model. 相似文献
7.
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
8.
We derive the formula for the variation of the magnetic axis in a collapsing oblique rotator. Our results of calculation show that the case of the magnetic axis coincident with the rotation axis is unstable, and the angle between the two axes will grow on any small perturbation. 相似文献
9.
脉冲星的偏振信息是理解脉冲星辐射区的重要手段，利用澳大利亚的64m射电望远镜进行大量的脉冲星观测，得到了一批脉冲星的偏振轮廓和偏振参数，编辑了几乎所有发表的脉冲星轮廓资料，系统总结了脉冲星圆偏辐射的规律，为理论上解释脉冲星辐射这一重要难题提供观测依据和物理限制，利用脉冲星作为探针，研究了银河系磁场结构和模型，确定了银河系BS磁场模型，发现了银河系上下反对称的环向磁场，并首次对星系尺度的发电机类型进行判别，证认出A0型发电机运行于银河系，发现了银晕中的垂直磁场和M31及银盘中的非常延展的磁场，探测到NGC2997星系中由内到外的旋涡磁场，并提出可能有两种发电机在这个星系的不同区域运行。 相似文献
10.
I. F. Malov 《Astronomy Letters》2008,34(10):699-703
At present, it is widely believed that anomalous X-ray pulsars (AXPs), soft gamma-ray repeaters (SGRs), rotational radio transients (RRATs), compact central objects (CCOs) in supernova remnants, and X-ray dim isolated neutron stars (XDINSs) belong to different classes of anomalous objects in which the central bodies are isolated neutron stars. Previously, we have shown that AXPs and SGRs can be described in terms of the drift model for parameters of the central neutron star typical of radio pulsars (rotation periods P ～ 0.1–1 s and surface magnetic fields B ～ 1011–1013 G). Here, we show that some of the peculiarities of the sources under consideration can be explained by their geometry (in particular, by the angle β between the rotation axis and the magnetic moment). If β ? 10° (an aligned rotator), the drift waves in the outer layers of the neutron star magnetosphere can account for the observed periodicity in the radiation. For large β (a nearly orthogonal rotator), the observed modulation of the radiation and its short bursts can be explained by mass accretion from the ambient medium (e.g., a relic disk). 相似文献
11.
Kurt Liffman 《Astrophysics and Space Science》2007,311(1-3):69-74
We discuss the star-disk electric circuit for a young stellar object (YSO) and calculate the expected torques on the star
and the disk. We obtain the same disk magnetic field and star-disk torques as given by standard magnetohydrodynamic (MHD)
analysis. We show how a short circuit in the star-disk electric circuit may produce a magnetically-driven jet flow from the
inner edge of a disk surrounding a young star.
An unsteady bipolar jet flow is produced that flows perpendicular to the disk plane. Jet speeds of order hundreds of kilometers
per second are possible, while the outflow mass loss rate is proportional to the mass accretion rate and is a function of
the disk inner radius relative to the disk co-rotation radius. 相似文献
12.
We investigate the Hall effect in a standard magnetized accretion disk which is accompanied by dissipation due to viscosity and magnetic resistivity.By considering an initial magnetic field,using the PLUTO code,we perform a numerical magnetohydrodynamic simulation in order to study the effect of Hall diffusion on the physical structure of the disk.Current density and temperature of the disk are significantly modified by Hall diffusion,but the global structure of the disk is not substantially affected.The changes in the current densities and temperature of the disk lead to a modification in the disk luminosity and radiation. 相似文献
13.
H. Riffert 《Astrophysics and Space Science》1980,71(1):195-201
We study the problem of magnetic field computation for a thin diamagnetic accretion disk around the oblique dipole of a magnetic neutron star. An analytic expression of the general solution is given, containing a free constant which describes the amount of magnetic flux through the hole in the middle of the disk. 相似文献
14.
A detailed theory of high-entropy supermassive oblique rotator is developed on the basis of the results of our previous work on the structure and evolution of supermassive rotating magnetic polytropes. Particular attention is paid to the problem of transformation of rotational energy into observable radiation. It is shown that a rather long duration of the quasi-stationary phase in combination with a considerably high value of magnetic dipole radiationL
md in comparison with that of thermal radiationL
th imposes substantial limitations on the character of the model. This combination is realized in the model of an oblique rotator with a rigid rotation and a poloidal magnetic field. Moreover, the magnetic energy must be comparable, in order of magnitude, with the module of the gravitational energy.The magnitude of the torque acting on the rotator by means of the magnetic field is influenced neither by the external plasma thrown out by the low-frequency radiation pressure, nor by the plasma ejected electrostatically, nor by that flowing out due to rotational instability. For definiteness we assume below that the rotational energy is lost by the rotator in the form of a low-frequency magnetic dipole radiation though many of the conclusions drawn in this paper are also valid for the case when the energy is liberated in some other forms (for example, in magnetohydrodynamic waves).Plasma outflow under rotational instability is considered in detail. This is a pulsating outflow. Near the light cylinder the plasma spreads out and begins to interact intensively with the magnetic dipole radiation. As a result, the particles are accelerated up to relativistic energies. Accelerated electrons radiate by synchrotron mechanism with the radiation maximum in the far infra-red region (Figure 4). Compton scattering of this radiation is in the X-ray and gamma regions. The character of the non-thermal radiation calculated accounts for the essential features of the observable radiation from quasars and active galactic nuclei.A secular variation of the magnetic dipole radiation in the course of evolution of a supermassive oblique rotator (Figure 1) with the account taken of the influence of magnetic dipole losses on the value of the angle between magnetic and rotational axes is discussed in the vacuum approximation substantiated in Section 3. For a wide interval of initial values of this angle the non-thermal (synchrotron and Compton) radiation increases in the course of a quasi-static contraction of the rotator, reaches its maximum at whichL
nthL
th, and then decreases considerably due to rapprochement of the magnetic and rotational axes. Such a behaviour ofL
nth corresponds to the expected secular change of the activity of galactic and quasar nuclei as a certain flaring up, reaching the maximum of their activity and subsequent dying out.Some essential properties of quasars and quasar-like phenomena in galactic and quasar nuclei are explained on the basis of the theory presented. As an illustration, the parameters of a supermassive rotator modelling the source of activity in quasar 3C 273 are calculated (Table I). The estimate of the frequency of occurrence of rotators withL
nth>L
th andL
nthL
th is in a reasonable correspondence with the statistics of active phenomena in the nuclei of galaxies of different morphological types.Observational tests for this theory are suggested. The most important one called upon to ascertain the presence in the nuclei of galaxies and quasars of a supermassive body as a source of their activity is the variability of infra-red radiation near its spectral maximum.Similarity and difference between a supermassive oblique rotator and a pulsar, the model of a quasar nucleus as a cluster of pulsars, an axysymmetric rotator and a low-entropy configuration (disk) as stages of the evolution of an oblique rotator as well as the problems of its genesis and fate are discussed at the end of the paper (Section 9). The main results of the paper are listed in Section 10. 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.
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
17.
In order to simulate evolution of a large-scale magnetic field in a barred galaxy possessing a gaseous halo we apply a three-dimensional
(3D) MHD numerical model. We solve a induction equation using a time-dependent velocity field of molecular gas resulting from
self-consistent 3D N-body simulations of a galactic disk. The gaseous halo rotates differentially co-rotating with the disk.
In our model we introduce the dynamo process causing the amplification of the magnetic field as well as the formation of field
structures high above the galactic disk. The simulated magnetic fields are used to construct the models of a high-frequency
(Faraday rotation-free) polarized radio emission that accounts for effects of projection and limited resolution, and is thus
suitable for direct comparison with observations. We found that the resultant magnetic field correctly reproduces the observed
structures of polarization B-vectors, forming coherent patterns well aligned with spiral arms and with the bar. The process
initializing a wave-like behavior of the magnetic field, which efficiently forms magnetic maxima between the spiral arms,
is demonstrated.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
18.
本文采用修正的粘滞定律及磁流体力学研究了薄吸积盘内区及外区的稳定性问题。运用微扰方法导出了色散方程，分析了四种情况下吸积盘的不稳定性，结果表明：在同时考虑磁场和修正的粘滞律时，吸积盘中存在着三种振荡模式，其中粘滞模式总是稳定的，磁声速模式（包括向里、向外传播两种模式）通常是不稳定的。这些结果为解释ＢＬ Ｌａｃ天体、Ｓｅｙｆｅｒｔ星系、类星体等活动星系核的光变现象提供了理论依据。 相似文献
19.
We show that radiation emitted from material falling toward a black hole or neutron star can be blue-shifted as well as red-shifted. Although the red shift can be arbitrarily large near a black hole, there is an upper limit for the blue shift of 1/2. Material incident toward the poles of a magnetic neutron star can simultaneously radiate red and blue-shifted lines. Near an oblique magnetic rotator, the red and blue shifts will show a sinusoidal variation. Such spectral variations are associated with SS 433. 相似文献
20.
We consider the generation of a magnetic field in the Galaxy by the electric currents excited by cosmic-ray particles in the disk and halo. We assume that the sources of relativistic particles are distributed continuously and uniformly in the Galactic disk, their total power is equal to the observed value, and the particles themselves undergo anisotropic diffusion in a homogeneous medium. We take into account the differential rotation of the Galactic disk but disregard the turbulence gyrotropy (the α effect). The strength of the generated magnetic field in our model is shown to strongly depend on the symmetry of the relativistic proton and thermal electron diffusion tensors, as well as on the relations between the tensor components. In particular, if the diffusion is isotropic, then no magnetic field is generated. For the independent tensor components estimated from observed parameters of the Galactic medium and with a simultaneous allowance made for the turbulent field dissipation processes, the mechanism under consideration can provide an observable magnetic-field strength of the order of several microgauss. This mechanism does not require any seed magnetic field, which leads us to suggest that relativistic particles can give an appreciable and, possibly, determining contribution to the formation of the global Galactic magnetic field. However, a final answer can be obtained only from a nonlinear self-consistent treatment, in which the symmetry and magnitude of the particle diffusion tensor components should be determined together with the calculation of the magnetic field. 相似文献