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Helioseismic measurements indicate that the solar tachocline is very thin, its full thickness not exceeding 4% of the solar
radius. The mechanism that inhibits differential rotation to propagate from the convective zone to deeper into the radiative
zone is not known, though several propositions have been made. In this paper we demonstrate by numerical models and analytic
estimates that the tachocline can be confined to its observed thickness by a poloidal magnetic field B
p of about one kilogauss, penetrating below the convective zone and oscillating with a period of 22 years, if the tachocline
region is turbulent with a diffusivity of η∼1010 cm2 s−1 (for a turbulent magnetic Prandtl number of unity). We also show that a similar confinement may be produced for other pairs
of the parameter values (B
p, η). The assumption of the dynamo field penetrating into the tachocline is consistent whenever η≳109 cm2 s−1.
Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1013389631585 相似文献
4.
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
5.
J.L. Johnson 《Astronomische Nachrichten》2011,332(8):841-845
We calculate the amount of angular momentum that thermal photons carry out of a viscous black hole accretion disk, due to the strong Doppler shift imparted to them by the high orbital velocity of the radiating disk material. While thermal emission can not drive accretion on its own, we show that along with disk heating it does nonetheless result in a loss of specific angular momentum, thereby contributing to an otherwise viscosity‐driven accretion flow. In particular, we show that the fraction of the angular momentum that is lost to thermal emission at a radius r in a standard, multi‐color disk is ∼0.4rs/r, where rs is the Schwarzschild radius of the black hole. We briefly highlight the key similarties between this effect and the closely related Poynting‐Robertson effect (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
6.
M. Yu. Piotrovich N. A. Silant’ev Yu. N. Gnedin T. M. Natsvlishvili 《Astrophysical Bulletin》2011,66(3):320-324
Various relations are found between the key parameters of black holes and active galactic nuclei. Some have a statistical
property, others follow from the theoretical consideration of the evolution of these objects. In this paper we use a recently
discovered empirical relation between the characteristic frequency of quasi-periodic oscillations of radiation ν
br
of black holes, their masses and matter accretion rates to determine the magnetic field strength B
H
at the black hole event horizon. Since the characteristic frequency can be determined from observations, the use of a new
relation for the estimations of magnetic field B
H
can yield more definite results, since we are decreasing the number of the unknown or poorly-determined parameters of objects
(it especially concerns the accretion rate Ṁ). The typical values which we have found are B
H
≃ 108G for the stellar mass black holes, and B
H
≃ 104G for the supermassive black holes. Besides, we demonstrate that if the linear polarization of an object is caused by the
radiation of a magnetized accretion disk, then the degree of observable polarization is p ∼ ν
br
−1/2. 相似文献
7.
The accretion of hot slowly rotating gas onto a supermassive black hole is considered. The important case where the velocities
of turbulent pulsations at the Bondi radius r
B are low, compared to the speed of sound c
s, is studied. Turbulence is probably responsible for the appearance of random average rotation. Although the angular momentum
at r
B is low, it gives rise to the centrifugal barrier at a depth r
c = l
2
/GM
BH ≪ r
B, that hinders supersonic accretion. The numerical solution of the problem of hot gas accretion with finite angular momentum
is found taking into account electron thermal conductivity and bremsstrahlung energy losses of two temperature plasma for
density and temperature near Bondi radius similar to observed in M87 galaxy. The saturation of the Spitzer thermal conductivity
was also taken into account. The parameters of the saturated electron thermal conductivity were chosen similar to the parameters
used in the numerical simulations of interaction of the strong laser beam radiation with plasma targets. These parameters
are confirmed in the experiments. It is shown that joint action of electron thermal conductivity and free-free radiation leads
to the effective cooling of accreting plasma and formation of the subsonic settling of accreting gas above the zone of a centrifugal
barrier. A toroidal condensation and a hollow funnel that separates the torus from the black hole emerge near the barrier.
The barrier divides the flow into two regions: (1) the settling zone with slow subKeplerian rotation and (2) the zone with
rapid supersonic nearly Keplerian rotation. Existence of the centrifugal barrier leads to significant decrease of the accretion
rate Ṁ in comparison with the critical Bondi solution for γ = 5/3 for the same values of density and temperature of the hot gas near Bondi radius. Shear instabilities in the torus and
related friction cause the gas to spread slowly along spirals in the equatorial plane in two directions.As a result, outer
(r > r
c) and inner (r < r
c) disks are formed. The gas enters the immediate neighborhood of the black hole or the zone of the internal ADAF flow along
the accretion disk (r < r
c). Since the angular momentum is conserved, the outer disk removes outward an excess of angular momentum along with part of
the matter falling into the torus. It is possible, that such outer Keplerian disk was observed by Hubble Space Telescope around
the nucleus of the M87 galaxy in the optical emission lines. We discuss shortly the characteristic times during which the
accretion of the gas with developed turbulence should lead to the changes in the orientation of the torus, accretion disk
and, possibly, of the jet. 相似文献
8.
Eugen Willerding 《Planetary and Space Science》1998,46(11-12)
In this paper we investigate both the global and the local hydrodynamics of axisymmetric accretion disks around young stellar objects under the simultaneous action of viscosity, self-gravity and pressure forces. For simplicity, we take for the global model a polytropic equation of state, make the infinitely thin disk approximation and characterize the surface density and temperature profiles in the disk as power laws in the radial distance r from the protostar. We solve the problem of the general density profile of a Keplerian disk showing that self-gravity could not be an important factor for the fast formation of the rocky cores of giant gaseous planets in our solar system. Under the hypothesis that the unperturbed rotation curve of the disk is nearly Keplerian throughout the radial extent, we can estimate with our polytropic model a lower limit for the resulting masses Md(r) of stable disks up to 100 AU. These masses are in the range of the so-called minimum mass solar nebular (d/Ms ≈ 0.01–0.02).By adopting a simplified viscosity model, where the height-integrated turbulent dynamical viscosity ν is a function of the surface density σ like η ∝ σΓ, we derive in the local shearing sheet model linearized evolution equations for small density perturbations describing both a diffusion process and the propagation of acoustic density waves. We solve a special initial value problem and calculate the appropriate Green's function. The analytical solutions so obtained describe in the case Γ < 0 the successive formation of quasi-stationary ring-shaped density structures in a disk with a definite mode of maximum instability, whereas in the case Γ > Γc the density wave equation describes the propagation of an “overstable” ring-shaped acoustic density wavelet to the outer ranges of the accretion disk. Whereas the group velocity of the wave packet is subsonic, the phase velocities of individual wave crests in the wave packet are supersonic. The mode of maximum instability, the growth rate and the number of growing waves in the wavelet are controlled by Γ and α. Our present knowledge concerning turbulent viscosity in protoplanetary disks is not sufficient to decide whether or not the case Γ > Γc is realized.The suggested structuring processes in the linear theory should initiate in the non-linear regime the formation of narrow ring-shaped density shock waves moving through the protoplanetary disk. These non-linear waves could produce extremely spatially and temporally heterogeneous temperature regions in the disk. We speculate that ring-shaped density waves, excited by inner boundary conditions and which have dominated the disk's evolution at early times, are responsible both for the fast growth of dust to planetesimals and at least for the rapid accretion of the rocky cores of giant gaseous planets in the protoplanetary accretion disk (shock wave trigger hypothesis). We derive provisional scaling rules for planetary systems regarding the spacing of orbits as a function of the mass ratio of the protoplanetary disk to the protostar. However, further analytical work and linear as well as nonlinear numerical simulations of density waves excited by inner boundary conditions are needed to consolidate the results and speculations of our linear wave mechanics in the future. 相似文献
9.
Results of ourmeasurements of the longitudinal magnetic field B
z
for the young star RWAur A are presented. B
z
measured from the so-called narrow component of the He I 5876 line varies in the range from −1.47 ± 0.15 to +1.10 ± 0.15
kG. Our data are consistent with a stellar rotation period of }~5.6 days and the model of two hot spots with opposite magnetic
field polarities spaced about 180° apart in longitude. Relative to the Earth, the spot with B
z
< 0 lies in the hemisphere above the midplane of the accretion disk, while the spot with B
z
> 0 is below the midplane. The upper limit for B
z
(at the 3σ level) obtained by averaging all observations is 180 G for the photosphere and 220 and 230 G for the Hα and [OI] 6300 line formation regions, respectively. We have also failed to detect a field in the formation region of broad
emission line components: the upper limit for B
z
is 600 G. In two of 11 cases, we have detected a magnetic field in the formation region of the blue absorption wing of the
Na I D doublet lines, i.e., in the wind from RW Aur A: B
z
= −180 ± 50 and −810 ± 80 G. The radial velocity of the photospheric lines in RW Aur A averaged over all our observations
is }~+10.5 km s−1, i.e., a value lower than that obtained by Petrov et al. (2001) ten years earlier by 5.5 km s−1. Therefore, we discuss the possibility that RW Aur is not a binary but a triple system. 相似文献
10.
Philip Hardee Yosuke Mizuno Ken-Ichi Nishikawa 《Astrophysics and Space Science》2007,311(1-3):281-286
A new general relativistic magnetohydrodynamics (GRMHD) code “RAISHIN” used to simulate jet generation by rotating and non-rotating
black holes with a geometrically thin Keplarian accretion disk finds that the jet develops a spine-sheath structure in the
rotating black hole case. Spine-sheath structure and strong magnetic fields significantly modify the Kelvin-Helmholtz (KH)
velocity shear driven instability. The RAISHIN code has been used in its relativistic magnetohydrodynamic (RMHD) configuration
to study the effects of strong magnetic fields and weakly relativistic sheath motion, c/2, on the KH instability associated with a relativistic, γ=2.5, jet spine-sheath interaction. In the simulations sound speeds up to
and Alfvén wave speeds up to ∼0.56c are considered. Numerical simulation results are compared to theoretical predictions from a new normal mode analysis of the
RMHD equations. Increased stability of a weakly magnetized system resulting from c/2 sheath speeds and stabilization of a strongly magnetized system resulting from c/2 sheath speeds is found. 相似文献
11.
The evolution of the current sheet in the electric current direction (in the guiding magnetic field direction) is studied
numerically in the 3-D particle-in-cell model with two current sheets and periodic boundary conditions. In the regime with
(where v
D and
are the electric current drift and electron thermal velocities, respectively) the current sheets are unstable owing to the
Buneman and kink instabilities and become strongly fragmented. During their evolution, in addition to an increase of the energy
of the electric field component in the guiding magnetic field direction, the energies of the electric field components in
the perpendicular direction are even more enhanced. In the current sheet the anomalous resistivity (η
anom/η
C∼7×105, where η
C is the classical resistivity) is generated and thus the magnetic field dissipates. Most of the dissipated magnetic energy
is transformed into the electron kinetic energy in the direction of the electric current. The associated electric field accelerates
the electrons from the tail of the distribution function. 相似文献
12.
13.
M. Yu. Piotrovich Yu. N. Gnedin T. M. Natsvlishvili N. A. Silant’ev 《Astronomy Letters》2010,36(6):389-395
We analyze the spectropolarimetric observations of 12 candidates for quasars from the spectroscopic database of the SDSS Catalog.
The magnetic fields of these objects are estimated in the context of a theory that includes the Faraday rotation of the polarization
plane on the mean free path of a photon in the outflow from an accretion disk. As a result, we have determined the column
density in the outflow, N
H ∼ 6 × 1023 cm−2, and the radial, B ∼ 1 G, and toroidal, B ∼ 600 G, magnetic fields. 相似文献
14.
A possible correlation between the magnetic and velocity fields has been analyzed based on the SOHO/MDI magnetograms and Dopplergrams.
It is found that the observed large-scale weak magnetic field (weaker than 50 G (gauss)) is correlated with the velocity statistically.
The curves of u⋅b with latitude, where u and b are the velocity and magnetic fields in a rectangular region (±15○ in longitude, ±45○ in latitude) on the Sun, show the same patterns in the years 2000, 2004, and 2007. The patterns indicate that u and b are positively correlated near the equator but are anti-correlated at the middle latitudes. For a strong magnetic field between
50 G and 3000 G, the curves of u⋅b with latitude show the same tendencies at the middle latitudes. Near the equator, however, the slope of the curve is positive
in 2000 and is negative in 2004 and 2007. In addition, we give an estimation for the amplitude of the cross helicity h
χ
(hc=[`(u·b)]h_{\chi}=\overline{\mathbf{u}\cdot\mathbf{b}}) inferred from the MDI data, which is of the order of 103 G m s−1 near the center of the solar disk. 相似文献
15.
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
16.
Hamid Reza Pakzad 《Astrophysics and Space Science》2011,333(1):247-255
Ion acoustic shock waves (IASWs) are studied in a plasma consisting of electrons, positrons and ions. Boltzmann distributed
positrons and superthermal electrons are considered in the plasma. The dissipation is taken into account the kinematic viscosity
among the plasma constituents. The Korteweg–de Vries–Burgers (KdV–Burgers) equation is derived by reductive perturbation method.
Shock waves are solutions of KdV–Burgers equation. It is observed that an increasing positron concentration decreases the
amplitude of the waves. Furthermore, in the existence of the kinematic viscosity among the plasma, the shock wave structure
appears. The effects of ion kinematic viscosity (η
0) and the superthermal parameter (k) on the ion acoustic waves are found. 相似文献
17.
K.C. Freeman 《Astrophysics and Space Science》1999,267(1-4):129-138
Away from the young disk, several classes of early type stars are found. They include (i) the old, metal-poor blue horizontal branch stars of the halo and the metal-poor tail of the thick disk; (ii) metal-rich young A stars in a rapidly rotating subsystem but with a much higher velocity dispersion than the A stars of the
young disk, and (iii) a newly discovered class of metal-poor young main sequence A stars in a subsystem of intermediate galactic rotation (Vrot ≈ 120 km s−1). The existence and kinematics of these various classes of early type stars provide insight into the formation of the metal-poor
stellar halo of the Galaxy and into the continuing accretion events suffered by our Galaxy.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
18.
We have investigated the influence of X-ray irradiation on the vertical structure of the outer accretion disk in low-mass
X-ray binaries by performing a self-consistent calculation of the vertical structure and X-ray radiation transfer in the disk.
Penetrating deep into the disk, the field of scattered X-ray photons with energy E ≳ 10 keV exerts a significant influence on the vertical structure of the accretion disk at a distance R ≳ 1010 cm from the neutron star. At a distance R ∼ 1011 cm, where the total surface density in the disk reaches Σ0 ∼ 20 g cm−2, X-ray heating affects all layers of an optically thick disk. The X-ray heating effect is enhanced significantly in the presence
of an extended atmospheric layer with a temperature T
atm ≈ (2–3) × 106 K above the accretion disk. We have derived simple analytic formulas for the disk heating by scattered X-ray photons using
an approximate solution of the transfer equation by the Sobolev method. This approximation has a ≲10% accuracy in the range
of X-ray photon energies E < 20 keV. 相似文献
19.
The distribution of radial (U) and rotational (V) velocities of red clump giants was studied as a function of their heights above the galactic plane. The stars of this type
were selected from the compiled catalogue of stellar proper motions and infrared photometry at the north galactic pole with
the use of the diagram “color-reduced proper motion.” According to the data on 1800 red clump giants located at heights from
1 to 3 kpc (mostly thick disk stars), mean kinematic parameters of the thick disk were determined: U
0 = −18 ± 2 km/s, V
0 = −56 ± 1 km/s, σ
U
= 72 ± 2 km/s, and σ
V
= 58 ± 1 km/s. The velocity of asymmetric drift V
0 and velocity variances σ
U
, σ
V
are shown to depend on heights above the galactic plane. 相似文献