共查询到20条相似文献，搜索用时 46 毫秒
1.
L. L. Kitchatinov 《Astronomy Reports》2008,52(3):247-255
The stability of a toroidal magnetic field in the rotating radiation zone of a star is analyzed to estimate the maximum possible magnitude of relic fields. Equations for small perturbations are obtained taking into account the finite diffusivity and the stabilizing effect of the subadiabatic stratification. The numerical solution of the eigenvalue problem indicates that the threshold field strength for the onset of instability in the radiation zone of the Sun is about 600 G. This figure sets an upper bound for the strength of the relic field. The assumption that magnetic instabilities are present in the solar radiation zone disagrees with the observed abundance of lithium. Our analysis of joint stability of toroidal field and nonuniform rotation shows that two-dimensional MHD solutions for the solar tachocline are stable against three-dimensional perturbations. 相似文献
2.
I. F. Malov 《Astronomy Reports》2009,53(5):472-476
A possible model for the pulsar PSR J1852+0040 associated with the supernova remnant Kes 79 and detected in place of a central compact object in this remnant is discussed. The main observational properties of the pulsar can be understood as consequences of its weak surface magnetic field (B s < 3 × 1011 G) and short rotational period (P ～ 0.1 s). Its X-ray emission is thermal, and is generated in a small region near the surface of the neutron star due to cooling of the surface as the surface accretes matter from a relict disk surrounding the pulsar. The radio emission is generated in the outer layers of the pulsar magnetosphere by the synchrotron (cyclotron) mechanism. The optical luminosity of J1852+0040 is estimated to be L opt < 1028 erg/s. If the spectral features in another central compact object, 1E 1207.4+5209, are interpreted as electron cyclotron lines, this provides evidence for a weak surface magnetic field for this neutron star as well (B < 6 × 1010 G). The hypothesis that all central compact objects have weak surface fields makes it possible to explain the number of detected central compact objects, the absence of pulsar-wind nebulae associated with these objects, and the fact that no pulsar has yet been detected at the position of SN 1987a. We suggest that, after the supernova remnant has dissipated, the central compact object becomes a weak X-ray source (XDINS), whose weak emission is also due to the weakness of its magnetic field. 相似文献
3.
We have calculated the degree and position angle of the polarization of radiation scattered in a magnetized, optically thin or optically thick envelope around a central source, taking into account Faraday rotation of the plane of polarization during the propagation of the scattered radiation and the finite size of the radiation source. The wavelength dependence of the degree of polarization can be used to estimate the magnetic field of the source (a star, the region around a neutron star, or a black hole), and we have used our calculations to estimate the magnetic fields in a number of individual objects: several hot O and Wolf-Rayet stars, compact objects in X-ray close binaries with black holes (SS 433, Cyg X-1), and supernovae. The spectrum of the linear polarization can be used to determine the magnetic field in the vicinity of a central supermassive black hole, where the polarized optical radiation is generated. In a real physical model, this value can be extrapolated to the region of the last stable orbit. In the future, the proposed technique will make it possible to directly estimate the magnetic field in the region of the last stable orbit of a supermassive black hole using X-ray polarimetry. 相似文献
4.
We explore the possibility of explaining Anomalous X-ray Pulsars (AXPs) and Soft Gammaray Repeaters (SGRs) in a scenario with fall-back magnetic accretion onto a young isolated neutron star. The X-ray emission of the pulsar in this case originates due to the accretion of matter onto the surface of the neutron star from a magnetic slab surrounding its magnetosphere. The spin-down rate of the neutron star expected in this picture is close to the observed value. We show that such neutron stars are relatively young and are going through the transition from the propeller state to the accretor state. The pulsar’s activity in gamma-rays is connected with its relative youth, and is enabled by energy stored in a non-equilibrium layer located in the crust of the low-mass neutron star. This energy can be released due to the mixing of matter in the neutron star crust with super heavy nuclei approaching its surface and becoming unstable. The fission of nuclei in the low-density region initiates chain reactions leading to a nuclear explosion. Outbursts are probably triggered by instability developing in the region where the matter accreted by the neutron star accumulates in the magnetic polar regions. 相似文献
5.
The integral equations of the direct problem of magnetic prospecting with respect to demagnetization are derived for a two-layer medium model; a method of determining the intensity of a secondary magnetic field for the case of magnetizing by a static field, whose potential is a harmonic function, is described. The respective integral equations are written based on the theory of a simple layer potential. An iteration algorithm for solving the three-dimensional structural inverse problem of magnetic prospecting with respect to demagnetization for the sampled medium model is proposed based on the modified method of local corrections. Its numerical implementation is less computer-intensive than, for example, the gradient methods of nonlinear minimization, the sampling accuracy being equal. Note that problems of electric and magnetic prospecting with respect to demagnetization and thermometry are mathematically equivalent for stationary fields [1]. Therefore, the obtained results can be used in interpreting the data of these methods. 相似文献
6.
The accretion of neutral gas (hydrogen and helium) onto a neutron star is studied. The gas is gravitationally captured into the magnetosphere of the star, where it is ionized by thermal radiation from the stellar surface and accelerated by the electric field at the light cylinder and in a tube of open magnetic lines. Particles accelerated at light cylinder generate gamma-ray, some particles move to the star and heat its polar regions, resulting in the emission of X-rays. Our calculations of the model parameters of the X-ray and gamma-ray radiation indicate that the radiation intensities should be sufficient to be observed. 相似文献
7.
Observations of the X-ray pulsar 4U 2206+54 obtrained over 15 years show that its period, which is now 5555 ± 9 s, is increasing dramatically. This behavior is difficult to explain using traditional scenarios for the spin evolution of compact stars. The observed spin-down rate of the neutron star in 4U 2206+54 is in good agreement with the value expected in a magnetic-accretion scenario, taking into account that, under certain conditions, the magnetic field of the accretion stream can affect the geometry and type of flow. The neutron star in this case accretes material from a dense gaseous slab with small angular momentum, which is kept in equilibrium by the magnetic field of the flow itself. A magnetic-accretion scenario can be realized in 4U 2206+54 if the magnetic-field strength at the surface of the optical counterpart to the neutron star is higher than 70 G. The magnetic field at the surface of the neutron star is 4 × 1012 G in this scenario, in agreement with estimates based on an analysis of X-ray spectra of the pulsar. 相似文献
8.
Mixing in the surface layers in association with internal waves during winter in the northwestern Bay of Bengal 总被引：1，自引：1，他引：0
The upper ocean has complex and variable temperature stratification, and the surface layers in the northwest Bay of Bengal
in winter indicate the presence of transient thermal inversions that wane with the advancement of the season. During winter,
the sea surface loses heat and the surface waters of the coastal regions of the east coast of India are fairly stratified
with the residual freshwater atop from the preceding southwest monsoonal discharge. The vertical stability favors the formation
and sustenance of temperature inversions. To investigate the mechanism and the influence of ubiquitous internal waves that
thrive on stability, a three-dimensional Princeton Ocean Model is configured for the east coast of India and is applied to
study the process in the surface layers in association with the internal waves. The model domain constitutes a variable curvilinear
grid, and the input fields comprise bathymetry, initial temperature and salinity, wind stress, air-sea heat fluxes and tidal
forcing at the open boundaries. The numerical experiments demonstrate that vertical stability alone cannot cause, support
or augment the internal wave oscillations, if the stratification is attributed to salinity only. Internal waves may therefore
be perceived in stable layers, essentially from temperature-induced stratification. Despite stratification and enough vertical
density gradient in the upper ocean, the conditions may not suit for the occurrence of internal waves due to thermal diffusive
processes that overpower the salinity gradients. The vertical spreading of heat due to double diffusion is believed to be
transparent to tidal forcing as the generation of internal waves is subdued even under density stratification. The model simulations
indicate that the horizontal convergence/divergence motions, required for the manifestation of internal waves at the surface
are inhibited in the presence of temperature inversion. The available SAR imageries in winter endorse the model simulations
to this effect. 相似文献
9.
The spin evolution of X-ray pulsars in high-mass X-ray binaries is discussed under various assumptions about the geometry and physical parameters of the accretion flow. The torque applied to the neutron star by the accretion flow and the equilibrium periods of the pulsars are estimated. It is shown that the observed spin evolution of the pulsars can be explained in a scenario in which the neutron star accretes material from a magnetized stellar wind. 相似文献
10.
The mechanism of magnetodipole braking of radio pulsars is used to calculate new values of the surface magnetic fields of neutron stars. The angles β between the spin axes and magnetic moments of the neutron stars were estimated for 376 radio pulsars using three different methods. It is shown that small inclinations of magnetic axes dominate. The equatorial magnetic fields for the considered sample of pulsars are calculated using the β values obtained. As a rule, these magnetic fields are a factor of a few higher than the corresponding values in known catalogs. 相似文献
11.
Known models proposed to explain the high space velocities of pulsars based on asymmetry of the transport coefficients of different sorts of neutrinos or electromagnetic radiation can be efficient only in the presence of high magnetic fields (to 1016 G) or short rotation periods for the neutron stars (of the order of 1 ms). This current study shows that the observed velocities are not correlated with either the pulsar periods or their surface magnetic fields. The initial rotation periods are estimated in a model for the magnetedipolar deceleration of their spin, aßsuming that the pulsar ages are equal to their kinematic ages. The initial period distribution is bimodal, with peaks at 5 ms and 0.5 s, and similar to the current distribution of periods. It is shown that asymmetry of the pulsar electromagnetic radiation is insufficient to give rise to additional acceleration of pulsars during their evolution after the supernova explosion that gave birth to them. The observations testify to deceleration of the motion, most likely due to the influence of the interstellar medium and interactions with nearby objects. The time scale for the exponential decrease in the magnetic field τD and in the angle between the rotation axis and magnetic moment τß are estimated, yielding τβ = 1.4 million years. The derived dependence of the transverse velocity of a pulsar on the angle between the line of sight and the rotation axis of the neutron star corresponds to the expected dependence for acceleration mechanisms associated with asymmetry of the radiation emitted by the two poles of the star. 相似文献
12.
The thermoelastic response due to a time-dependent rectangular heat source in a semi-infinite medium is analyzed. The problem originates from studies of nuclear waste repositories in rock. Canisters containing heat-emitting nuclear waste are deposited over a large rectangular area deep below the ground surface. The solution for a time-dependent heat source is obtained from the corresponding instantaneous heat source by superposition. The thermoelastic problem for the instantaneous rectangular heat source in a infinite surrounding is solved exactly. An important step is the introduction of so-called quadrantal heat sources. The solution for the rectangle is obtained from four quadrantal solutions. The solution for the quadrantal heat source depends on the three dimelasionless coordinates only. Time occurs in the scale factors only. The condition of zero normal and shear stresses at the ground surface is fulfilled by using a mirror heat source and a boundary solution. The boundary solution accounts for the residual normal stress at the ground surface. Using a Hertzian potential, a surprisingly simple solution is obtained. The final analytical solution is quite tractable considering the complexity of the initial problem. The solution may be used to test numerical models for coupled thermoelastic processes. It may also be used in more detailed numerical simulations of the process near the heat sources as boundary conditions to account for the three-dimensional global process. 相似文献
13.
Using the “Scenario Machine” (a computer code based on the Monte-Carlo method, developed to calculate the evolution of a large ensemble of binaries), we have carried out population-synthesis calculations for X-ray binaries for the purpose of modeling the X-ray luminosity functions in various type galaxies. These calculations were focused on the evolution of magnetized neutron stars. The X-ray luminosity function is not universal, and depends on the star formation rate in the galaxy. In theoretical models, it is very important to take into account the evolution of the binaries and their lifetimes in their X-ray stages. We calculated the cumulative and differential X-ray luminosity functions in galaxies with a constant star formation rate, the cumulative luminosity functions for various time intervals since the peak star formation, and curves describing the evolution of the X-ray luminosity after a star formation burst in the galaxy. 相似文献
14.
E. De Grave R. M. Persoons D. G. Chambaere R. E. Vandenberghe L. H. Bowen 《Physics and Chemistry of Minerals》1986,13(1):61-67
Four synthetic lepidocrocite samples with a significant difference in crystallinity have been studied by the 57Fe Mössbauer effect technique at temperatures ranging between 12 K and 360 K. In the magnetically ordered region, the spectra display broad hyperfine field distributions, the profile of which could be derived numerically. In a broad transition region of approximately 20 K, an additional doublet must be included in the fitting model. From Mössbauer thermoscanning measurements at zero-velocity and with and without the application of an external magnetic field, it is found that the doublet is due to Fe3+ species in a paramagnetic state. Neither the hyperfine field nor the Néel temperature distributions are markedly affected by the crystallinity. In the pure paramagnetic state, the broadened doublets are best described by a distribution of quadrupole splittings in the range 0.3–1.9 mm/s. By numerical manipulations of the derived probability profiles it was possible to distinguish between the iron species in the outer surface layers of the γ-FeOOH particles and those in the innermost layers, the relative amount of each being correlated with the measured surface areas. The centre shift is quite uniform and its temperature dependence, determined in detail for one of the samples, is perfectly described by the Debije approximation for the second-order Doppler shift. From the obtained Debije temperature, a Mössbauer fraction f of 0.79 at room temperature was calculated, which is in good agreement with the results obtained for a reference lepidocrocite/hematite mixture. Finally, line shape simulations aimed to explain the external field spectrum at 180 K provided strong indications that both the sign and the asymmetry parameter of the electric field gradient are non-uniform as well. 相似文献
15.
M. O Olorunfemi A. A Adepelumi D. E Falebita O. A Alao 《Arabian Journal of Geosciences》2013,6(5):1657-1667
Spectral analysis method was applied to aeromagnetic data obtained for Ikogosi warm spring (IWS) area of southwestern Nigeria. This was done with the objective of determining the bottom of the magnetized crust called Curie point depth (CDP) and understand the nature and extent of the local geothermal system at depth beneath IWS. The depth to the centroid, Z o, of the deepest distribution of the magnetic dipoles was obtained by computing least-squares fit to the lowest-frequency segment of the azimuthally averaged log power spectrum. The average depth to the top of the deepest crustal block was computed as the depth to the top, Z t, of the second lowest-frequency segment of the spectrum. The depth to the bottom of the deepest magnetic dipoles, the inferred Curie point depth, was then calculated from Z b?=?2Z o???Z t. The Curie depth estimates for IWS range between 4.68 and 11.38 km (below sea level). We also estimate the heat flow and Curie temperature using a one-dimensional conductive heat transport model. The average heat flow, 42 mW m?2, and geothermal gradient, 32°C/km, obtained suggest a low enthalpy thermal regime. The Curie temperature for the region varies between 153°C and 350°C. Also, an inverse linear relationship between heat flow and Curie depths was determined. Good agreement between the Curie point depths derived from heat flow data and magnetic data suggests that the Curie point depth analysis is useful to estimate the regional thermal structure and the tectonic settings. 相似文献
16.
Abstractthe effect of the large-scale magnetic fields generated by the solar dynamo on the radiation flux issuing from the convection zone is studied. A governing equation describing convective heat transfer is obtained in the framework of mean-field magnetohydrodynamics, with account for the influence of magnetic fields and differential rotation on the energy budget of the convection zone. The principal effects are illustrated using a one-dimensional numerical model. Calculations indicate that the influence of large-scale magnetic fields can modulate the solar irradiance with a relative amplitude of ～0.07%. 相似文献
17.
We have developed a three-dimensional numerical model and applied it to simulate plasma flows in semi-detached binary systems
whose accretor possesses a strong intrinsic magnetic field. The model is based on the assumption that the plasma dynamics
are determined by the slow mean flow, which forms a backdrop for the rapid propagation of MHD waves. The equations describing
the slow motion of matter were obtained by averaging over rapidly propagating pulsations. The numerical model includes the
diffusion of magnetic field by current dissipation in turbulent vortices, magnetic buoyancy, and wave MHD turbulence. A modified
three-dimensional, parallel, numerical code was used to simulate the flow structure in close binary systems with various accretor
magnetic fields, from 105 to 108 G. The conditions for the formation of the accretion disk and the criteria distinguishing the two types of flow corresponding
to intermediate polars and polars are discussed. 相似文献
18.
The evolution of the angle between the magnetic moment and rotation axis of radio pulsars (inclination angle) is considered taking into account the presence of a non-dipolar magnetic field at the neutron-star surface and superfluid neutrons in the stellar interior. It is assumed that the total loss of angular momentum by the pulsar can be represented as a sum of magnetodipole and current losses. The neutron star is treated as a two-component system consisting of a charged component (including protons and electrons, as well as the crust, which is rigidly coupled with them, and normal neutrons) and a superfluid core. The components interact through scattering of degenerate electrons on magnetized Feynman-Onsager vortices. If a superfluid core is absent, then, in spite of the presence of stable equilibrium inclination angles, the rate with which these are reached is so slow that most pulsars do not have sufficient time to approach them during their lifetimes. The presence of superfluid neutrons results, first, in faster evolution of the inclination angle and, second, in the final stage of the evolution being either an orthogonal or a coaxial state. The proposed model fits the observations better in the case of small superfluid cores. 相似文献
19.
The equation of heat conduction is solved for a horizontally stratified medium initially at constant temperature and subject to a step-function surface temperature change. The problem is solved by Laplace transformation and by applying a concept of further stratifications of the medium into unitary layers of constant ratio of thickness to the square root of thermal diffusivity. The solution takes the form of an infinite sum of complementary error functions, with coefficients given by recursion relations, is suitable for numerical applications and offers an attractive alternative to harmonic and quasi transient approaches in calculating the penetration of transient surface temperature variations into a layered medium. Use of the theorem of superposition yields a general expression for an arbitrary surface temperature function. The utility of the theory is illustrated by modelling examples of palaeoclimatically induced subsurface temperature and heat flow perturbations. 相似文献
20.
外壁恒温条件下冻结管壁热流密度变化规律数值计算研究 总被引：5，自引：0，他引：5
应用相似理论将冻结温度场方程和参数无量纲化,然后建立无量纲的数值计算模型,研究了在外表面恒温情况下冻结管壁的无量纲热流密度与各无量纲影响参数间的关系.结果表明:无量纲热流密度随未冻土与冻土的导热系数之比线性增加,与冻土与未冻土的容积比热之比成线性关系,随无量纲结冰潜热线性增长,与无量纲盐水温度成线性关系;可表示为无量纲时间的负三分之一次方的线性函数.最后对167706次数值计算结果进行了回归分析,得到具有较高精度的回归计算公式. 相似文献