共查询到20条相似文献，搜索用时 15 毫秒
1.
E. I. Staritsin 《Astronomy Reports》2017,61(5):450-460
Partial mixing of material in the radiative envelopes and convective cores of rotating main sequence stars with masses of 8 and 16 M ⊙ is considered as a function of the inital angular momentum of the stars. Losses of rotational kinetic energy to the generation of shear turbulence in the radiative envelope and the subsequent mixing of material in the envelope are taken into account. With an initial equatorial rotational velocity of 100 km/s, partial mixing develops in the upper part of the layer with variable chemical composition and the lower part of the chemically homogeneous radiative envelope. When the initial equatorial rotational velocity is 150–250 km/s, the joint action of shear turbulence and semi-convection leads to partial mixing in the radiative envelope and central parts of the star. The surface abundance of helium is enhanced, with this effect increasing with the angular momentum of the star. With an initial equatorial rotational velocity of 250 km/s, the ratio of the surface abundances of helium and hydrogen grows by ~30% and ~70% toward the end of the main-sequence evolution of an 8 M ⊙ and 16 M ⊙ star, respectively. The transformation of rotational kinetic energy into the energy of partial mixing increases with the angular momentum of the star, but does not exceed ~2%?3% in the cases considered. 相似文献
2.
L. L. Kitchatinov 《Astronomy Reports》2006,50(6):512-516
Global meridional flows in stars transport angular momentum, thus giving rise to nonuniform rotation. The pattern of differential rotation associated with slow meridional circulation depends on the direction of this circulation. A flow directed from the poles to the equator at the surface and from the equator to the poles in deep layers results in relatively fast rotation of the equatorial zone. If the circulation is directed oppositely, the angular velocity increases from the equator to the poles. Relatively fast rotation at the poles may also result from fast circulation, irrespective of its direction. A simple illustrative explanation is given here to these results. Analytical estimates are supported by numerical calculations. The time variations in the meridional flow observed on the Sun should contribute to torsional oscillations. 相似文献
3.
E. I. Staritsin 《Astronomy Reports》2014,58(11):808-824
The evolution of rapidly rotating 8, 4, and 2 M ⊙ main-sequence stars is considered together with hydrodynamical transfer in their interiors. The conditions under which turbulent erosion, semiconvection, and shear turbulence lead to partial mixing of the matter in the radiative envelope and central regions of the stars are determined. The enhancement of the surface helium abundance with time depends on both the intensity of partial mixing in their interiors and mass loss by the stellar wind. The ratio of the number densities of helium and hydrogen at the surface can rise by the end of main-sequence stage by ～30% for a 8 M ⊙ star and ～10?20% for a 4 M ⊙ star, depending on the mass-loss rate. Partial mixing of the matter in the radiative envelope and in the central region of the star can provide an explanation for the observed enhancement of the atmospheric helium abundances of early B stars toward the end of their main-sequence evolution. The enhancement of the surface helium abundance in a 2 M ⊙ star is so small that it cannot be detected, and is appreciably lower than the enhancement beneath the surface. 相似文献
4.
V. M. Danilov 《Astronomy Reports》2000,44(5):298-308
In dynamical models for open clusters, virial equilibrium is not achieved over the violent relaxation time scale τvr. The stars form an equilibrium distribution in (?, ?ζ, l) space, where ? and l are the energy and angular momentum per unit stellar mass in the combined field of the Galaxy and cluster and ?ζ is the energy of motion perpendicular to the Galactic plane per unit mass of cluster stars in the gravitational field of the Galaxy. This distribution of stars changes little when t>τvr. The stellar phase-space distribution corresponding to this type of equilibrium and the regular cluster potential vary periodically (or quasi-periodically) with time. This phase-space equilibrium is probably possible due to an approximate balance in the stellar transitions between phase-space cells over times equal to the oscillation period for the regular cluster field. 相似文献
5.
We transformed radial velocities compiled from more than 1400 published sources, including the Geneva-Copenhagen survey of the solar neighborhood (CORAVEL-CfA), into a uniform system based on the radial velocities of 854 standard stars in our list. This enabled us to calculate the average weighted radial velocities for more than 25000 HIPPARCOS stars located in the local Galactic spiral arm (Orion arm) with a median error of ±1 km/s. We use these radial velocities together with the stars’ coordinates, parallaxes, and proper motions to determine their Galactic coordinates and space velocities. These quantities, along with other parameters of the stars, are available from the continuously updated Orion Spiral Arm Catalogue (OSACA) and the associated database. We perform a kinematic analysis of the stars by applying an Ogorodnikov-Milne model to the OSACA data. The kinematics of the nearest single and multiple main-sequence stars differ substantially. We used distant (-r ≈ 0.2 kpc) stars of mixed spectral composition to estimate the angular velocity of the Galactic rotation, ωo = ?25.7 ± 1.2 kms?1 kpc?1, and the vertex deviation, l = 13° ± 2°, and detected a negative K effect. This negative K effect is most conspicuous in the motion of A0–A5 giants and is equal to K = ?13.1 ± 2.0 kms?1 kpc?1. 相似文献
6.
The dynamical, thermal, and chemical evolution of gas in protogalaxies with non-zero angular momentum is considered. It is
shown that, in protogalaxies with a total mass (dark and baryonic) of M = 107
M⊙ at redshifts z = 12 whose gas has rotational angular momentum (spin parameter λ ≳ 0.005), a disk-like structure forms during the initial collapse of the galaxy, in contrast to non-rotating protogalaxies,
whose collapse is spherically symmetric. The existence of initial angular momentumfor gas in protogalaxies increases the cooling
time of the gas, delaying the formation of the first stars. Increasing the rotational angular momentum of the gas leads to
cooling of the gas to lower temperatures (T < 100 K), at which HD molecules dominate in the cooling, while the total mass of cool gas (T < 1000 K) is decreased. The stability of disk-like structures in the central regions of protogalaxies is analyzed. It is
shown that the disk that is formed is gravitationally unstable, and multiple fragmentation at various distances from the center
is possible when the initial rotational angular momentum of the protogalaxy is increased. The possible birth of several stars
in the first protogalaxies is discussed. 相似文献
7.
Possible paths for the formation of Ap/Bp stars—massive main-sequence stars with strong magnetic fields—are analyzed based
on modern theories for the evolution of single and binary stars. Assuming that the strong magnetic fields of these stars are
the main reason for their comparatively slow axial rotation and the observed anomalies in the chemical compositions of their
atmospheres, possible origins for these high magnetic fields are considered. Analysis of several possible scenarios for the
formation of these stars leads to the conclusion that their surface magnetic fields are probably generated in the convective
envelopes of the precursor stars. These precursors may be young, single stars with masses 1.5–3 M
⊙ that formed at the peripheries of forming star clusters and ended their accretion at the Hayashi boundary, or alternatively
close binaries whose components have convective envelopes, whose merger leads to the formation of an Ap/Bp star. Arguments
are presented supporting the view that the merger of close binaries is the main channel for the formation of Ap/Bp stars,
and a detailed analysis of this scenario is presented. The initial major axes of the merging binary systems must be in the
range 6–12 R
⊙, and the masses of their components in the range 0.7–1.5 M
⊙. When the merging components possess developed convective envelopes and fairly strong initial magnetic fields, these can
generate powerful magnetic fields “inherited” by the products of the merger—Ap/Bp stars. The reason the components of the
close binaries merge is a loss of angular momentum via the magnetic stellar winds of the components. 相似文献
8.
The formation of neutron stars in the closest binary systems (P orb<12 h) gives the young neutron star/pulsar a high rotational velocity and energy. The presence of a magnetic field of 3×1011–3×1013 G, as is observed for radio pulsars, enables the neutron star to transfer ～1051 erg of its rotational energy to the envelope over a time scale of less than an hour, leading to a magnetorotational supernova explosion. Estimates indicate that about 30% of all type-Ib,c supernovae may be the products of magnetorotational explosions. Young pulsars produced by such supernovae should exhibit comparatively slow rotation (P rot>0.01 s), since a large fraction of their rotational angular momentum is lost during the explosion. The magnetorotational mechanism for the ejection of the envelope is also reflected by the shape of the envelope. It is possible that the Crab radio pulsar is an example of a product of a magnetorotational supernova. A possible scenario for the formation of the close binary radio pulsar discovered recently by Lyne et al. is considered. 相似文献
9.
Krauz V. I. Mitrofanov K. N. Voitenko D. A. Astapenko G. I. Markoliya A. I. Timoshenko A. P. 《Astronomy Reports》2019,63(2):146-160
Results from experiments on the radial distribution of the magnetic fields in axial plasma flows formed during the compression of a plasma–current sheath carried out at the KPF-4-PHOENIX plasmafocus installation are presented. The plasma flows were generated in a discharge with stationary filling of the chamber with a working gas of argon or hydrogen, and also with a pulsed injection of argon. Analysis of the radial profiles of the magnetic field distribution and their time variations are used to localize regions of trappedmagnetic field, as well as regions where a return current flows at the periphery of the plasma flow. It is shown that the transverse (radial) size of the plasma flow depends on the density of the ambient medium (background gas) through which it propagates. These experiments were carried out in the framework of a project on laboratory simulations of non-relativistic jets from young stars.
相似文献10.
We study how deviations from spherical symmetry of a system, produced by angular momentum, and shear stress, modify the spherical collapse model parameters, as linear density threshold for collapse of the non-relativistic component (δc) and its virial overdensity (ΔV), in Einstein–de Sitter and ΛCDM models. We modify the spherical collapse model to take account of the shear term and angular momentum term. We find that the non-spherical terms change the non-linear evolution of the system and that the collapse stops “naturally” at the virial radius. Moreover, values of the linear overdensity parameter and of ΔV are modified with respect to the standard spherical collapse model. 相似文献
11.
We investigate the physical characteristics of single, rapidly rotating white dwarfs, which could form as a result of a merger of two white dwarfs with different masses and filled Roche lobes, due to the radiation of gravitational waves. When the merging of the binary components occurs without loss of mass and angular momentum, the merger products are subject to secular instability, and the density in their cores does not exceed ～108 g/cm3. Models are constructed for rapidly rotating neutron stars, which could form after the collapse of rotating iron cores of evolved massive stars. Dynamically unstable neutron-star models are characterized by a shift of the maximum density from the rotational axis. The total momentum of such neutron stars is about half the maximum possible momentum for the evolved cores of massive stars. 相似文献
12.
Accretion disks in binary systems can experience hydrodynamical influences at both their inner and outer edges. The former is typical for protoplanetary disks around young T Tauri stars, while the latter is typical for circumstellar disks in close binaries. This influence excites perturbations with various scales and amplitudes in the disk. The nonlinear evolution of perturbations with a finite, but small amplitude against the background of a sub-Keplerian flow is investigated. Nonlinear effects at the fronts of perturbation waves lead to the formation of discontinuities in the density and radial velocity; i.e., to formation of shocks. The tangential flow in the neighborhood of the shock becomes equivalent to a flow in a boundary layer. Due to an instability of the tangential flow, the disk becomes turbulent. The characteristics of the turbulence depend on the parameters of the perturbations, but the Shakura–Syunyaev α parameter does not exceed ~0.1. 相似文献
13.
Under certain conditions, stars close to intermediate-mass black holes (IMBHs) can form close binary systems with these objects, in which the Roche lobe can be filled by the star and intense accretion of the star’s matter onto the IMBH is possible. Recently, accreting IMBHs have been associated with hyperluminous X-ray sources (HLXs), whose X-ray luminosities can exceed 1041 erg/s. In this paper, the evolution of star—IMBH binary systems is investigated assuming that the IMBH mainly accretes the matter of its companion star, and that the presence of gas in the vicinity of the IMBH does not appreciably affect changes in the orbit of the star. The computations take into account all processes determining the evolution of ordinary binary systems, as well as the irradiation of a star by hard radiation during the accretion of its matter onto the IMBH. The absorption of external radiation in the stellar envelope was calculated applying the same formalism that is used to calculate the opacity of the stellar matter. The computations also assumed that, if the characteristic time for the mass transfer is less than the thermal time scale of the star, there is no exchange betwween the orbital angular momentum of the system and the angular momentum of the matter flowing onto the IMBH.
Numerical simulations have shown that, under these assumptions, three types of evolution are possible for such a binary system, depending on the mass of the IMBH and the star, as well as on the star’s initial distance from the IMBH. The first type ends with the destruction of the star. For low-mass main sequence (MS) stars, only this option is realized, even in the case of large initial distances from IMBH. For massive MS stars, the star is also destroyed if the mass of the IMBH is high and the initial distance of the star from the IMBH is sufficiently small.
The second type of evolution can occur for massive MS stars, which are initially located farther from the IMBH than in the first type of evolution. In this case, the massive star fills its Roche lobe during its evolutionary expansion, after which a stage of intense mass transfer begins. It is in this phase of the evolution that the star- IMBH system can manifest itself as a HLX, when its X-ray luminosity LX exceeds 1041 erg/s for a fairly long time. Numerical simulations show that the initial mass of the donor star in systems with MBH = (103?105)M⊙ must be close to ～10 M⊙ in this case. The characteristic duration of the HLX stage is 30 000–70 000 years. For smaller initial donor masses close to ～5M⊙, LX does not reach 1041 erg/s in the stage of intense mass transfer, but can exceed 1040 erg/s. The duration of this stage of evolution is 300 000–800 000 years. A characteristic feature of this second type of evolution is an increase in the orbital period of the system over time. As a result, after a period of intense mass loss, the star “retreats” inside the Roche lobe. A remnant of the star in the form of a white dwarf is left behind, and can end up fairly far from the IMBH.
The third type of evolution can occur for massive MS stars that are initially even farther from the IMBH, as well as for massive stars that are already evolved at the initial time. In this case, conservative mass exchange in the presence of intense stellar wind leads to the star moving away from the IMBH, without filling its Roche lobe at all. For massive stars with sufficiently strong stellar winds (for example, stars with masses ～50M⊙), the accretion rate of matter onto the IMBH in this case can reach values that are characteristic of HLXs. As in the case of the second type of evolution, the stellar remnant can remain at a fairly large distance from the IMBH.
相似文献14.
G. I. Alsop 《Geological Journal》1992,27(3):271-283
Internal regions of orogenic belts may be characterized by an alignment of fold axes with mineral elongation lineations. This relationship is commonly interpreted as representing progressive tightening and rotation towards the shear direction of early buckle folds, the hinges of which were initiated orthogonal to this direction. Detailed structural analysis of lower amphibolite facies Dalradian metasediments of the Ballybofey (fold) Nappe, north-west Ireland, shows that an intense S3 schistosity is developed axial planar to mesoscopic and minor F3 folds. In areas of low D3 strain, F3 fold axes plunge gently towards the north-east, whereas in regions of greater strain plunges are towards the south-east subparallel to the constant mineral lineation. Minor folds which initiated at angles of 70–80° from the mineral lineation subsequently rotated towards the shear direction in a consistent clockwise sense. Progressive and variable non-coaxial deformation oblique to the original mean F3 orientation has resulted in a unimodal distribution pattern of fold axes. Analysis of the angular rotation of fold axes enables estimates of the bulk shear strain to be evaluated and models of progressive deformation to be assessed. 相似文献
15.
Data from the revised Geneva-Copenhagen catalog are used to study the influence of radial migration of stars on the age dependences
of parameters of the velocity ellipsoids for nearby stars in the thin disk of the Galaxy, assuming that the mean radii of
the stellar orbits remain constant. It is demonstrated that precisely the radial migration of stars, together with the negative
metallicity gradient in the thin disk, are responsible for the observed negative correlation between the metallicities and
angular momenta of nearby stars, while the angular momenta of stars that were born at the same Galactocentric distances do
not depend on either age or metallicity. The velocity components of the Sun relative to the Local Standard of Rest derived
using data for stars born at the solar Galactocentric distance are (U
⊙, V
⊙, W
⊙)
LSR
= (5.1 ± 0.4, 7.9 ± 0.5, 7.7 ± 0.2) km/s. The two coordinates of the apex of the solar motion remain equal to 〈l
⊙〉 = 70° ± 7° and 〈b
⊙〉 = 41° ± 2°, within the errors. The indices for the power-law age dependences of them ajor, middle, and minor semi-axes become
0.26±0.04, 0.32±0.03, and 0.07±0.03, respectively. As a result, with age, the velocity ellipsoid for thin-disk stars born
at the solar Galactocentric distance increases only in the plane of the disk, remaining virtually constant in the perpendicular
direction. Its shape remains far from equilibrium, and the direction of the major axis does not change with age: the ellipsoid
vertex deviation remains constant and equal to zero within the errors (〈L〉 = 0.7° ± 0.6°, 〈B〉 = 1.9° ± 1.1°). Such a small increase in the velocity dispersion perpendicular to the Galactic plane with age can probably
be explained by “heating” of the stellar system purely by spiral density waves, without a contribution from giant molecular
clouds. 相似文献
16.
We analyze the X-ray emission and chromospheric activity of late-type F, G, and K stars studied in the framework of the HK project. More powerful coronas are possessed by stars displaying irregular variations of their chromospheric emission, while stars with cyclic activity are characterized by comparatively modest X-ray luminosities and ratios of the X-ray to bolometric luminosity L X/L bol. This indicates that the nature of processes associated with magnetic-field amplification in the convective envelope changes appreciably in the transition from small to large dynamo numbers, directly affecting the character of the (α-Ω) dynamo. Due to the strong dependence of both the dynamo number and the Rossby number on the speed of axial rotation, earlier correlations found between various activity parameters and the Rossby number are consistent with our conclusions. Our analysis makes it possible to draw the first firm conclusions about the place of solar activity among analogous processes developing in active late-type stars. 相似文献
17.
E. I. Staritsin 《Astronomy Reports》2001,45(6):467-474
The evolution of a rotating main-sequence star with a mass of 16M ⊙ is studied. The flow of material in the radiative envelope is turbulent. When the exchange of energy and chemical elements is taken into account, this turbulence transfers helium and momentum from the convective core to the radiative envelope. The helium content at the stellar surface is enhanced toward the end of the evolution on the main sequence by 0.006–0.306, depending on the character of the star’s initial rotation and the horizontal coefficient of the turbulent viscosity. 相似文献
18.
I. F. Malov 《Astronomy Reports》2001,45(5):389-394
Various mechanisms for the loss of angular momentum of neutron stars are analyzed. Theoretical predictions about the evolution of the period are compared with the observed distribution of pulsars on the log\(\dot P\)log(P) diagram. Pulsars with short periods (P≤0.1 s) cannot be fit well by any of the models considered. Their braking index is n=?1, which requires the development of a new braking mechanism. The evolution of pulsars with P>1.25 s is described by the law \(\dot P \propto P^2\), probably due to processes internal to the neutron stars. The observational data for pulsars with 0.1<P≤1.25 s can be fit with a hybrid model incorporating internal processes and magnetic-dipole losses. The magnetic fields in pulsar catalogs should be recomputed in accordance with the results obtained. For example, the magnetic fields obtained for two magnetars with P=5.16 s and P=7.47 s are B s =1.7×1013 and 2.9×1013 G, which are lower than the critical field Bcr=4.4×1013 G. For a substantial fraction of pulsars, their characteristic ages \(\tau = P/2\dot P\) cannot serve as measures of their real ages. 相似文献
19.
Several scenarios for the formation of accretion and decretion disks in single and binary Ae and Be stars are proposed. It is shown that, in order for a rapidly rotating main-sequence Be star to lose mass via a disk, the star’s rotation must be quasi-rigid-body. Estimates show that such rotation can be maintained by the star’s magnetic field, which is probably a relict field. The evolution of single Be main-sequence stars is numerically simulated allowing for mass loss via the stellar wind and rotational mass loss assuming rigid-body rotation. The stellar wind is the factor that determines the maximum mass of Be stars, which is close to 30M ⊙. The evolution of Be stars in close binaries is analyzed in the approximation adopted in our scenario. Long gamma-ray bursts can be obtained as a result of the collapse of rapidly rotating oxygen—neon degenerate dwarfs—the accreting companions of Be stars—into neutron stars. 相似文献
20.
The evolution of close binary systems containing Wolf-Rayet (WR) stars and black holes (BHs) is analyzed numerically. Both the stellar wind from the donor star itself and the induced stellar wind due to irradiation of the donor with hard radiation arising during accretion onto the relativistic component are considered. The mass and angular momentum losses due to the stellar wind are also taken into account at phases when the WR star fills its Roche lobe. It is shown that, if a WR star with a mass higher than ～10M ⊙ fills its Roche lobe in an initial evolutionary phase, the donor star will eventually lose contact with the Roche lobe as the binary loses mass and angular momentum via the stellar wind, suggesting that the semi-detached binary will become detached. The star will remain a bright X-ray source, since the stellar wind that is captured by the black hole ensures a near-Eddington accretion rate. If the initial mass of the helium donor is below ～5M ⊙, the donor may only temporarily detach from its Roche lobe. Induced stellar wind plays a significant role in the evolution of binaries containing helium donors with initial masses of ～2M ⊙. We compute the evolution of three observed WR-BH binaries: Cyg X-3, IC 10 X-1, and NGC 300 X-1, as well as the evolution of the SS 433 binary system, which is a progenitor of such systems, under the assumption that this binary will avoid a common-envelope stage in its further evolution, as it does in its current evolutionary phase. 相似文献