The role of close passages of galaxies and the asymmetry of their dark haloes in the formation of their spiral patterns

The influence of close passages of galaxies on the shapes of disk galaxies and the distribution of stars in them is studied for several types of interactions in the framework of the restricted N-body problem. Depending on the conditions adopted, either two spiral density waves or ring structures are formed in the stellar disk of the galaxy. These structures can generate star formation fronts with the corresponding shape, as are observed in disk galaxies. Our calculations can also be applied to study the influence of the passage of a nearby star on a protoplanetary disk. The formation of ring structures there could specify the type of planet formation in the outer regions of the planetary system and the distribution of semimajor axes for the planetary orbits. We use the same model to study the generation and evolution of spiral density waves in the stellar disks of galaxies as a result of the recently found asymmetry of the gravitational potential in the massive dark haloes in disk galaxies. The dipole component of the gravitational field of the halo can continuously permanently generate the spiral structure in disk galaxies.     

Hydrodynamical Aspects of the Formation of Spiral–Vortical Structures in Rotating Gaseous Disks

This paper is dedicated to numerical simulations of spiral–vortical structures in rotating gaseous disks using a simple model based on two-dimensional, non-stationary, barotropic Euler equations with a body force. The results suggest the possibility of a purely hydrodynamical basis for the formation and evolution of such structures. New, axially symmetric, stationary solutions of these equations are derived that modify known approximate solutions. These solutions with added small perturbations are used as initial data in the non-stationary problem, whose solution demonstrates the formation of density arms with bifurcation. The associated redistribution of angular momentum is analyzed. The correctness of laboratory experiments using shallow water to describe the formation of large-scale vortical structures in thin gaseous disks is confirmed. The computations are based on a special quasi-gas-dynamical regularization of the Euler equations in polar coordinates.     

Nonlinear Rayleigh-Taylor instability in partially ionized plasma and the equatorial spread-F

The nonlinear evolution of the collisional gravitation induced RayleighTaylor (R-T) instability in the equatorialF region is investigated taking into account the finite larmor radius (FLR) effects and the complete ion inertial term in ion equation of motion. A special class of coherent weakly nonlinear modes as solutions to the wave equation describing R-T instability driven modes is obtained. The leading nonlinear effects in the wave equation are found to appear throughV _l, the ion diamagnetic drift which essentially gives the FLR corrections. It is shown that the R-T modes in the equatorialF region can evolve into coherent, nonlinear, almost sinusoidal, stationary wave structures. These structures are found to travel with a constant phase velocity and to have slightly distorted sinusoidal shapes. These results seem to have a good agreement with many of the recent rocket and satellite observations of the equatorial spreadF irregularities.     

The development of large-scale instability in Keplerian stellar accretion disks

Our previous studies of large-scale vortical flows arising in shear flows of stellar accretion disks with Keplerian azimuthal velocity distributions as a result of the development of small perturbations are continued. The development of large-scale instability in an accretion disk is investigated via mathematical modeling. One result obtained is the change of the disk flow structure due to the formation of large vortices. In the limiting case, sufficiently long evolution leads to the formation of several asymmetric spiral structures of the flow of disk matter. The presence of large-scale structures leads to angular-momentum redistribution in the disk.     

The formation of polarized lines: Allowance for the Hanle effect

The paper formulates the standard theory for the transport of polarized radiation in the presence of resonance scattering in an atmosphere with a weak magnetic field, so that the Zeeman splitting is small compared to the Doppler line width. For an atmosphere with conservative scattering, this reduces to the Milne problem, which consists of computing a polarized radiation field in a medium with sources lying in infinitely deep layers. In the approximation of complete frequency redistribution, the problem reduces to solving a Wiener-Hopf integral equation for a (6×6)-matrix source function. Asymptotic and numerical solutions for the standard problem are obtained, including solutions for the Milne problem, for the case of a Doppler absorption profile. The line polarization profiles for the emergent flux at various angular distances from the disk center are derived, and the dependence of the limiting degree of polarization (at the line center at the disk edge) on the direction of the magnetic field is computed. For nearly conservative scattering with photon destruction probability ε?1, the limiting degree of polarization varies with ε in accordance with a simple empirical law similar to that found earlier for a medium with zero magnetic field.     

Scattering of cyclotron radiation in a moving plasma

We consider the scattering of cyclotron radiation in a plasma moving along a homogeneous magnetic field. The equation of radiation transfer in a co-moving frame is derived and two limiting cases are pointed out. In the first case of a “small” velocity gradient, the total Doppler frequency shift due to variations in the plasma velocity over the flow is much smaller than the width of the line. The second, opposite, case of a “large” velocity gradient is analogous to the Sobolev approximation in the theory of moving stellar envelopes. The solution of the transfer equation for a wind-type flow illuminated by radiation of a given intensity is obtained in the latter case, when the influence of the plasma motion on cyclotron scattering is most important. It is shown that cyclotron scattering in a moving plasma differs from the known (and qualitatively similar) problems of resonance scattering in moving stellar envelopes and cyclotron scattering in a motionless plasma permeated by an inhomogeneous magnetic field. In particular, a symmetric absorption band with residual intensity proportional to the velocity gradient appears in the spectrum of the outgoing radiation, while in these two other problems, the depth of the corresponding spectral features cannot exceed half the continuum level. Detailed qualitative analysis reveals that this difference is due to the particular form of the frequency redistribution for cyclotron scattering.     

BVRI surface photometry of the galaxy NGC 3726

We present BVRI surface photometry of the late-type spiral galaxy NGC 3627. The distributions of the color indices and extinction-independent Q indices show that the observed photometric asymmetry in the inner part of the galaxy, including the bar, is due to an asymmetric distribution of absorbing material. The bluest regions of star formation are located in a ring surrounding the bar. The background-subtracted color indices of individual blue knots are used to estimate the ages of young stellar aggregates. In combination with previously published photometric data, our measurements indicate that the R-band profile of the disk is rather flat in its inner part (r<50″) and becomes steeper further from its center. We estimate the mass of the disk and dark halo by decomposing the rotation curve. The mass-to-light ratio M/L _B for the stellar disk is ≈1.4. The galaxy possesses a massive dark halo; however, the mass of the disk exceeds that of the halo in the inner part of the galaxy, which displays a regular spiral structure.     

New aspects of rhomb structures

The acute angles of 48 rhombic, trapezoidal and triangular grabens, horsts and upthrusts throughout the world were determined. The results indicate an angular spread of 7—73°. In most structures the two acute angles differ considerably. Most rhomb structures are grabens which show affinities to seismic faults, and only a few are horsts and upthrusts. In this connection modes of static crustal fracture propagation are compared to dynamic propagation. Rhomb structures occur along strike-slip faults and in rifts. These results question the universality of the pull-apart model as a mechanism for rhomb structure development. The two mechanisms that can lead to the formation of rhomb structures are (1) interaction of en échelon or non-coplanar fractures by shear and extensional modes and (2) fracture bifurcation in an extensional mode. The two mechanisms may result in faults of different shapes characterized by curved and straight boundaries, respectively. The grabens and horsts in extensional regions are the consequence of early fracture and later vertical displacements.     

The development of large-scale instability in stellar accretion disks and its influence on the redistribution of angular momentum

The paper continues our studies of large-scale instability arising during shearmotions in stellar accretion disks due to the development of small perturbations. The evolution of a local perturbation introduced into the outer part of a stationary accretion disk is modeled mathematically. The possible formation of large-scale structures that propagate throughout the disk, leading to an appreciable redistribution of angular momentum, is demonstrated.     

Wolf-Rayet stars with relativistic companions

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.     

The stellar structure of irregular galaxies. Face-on galaxies

Stellar photometry of nearby irregular galaxies of the Local Group is used to identify and study the young and old stellar populations of these galaxies. An analysis of the spatial distributions of stars of different ages in face-on galaxies shows that the young stellar populations in irregular galaxies are concentrated toward the center, and form local inhomogeneities in star-forming regions, while the old stellar populations—red giants—form extended structures around the irregular galaxies. The sizes of these structures exceed the visible sizes of the galaxies at the 25^m/arcsec² isophote by a factor of two to three. The surface density of the red giants decreases exponentially from the center toward the edge, similar to the disk components in spiral galaxies.     

土质围岩开挖破坏模式研究

结合大量土质隧道围岩破坏调研资料和室内模型试验,研究土质围岩破坏的位置和形态,以破坏位置和特点探讨了土质围岩的破坏模式,并对各破坏模式进行了定义。研究表明,土质围岩破坏模式有未支护段破坏模式、掌子面破坏模式、未支护段和掌子面同时破坏模式及不发生破坏4种;未支护段破坏模式可分为自稳破坏、进尺破坏和不破坏;未支护段自稳破坏模式主要受洞形跨度和跨高、围岩条件控制,而进尺破坏模式主要受进尺和围岩条件控制;掌子面破坏模式的破坏面可近似为对数螺旋曲面或斜平面,未支护段破坏模式的破坏面可近似为抛物线曲面,未支护段和掌子面同时破坏模式的破坏面近似为数螺旋曲面或斜平面和抛物线曲面相交。     

Mathematical modeling of the structure of an accretion disk in a stellar binary system

Numerical simulations of gas-dynamical processes taking place in the accretion disk of a stellar binary system are presented. The initial state of the disk is an equilibrium gaseous configuration. Mechanisms for the development of spiral waves and associated variations in the angular momentum of the gas are considered. The influence of the ratio of the binary-component masses and the initial disk configuration are investigated. It is concluded that the existence of a steady-state disk is impossible without a flow of gas from the donor star.     

The resolvent relation of Sobolev for vector integral transfer equations in a half-space

The semi-inverse factorization method for the derivation of the Sobolev resolvent relation (SRR) for the transfer of radiation in a homogeneous half-space is proposed. The SRR is extended to vector, integral transfer equations. The method is compared with the non-linear factorization equation method. The application of the SRR to the transfer of resonance radiation is discussed.     

Equilibrium phase-space density distribution in numerical dynamical models of open clusters

In dynamical models for open clusters, virial equilibrium is not achieved over the violent relaxation time scale τ_vr. 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>τ_vr. 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.     

Disk accretion onto a magnetized star

An exact solution is found for the interaction of a rotating magnetic field that is frozen into a star with a thin, highly conducting accretion disk. The disk pushes the magnetic-field lines towards the star, compressing the stellar dipole magnetic field. At the corotation radius, where the Keplerian and stellar rotational frequencies are equal, a current loop appears. Electric currents flow in the magnetosphere only along two particular magnetic surfaces, which connect the corotation region and the inner edge of the disk with the stellar surface. It is shown that a closed current surface encloses the magnetosphere. The disk rotation is stopped at some distance from the stellar surface, equal to 0.55 of the corotation radius. The accretion from the disk spins up the stellar rotation. The angular momentum transferred to the star is determined.     

广义Gibson饱和地基中刚性圆板的扭转柔度系数

考虑到地基在竖直方向上的非均匀性,结合扭转振动的特点,建立了简谐扭转动荷载作用时剪切模量随深度增大的广义Gibson饱和地基的动力方程,通过积分变换求解了动力方程。考虑到半空间地基表面处应力自由、埋置圆板所在平面为混合边界和无穷深度处为波的辐射边界等条件,得到了广义Gibson饱和地基中刚性圆板扭转振动时的对偶积分方程,通过合适的变换转化成了一个第2类Fredholm积分方程,求解了相应的动力响应问题。对比静扭距作用时的荷载-位移关系,给出了动力柔度系数和扭转角位移幅值的表达式,并把所研究的问题进行退化且与前人成果进行了对比。数值研究表明：当基础的埋置深度小于5倍基础半径时,广义Gibson饱和地基中埋置基础的扭转振动存在明显的边界层现象,且埋置深度越小,边界层现象越明显。     

整体式螺旋钻杆的研制及应用

针对焊接式螺旋钻杆配合风压钻进松软煤层时,易出现螺旋翼片开裂、脱焊及不耐磨等现象,导致成孔难甚至报废等问题,通过铣屑方式在厚壁管材中加工出螺旋槽,改进钻杆接头,对螺纹进行表面处理,研制出整体式螺旋钻杆。简要介绍了整体式螺旋钻杆设计和加工流程。在淮南潘一矿的试验结果表明:整体式螺旋钻杆具有良好的综合机械性能,成孔能力强,使用寿命长,既可替代焊接式螺旋钻杆在松软煤层中进行风压钻进,也可替代常规外平钻杆施工岩石钻孔。      

Natural vibration characteristics of gravity structures

A forced vibration procedure is presented to estimate fundamental and higher frequencies of vibrations and associated mode shapes of gravity structures. The gravity structures considered are retaining walls and gravity dams. The validity of the proposed procedure is tested on three test problems of varying complexity for which the natural vibration frequencies and mode shapes either have known analytical solutions or have been determined via numerical means/field tests by others. Also included are the results of natural vibration frequencies and associated mode shapes for a spillway control structure located near the abutment end of an embankment dam obtained using the proposed procedure. For all problems considered, fundamental frequency and mode shape results using the proposed procedure are compared with the results obtained using an alternative procedure in which static deflections due to the structure's own weight are used as the starting point for free vibrations by setting the gravity vector to zero. All results compare well. The merits of the proposed procedure are discussed. Published in 2006 by John Wiley & Sons, Ltd.     

Corotation resonance and the radial distribution of oxygen in a spiral galaxy

A theory is proposed for the formation of the bimodal radial distribution of oxygen in the Galaxy, which displays a steep abundance gradient in the inner region and becomes flat in the middle and outer regions. The formation of this structure is associated with the following effects. First, type II supernovae, which are sources of oxygen, are concentrated in the spiral arms. Second, the enrichment rate is proportional to the rate at which a chemical element enters the spiral arm, which has the form of a density wave. This implies that the rate of oxygen production is substantially suppressed in most of the corotation circle due to the low relative velocity of the Galactic material with respect to the spiral arms and the low rate at which the arms are crossed by the Galactic material. Third, the corotation radius is near the Sun. The net result of these effects combined with turbulent diffusion is the development of the structure discussed in the paper.