白矮星研究的某些进展

综合叙述了白矮星诞生率研究的近况，详细介绍了ＤＡ，非ＤＡ型白矮星质量，质量分布以及确定质量的方法，对白矮星光度函数作了较为全面的回顾，指出了一些目前白矮星研究工作中仍存在的问题。     

White Dwarfs with Anisotropic Pressure in Postnewtonian Gravity

The theory of equilibrium of nonrotating, spherically symmetric neutron stars and white dwarfs is revisited. We propose that the condition for equilibrium is that the total energy is a minimum, for given baryon number, amongst configurations fulfilling Einstein's equations. The isotropy of the stress tensor is not assumed 'a priori'. Using this condition it is shown that, in post-Newtonian gravity, there are equilibrium configurations of white dwarfs, with local anisotropy, having mass above the Chandrasekhar limit. This revised version was published online in July 2006 with corrections to the Cover Date.     

巡天项目中的白矮星搜寻

首先,描述了白矮星的颜色特点和光谱型分类。白矮星是大多数恒星的终极状态,它会随着时间的推移而逐渐冷却。根据表面大气成分,它被划分成DA和DB等不同的光谱型。对某一颗白矮星来说,其光谱型分类可能会随着时间的推移而发生变化,这主要是由于其大气包层中的元素含量被对流过程改变引起的。为了对各类光谱型有更深入的研究,样本的完备性不可或缺。得益于近十几年来大型巡天项目的开展(如斯隆数字巡天、郭守敬望远镜的光谱巡天等),目前通过光谱证认的白矮星已超过4×10~4颗,其中绝大多数是H主导的DA型白矮星,另外还包含2 000多颗He主导的DB白矮星。此外,GALEX项目和GAIA卫星分别提供了有价值的白矮星紫外数据、空间位置和速度等信息。这些数据和信息对于白矮星的研究工作起到了非常大的促进作用。随后,对白矮星的大气参数、质量和运动学等内容作了简单的总结。人们利用模板匹配测量了绝大多数已知白矮星的大气参数,并获得了DB型白矮星的有效温度与紫外颜色间的线性关系。基于斯隆数字巡天的光谱数据,白矮星的质量分布和质量-半径关系也得到了较深入的研究。小质量白矮星具有较大的速度弥散,而大质量的则相反。最后,讨论了白矮星的形成和演化机制,这些机制都是基于现有的理论模型得到的。随着巡天项目中白矮星样本的增加,现有的模型会逐渐被修正和完善,新的模型也将随之建立。     

行星际闪烁单站单频数据处理

太阳风是日地空间的主要物质来源,太阳风的观测对日地空间环境及地球物理的研究具有重要意义。地基行星际闪烁观测是监测太阳风风速,测量太阳风等离子体不规则结构,研究遥远致密射电源角径的重要且有效的方法。介绍了行星际闪烁地基观测的单站单频模式的基本理论,针对单站单频模式观测的数据处理及自编软件。本文的工作是为行星际闪烁单站双频系统数据分析处理作前期准备。     

Particle Acceleration in the Presence of Weak Turbulence at an X-Type Neutral Point

We simulate the likely noisy situation near a reconnection region by superposing many 2D linear reconnection eigenmodes. The superposition of modes on the steady state X-type magnetic field creates multiple X- and O-type neutral points close to the original neutral point and so increases the size of the non-adiabatic region. We study test particle trajectories of initially thermal protons in these fields. Protons become trapped in this region and are accelerated by the turbulent electric field to energies up to 1 MeV in time scales relevant to solar flares. Higher energies are achieved due to the interaction of particles with increasingly turbulent electric and magnetic fields.     

Secular Solutions at Triangular Equilibrium Point in the Generalized Photogravitational Restricted Three Body Problem

In this paper we have found secular solutions at the triangular equilibrium point in the generalized photogravitational restricted three body problem. The problem is generalised in the sense that smaller primary is an oblate spheroid and more massive primary as source of radiation. The triangular point has long or short-period retrograde elliptical orbits. The critical mass parameter decreases with the increase in oblateness and radiation pressure. This revised version was published online in July 2006 with corrections to the Cover Date.     

Fast Magnetoacoustic Waves in a Fan Structure Above a Coronal Magnetic Null Point

We analyze the 26 November 2005 solar radio event observed interferometrically at frequencies of 244 and 611 MHz by the Giant Metrewave Radio Telescope (GMRT) in Pune, India. These observations are used to make interferometric maps of the event at both frequencies with the time cadence of 1 s from 06:50 to 07:12 UT. These maps reveal several radio sources. The light curves of these sources show that only two sources at 244 MHz and 611 MHz are well correlated in time. The EUV flare is more localized with flare loops located rather away from the radio sources. Using SoHO/MDI observations and potential magnetic field extrapolation we demonstrate that both the correlated sources are located in the fan structure of magnetic field lines starting from a coronal magnetic null point. Wavelet analysis of the light curves of the radio sources detects tadpoles with periods in the range P=10?–?83 s. These wavelet tadpoles indicate the presence of fast magnetoacoustic waves that propagate in the fan structure of the coronal magnetic null point. We estimate the plasma parameters in the studied radio sources and find them consistent with the presented scenario involving the coronal magnetic null point.     

点在球面多边形内外的判定方法及应用

精确判断卫星进出南大西洋辐射异常区(SAA)对于保护卫星上重要仪器、延长卫星寿命和制定观测计划至关重要,这就是所谓点定位问题,简单说来就是判定球面上一点P是否位于指定的球面区域R内的问题。传统的研究方法是把球面简化为平面,研究点与平面多边形的关系。但SAA区域为球面上的区域,用平面简化得出的点定位误差较大。提出判断点在球面区域内外的算法,处理了可能出现的奇异情况,实现了点在球面区域内的准确定位。     

Observation of a 3d Magnetic Null Point in the Solar Corona

A saddle-like structure is clearly discerned in SOHO EIT images in the Fe wavebands in the active region NOAA 8113, from 6 to 10 December 1997, when the active region reached the solar limb. It appears after the emergence of a small patch of parasitic polarity near the footpoints of the loops forming the outer boundary of an arcade. There is a 3D null point in the center of the saddle. The height of the null is in good agreement with the relationship between the parasitic flux and the background magnetic field. In spite of the fact that a saddle-like or hyperbolic magnetic configuration is usually assumed as the necessary condition for magnetic field-line reconnection and magnetic energy release, no additional heating is observed inside the saddle structure. Moreover, interior parts of the saddle seem to be darker than the neighbouring loops of the arcade.     

Stability of the Triangular Libration Points in the Unrestricted Planar Problem of a Symmetric Rigid Body and a Point Mass

In papers (Godziewski and Maciejewski, 1998a, b, 1999), we investigate unrestricted, planar problem of a dynamically symmetric rigid body and a sphere. Following the original statement of the problem by Kokoriev and Kirpichnikov (1988), we assume that the potential of the rigid body is approximated by the gravitational field of a dumb-bell. The model is described in terms of a 2D Hamiltonian depending on three parameters.In this paper, we investigate the stability of triangular equilibria permissible by the dynamics of the model, under the assumption of low-order resonances. We analyze all resonances of order smaller than four, and we examine the stability with application of theorems by Markeev and Sokolsky. These are the possible following cases: the non-diagonal resonance of the first order with two null characteristic frequencies (unstable); resonances of the first order with one nonzero frequency (diagonal and non-diagonal, stable and unstable); the second-order resonance, which is non-diagonal and stable, and the third-order resonance which is generically unstable, except for three points in the parameters' space, corresponding to stable equilibria.We discuss a perturbed version of Kokoriev and Kirpichnikov model, and we find that if the perturbation is small and depends on the coordinates only, the triangular equilibria persist, except if for the unperturbed equilibria the first-order resonance occurs. We show that the resonances of the order higher than two are also preserved if the perturbation acts.     

Behaviour of Magnetoacoustic Waves in the Neighbourhood of a Two-Dimensional Null Point: Initially Cylindrically Symmetric Perturbations

The propagation of magnetoacoustic waves in the neighbourhood of a 2D null point is investigated for both β=0 and β ≠ 0 plasmas. Previous work has shown that the Alfvén speed, here v _A ∝r, plays a vital role in such systems and so a natural choice is to switch to polar coordinates. For β=0 plasma, we derive an analytical solution for the behaviour of the fast magnetoacoustic wave in terms of the Klein–Gordon equation. We also solve the system with a semi-analytical WKB approximation which shows that the β=0 wave focuses on the null and contracts around it but, due to exponential decay, never reaches the null in a finite time. For the β ≠ 0 plasma, we solve the system numerically and find the behaviour to be similar to that of the β=0 system at large radii, but completely different close to the null. We show that for an initially cylindrically-symmetric fast magnetoacoustic wave perturbation, there is a decrease in wave speed along the separatrices and so the perturbation starts to take on a quasi-diamond shape; with the corners located along the separatrices. This is due to the growth in pressure gradients that reach a maximum along the separatrices, which in turn reduces the acceleration of the fast wave along the separatrices leading to a deformation of the wave morphology.     

Dissipative-acoustic instability in accretion disks at a nonlinear stage

Nonstationary hydrodynamic models of a viscous accretion disk around a central compact object were constructed. Two different numerical methods (TVD and SPH) are used to study the dynamics of dissipatively unstable acoustic perturbations at the nonlinear stage in terms of the standard α-disk model. The standard disk accretion in the Shakura-Sunyaev model is unstable against acoustic waves for various parameters of the system. If the α parameter, which specifies the level of turbulent viscosity, exceeds α?0.03, then a complex nonstationary system of small-scale weak shock waves is formed. The growth rate of the perturbations is higher in the central disk region. For α?0.2, the relative shock amplitude can exceed 50% of the equilibrium disk parameters. The reflection of waves from the disk boundaries and their nonlinear interaction are important factors that can produce unsteady accretion. The luminosity of such a disk undergoes quasi-periodic oscillations at a level of several percent (?5%) of the equilibrium level.     

RFI Test Observations at a Candidate Ska Site in China

Radio frequency interference (RFI) test observations were carried out at one of the candidate Square Kilometre Array (SKA) sites in Guizhou province, following the “RFI Measurement Protocol for Candidate SKA Sites” (hereafter RFI protocol). All data (raw and calibrated) are preserved in some suitable format, such as that set by the international RFI working group of the Site Evaluation and Selection Committee (SESC). An RFI test in December 2003 was performed according to Mode 1 of the RFI Protocol, in order to identify technical difficulties which might arise during a co-ordinated RFI measurement campaign over a period of 1 year. In this paper we describe the current equipment, observational technique and data presentation. The preliminary results demonstrate that the RFI situation at Dawodang depression in Guizhou province makes it quite a promising location for the proposed SKA. Furthermore, the first session of the RFI monitoring program, which was made in May 2004, showed that a complete RFI measurement including both modes 1 and 2 of the RFI Protocol would take about 2 weeks. The possible ways to minimize some limitations of the current equipment are also discussed, which will enable us to meet the RFI protocol.     

A New Look at Mode Conversion in a Stratified Isothermal Atmosphere

Recent numerical investigations of wave propagation near coronal magnetic null points (McLaughlin and Hood: Astron. Astrophys. 459, 641, 2006) have indicated how a fast MHD wave partially converts into a slow MHD wave as the disturbance passes from a low-β plasma to a high-β plasma. This is a complex process and a clear understanding of the conversion mechanism requires the detailed investigation of a simpler model. An investigation of mode conversion in a stratified, isothermal atmosphere with a uniform, vertical magnetic field is carried out, both numerically and analytically. In contrast to previous investigations of upward-propagating waves (Zhugzhda and Dzhalilov: Astron. Astrophys. 112, 16, 1982a; Cally: Astrophys. J. 548, 473, 2001), this paper studies the downward propagation of waves from a low-β to high-β environment. A simple expression for the amplitude of the transmitted wave is compared with the numerical solution.     

Accretion by a Neutron Star Moving at a High Kick Velocity in the Supernova Ejecta

1 INTRODUCTION The classical problem of spherical accretion onto a compact object, e.g., a black hole or a neutron star (NS), has been studied by many authors. A hydrodynamical solution was presented by Bondi 1952, who showed that inside a certain capture…     

Cataclysmic variables in a white dwarf survey at the South Galactic Pole

During a photometric and spectrophotometric survey of 200 white dwarf candidates with m_pg<15^m.0 in a field around the South Galactic Pole two new cataclysmic variables have been identified and new observations of one already known object have been accumulated. Observations in the visible and UV-region show variability and differences in spectral type. If compared to the numbers of cataclysmic variables/white dwarfs as computed by Ritter and Burkert (1985) there is a shortage of a factor of 5.Paper presented at the IAU Colloquium No. 93 on Cataclysmic Variables. Recent Multi-Frequency Observations and Theoretical Developments, held at Dr. Remeis-Sternwarte Bamberg, F.R.G., 16–19 June, 1986.     

Observation of infrared lines in a prominence at 1–5 microns

We have observed 11 hydrogen and 3 helium lines in a quiescent prominence; none have been previously observed except for the He 10830 line. In contrast to Zirker's results based on longer-wavelength infrared lines, we find no net velocity shift, Doppler widths narrower than his by more than 30%, and a hydrogen excitation temperature 3 times higher. From the line intensity method, we find a helium temperature of 5300(±200) K and a hydrogen temperature of 10 900(±1900) K, in reasonable agreement with the findings of visible line studies. We conclude that helium lines are formed in the central region while hydrogen lines are formed around the edges of the prominence.Visiting Astronomer at the National Solar Observatory, a division of the National Optical Astronomy Observatories, which are operated by the Association of Universities for Research in Astronomy, under contract with the National Science Foundation.