$\textstyle \parbox{150mm}{
{\LARGE\bf Chinese Journal of Astronomy and Astrophysics}\\
}$

2007, Vol.7, No.4 (August 20, 2007)


Contents


RESEARCH PAPERS

The Sun


Stars


Stellar Clusters


Extragalactic Astronomy


Celestial Mechanics




Abstract


Observational Features of Large-Scale Structures as Revealed by the Catastrophe Model of Solar Eruptions

Jun Lin

National Astronomical Observatories / Yunnan Observatory, Chinese Academy of Sciences, Kunming 650011; jlin@ynao.ac.cn
Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA

Abstract
Large-scale magnetic structures are the main carrier of major eruptions in the solar atmosphere. These structures are rooted in the photosphere and are driven by the unceasing motion of the photospheric material through a series of equilibrium configurations. The motion brings energy into the coronal magnetic field until the system ceases to be in equilibrium. The catastrophe theory for solar eruptions indicates that loss of mechanical equilibrium constitutes the main trigger mechanism of major eruptions, usually shown up as solar flares, eruptive prominences, and coronal mass ejections (CMEs). Magnetic reconnection which takes place at the very beginning of the eruption as a result of plasma instabilities/turbulence inside the current sheet, converts magnetic energy into heating and kinetic energy that are responsible for solar flares, and for accelerating both plasma ejecta (flows and CMEs) and energetic particles. Various manifestations are thus related to one another, and the physics behind these relationships is catastrophe and magnetic reconnection. This work reports on recent progress in both theoretical research and observations on eruptive phenomena showing the above manifestations. We start by displaying the properties of large-scale structures in the corona and the related magnetic fields prior to an eruption, and show various morphological features of the disrupting magnetic fields. Then, in the framework of the catastrophe theory, we look into the physics behind those features investigated in a succession of previous works, and discuss the approaches they used.

Key Words:   Sun: large-scale magnetic structures -- Sun: eruptive processes -- Sun: theory and observations -- magnetic reconnection and current sheets
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Are Homologous Radio Bursts Driven by Solar Post-Flare Loops?

Min Wang1, Rui-Xiang Xie1, Yi-Hua Yan2 and Yu-Ying Liu2

1 National Astronomical Observatories/Yunnan Observatory, Chinese Academy of Sciences, Kunming 650011; wm@ynao.ac.cn
2 National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012

Abstract
Three particularly complex radio bursts (2001 October 19, 2001 April 10 and 2003 October 26) obtained with the spectrometers (0.65-7.6GHz) at the National Astronomical Observatories, Chinese Academy of Sciences (NAOC, Beijing and Yunnan) and other instruments (NoRH, TRACE and SXT) are presented. They each have two groups of peaks occurring in different frequency ranges (broad-band microwave and narrow-band decimeter wavelengths). We stress that the second group of burst peaks that occurred in the late phase of the flares and associated with post-flare loops may be homologous radio bursts. We think that they are driven by the post-flare loops. In contrast to the time profiles of the radio bursts and the images of coronal magnetic polarities, we are able to find that the three events are caused by the active regions including main single-bipole magnetic structures, which are associated with multipole magnetic structures during the flare evolutions. In particular, we point out that the later decimetric radio bursts are possibly the radio counterparts of the homologous flares (called ``homologous radio bursts'' by us), which are also driven by the single-bipole magnetic structures. By examining the evolutions of the magnetic polarities of sources (17GHz), we could presume that the drivers of the homologous radio bursts are new and/or recurring appearances/disappearances of the magnetic polarities of radio sources, and that the triggers are the magnetic reconnections of single-bipole configurations.

Key Words:   Sun: radio radiation -- Sun: magnetic fields
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Linear Correlations between Peak Frequency of Gyrosynchrotron Spectrum and Photosphere Magnetic Fields

Ai-Hua Zhou and Jian-Ping Li

Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008; zhouah@pmo.ac.cn

Abstract
The gyrosynchrotron spectra are computed in a nonuniform magnetic field case, taking into account the self- and gyroresonance absorption. It is found that the peak frequency $\nu
_p$ of the gyrosynchrotron spectrum systematically increases with the increasing photosphere magnetic field strength B 0 and increasing viewing angle $\theta$. It is also found for the first time that there are good positive linear correlations between $\nu
_p$ and B 0, and between $\log\nu _p$ and $\log \theta$, with linear correlation coefficient 0.99 between $\nu
_p$ and B 0 and 0.95 between $\log\nu _p$ and $\log \theta$. We apply the correlations to analyze two burst events observed with OVSA and find that the evolution tendencies of the photosphere magnetic field strength B 0 estimated from the above expression are comparable with the observational results of SOHO/MDI. We also give a comparison of the diagnostic results of coronal magnetic field strength in both uniform and nonuniform source models.

Key Words:   Sun: flare -- sun: radio emission -- Sun: magnetic field
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An Annular Gap Acceleration Model for $\gamma$-ray Emission of Pulsars

Guo-Jun Qiao1, Ke-Jia Lee1, Bing Zhang2, Hong-Guang Wang3 and Ren-Xin Xu1

1 Department of Astronomy, Peking University, Beijing 100871; gjn@pku.edu.cn
2 Department of Physics, University of Nevada, Las Vegas, Nevada 89154, USA
3 Center for Astrophysics, Guangzhou University, Guangzhou 510400

Abstract
If the binding energy of the pulsar's surface is not so high (the case of a neutron star), both negative and positive charges will flow out freely from the surface of the star. An annular free flow model for $\gamma$-ray emission of pulsars is suggested. It is emphasized that: (1) Two kinds of acceleration regions (annular and core) need to be taken into account. The annular acceleration region is defined by the magnetic field lines that cross the null charge surface within the light cylinder. (2) If the potential drop in the annular region of a pulsar is high enough (normally the case for young pulsars), charges in both the annular and the core regions could be accelerated and produce primary gamma-rays. Secondary pairs are generated in both regions and stream outwards to power the broadband radiations. (3) The potential drop grows more rapidly in the annular region than in the core region. The annular acceleration process is a key process for producing the observed wide emission beams. (4) The advantages of both the polar cap and outer gap models are retained in this model. The geometric properties of the $\gamma$-ray emission from the annular flow are analogous to that presented in a previous work by Qiao et al., which match the observations well. (5) Since charges with different signs leave the pulsar through the annular and the core regions respectively, the current closure problem can be partially solved.

Key Words:   pulsars: general -- pulsars: -- radiation mechanisms: non-thermal -- stars: neutron -- elementary particles
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GRB Energies and $E_{\gamma}-E_{\rm peak}$ Correlation with the Jet Expanding Laterally at the Sound Speed

Xiao-Hong Zhao1,2 and Jin-Ming Bai1

1 National Astronomical Observatories / Yunnan Observatory, Chinese Academy of Sciences, Kunming 650011; zhaoxh@ynao.ac.cn
2 Graduate School of Chinese Academy of Sciences, Beijing 100049

Abstract
A Gamma-ray burst (GRB) is generally believed to be a jet with a small opening angle, this opening angle is usually derived with the afterglow light curve break time using an analytical method. Here we show that the method is not accurate. Using the set of equations of hydrodynamic evolution with the sideways expansion at the local sound speed derived by previous authors and the observed light curve break times, we numerically derive the initial opening angles. Then the collimation-corrected energies ($E_{\gamma}$) for a sample of GRBs are calculated. They are found to show a wide spread, suggesting that the previously declared clustering by some authors may not exist. Also, the $E_{\rm peak}-E_{\gamma}$ relation, claimed by some other authors ($E_{\rm peak}$ is the spectral peak energy), is found still to hold, with a slightly stronger correlation.

Key Words:   gamma-ray: bursts -- methods: numerical -- ISM: jets
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The Synchrotron-self-Compton Radiation Accompanying Shallow Decaying X-Ray Afterglow: the Case of GRB 940217

Da-Ming Wei1,2,3 and Yi-Zhong Fan1,2,4,5

1 Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008; dmwei@pmo.ac.cn
2 National Astronomical Observatories, Chinese Academy of Sciences, Beijing, 100012
3 Joint Center for Particle Nuclear Physics and Cosmology of Purple Mountain Observatory - Nanjing University, Nanjing 210008
4 The Racah Inst. of Physics, Hebrew University, Jerusalem 91904, Israel
5 Golda Meir Fellow

Abstract
High energy emission (> tens MeV) of Gamma-Ray Bursts (GRBs) provides an important clue on the physical processes occurring in GRBs that may be correlated with the GRB early afterglow. A shallow decline phase has been well identified in about half of Swift Gamma-ray Burst X-ray afterglows. The widely considered interpretation involves a significant energy injection and possibly time-evolving shock parameter(s). We calculate the synchrotron-self-Compton (SSC) radiation of such an external forward shock and show that it could explain the well-known long term high energy (i.e., tens MeV to GeV) afterglow of GRB 940217. We propose that cooperation of Swift and GLAST will help to reveal the nature of GRBs.

Key Words:   gamma-rays: bursts -- ISM: jets and outflows -- radiation mechanisms: nonthermal
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X-Ray Afterglows of GRBs 050318 and 060124 and their External Shock Origin

Jin Zhang1,4,Yi-Ping Qin2,3, Fu-Wen Zhang1,4 - Bin-Bin Zhan1,4

1 National Astronomical Observatories / Yunnan Observatory, Chinese Academy of Sciences, Kunming 650011; jinzhang@ynao.ac.cn
2 Center for Astrophysics of Guangzhou University, Guangzhou 510006
3 Physics Department, Guangxi University, Nanning 530004
4 Graduate School of Chinese Academy of Sciences, Beijing 100049

Abstract

The observations with Swift X-ray telescope (XRT) challenge the conventional gamma-ray burst model in many aspects. The XRT light curves are generally composed of four consecutive segments, i.e., a steep decay segment, a shallow decay segment, a normal decay segment, a jet-like steep decay segment, and sometimes erratic flares as well. The physical origin of the X-ray emission is highly debatable. We focus here on the physical origin of the X-ray emissions of GRBs 050318 and 060124. We present the XRT light curves and spectra of the two bursts. The light curve decay slopes of the two bursts are normal, and their relations to the spectral indices are consistent with the prediction of the standard forward shock model. The multi-wavelength light curves at 0.5keV, 1.0keV, 2.0keV and 4.0keV can be reproduced by this model with an isotropic kinetic energy $E_{\rm {k}}=2.2\times10^{52}$erg, $\epsilon_{\rm {e}}=0.04$, $\epsilon_{B}=0.01$ for GRB 050318 and $E_{\rm {k}}=4.2\times10^{53}$erg, $\epsilon_{\rm {e}}=0.05$, $\epsilon_{B}=0.01$ for GRB 060124. These facts suggest that the normal decay phases of the X-rays for the two bursts are of the forward shock origin.

Key Words:   gamma-rays -- bursts-individual: GRB 050318, GRB 060124

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A Statistical Analysis of Radio Pulsar Timing Noise

A. E. Chukwude

Department of Physics & Astronomy, University of Nigeria, Nsukka
Hartebeesthoek Radio Astronomy Observatory (HartRAO), P.O. Box 443, Krugersdorp 1740, South Africa; aus-chukwude@yahoo.com

Abstract
We present an analysis of the timing observations on 27 radio pulsars, collected at Hartebeesthoek Radio Astronomy Observatory (HartRAO), with time spans ranging between $\sim$ 9 and 14yr. Our results show that the measured pulsar frequency second derivatives are non-stationary. Both the magnitude and the sign of the $\ddot{\nu}$ values depend upon the choice of epoch and data span. A simple statistical analysis of the observed second time derivative of the pulse frequency ( $\ddot{\nu}_{\mathrm{obs}}$) of a large sample of 391 (25 HartRAO and 366 Jodrell Bank Observatory) pulsars reveals that $\ddot{\nu}$ is only marginally correlated with both the pulsar spin-down rate ($\dot{P}$) and the characteristic age ($\tau_{c}$). We find correlation coefficients of $\sim$ 0.20 and -0.30 between the measured braking indices and, respectively, $\dot{P}$ and $\tau_{c}$. This result reaffirms earlier conclusions that the braking indices of most radio pulsars, obtained through the standard timing technique, are strongly dominated by sustained random fluctuations in the observed pulse phase.

Key Words:   methods: statistical -- neutron -- pulsars: general
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Measuring the Distance of VX Sagittarii with SiO Maser Proper Motions

Xi Chen1,2, Zhi-Qiang Shen1 and Ye Xu1

1 Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030; chenxi@shao.ac.cn
2 Department of Earth Sciences, National Taiwan Normal University, 88 Sec. 4, Ting-Chou Rd., Taipei 116, Taiwan

Abstract
We report on 43GHz v=1, J=1-0 SiO maser proper motions in the circumstellar envelope of the M-type semi-regular variable star VX Sgr, observed by Very Long Baseline Array (VLBA) at 3 epochs during 1999 April-May. Applying the statistical parallax analysis to these proper motions, we estimated a distance of VX Sgr of 1.57$\pm$0.27kpc, which is consistent with that based on the proper motions of H2O masers, or on the assumption that VX Sgr belongs to the Sgr OB1 association. At this distance, VX Sgr can be classified as a red supergiant. Comparing the statistical parallax method with those of model fitting and annual parallax, we think that the statistical parallax method may be a good way of estimating SiO maser distances at present.

Key Words:   methods: statistical -- masers -- stars: individual (VX Sgr) -- stars: distances
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A Model for Contact Binary Systems

Han-Feng Song1,2, Run-Qian Huang1 and Shao-Lan Bi1

1 National Astronomical Observatories / Yunnan Observatory, Chinese Academy of Sciences, Kunming 650011; rqhuang@ynao.ac.cn
2 Graduate School of Chinese Academy of Sciences, Beijing 100049

Abstract
A model for contact binary systems is presented, which incorporates the following special features: a) The energy exchange between the components is based on the understanding that the energy exchange is due to the release of potential, kinetic and thermal energies of the exchanged mass. b) A special form of mass and angular momentum loss occurring in contact binaries is losses via the outer Lagrangian point. c) The effects of spin, orbital rotation and tidal action on the stellar structure as well as the effect of meridian circulation on the mixing of the chemical elements are considered. d) The model is valid not only for low-mass contact binaries but also for high-mass contact binaries. For illustration, we used the model to trace the evolution of a massive binary system consisting of one $12M_{\odot}$ and one $5M_{\odot}$ star. The result shows that the start and end of the contact stage fall within the semi-detached phase during which the primary continually transfers mass to the secondary. The time span of the contact stage is short and the mass transfer rate is very large. Therefore, the contact stage can be regarded as a special part of the semi-detached phase with a large mass transfer rate. Both mass loss through the outer Lagrangian point and oscillation between contact and semi-contact states can occur during the contact phase, and the effective temperatures of the primary and the secondary are almost equal.

Key Words:   stars: binaries -- stars: evolution
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Rotation Periods of Nine ROSAT Selected Solar-Type Stars

Li-Feng Xing1,2, Shuang-Yi Zhao3, Wei Su2, Yan Shen1, Xiao-Bin Zhang2 and Jian-Yan Wei2

1 Department of Technology and Physics, Zhengzhou University of Light Industry, Zhengzhou 450002; lfxing@bao.ac.cn
2 National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012
3 Department of Physics, Xingtai University, Xingtai 054001

Abstract
We monitored 16 X-ray selected young solar-type stars for light variation and found appreciable periodic light variability with amplitudes of a few hundredths of a magnitude in nine of the objects. Using the method of Phase Dispersion Minimization (PDM) and Fourier analysis (software PERIOD04), the rotation periods of these stars were determined from the photometric data. The rotation periods of all nine stars are shorter than about 3days. It is suggested that, as with the Pleiades cluster, small amplitude light variations are quite common among young solar-type stars with rotation periods around 3 days or less. This gives further evidence for the spin up of solar-type stars predicted by models of angular momentum evolution of pre-main sequence stars.

Key Words:   stars: late-type -- stars: pre-main sequence -- stars: rotation
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Velocity Curve Analysis of Spectroscopic Binary Stars AI Phe, GM Dra, HD 93917 and V502 Oph by Nonlinear Regression

K. Karami1,2,3 and R. Mohebi3

1 Department of Physics, University of Kurdistan, Pasdaran St., P. O. Box 66177-15175, Sanandaj, Iran;
2 Research Institute for Astronomy $\&$ Astrophysics of Maragha (RIAAM), P. O. Box 55134-441, Maragha, Iran
3 Institute for Advanced Studies in Basic Sciences (IASBS), Gava Zang, P. O. Box 45195-1159, Zanjan, Iran

Abstract
We introduce a new method to derive the orbital parameters of spectroscopic binary stars by nonlinear least squares of (o-c). Using the measured radial velocity data of the four double lined spectroscopic binary systems, AI Phe, GM Dra, HD 93917 and V502 Oph, we derived both the orbital and combined spectroscopic elements of these systems. Our numerical results are in good agreement with the those obtained using the method of Lehmann-Filhés.

Key Words:   stars: binaries: eclipsing -- stars: binaries: spectroscopic
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A New Feature Vector Using Selected Line Spectra for Pulsar Signal Bispectrum Characteristic Analysis and Recognition

Zhen-Hua Xie1, Lu-Ping Xu1, Guang-Ren Ni2 and Yan Wang1

1 School of Electronic Engineering, Xidian University, Xi'an 710071; zhhxie@mail.xidian.edu.cn
2 National Time Service Center, Chinese Academy of Sciences, Xi'an 710600

Abstract
Average pulse profiles of pulsar signals are analyzed using the bispectrum technique. The result shows that there are nonlinear phase couplings between the two frequency axes of the bispectrum charts, which indicate nonlinear factors in the generation and propagation of pulsar signals. Bispectra can be used as feature vectors of pulsar signals because of their being translation invariant. A one-dimension selected line spectrum algorithm for extracting pulsar signal characteristic is proposed. Compared with selected bispectra, the proposed selected line spectra have the maximum interclass separability measurements from the point of view of the whole one-dimension feature vector. Recognition experiments on several pulsar signals received at several frequency bands are carried out. The result shows that the selected line spectrum algorithm is suitable for extracting pulsar signal characteristics and has a good classification performance.

Key Words:   pulsars: general--stars: magnetic fields--stars: fundamental parameters (classification)
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Velocity Distance of the Open Cluster M11

Jun-Liang Zhao and Li Chen

Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030; jlzhao@center.shao.ac.cn

Abstract
On the reasonable hypothesis that the internal motions of member stars of a cluster are random and isotropic, a method which can be used to estimate the velocity distance of the cluster and its uncertainty is developed. The velocity distance so determined is an absolute distance estimate, and is completely independent of the (widely used) luminosity distance, which is a relative distance estimate. Using the published high-accuracy observational data of radial velocities and proper motions of the stars in the open cluster M11 region, we have determined the distance of M11 to be $1.89\pm 0.52$kpc. This is in quite good agreement with the published luminosity distances of the cluster. We briefly discuss the problems concerned, including the sources of errors in the method and its applicable range.

Key Words:   open cluster--radial velocity--proper motion --luminosity distance--velocity distance
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Disk Thicknesses and Some Parameters of 108 Non-Edge-On Spiral Galaxies

Tao Hu1,2,3, Qiu-He Peng1,4,5 and Hong-Shi Zong2

1 Department of Astronomy, Nanjing University, Nanjing 210093; taohu@nju.edu.cn
2 Department of Physics, Nanjing University, Nanjing 210093
3 Department of Physics and Electronics, Zhangzhou Normal University, Zhangzhou 363000
4 Joint Astrophysics Center of Chinese Academy of Sciences-Peking University, Beijing 100871
5 Open Laboratory of Cosmic Ray and High Energy Astrophysics, Chinese Academy of Sciences, Beijing 100039

Abstract
We present disk thicknesses, some other parameters and their statistics of 108 non-edge-on spiral galaxies. The method for determining the disk thickness is based on solving Poisson's equation for a disturbance of matter density in three-dimensional spiral galaxies. From the spiral arms found we could obtain the pitch angles, the inclination of the galactic disk, and the position of the innermost point (the forbidden region with radius r0 to the galactic center) of the spiral arm, and finally the thickness.

Key Words:   galaxy: disk -- galaxies: fundamental parameters -- galaxies: spiral -- galaxies: structure
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Stellar and HI Mass Functions Predicted by a Simple Preheating Galaxy Formation Model

Zhi-Jian Luo1 and Cheng-Gang Shu2,3

1 Department of Physics, Jiangxi Normal University, Nanchang 330022; zjluo@center.shao.ac.cn
2 Joint Center for Astrophysics, Shanghai Normal University, Shanghai 200234
3 Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030

Abstract
According to the new preheating mechanism of galaxy formation suggested by Mo et al., we construct a simple model of formation of disk galaxies within the current paradigm of galaxy formation. It incorporates preheating, gas cooling, bulge formation and star formation. The predicted stellar and HI mass functions of galaxies are discussed and compared with the observations. It is found that our model can roughly match both the observed galaxy luminosity function and the observed HI-mass function.

Key Words:   galaxies: halos -- galaxies: formation -- galaxies: bulges -- galaxies: cooling flows
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The Multi-Wavelength Quasar Survey IV. Quasars in the Leo Cluster

Yang Chen1, Yu Bai1, Xiang-Tao He1, Jiang-Hua Wu2, Jing Wang2, Qing-Kang Li1, Yan-Chun Sun1, Richard F. Green3, and Wolfgang Voges4

1 Department of Astronomy, Beijing Normal University, Beijing 100875; cheny@bnu.edu.cn
2 National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012
3 Large Binocular Telescope Observatory, NOAO, Tucson, AZ85721, USA
4 Max-Planck-Institute für Extraterrestrische Physik, D-85740 Garching, Germany

Abstract
We aim to provide a quasar sample that is more complete than any previous surveys by using a combined selection technique to reduce the selection effects. Here we present the observational results for the X-ray candidates in the field of the Leo Cluster. We found 33 X-ray AGNs in this field of which 10 are new discoveries. The X-ray data and optical spectra of these AGNs are given. We also study the near-IR properties of the X-ray-selected AGNs by using the data from 2MASS. Most of the AGNs in our sample span the color range 0.0<B-J<2.5, 1.0<J-Ks<2.0 and 0.5<H-Ks<1.2.

Key Words:   galaxies: active: individual (Leo Cluster) -- X-rays: galaxies: quasars
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A Survey of Newtonian Core-Shell Systems with Pseudo High Order Symplectic Integrator and Fast Lyapunov Indicator

Jun-Fang Zhu, Xin Wu and Da-Zhu Ma

Department of Physics, Nanchang University, Nanchang 330031; xwu@ncu.edu.cn

Abstract
Newtonian core-shell systems, as limiting cases of relativistic core-shell models under the two conditions of weak field and slow motion, could account for massive circumstellar dust shells and rings around certain types of star remnants. Because this kind of systems have Hamiltonians that can be split into a main part and a small perturbing part, a good choice of the numerical tool is the pseudo 8th order symplectic integrator of Laskar & Robutel, and, to match the symplectic calculations, a good choice of chaos indicator is the fast Lyapunov indicator (FLI) with two nearby trajectories proposed by Wu, Huang & Zhang. Numerical results show that the FLI is very powerful when describing not only the transition from regular motion to chaos but also the global structure of the phase space of the system.

Key Words:   celestial mechanics -- chaos -- methods: numerical
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ChJAA, 2007, Vol.7, No.4

Chinese Journal of Astronomy and Astrophysics: Online Edition

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