2003, Vol.3, No.4 (August 20, 2003)


Contents

Invited Review
  • Measuring the Fundamental Parameters of Hot Hydrogen-Rich White Dwarfs   p.287
    M. A. Barstow 
    Abstract | | PDF file (1070 KB) | PS file *.gz  (1831 KB) 
Letter
The Galaxy
Cosmology
  • A Check on the Cardassian Expansion Model with Type-Ia Supernovae Data   p.341
    Li Cao 
    Abstract | | PDF file (593 KB) | PS file *.gz  (190 KB) 
Extragalactic Astronomy
  • On the Spectral Index-Flux Density Relation for Large Samples of Radio Sources   p.347
    Xi-Zhen Zhang, W. Reich, P. Reich and R. Wielebinski
    Abstract | | PDF file (483 KB) | PS file *.gz  (487 KB) 
Stars
  • A Study of Neutron Star Structure in Strong Magnetic Fields that includes Anomalous Magnetic Moments  p.359
    Guang-Jun Mao, Akira Iwamoto and Zhu-Xia Li
    Abstract | | PDF file (256 KB) | PS file *.gz  (332 KB)  
The Sun
  • Near Real-Time Calculation of Ionospheric Electric Fields and Currents Using GEDAS   p.375
    H. Shirai, Y. Kamide, E. A. Kihn, B. Hausman, M. Shinohara, H. Nakata, M. Isowa, T. K. Takada, Y. Watanabe and S. Masuda
    Abstract | | PDF file (223 KB) | PS file *.gz  (320 KB) 
  • Solar Partial N-burst   p.381
    Zong-Jun Ning, Yu-Ying Liu, Qi-Jun Fu and Fu-Ying Xu   
    Abstract | PDF file (1070 KB) |  PS file *.gz  (469 KB) 


Abstract


Measuring the Fundamental Parameters of Hot Hydrogen-Rich White Dwarfs
 

M. A. Barstow


Affiliation:  Department of Physics and Astronomy, University of Leicester,
University Road, Leicester, LE1 7RH, UK

E-mail:    mab@star.le.ac.uk 

Abstract

This review considers the observations of hot, hydrogen-rich white dwarf stars, with particular reference to measurements of temperature, surface gravity and composition. Spectroscopic data from a variety of wavelength ranges are required for this work and, in particular, the important contributions from optical, ultraviolet and extreme ultraviolet studies are discussed. Using the values of Teff and log g determined for an individual white dwarf, estimates of mass and radius might be derived from the theoretical mass-radius relation. The issue of the accuracy of the theoretical mass-radius calculations and the prospects for making empirical tests using observational data are outlined.

Key words: stars: white dwarfs --- ultraviolet --- spectroscopy --- mass --- radius

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Discovery of Four Peculiar Variable Stars   

Xiao-Bin Zhang (1,2) and Rong-Xian Zhang (1)

Affiliation:  (1) National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012
             (2) CAS-PKU Joint Beijing Astrophysics Center, Beijing 100871

E-mail:   xzhang@bao.ac.cn     (corresponding author)

Abstract

Four stars, GSC 1258--0143, GSC 1986--1665, GSC 3045--0892 and GSC 2983--1597 were found to be new variables during a campaign of CCD photometric monitoring of short-period eclipsing binary stars. The variabilities of these new variables are reported. The main characteristics and probable classifications of the variables are discussed through a preliminary analysis on their light curves. Among these stars, GSC 1258--0143 is classified as a RR Lyr star of sub-type a. Its pulsation period is determined to be about 0.5206 days. The other three stars, are definite variables, but their periods and types of light variations remain unknown.

Key words: techniques: photometric --- stars: variables: RR Lyr --- stars: individual: GSC 1258--0143, GSC 1986--1665, GSC 3045--0892, GSC 2983--1597

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Non-LTE Analysis of the Sodium Abundance of Metal-Poor Stars in the Galactic Disk and Halo

Yoichi Takeda (1), Gang Zhao (2), Masahide Takada-Hidai (3), Yu-Qin Chen (2),
Yu-ji Saito
(4) and Hua-Wei Zhang (5,6) 

Affiliation:   (1) Komazawa University, Komazawa, Setagaya, Tokyo 154-8525, Japan
              (2) National Astronomical Observatories, CAS (NAOC), Beijing 100012, China
              (3) Liberal Arts Education Center, Tokai University, Hiratsuka, Kanagawa 259-1292, Japan
              (4) Department of Physics, Faculty of Science, Tokai University, Hiratsuka, Kanagawa 259-1292, Japan
              (5) Department of Astronomy, School of Physics, Peking University, Beijing 100871
              (6) CAS-Peking University Joint Beijing Astrophysical Center, Beijing 100871

E-mail:   takedayi@cc.nao.ac.jp (corresponding author)  

Abstract

We performed an extensive non-LTE analysis of the neutral sodium lines of Na I 5683/5688, 5890/5896, 6154/6161, and 8183/8195 in disk/halo stars of types F--K covering a wide metallicity range (4 $\la$[Fe/H]$\lsim$ +0.4), using our own data as well as data collected from the literature. For comparatively metal-rich disk stars (1 $\la$[Fe/H]$\la$ +0.4) where the weaker 6154/6161 lines are the best abundance indicators, we confirmed [Na/Fe] $\sim$ 0 with an ``upturn'' (i.e., a shallow/broad dip around 0.5 $\lsim$ [Fe/H] $\lsim$ 0) as already reported in previous studies. For the metal-deficient halo stars, where the much stronger 5890/5896 or 8183/8195 lines subject to considerable (negative) non-LTE corrections amounting to 0.5 dex have to be used, our analysis suggests mildly ``subsolar'' [Na/Fe] values down to  $\sim$ 0.4 (with a somewhat large scatter of $\sim$±0.2dex) on the average at the typical halo metallicity of [Fe/H] $\sim$  2, followed by a rise again to a near-solar ratio of [Na/Fe] $\sim$ 0 at the very metal-poor regime [Fe/H]  $\sim$  4. These results are discussed in comparison with the previous observational studies along with the theoretical predictions from the available chemical evolution models.

Key words: Galaxy: evolution --- radiative transfer --- stars: abundances --- stars: atmospheres --- stars: late-type

PDF file (511 KB) | gzipped PS file (1103 KB) | Table E1 , Table E2 , Readme  | Back to Contents


A Check on the Cardassian Expansion Model with Type-Ia Supernovae Data

Li Cao

Affiliation: National Astronomical Observatories, Chinese Academy of Sciences,
Beijing 100012
                          
E-mail:   caoli@bao.ac.cn 

Abstract

We use the magnitude-redshift relation for the type Ia supernova data compiled by Riess et al. to analyze the Cardassian expansion scenario. This scenario assumes the universe to be flat, matter dominated, and accelerating, but contains no vacuum contribution. The best fitting model parameters are H0 = 65.3 ksm, n =0.35 and $\Omega_m$=0.05. When the highest redshift supernova, SN 1997ck, is excluded, H0 remains the same, but n becomes 0.20 and $\Omega_m$, 0.15, and the matter density remains unreasonably low. Our result shows that this particular scenario is strongly disfavoured by the SNeIa data.

Key words: supernovae: general --- cosmology: theory --- distances and redshifts

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On the Spectral Index-Flux Density Relation for Large Samples of Radio Sources

Xi-Zhen Zhang
(1), W. Reich (2), P. Reich (2) and R. Wielebinski (2)

Affiliation:   (1) National Astronomical Observatories, CAS (NAOC), Beijing 100012, China
              (2) Max-Planck-Institut f\"ur Radioastronomie, Auf dem H\"ugel 69, D-53121 Bonn, Germany

E-mail:   zxz@bao.ac.cn  (coressponding author)
  
Abstract

We present new statistical results on the spectral index--flux density relation for large samples of radio sources using archival data of the most sensitive surveys, such as 6C, Miyun, WENSS, B3, NVSS, GB87. Instrumental selection effects and the completeness of the catalogs are discussed. Based on the spectral indices calculated for about 200000 sources from the WENSS (327 MHz) and NVSS (1.4 GHz) catalogs, we obtained (1) The median spectral index increases from $\rm\alpha_{\rm med} \sim -0.9 $ to $\rm\alpha_{\rmmed}\sim -0.8 $ ($ S_\nu \propto \nu^{\alpha}$), while S327 flux densities decrease from 0.1 Jy down to 25 mJy. The median spectral indices nearly show no variation within the error bars when the flux density is larger than 0.1 Jy. (2) A dependence of the fraction of ultra-steep spectrum sources (USS, $\rm -1.5 \le \alpha < -1.0 $), steep spectrum sources (SSS, $\rm -1.0 \le \alpha <-0.5 $) and flat spectrum sources (FSS, $\rm -0.5 \le \alpha \le 0.0 $) is partly responsible for the spectral flattening. Another contribution to the spectral flattening comes from the variation of $\alpha_{\rm med}$ of steep spectrum sources ($\alpha < -0.5$) themselves which increases with decreasing flux densities. (3) The spectral flattening for faint sources (down to S327 $\sim$ 20 mJy) with steep spectra ($\alpha < -0.5$) suggests that $\alpha_{\rm med}$ is correlated with luminosity rather than redshift according to the Condon' model. (4) A strong spectral selection effect occurs when spectral indices are calculated from samples with a large frequency separation.

Key words: radio continuum: general --- methods: statistical --- cosmology

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A Study of Neutron Star Structure in Strong Magnetic Fields that includes Anomalous Magnetic Moments

Guang-Jun Mao (1,2), Akira Iwamoto (1) and Zhu-Xia Li (3)


Affiliation:   (1) Japan Atomic Energy Research Institute, Tokai,
Naka, Ibaraki 319-1195, Japan
              (2) Institute of High Energy Physics, Chinese Academy of Science, Beijing 100039
              (3) China Institute of Atomic Energy, Beijing 102413
                         
E-mail:   maogj@mail.ihep.ac.cn   (corresponding author)

Abstract

We study the effect of strong magnetic fields on the structure of neutron star. We find that if the interior field is on the same order as the surface field currently observed, then the influences of the field on the star's mass and radius are negligible; if the field is as large as that estimated from the scalar virial theorem, then considerable effects will be induced. The maximum mass of the star will be increased substantially while the central density is greatly reduced. The radius of a magnetic star can be larger by about 10% $\sim$ 20% than a nonmagnetic star of the same mass.

Key words: stars: neutron stars --- stars: magnetic fields --- equation of state

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Near Real-Time Calculation of Ionospheric Electric Fields and Currents Using GEDAS

H. Shirai (1,2), Y. Kamide (1), E. A. Kihn (3), B. Hausman (4), M. Shinohara (1,5), H. Nakata (1,6), M. Isowa (1), T. K. Takada (1), Y. Watanabe (1), and S. Masuda (1)

Affiliation:    (1) Solar-Terrestrial Environment Laboratory, Nagoya University,
Toyokawa, Aichi 442-8507, Japan
              (2) Now at Ichinoseki National College of Technology, Ichinoseki,
Iwate 021-8511, Japan
              (3) NOAA National Geophysical Data Center, Boulder, Colorado 80303-3328, U.S.A.
              (4) Department of Physics and Astronomy, Rice University,
Houston, Texas 77005-1892, U.S.A.
              (5) Now at Communications Research Laboratory, Koganei, Tokyo 184-8795, Japan
              (6) Now at Chiba University, Chiba, Chiba 263-8522, Japan
                    
E-mail:      (corresponding author)

Abstract

This paper presents the recent progress in our project of estimating near real-time electric fields and currents in the ionosphere through our computer system called the Geospace Environment Data Analysis System (GEDAS). We show a new technique in which data from ground magnetometers are collected by the system and used as input for the KRM and AMIE programs to calculate the distribution of ionospheric electric fields and currents, as well as of other ionospheric parameters, such as electric potential patterns. One of the goals of this project is to specify ionospheric processes. Examples of the near real-time calculation and the data flow of our scheme are presented.

Key words: data analysis --- solar-terrestrial relations

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Solar Partial N-burst

Zong-Jun Ning (1), Yu-Ying Liu (1), Qi-Jun Fu (1) and Fu-Ying Xu (2)

Affiliation:  (1) National Astronomical Observatories, CAS (NAOC), Beijing 100012, China
              (2) Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing
210008
                    
E-mail:   (corresponding author)

Abstract

We present a new sub-class of type III solar radio burst at the high frequencies around 6.0 GHz. In addition to a descending and an ascending branch on the dynamic spectrum, it has an inverted morphology different from the simple type U-burst. We call it ``partial N-burst'' because it is interpreted as the known N-burst minus its first branch. The partial N-burst presented here was detected among a reverse slope type III (RS-III) burst group prior to the type V solar radio continuum and was simultaneously recorded by two spectrometers at the National Astronomical Observatories, Chinese Academy of Sciences (NAOC, 5.207.60 GHz) and at Purple Mountain Observatory (PMO, 4.507.50 GHz) on 1999 August 25. After the N-burst and M-burst, the partial N-burst is a third piece of evidence for a magnetic mirror effect in solar radio observation, when the same electron is reflected at a pinched foot of a flare loop.

Key words: Sun: radio radiation --- Sun: magnetic fields

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ChJAA, 2003, Vol.3, No.4

Chinese Journal of Astronomy and Astrophysics: Online Edition

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