| 2002,
Vol.2, No.3 (June 20, 2002)
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Letter
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Extragalactic astronomy
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Stellar cluster
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The Galaxy
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Stars
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The Sun
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IRAS 01005+7910: a High Galactic Latitude Post-AGB Star Jing-Yao Hu E-mail: hjy@bao.ac.cn
Abstract |
Positions and Spectral Energy Distributions of 41 Star Clusters in M33 Jun Ma,Xu Zhou,Jian-Sheng Chen,Hong Wu,Zhao-Ji Jiang,Sui-Jian Xue and Jin Zhu E-mail: majun@vega.bac.pku.edu.cn Abstract |
The Variability of Hardness Ratio 1 observed by ROSAT: Narrow-Line Seyfert 1 Galaxies versus Broad-Line Seyfert 1 Galaxies Lin-Peng Cheng, Jian-Yan Wei and Yong-Heng Zhao E-mail: clp@lamost.bao.ac.cn
Abstract |
| Determination of the Proper Motions and Membership of the Globular Cluster M3 and of its Orbit in the Galaxy Zhen-Yu Wu, Jia-Ji Wang and Li Chen E-mail: wuzy@center.shao.ac.cn Abstract We examine 14 plates of the globular cluster M3 (NGC 5272) taken with the 40 cm refractor at the Sheshan station of Shanghai Astronomical Observatory. The plates span over a period of about 77 years. The positions and absolute proper motions of eight stars in the Hipparcos Catalogue and of 49 stars in the Tycho-2 Catalogue are used as the reference frame. The astrometric reduction is made with the central overlapping principle. The absolute proper motions of 534 stars in a region of about 100'×100' around the cluster are measured. With the new proper motion data the membership probabilities of the stars are determined. The average absolute proper motion obtained for the cluster is -0.06±0.30 mas yr-1 in R.A. and -2.6±0.30 mas yr-1 in Decl. By combining this result with the known distance and radial velocity of the cluster, we also obtained the Galactic orbit of M3 for a chosen three-component Galactic potential. PDF file (302 KB) | gzipped PS file (456 KB) | Back to Contents |
History of Star Formation and Chemical Enrichment in the Milky Way Disk Rui-Xiang Chang,Cheng-Gang Shu and Jin-Liang Hou E-mail: crx@center.shao.ac.cn Abstract Based on a physical treatment of the star formation law similar to that given by Efstathiou, we have improved our two-component chemical evolution model for the Milky Way disk. Two gas infall rates are compared, one exponential, one Gaussian. It is shown that the star formation law adopted in this paper depends more strongly on the gas surface density than that in Chang et al. It has large effects on the history of star formation and gas evolution of the whole disk. In the solar neighborhood, the history of chemical evolution and star formation is not sensitive to whether the infall rate is Gaussian or exponential. For the same infall time scale, both forms predict the same behavior for the current properties of the Galactic disk. The model predictions do depend on whether or not the infall time scale varies with the radius, but current available observations cannot decide which case is the more realistic. Our results also show that it would be inadequate to describe the gradient evolution along the Galactic disk by only one word ``flatter'' or ``steeper'', as was suggested by Hou et al. and Chiapinni et al. We point out that both the absolute value and the evolution of the abundance gradient may be different in the inner and outer regions. PDF file (378 KB) | gzipped PS file (673 KB) | Back to Contents |
An Exact Anisotropic Quark Star Model M. K. Mak and T. Harko E-mail: mkmak@vtc.edu.hk Abstract We present an exact analytical solution of the gravitational field equations describing a static spherically symmetric anisotropic quark matter distribution. The radial pressure inside the star is assumed to obey a linear equation of state, while the tangential pressure is a complicated function of the radial coordinate. In order to obtain the general solution of the field equations a particular density profile inside the star is also assumed. The anisotropic pressure distribution leads to an increase in the maximum radius and mass of the quark star, which in the present model is around three solar masses. PDF file (204 KB) | gzipped PS file (159 KB)| Back to Contents |
Near-Infrared Imaging Observations of the Orion A-W Star Forming Region Jun-Jie Wang E-mail: wangjj@bao.ac.cn Abstract We carried out near-infrared imaging observations of the Ori A-W region using the Italian 1.5 m TIRGO infrared telescope at Gornergrat. A group of infrared objects is visible on the K band image, including an IRAS source (IRS 1). From its IRAS flux density the IRAS luminosity is derived to be 45  , which shows that
IRS 1 is a low-mass protostar. By superimposing the position of
the VLA H2O maser on the K image, we can identify the less evolved
object IRS 1 as the excitation source of the H2O maser,
within a
projected distance of 470 AU. This would be evidence that the maser effect is associated with the youngest phase of stellar evolution.
The first probable HH object candidate in the Ori A-W region is discovered from the
H2O S(1) 1
 0 observation. Comparing the
position of the H2O maser with the direction of the molecular
hydrogen emission in the region, we suggest that the observed
H2O maser could be tracing the circumstellar disk of
IRS 1.PDF file (585 KB) | gzipped PS file (510 KB) | Back to Contents |
Magnetic Field Strengths and Structures from Radio Observations of Solar Active Regions Chang-Xi Zhang, G. B. Gelfreikh and Jing-Xiu Wang E-mail: zhcx@vega.bac.pku.edu.cn Abstract Radio observations of some active regions (ARs) obtained with the Nobeyama radioheliograph at  = 1.76 cm
are used for estimating the magnetic field strength in the upper
chromosphere, based on thermal bremsstrahlung. The results are compared
with the magnetic field strength in the photosphere from observations
with the Solar
Magnetic Field Telescope (SMFT) at Huairou Solar Observing Station of
Beijing Astronomical Observatory. The difference in the magnetic field
strength between the two layers seems reasonable. The solar radio maps
of active regions obtained with the Nobeyama radioheliograph, both in
total intensity (I-map) and in circular
polarizations (V-map), are compared with the optical magnetograms
obtained with the SMFT. The comparison between the radio map in circular
polarization and the longitudinal photospheric magnetogram of a plage
region suggests that the radio map in circular polarization is a kind of
magnetogram of the upper chromosphere. The
comparison of the radio map in total intensity with the photospheric
vector magnetogram of an AR shows that the radio map in total intensity
gives indications of magnetic loops in the corona, thus we have a method
of defining the coronal magnetic structure from the radio I-maps
at
= 1.76 cm.
Analysing the I-maps, weidentified three components: (a) a compact bright source; (b) a narrow elongated structure connecting two main magnetic islands of opposite polarities (observed in both the optical and radio magnetograms); (c) a wide, diffuse, weak component that corresponds to a wide structure in the solar active region which shows in most cases an S or a reversed S contour, which is probably due to the differential rotation of the Sun. The last two components suggest coronal loops on different spatial scales above the neutral line of the longitudinal photospheric magnetic field. PDF file (486 KB) | gzipped PS file (503 KB) | Back to Contents |
Line Broadening in a Limb Flare: Derivation of Macro-turbulent Velocity Fields Ying Liu and Ming-De Ding E-mail: dmd@nju.edu.cn Abstract The line profiles of H 
in a limb flare on 1998 November 11 appear to be unusually broadened. It is considered that
macro-turbulence (or macroscopic mass motions) may be one of the main causes. We use an inversion technique to extract the probability
distribution of the line-of-sight velocity in the flare. There exist some differences between the velocity distributions deduced from
H
and from Ca II
8542, which may be because
the two lines depend differently on the temperature and velocity. Since the loop density is high, we obtain a rather short cooling time
(several tens of seconds) from the hot X-ray loops to the cool loops visible in
H. Possible origins of the large scale motions are discussed.PDF file (236 KB) | gzipped PS file (396 KB) | Back to Contents |
An Electron Density Model above the Sunspot from a Mapping of NOAA 7260 at 17 GHz Xing-Feng Yu and Jin-Xing Yao E-mail: pmoyxf@263.net Abstract The brightness temperature distribution of microwave emission in a solar active region generally shows a ring structure, with a dip at the centre. However, no dip was found in the Nobeyama Radioheliograph left handed circular polarization (LCP) image on 1992 August 18; instead, there was a peak. This is a completely LCP source with zero right-handed circular polarization (RCP). We examine this structure in terms of the joint effect of gyroresonance and bremsstrahlung mechanism with a raised electron density above the central part of the sunspot, and the commonly assumed temperature and vertical dipole magnetic field models. The raised electron density is found to be 1.4×1011 cm-3 at the chromosphere base. PDF file (167 KB) | gzipped PS file (193 KB) | Back to Contents |
V colors
and find that most of them are blue, which is consistent with previous findings.