A principal component analysis of the equivalent widths of spectral lines of star clusters

Using principal component analysis, we have studied the equivalent widths of optical spectral lines of 41 Galactic globular clusters and 22 young stellar clusters. We have discovered that some characteristic spectral lines, such as the CN, Ca IIK, Ca IIH and MgI+MgII lines, are rather sensitive to metallicity. We have also found some lines that may be used as indicators of age, e.g. Hδ, Hγ, Hβ and H. These lines can help us to disentangle the coupling effects of age and metallicity in the clusters.     

Analysis of high-resolution spectra of mildly metal-poor stars I. Spectral data and equivalent widths

A number of high resolution and high signal-to-noise ratio spectra of mildly metal-poor stars were obtained with the coudé echelle spectrograph attached to the 2.16m telescope at Xinglong Observation Base of Beijing Astronomical Observatory. The spectra of seven mildly metal-poor stars were processed with the MIDAS software package, and the equivalent widths of their spectral lines were measured. The measured equivalent widths together with an error analysis are presented.     

Propagation of disturbances in the solar chromosphere and corona (II)

The first part of this article presents an analytic discussion of the linear properties of magnetohydrodynamic (MHD) wave propagation. Then, with a 2-dimensional, time-dependent, compressible MHD simulation subject to a self-consistent non-isothermal, non-uniform initial state, we study numerically the global propagation process following an initial pressure pulse applied at the base of the chromosphere. Our numerical results indicate that, if the pulse is applied near the pole, there are two modes, one fast, one slow of magnetoacoustic waves; whereas if the pressure pulse is applied near the equator, there is a fast mode and a standing disturbance located near the source. These results may help interpret the wave events observed by SOHO/EIT.     

On the multiple jet flows in Jupiter''s atmosphere

Measurements by the Galileo probe in Jupiter's deep atmosphere support the possibility that the mean zonal multiple-jet flows in Jupiter's atmosphere are deep rooted. As a consequence of Jupiter's high rotation rate, the primary dynamics of the zonal flows must be geostrophic, i.e., the dynamic balance is largely between the Coriolis and pressure forces. This paper describes a new analytical theory for the generation of zonal multiple-jet flows on the basis of the nonlinear interaction of slowly traveling, nearly two-dimensional and non-axisymmetric geostrophic waves. An explicit analytical expression for the geostrophic waves is obtained as the leading-order solution of the weakly nonlinear problem. In the high-order problem taking into account of nonlinear effects, an analytical expression for an alternating multiple-jet flow is derived. Implications of the theory for Jupiter and other planets are discussed.     

Are gamma-ray bursts due to isotropic fireballs or cylindrical jets?

Afterglows from most gamma-ray bursts, such as GRBs 970228, 970508, 971214, 980329 and 980703, decay slowly and steadily. These bursts are widely believed to be due to isotropic fireballs. However, they in fact can also be well fitted by a cylindrical jet model. It is proposed that these bursts are probably generated by cylindrical jets, and not necessarily by isotropic fireballs.     

The infrared variability of QSO 3C 273

The near-infrared data of the quasar 3C273 are collected from available literature and are used to analyze its variability properties. The largest amplitudes of variations are ΔJ = 1.0, ΔH = 0.9, and ΔK = 0.86. The color indices are J − H = 0.82, J − K = 1.96, and H − K = 1.13. Analysis with the discrete correlation function (DCF) method indicates that the variability between any two infrared bands is correlated without any time lag. The relation between the color index and brightness is also investigated and the result indicates that the spectrum steepens when the source first dims, while it flattens after the source has dimmed to a some extent, suggesting that the emission of 3C 273 at near-IR consists of at least two components. Some discussions are presented.     

Polarization observations of four southern pulsars at 1560 MHz

We present some interesting results from the mean pulse polarization observations of four southern pulsars made at the Australian National Radio Astronomy Observatory, Parkes, using the 64-m telescope in June and July, 1988. The 2 × 16 × 5 MHz filter system from Jodrell Bank has proved excellent in de-dispersing the pulse signals and measuring their polarization properties. We give the data for the four pulsars in some detail and discuss their spectral behaviour.     

Magnetic fields at the surfaces of stars

Summary Magnetic fields have now been detected in stars in several parts of the Hertzsprung-Russell diagram. Roughly dipolar fields ranging in strength between 3× 10² and 3×10⁴ G are found in many chemically peculiar A and B main sequence stars. Dipolar fields are also found in some 2–3% of white dwarfs, but with strengths between 1×10⁶ and 5×10⁸ G. In both these types of stars, the observed fields vary as the underlying star turns, but do not change in a secular manner. In solartype stars, structurally complex fields of a few kG are found with filling factors of the order of 0.1 to 0.8. Further indirect evidence of fields in cool main sequence stars is provided by detection of visible and ultraviolet line emission (chromospheric activity), x radiation (coronal matter), and giant starspots. In this review, we survey the observations of stellar magnetism in all these types of stars, as well as efforts to model the observed magnetic fields and associated photospheric peculiarities and activity.This article was processed by the author using the Springer-Verlag T_EX AAR macro package 1991.     

Describing neutron stars by the relativistic mean field theory

Neutron stars are studied in the framework of the relativistic mean field theory of interacting nucleons, hyperons, and mesons. Within the hadronic freedom, the cores of neutron stars are found to be dominated by hyperons when the density is sufficiently high. The influence of hyperon coupling constants on the transition from a neutron star to a hyperon-dominated strange neutron star is also investigated. It is found that the transition density gets its minimum value when the ratio of hyperon coupling constant to nucleon's takes the value of 0.65, and the calculated maximum mass of the neutron star is 1.4 M which lies within the range of the observational results.     

A rope-shaped solar filament and a IIIb flare

On September 14–18, 2000, a medium-small solar active region was observed at Ganyu Station of Purple Mountain Observatory. Its spots were not large, but it had a peculiar active filament. On Sep.16, a flare of importance III_b with rather intense geophysical effects was produced. Our computation of the magnetic structure of the active region reveals that the rope-shaped filament was concerned with a low magnetic arc close to magnetic neutral line. An intense shear of magnetic field occurred near magnetic rope. The QSL analysis shows that a 3-D magnetic reconnection might appear in the vicinity of filament, and this can be used to interpret the formation of a large flare.