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- The Korean Astronomical Society (The Korean Astronomical Society)
- 계간 (Quarterly)
- ISSN : 1225-4614 (ISSN : 1225-4614)
1225-4614
- DB구축현황 : 1,211건 (DB Construction : 1,211 Articles)
총 게시글 1,211건
페이지 53/122
521
- CHEN YANG
- Journal of the Korean astronomical society = 천문학회지
- 38, n.2
- pp.211-214
- 2005
- 원문 바로보기
We performed a spatially resolved spectroscopic study of the thermal composite supernova remnant 3C 391 by the Chandra observation. Broad- and narrow-band X-ray images show a southeast-northwest elongated morphology and unveil a highly clumpy structure of the remnant. The spectral analysis for. the small-scale features indicates normal metal abundance and uniform temperature for the interior gas. The properties of the hot gas are largely in agreement with the cloudlet evaporation model as a main mechanism for the 'thermal composite' X-ray appearance, though radiative rim and thermal conduction may also be effective. An unresolved X-ray source, with a power-law spectrum, is observed on the northwest border. The equivalent width images reveal a faint finger-like protrusion in Si and S lines out of the southwest radio border.
522
- GALAZUTDINOV GAZINUR
- Journal of the Korean astronomical society = 천문학회지
- 38, n.2
- pp.215-218
- 2005
- 원문 바로보기
An atlas of high resolution (<TEX>${\lambda}/{\Delta}{\lambda}$</TEX>=45,000) profiles of interstellar atomic lines of K I (7665, 7699 <TEX>${\AA}$</TEX>), Na I (D 1, D2), Ca II (H, K), Ca I (4227 <TEX>${\AA}$</TEX>), molecular structures of CH, CH+, CN and the major diffuse interstellar bands at 5780 and 5797 <TEX>${\AA}$</TEX> based on <TEX>${\~}$</TEX>300 echelle spectra of <TEX>${\~}$</TEX>200 OB stars is presented. Relationships between the reddenings, distances and equivalent widths of NaI, CaII, KI, CH, CH+, CN and diffuse bands are discussed. The equivalent width of K I (7699 <TEX>${\AA}$</TEX>) as well as of CH4300 <TEX>${\AA}$</TEX> / correlate very tightly with E(B- V) in contrast to the features of neutral sodium, ionized calcium and the molecular ion CH+. The equivalent widths of the Hand K lines of Call grow with distance at a rate <TEX>${\~}$</TEX>250m<TEX>${\AA}$</TEX> per 1 kpc. A similar relation for NaI is much less tight. The strengths of neutral potassium lines, molecular features and diffuse interstellar bands do not correlate practically with distance. These facts suggest that ionized calcium fills the interstellar space quite homogeneously while the other carriers mentioned above, especially K I, CH and these of diffuse bands occupy more and more compact volumes, also filled with dust grains. Apparently the carriers of narrow diffuse bands are spatially correlated with simple molecules and dust grains - all abundant in the so-called 'zeta' type clouds. The same environment seems to be hostile to the carriers of broad diffuse interstellar bands (DIEs) (like 5780 or 6284) and -to a certain extent - also to CaII, NaI and CH+.
523
- VALYAVIN G.
- Journal of the Korean astronomical society = 천문학회지
- 38, n.2
- pp.283-287
- 2005
- 원문 바로보기
The slow evolution of global magnetic fields and other dynamical processes in atmospheres of CP magnetic stars lead to the development of induced electric currents in all conductive atmospheric layers. The Lorentz force, which results from the interaction between a magnetic field and the induced currents, may modify the atmospheric structure and provide insight into the formation and evolution of stellar magnetic fields. This modification of the pressure-temperature structure influences the formation of absorption spectral features producing characteristic rotational variability of some spectral lines, especially the Balmer lines (Valyavin et al., 2004 and references therein). In order to study these theoretical predictions we began systematic spectroscopic survey of Balmer line variability in spectra of brightest CP magnetic stars. Here we present the first results of the program. A0p star <TEX>$\Theta$</TEX> Aur revealed significant variability of the Balmer profiles during the star's rotation. Character of this variablity corresponds to that classified by Kroll (1989) as a result of an impact of significant Lorentz force. From the obtained data we estimate that amplitudes of the variation at H<TEX>$\alpha$</TEX>, H<TEX>$\beta$</TEX>, H<TEX>$\gamma$</TEX> and H<TEX>$\delta$</TEX> profiles reach up to <TEX>$2.4\%$</TEX>during full rotation cycle of the star. Using computation of our model atmospheres (Valyavin et al., 2004) we interpret these data within the framework of the simplest model of the evolution of global magnetic fields in chemically peculiar stars. Assuming that the field is represented by a dipole, we estimate the characteristic e.m.f. induced by the field decay electric current (and the Lorentz force as the result) on the order of <TEX>$E {\~} 10^{-11}$</TEX> cgs units, which may indicate very fast (< < <TEX>$10^{10}$</TEX> years) evolution rate of the field. This result strongly contradicts the theoretical point of view that global stellar magnetic fields of CP stars are fossil and their the characteristic decay time of about <TEX>$10^{10}$</TEX> yr. Alternatively, we briefly discuss concurring effects (like the ambipolar diffusion) which may also lead to significant atmospheric currents producing the observable Lorentz force.
524
- KAWASAKI WATARU
- Journal of the Korean astronomical society = 천문학회지
- 38, n.2
- pp.141-144
- 2005
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Based on optical galaxy data, we executed a systematic search for galaxy clusters around the 15 steady unidentified EGRET GeV gamma-ray sources in high Galactic-latitude sky ([b] > <TEX>$30^{\circ}$</TEX>). We found a strong correlation with 3.7<TEX>$\sigma$</TEX> level between close cluster pairs (merging cluster candidates) and the unidentified EGRET sources, though, in contrast, no correlation with single clusters. This result implies that merging clusters of galaxies are a possible candidate for the origin of high galactic-latitude, steady unidentified EGRET gamma-ray sources.
525
- KANG HYESUNG
- Journal of the Korean astronomical society = 천문학회지
- 37, n.4
- pp.225-232
- 2004
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Shocks are ubiquitous in astrophysical environments and cosmic-rays (CRs) are known to be accelerated at collisionless shocks via diffusive shock acceleration. It is believed that the CR pressure is important in the evolution of the interstellar medium of our galaxy and most of galactic CRs with energies up to <TEX>${\~}\;10^{15}$</TEX> eV are accelerated by supernova remnant shocks. In this contribution we have studied the CR acceleration at shocks through numerical simulation of 1D, quasi-parallel shocks for a wide range of shock Mach numbers and shock speeds. We show that CR modified shocks evolve to time-asymptotic states by the time injected particles are accelerated to moderately relativistic energies, and that two shocks with the same Mach number, but with different shock speeds, evolve qualitatively similarly when the results are presented in terms of a characteristic diffusion length and diffusion time. We find that <TEX>$10^{-4} - 10^{-3}$</TEX> of the particles passed through the shock are accelerated to form the CR population, and the injection rate is higher for shocks with higher Mach number. The time asymptotic value for the CR acceleration efficiency is controlled mainly by shock Mach number, and high Mach number shocks all evolve towards efficiencies <TEX>${\~}50\%$</TEX>, regardless of the injection rate and upstream CR pressure. We conclude that the injection rates in strong quasi-parallel shocks are sufficient to lead to significant nonlinear modifications to the shock structures, implying the importance of the CR acceleration at astrophysical shocks.
526
- LAZARIAN A.
- Journal of the Korean astronomical society = 천문학회지
- 37, n.5
- pp.563-570
- 2004
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Turbulence is a crucial component of dynamics of astrophysical fluids dynamics, including those of ISM, clusters of galaxies and circumstellar regions. Doppler shifted spectral lines provide a unique source of information on turbulent velocities. We discuss Velocity-Channel Analysis (VCA) and its offspring Velocity Coordinate Spectrum (VCS) that are based on the analytical description of the spectral line statistics. Those techniques are well suited for studies of supersonic turbulence. We stress that a great advantage of VCS is that it does not necessary require good spatial resolution. Addressing the studies of mildly supersonic and subsonic turbulence we discuss the criterion that allows to determine whether Velocity Centroids are dominated by density or velocity. We briefly discuss ways of going beyond power spectra by using of higher order correlations as well as genus analysis. We outline the relation between Spectral Correlation Functions and the statistics available through VCA and VCS.
527
- ALAWY A. EL-BASSUNY
- Journal of the Korean astronomical society = 천문학회지
- 37, n.3
- pp.119-129
- 2004
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A new method has been developed to solve the star cluster membership problem. It is based on synthetic photometry employing the Black Body concept as stellar radiation simulator. Synthetic color-magnitude diagram is constructed showing the main sequence band and the positions of binary star systems of combinations of various components through different photometric tracks. The method has been applied to the Hyades. The cluster membership problem has been re-appraised for the cluster (both single and binary) stars. For the binary members, the components' spectral types have been derived by the method. The results obtained agree very well with those found in literature, The method is simpler than the others and can be developed to undertake other cases as multiple star systems.
528
- KIM YEON-HAN
- Journal of the Korean astronomical society = 천문학회지
- 37, n.4
- pp.171-177
- 2004
- 원문 바로보기
X-ray plasma ejections often occurred around the impulsive phases of solar flares and have been well observed by the SXT aboard Yohkoh. Though the X-ray plasma ejections show various morphological shapes, there has been no attempt at classifying the morphological groups for a large sample of the X-ray plasma ejections. In this study, we have classified 137 X-ray plasma ejections according to their shape for the first time. Our classification criteria are as follows: (1) a loop type shows ejecting plasma with the shape of loops, (2) a spray type has a continuous stream of plasma without showing any typical shape, (3) a jet type shows collimated motions of plasma, (4) a confined ejection shows limited motions of plasma near a flaring site. As a result, we classified the flare-associated X-ray plasma ejections into five groups as follows: loop-type (60 events), spray-type (40 events), jet-type (11 events), confined ejection (18 events), and others (8 events). As an illustration, we presented time sequence images of several typical events to discuss their morphological characteristics, speed, CME association, and magnetic field configuration. We found that the jet-type events tend to have higher speeds and better association with CMEs than those of the loop-type events. It is also found that the CME association (11/11) of the jet-type events is much higher than that (5/18) of the confined ejections. These facts imply that the physical characteristics of the X-ray plasma ejections are closely associated with magnetic field configurations near the reconnection regions.
529
- HENRIKSEN MARK
- Journal of the Korean astronomical society = 천문학회지
- 37, n.5
- pp.299-305
- 2004
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We find evidence of a hard X-ray excess above the thermal emission in two cool clusters (Abell 1750 and IC 1262) and a soft excess in two hot clusters (Abell 754 and Abell 2163). Our modeling shows that the excess components in Abell 1750, IC 1262, and Abell 2163 are best fit by a steep power law indicative of a significant non-thermal component. In the case of Abell 754, the excess emission is thermal, 1 ke V emission. We analyze the dynamical state of each cluster and find evidence of an ongoing or recent merger in all four clusters. In the case of Abell 2163, the detected, steep spectrum, non-thermal X-ray emission is shown to be associated with the weak merger shock seen in the temperature map. However, this shock is not able to produce the flatter spectrum radio halo which we attribute to post-shock turbulence. In Abell 1750 and IC 1262, the shocked gas appears to be spatially correlated with non-thermal emission suggesting cosmic-ray acceleration at the shock front.
530
- KIM YOONBAI
- Journal of the Korean astronomical society = 천문학회지
- 37, n.1
- pp.1-14
- 2004
- 원문 바로보기
Basic idea of Randall-Sundrum brane world model I and II is reviewed with detailed calculation. After introducing the brane world metric with exponential warp factor, metrics of Randall-Sundrum models are constructed. We explain how Randall-Sundrum model I with two branes makes the gauge hierarchy problem much milder, and derive Newtonian gravity in Randall-Sundrum model II with a single brane by considering small fluctuations.