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- The Korean Astronomical Society (The Korean Astronomical Society)
- 계간 (Quarterly)
- ISSN : 1225-4614 (ISSN : 1225-4614)
1225-4614
- DB구축현황 : 1,227건 (DB Construction : 1,227 Articles)
총 게시글 1,227건
페이지 69/123
681
- YIM KI-JEONG
- Journal of the Korean astronomical society = 천문학회지
- 35, n.2
- pp.75-85
- 2002
- 원문 바로보기
We present N-body simulations of globular clusters including gravitational field of the Galaxy, in order to study effects of tidal field systematically on the shape of outer parts of globular clusters using NBODY6. The Galaxy is assumed to be composed of central bulge and outer halo. We mvestigate the cluster of multi-mass models with a power-law initial mass function (IMF) starting with different initial masses, initial number of particles, different slopes of the IMF and different orbits of the cluster. We have examined the general evolution of the clusters, the shape of outer parts of the clusters, density profiles and the direction of tidal tails. The density profiles appear to become somewhat shallower just outside the tidal boundary consistent with some observed data. The position angle of the tidal tall depends on the location in the Galaxy as well as the direction of the motion of. clusters. We found that the clusters become more elongated at the apogalacticon than at the pengalacticon. The tidal tails may be used to trace the orbital paths of globular clusters.
682
- LEE YOUNGUNG
- Journal of the Korean astronomical society = 천문학회지
- 35, n.2
- pp.97-103
- 2002
- 원문 바로보기
We have mapped 1 <TEX>$deg^2$</TEX> region toward a high latitude cloud MBM 40 in the J = 1 - 0 transition of <TEX>$^{12}CO$</TEX> and <TEX>$^{13}CO$</TEX>, using the 3 mm SIS receiver on the 14 m telescope at Taeduk Radio Astronomy Observatory. We used a high resolution autocorrelator to resolve extremely narrow CO linewidths of the molecular gas. Though the linewidth of the molecular gas is very narrow (FWHP < 1 km <TEX>$s^{-1}$</TEX> ), it is found that there is an evident velocity difference between the middle upper part and the lower part of the cloud. Their spectra for both of <TEX>$^{12}CO$</TEX> and <TEX>$^{13}CO$</TEX> show blue wings, and the position-velocity map shows clear velocity difference of 0.4 km <TEX>$s^{-1}$</TEX> between two parts. The mean velocity of the cloud is 3.1 km <TEX>$s^{-1}$</TEX>. It is also found that the linewidths at the blueshifted region are broader than those of the rest of the cloud. We confirmed that the visual extinction is less than 3 magnitude, and the molecular gas is translucent. We discussed three mass estimates, and took a mass of 17 solar masses from CO integrated intensity using a conversion factor <TEX>$2.3 {\times} 10^{20} cm^{-2} (K\;km s^{-1})^{-1}$</TEX>. Spatial coincidence and close morphological similarity is found between the CO emission and dust far-infrared (FIR) emission. The ratio between the 100 f.Lm intensity and CO integrated intensity of MBM 40 is 0.7 (MJy/sr)/(K km <TEX>$s^{-1}$</TEX>), which is larger than those of dark clouds, but much smaller than those of GMCs. The low ratio found for MBM 40 probably results from the absence of internal heating sources, or significant nearby external heating sources.
683
- KANG HYESUNG
- Journal of the Korean astronomical society = 천문학회지
- 35, n.4
- pp.159-174
- 2002
- 원문 바로보기
Cosmological hydrodynamic simulations of large scale structure in the universe have shown that accretion shocks and merger shocks form due to flow motions associated with the gravitational collapse of nonlinear structures. Estimated speed and curvature radius of these shocks could be as large as a few 1000 km/s and several Mpc, respectively. According to the diffusive shock acceleration theory, populations of cosmic-ray particles can be injected and accelerated to very high energy by astrophysical shocks in tenuous plasmas. In order to explore the cosmic ray acceleration at the cosmic shocks, we have performed nonlinear numerical simulations of cosmic ray (CR) modified shocks with the newly developed CRASH (Cosmic Ray Amr SHock) numerical code. We adopted the Bohm diffusion model for CRs, based on the hypothesis that strong Alfven waves are self-generated by streaming CRs. The shock formation simulation includes a plasma-physics-based 'injection' model that transfers a small proportion of the thermal proton flux through the shock into low energy CRs for acceleration there. We found that, for strong accretion shocks, CRs can absorb most of shock kinetic energy and the accretion shock speed is reduced up to <TEX>$20\%$</TEX>, compared to pure gas dynamic shocks. For merger shocks with small Mach numbers, however, the energy transfer to CRs is only about <TEX>$10-20\%$</TEX> with an associated CR particle fraction of <TEX>$10^{-3}$</TEX>. Nonlinear feedback due to the CR pressure is insignificant in the latter shocks. Although detailed results depend on models for the particle diffusion and injection, these calculations show that cosmic shocks in large scale structure could provide acceleration sites of extragalactic cosmic rays of the highest energy.
684
- KANG YaNG-WOO
- Journal of the Korean astronomical society = 천문학회지
- 35, n.2
- pp.87-95
- 2002
- 원문 바로보기
We have conducted VI CCD photometry of the open cluster NGC 6819 in order to understand the effects of dynamical evolution in old open clusters. Our photometry covers 18' <TEX>$\times$</TEX> 18' on the sky, centered on the cluster, which seems to cover the whole cluster field. Our photometry reaches down to V <TEX>$\approx$</TEX> 20.5, which allows us to analyze the luminosity function and spatial distribution of stars brighter than Mv <TEX>$\approx$</TEX> 8.5. There is a clear evidence for mass segregation in NGC 6819, i.e., the giants and upper main-sequence stars are concentrated in the inner regions, whereas the lower main-sequence stars distribute almost uniformly throughout the cluster. The luminosity function of the main-sequence stars of NGC 6819 is almost flat. The flat luminosity function indicates that a large number of low mass stars has escaped from the cluster unless its initial mass function is much different from the Salpeter type (<TEX>${\phi}(m){\propto} m^{-(1+x)},x = 1.35$</TEX>).
685
- MOON Y.-J.
- Journal of the Korean astronomical society = 천문학회지
- 35, n.3
- pp.143-149
- 2002
- 원문 바로보기
In this paper we present a methodology to derive the temporal change of the magnetic shear angle from a series of vector magnetograms, with a high time cadence. This method looks for the minimum change of the shear angle between a pair of magnetograms, free from the <TEX>$180^{\circ}$</TEX> ambiguity, and then accumulates this change over many successive pairs to derive the temporal change of magnetic shear. This methodology will work well if only the successive magnetograms occurred in an active region are well aligned and its helicity sign is reasonably determined. We have applied this methodology to a set of vector magnetograms of NOAA Active Region 9661 on October 19, 2001 by the new digital magnetograph at the Big Bear Solar Observatory (BBSO). For this work we considered well aligned magnetograms whose cross-correlation values are larger than 0.95. As a result, we have confirmed the recent report of Wang et al. that there was the abrupt shear change associated with the X1.6 flare. It is also demonstrated that the shear change map can be an useful tool to highlight the local areas that experienced the abrupt shear change. Finally, we suggest that this observation should be a direct support of the emergence of sheared magnetic fields.
686
- YUSHCHENKO ALEXANDER
- Journal of the Korean astronomical society = 천문학회지
- 35, n.4
- pp.209-220
- 2002
- 원문 바로보기
In order to increase the completeness of the investigations of stellar abundances, we can use spectrum synthesis method, new atomic data and observation of stellar spectra with resolution comparable to solar spectral atlases. We made a brief review of main problems of these three ways. We present new results of abundance determinations in the atmospheres of four stars. The first is the implementation of new atomic data to well known Przybylski's star. We show that the number of spectral lines, which can be identificated in the spectrum of this star, can be significantly higher. The second example is the investigation of <TEX>$\zeta$</TEX> Cyg. We found the abundances of 51 elements in the atmosphere of this mild barium star. The third example is halo star HD221170. Our preliminary abundance pattern consists of 42 elements. The heaviest elements in this pattern are U and Th. The last star is the spectroscopic binary HD153720. The number of elements investigated in the spectra of components of this star is not large, but the results show that the components are Am-stars.
687
- OH SU YEON
- Journal of the Korean astronomical society = 천문학회지
- 35, n.3
- pp.151-157
- 2002
- 원문 바로보기
From the data of solar wind observation by ACE spacecraft orbiting the Earth-Sun Lagrangian point, we selected 48 forward interplanetary shocks(IPSs) occurred in 2000, maximum solar activity period. Examining the profiles of solar wind parameters, the IPSs are classified by their shock drivers. The significant shock drivers are the interplanetary coronal mass ejection(ICME) and the high speed stream(HSS). The IPSs driven by the ICMEs are classified into shocks driven by magnetic clouds and by ejectas based on the existence of magnetic flux rope structure and magnetic field strength. Some IPSs could be formed as the blast wave by the smaller energy and shorter duration of shock drivers such as type II radio burst. Out of selected 48 forward IPSs, <TEX>$56.2\%$</TEX> of the IPSs are driven by ICME, <TEX>$16.7\%$</TEX> by HSS, and <TEX>$16.7\%$</TEX> of the shocks are classified into blast-wave type shocks. However, the shock drivers of remaining <TEX>$10\%$</TEX> of the IPSs are unidentified. The classification of the IPSs by their driver is a first step toward investigating the critical magnitudes of the IPS drivers commencing the magnetic storms in each class.
688
- LEE CHANG WON
- Journal of the Korean astronomical society = 천문학회지
- 35, n.4
- pp.187-196
- 2002
- 원문 바로보기
A spectral line survey is performed from 159.7 to 164.7 GHz toward Orion-KL, as an extension of our previous line survey from 138.3 to 150.7 GHz with the same 14 m radio telescope of Taeduk Radio Astronomy Observatory. Typical system temperatures were 260 - 1000 K to achieve a sensitivity of about 0.02 - 0.04 K in TA unit. A total of 63 line spectra are detected in this survey. Among them, 54 lines lines are found to be the first detections towards an astronomical source and only 9 spectral lines have been previously identified from other observations. Forty-eight of 54 lines are believed to be from the known transitions of the known molecules, while 6 lines are 'unidentified'. All detected lines are found to be from a total of 10 molecular species and their isotopic variants. The molecular species with most numerous detected transitions are <TEX>$HCOOCH_3$</TEX> (22), followed by <TEX>$CH_3OCH_3$</TEX> (7), <TEX>$C_2H_5CN$</TEX> (7), and <TEX>$SO_2$</TEX> (6). The LTE rotation diagram analysis using all homogeneous data with those from previous survey gives more reliable determination of physical quantities. The derived values of the rotation temperatures and column densities for <TEX>$HCOOCH_3$</TEX>, <TEX>$CH_3OCH_3$</TEX>, and <TEX>$SO_2$</TEX> are are 75 <TEX>$\~$</TEX> 197 K and <TEX>$1.5 {\~}18 {\times} 10^{15}\;cm^{-2}$</TEX>, respectively.
689
- KIM SANG CHUL
- Journal of the Korean astronomical society = 천문학회지
- 35, n.1
- pp.9-28
- 2002
- 원문 바로보기
We present UBVI CCD photometry of the stellar contents and globular cluster(GC) candidates in the spiral galaxy NGC 300 in the Sculptor group. Color-magnitude diagrams for 18 OB associations having more than 30 member stars are presented. The slope of the initial mass function for the bright stars in NGC 300 is estimated to be <TEX>${\Gamma}= -2.6{\pm} 0.3$</TEX>. Assuming the distance to NGC 300 of (m - M)o = 26.53 <TEX>$\pm$</TEX> 0.07, the mean absolute magnitude of three brightest blue stars is obtained to be < <TEX>$M_v^{BSG}$</TEX> (3) > = -8.95 mag. We have performed search for GCs in NGC 300 and have found 17 GC candidates in this galaxy. Some characteristics of these GC candidates are discussed.
690
- CHANG HEON-YOUNG
- Journal of the Korean astronomical society = 천문학회지
- 35, n.3
- pp.123-130
- 2002
- 원문 바로보기
An elliptical accretion disk may be formed by tidally disrupted debris of a flying-by star in an active galactic nucleus (AGN) or by tidal perturbation due to a companion in a binary black hole system. We investigate the iron K<TEX>$\alpha$</TEX> line profiles expecting from a geometrically thin, relativistic, elliptical disk in terms of model parameters, and find that a broad and skewed line profile can be reproduced well. Its shape is variable to the model parameters, such as, the emissivity power-law index, the ellipticity of the disk, and the major axis orientation of the elliptical accretion disk. We suggest that our results may be useful to search for such an elliptical disk and consequently the tidal disruption event.