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한국천문학회지

1968년 ~ 2023년까지 1,187 건한국천문학회지를 격월간 확인하실 수 있습니다.

  • The Korean Astronomical Society (The Korean Astronomical Society)
  • 계간 (Quarterly)
  • ISSN : 1225-4614 (ISSN : 1225-4614)
  • DB구축현황 : 1,187건 (DB Construction : 1,187 Articles)
안내사항
총 게시글 1,187 페이지 65/119
641
  • 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.
642
  • 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>).
643
  • 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.
644
  • AHN SANG-HYEON
  • Journal of the Korean astronomical society = 천문학회지
  • 35, n.4
  • pp.175-185
  • 2002
  • 원문 바로보기
Almost half of primeval galaxies show P-Cygni type profiles in the Ly<TEX>$\alpha$</TEX> emission line. The main underlying mechanism for the profile formation in these systems is thought to be the frequency re-distribution of the line photons in expanding scattering media surrounding the emission source. A Monte Carlo code is developed to investigate the Ly<TEX>$\alpha$</TEX> line transfer in an optically thick and moving medium with a careful consideration of the scattering in the damping wings. Typical column densities and expansion velocities of neutral hydrogen investigated in this study are <TEX>$N_{H1}{\~}10^{17-20}\;cm^{-2}$</TEX> and <TEX>${\Delta}V{\~} 100 km\;s^{-1}$</TEX>. We investigate the dependence of the emergent profiles on the kinematics and on the column density. Our numerical results are applied to show that the damped Ly<TEX>$\alpha$</TEX> absorbers may possess an expanding H I supershell with bulk flow of <TEX>${\~}200 km\;s^{-l}$</TEX> and H I column density <TEX>$N_{H1}{\~}10^{19}\;cm^{-2}$</TEX>. We briefly discuss the observational implications.
645
  • PARK BYEONG-GON
  • Journal of the Korean astronomical society = 천문학회지
  • 35, n.4
  • pp.197-208
  • 2002
  • 원문 바로보기
Five contemporary pre-main sequence (PMS) evolution model grids are compared with the photo-metric data for a nearly complete sample of low-mass members in NGC 2264. From amongst the grids compared, the models of Baraffe et al. (1998) prove to be the most reliable in mass-age distribution. To overcome the limited mass range of the models of Baraffe et al. we derived a simple transformation relation between the mass of a PMS star from Swenson et al. (1994) and that from Baraffe et al., and applied it to the PMS stars in NGC 2264 and the Orion nebula cluster (ONC). The resulting initial mass function (IMF) of the ONC shows that the previous interpretation of the IMF is not a real feature, but an artifact caused by the evolution models adopted. The IMFs of both clusters are in a good agreement with the IMF of the field stars in the solar neighborhood. This result supports the idea proposed by Lada, Strom, & Myers (1993) that the field stars originate from the stars that are formed in clusters and spread out as a result of dynamical dissociation. Nevertheless, the IMFs of OB associations and young open clusters show diverse behavior. For the low-mass regime, the current observations suffer from difficulties in membership assignment and sample incompleteness. From this, we conclude that a more thorough study of young open clusters is necessary in order to make any definite conclusions on the existence of a universal IMF.
646
  • KIM KANG-MIN
  • Journal of the Korean astronomical society = 천문학회지
  • 35, n.4
  • pp.221-227
  • 2002
  • 원문 바로보기
Cassegrain interface module (CIM) of the fiber-fed high resolution echelle spectrograph has been designed and manufactured for the 1.8 m reflector at the Bohynsan Optical Astronomy Observatory. We also constructed a long slit spectrograph attached to this CIM, which would replace the earlier rather inefficient medium dispersion spectrometer. We present detailed description for design and manufacturing concepts of the CIM which consists of a slit assembly, slit monitoring system, calibration lamp system and a long slit spectrograph, in order to provide how the overall system and each part. are constructed. The preliminary performance test carried out so far seems to indicate a successful result.
647
  • LEE Ho-GYU
  • Journal of the Korean astronomical society = 천문학회지
  • 35, n.2
  • pp.105-110
  • 2002
  • 원문 바로보기
We present the results of VLA <TEX>$NH_3$</TEX> (1,1) and (2,2) line observations of the young-stellar object (YSO) IRAS 19550+3248. The integrated intensity map of the <TEX>$NH_3$</TEX> (1,1) line shows that there are two ammonia cores in this region; core A which is associated with the YSO, and core B which is diffuse and located at the northeast of core A. Core A is compact and elongated along the east-west direction (0.07 pc<TEX>$\times$</TEX>0.05 pc) roughly perpendicular to the molecular outflow axis. Core B is diffuse and extended (0.18 pc<TEX>$\times$</TEX>0.07 pc). <TEX>$NH_3$</TEX> (2,2) line is detected only toward core A, which indicates that it is hotter (~ 15 K), presumably due to the heating by the YSO. The <TEX>$NH_3$</TEX> (1,1) line toward core A is wide (<TEX>${\Delta}v{\ge} 3 km s^{-l}$</TEX>) and appears to have an anomalous intensity ratio of the inner satellite hyperfine lines. The large line width may be attributed to the embedded YSO, but the hyperfine anomaly is difficult to explain. We compare the results of <TEX>$NH_3$</TEX> observations with those of previous CS observations and find that the CS emission is detected only toward core A and is much more extended than the <TEX>$NH_3$</TEX> emission.
648
  • ANN HONG BAE
  • Journal of the Korean astronomical society = 천문학회지
  • 35, n.1
  • pp.29-34
  • 2002
  • 원문 바로보기
We have conducted UBVI CCD photometry of an intermediate-age open cluster NGC 559 to investigate the effect of dynamical evolution on the stellar distributions in NGC 559. Our photometry allows better estimates of distance and age of the cluster owing to much deeper photometry (V <TEX>$\le$</TEX> 21) than previous ones. It is found that the luminosity function and mass function as well as the spatial stellar distributions are affected by the dynamical evolution. Mass segregation leads to the central concentration of the high mass stars, which results in the flattened mass function inside the half mass radius.
649
  • HAN CHEONGHO
  • Journal of the Korean astronomical society = 천문학회지
  • 35, n.1
  • pp.35-40
  • 2002
  • 원문 바로보기
Gaudi, Naber & Sackett pointed out that if an event is caused by a lens system containing more than two planets, all planets will affect the central region of the magnification pattern, and thus the existence of the multiple planets can be inferred by detecting additionally deformed anomalies from intensive monitoring of high magnification microlensing events. Unfortunately, this method has important limitations in identifying the existence of multiple planets and determining their parameters (the mass ratio and the instantaneous projected separation) due to the degeneracy of the resulting light curve anomalies from those induced by a single planet and the complexity of multiple planet lensing models. In this paper, we propose a new channel to search for multiple planets via microlensing. The method is based on the fact that the lensing light curve anomalies induced by multiple planets are well approximated by the superposition of those of the single planet systems where the individual planet-primary pairs act as independent lens systems. Then, if the source trajectory passes both of the outer deviation regions induced by the individual planets, one can unambiguously identify the existence of the multiple planets. We illustrate that the probability of successively detecting light curve anomalies induced by two Jovian-mass planets located in the lensing zone through this channel will be substantial. Since the individual anomalies can be well described by much simpler single planet lensing models, the proposed method has an important advantage of allowing one to accurately determine the parameters of the individual planets.
650
  • LEE SUNGHO
  • Journal of the Korean astronomical society = 천문학회지
  • 35, n.3
  • pp.111-121
  • 2002
  • 원문 바로보기
We have measured the correlation functions of the optically selected clusters of galaxies in the Abell and the APM catalogs, and of the X-ray clusters in the X-ray-Brightest Abell-type Clusters of galaxies (XBACs) catalog and the Brightest Clusters Sample (BCS). The same analysis method and the same method of characterizing the resulting correlation functions are applied to all observational samples. We have found that the amplitude of the correlation function of the APM clusters is much higher than what has been previously claimed, in particular for richer subsamples. The correlation length of the APM clusters with the richness R <TEX>$\ge$</TEX> 70 (as defined by the APM team) is found to be <TEX>$r_0 = 25.4_{-3.0}^{+3.1}\;h^{-1}$</TEX> Mpc. The amplitude of correlation function is about 2.4 times higher than that of Croft et al. (1997). The correlation lengths of the Abell clusters with the richness class RC <TEX>$\ge$</TEX> 0 and 1 are measured to be <TEX>$r_0 = 17.4_{-1.1}^{+1.2}$</TEX> and <TEX>$21.0_{-2.8}^{+2.8}\;h^{-1}$</TEX> Mpc, respectively, which is consistent with our results for the APM sample at the similar level of richness. The richness dependence of cluster correlations is found to be <TEX>$r_0= 0.40d_c + 3.2$</TEX> where <TEX>$d_c$</TEX> is the mean intercluster separation. This is identical in slope with the Bahcall & West (1992)'s estimate, but is inconsistent with the weak dependence of Croft et al. (1997). The X-ray bright Abell clusters in the XBACs catalog and the X-ray selected clusters in the BCS catalog show strong clustering. The correlation length of the XBACs clusters with <TEX>$L_x {\ge}0.65{\times} 10^{44}\;h^{-2}erg\;s^{-1}$</TEX> is <TEX>$30.3_{-6.5}^{+8.2}\;h^{-1}$</TEX> Mpc, and that of the BCS clusters with <TEX>$L_x {\ge}0.70{\times} 10^{44}\;h^{-2}erg\;s^{-1}$</TEX> is <TEX>$30.2_{-8.9}^{+9.8}\;h^{-1}$</TEX> Mpc. The clustering strength of the X-ray clusters is much weaker than what is expected from the optical clusters.