• 천문학회지
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• 제13권1호
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• pp.15-26
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• 1980

The BV-photographic photometry was made for 1714 stars (V<19.5) in NGC 6752. The C-M diagram of this cluster shows an unusually extended blue horizontal branch $(V=13.5{\sim}17.8)$ with a wide gap $(V=16{\sim}16.7)$ and the well defined giant branch with gaps at V=13.85 and 16.2. The turnoff point is defined at $V=17.25{\pm}0.15$ and (B-V) = $0.46{\pm}0.02$. If we take $V_{HB}=13.85$ for NGC 6752, then ${\Delta}V=2.80,\;(B-V)_{0,g}=0.76\;and\;{\Delta}V_{TO}=3.40$ and the chemical abundance is estimated to be [Fe/H]=-1.67 or $Z=4.3{\times}10^{-4}\;and\;Y=0.26$. Some other physical parameters of this cluster are derived and compared with those for the well observed clusters M 3, M 13, M 15 and M 92.

• 천문학회보
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• 제39권1호
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• pp.61.2-61.2
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• 2014

We present a set of fiducial sequences of horizontal-branch stars in bright Galactic globular clusters, which have previously been observed in the Sloan Digital Sky Survey (SDSS). We derive fiducial lines on color-magnitude diagrams in multiple color indices (g - r, g - i, g - z, and u - g), after rejecting foreground and background objects as well as RR Lyrae variables utilizing these color indices. We compare our fiducial sequences with model predictions from Yonsei-Yale evolutionary tracks and BaSel spectral libraries, and find a satisfactory agreement between them in terms of their color-magnitude relations, except in u - g. We also compare theoretical models to color-magnitude diagrams of two open clusters (M67 and NGC 6791). Based on our best available cluster distance and reddening, we find that the mass of red clump (RC) stars in NGC 6791 is about a factor of two smaller than an earlier estimate from the application of asteroseismic scaling relations for solar-like oscillations. The smaller RC mass implies an enhanced mass loss along the red giant branch, which is in accordance with other compelling evidences found in this metal-rich system. Our estimated luminosity of RC stars in NGC 6791 is about 0.2 mag fainter than in earlier investigations based on solar-metallicity calibrations, and results in ~10% reduction in the RC-based distance estimation, when applied to metal-rich systems such as in the Galactic bulge.

• 천문학회보
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• 제36권2호
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• pp.143.1-143.1
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• 2011

Deep Ca,b,y images obtained from the CTIO 4m Blaco telescope are used to investigate the multiple stellar populations of red giant branch (RGB) and sub-giant branch (SGB) in Galactic globular clusters NGC 6656 and NGC 6723. For NGC 6656, confirming the result of Lee et al. (2009), we find two discrete populations of the RGB stars of which mean color separation is about 0.2 mag in hk[=(Ca-b)-(b-y)] index. Furthermore, we also find the bimodel distribution of the SGB stars in (hk, y) color-magnitude diagram. A new finding is that the (hk, y) color-magnitude diagram of NGC 6723 shows two distinct RGB stars with different calcium abundances of which mean color separation is about 0.12 mag in hk index. This multiple stellar feature has not been observed in previous observation, suggesting that NGC 6723 may also be a possible relic of dwarf galaxies that merged into the Milky Way in the past. Thus our result adds further constraints to the merging scenario of the Galaxy formation. Unfortunately, the split of SGB stars in NGC 6723 is not obvious. We will present some statistical results to compare properties of two populations in two clusters.

• 천문학회지
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• 제28권2호
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• pp.169-175
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• 1995

Deep V I CCD photometry of the Pegasus dwarf irregular galaxy shows that the tip of the red giant branch (RGB) is located at I = $21.15{\pm}0.10$ mag and (V - I) = $1.58{\pm}0.03$. Using the I magnitude of the tip of the RGB (TRGB), the distance modulus of the Pegasus galaxy is estimated to be $(m\;-\;M)_o\;=\;25.13{\pm}0.11$ mag (corresponding to a distance of d = $1060{\pm}50$ kpc). This result is in a good agreement with the recent distance estimate based on the TRGB method by Aparicio [1994, ApJ, 437, L27],$ (m\;-\;M)_o$ = 24.9 (d = 950 kpc). However, our distance estimate is much smaller than that based on the Cepheid variable candidates by Hoessel et al.[1990, AJ, 100, 1151], $(m\;-\;M)_o\;=\;26.22{\pm}0.20$ (d = $1750{\pm}160$ kpc) mag. The color-magnitude diagram illustrates that the Cepheid candidates used by Hoessel et al.are not located in the Cepheid instability strip, but in the upper part of the giant branch. This result shows that the Cepheid candidates studied by Hoessel et al.are probably not Cepheids, but other types of variable stars. Taking the average of our distance estimate and Aparicio's, the distance to the Pegasus galaxy is d= $1000{\pm}80$ kpc. Considering the distance and velocity of the Pegasus galaxy with respect to the center of the Local Group, we conclude that the Pegasus galaxy is probably a member of the Local Group.

• 천문학논총
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• 제30권2호
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• pp.255-259
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• 2015

Recent narrow-band Ca photometry discovered two distinct red giant branch (RGB) populations in some massive globular clusters (GCs) including M22, NGC 1851, and NGC 288. In order to investigate the differences in light/heavy elements abundances between the two subpopulations, we have performed low-resolution spectroscopy for stars on the two RGBs in these GCs. We find a significant difference (more than $4{\sigma}$) in calcium abundance from the spectroscopic HK' index for both M22 and NGC 1851. We also find a more than $8{\sigma}$ difference in CN band strength between the Ca-strong and Ca-weak subpopulations. For NGC 288, however, we detect the presence of a large difference only in the CN strength. The calcium abundances of the two subpopulations in this GC are identical within errors. We also find interesting differences in CN-CH relations among these GCs. While CN and CH indices are correlated in M22, they show an anti-correlation in NGC 288. However, NGC 1851 shows no difference in CH between two groups of stars having different CN strengths. The CN bimodality in these GCs could be explained by pollution from intermediate-mass asymptotic giant branch stars and/or fast-rotating massive stars. For the presence or absence of calcium bimodality and the differences in CN-CH relations, we suggest these would be best explained by how strongly type II supernovae enrichment has contributed to the chemical evolutions of these GCs.

• 천문학회보
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• 제38권2호
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• pp.75-75
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• 2013

Recent observations have shown that some massive globular clusters (GCs) host multiple stellar populations having different heavy element abundances enriched by supernovae. They usually accompany multiple red giant branches (RGBs) in the color-magnitude diagrams (CMDs), and are distinguished from most of the other GCs which display variations only in light element abundances. In order to investigate the star formation histories of these peculiar GCs, we have constructed synthetic CMDs based on the updated versions of Yonsei.Yale ($Y^2$) isochrones and horizontal branch evolutionary tracks which include the cases of enhancements in both helium and the total CNO abundances. To estimate ages and helium abundances of subpopulations in each GC, we have compared our models with the observations on the Hess diagram by employing a ${\chi}^2$ minimization technique. In this talk, we will present our progress in the population modeling for these GCs with multiple RGBs.

• 천문학회지
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• 제48권2호
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• pp.93-104
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• 2015

WFIRST microlensing observations will return high-precision parallaxes, σ(π) . 0.3 µas, for the roughly 1 million stars with H < 14 in its 2.8 deg² field toward the Galactic bulge. Combined with its 40,000 epochs of high precision photometry (∼ 0.7 mmag at H_vega = 14 and ∼ 0.1 mmag at H = 8), this will yield a wealth of asteroseismic data of giant stars, primarily in the Galactic bulge but includindvvvvvg a substantial fraction of disk stars at all Galactocentric radii interior to the Sun. For brighter stars, the astrometric data will yield an external check on the radii derived from the two asteroseismic parameters, the large-frequency separation <∆ν_nl> and the frequency of maximum oscillation power ν_max, while for the fainter ones, it will enable a mass measurement from the single measurable asteroseismic parameter ν_max. Simulations based on Kepler data indicate that WFIRST will be capable of detecting oscillations in stars from slightly less luminous than the red clump to the tip of the red giant branch, yielding roughly 1 million detections.

• Clinical and Experimental Pediatrics
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• 제46권3호
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• pp.302-307
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• 2003

환아는 3개월된 영아로 발병 10일째에 본원에 내원하여 초기에 적절한 진단과 치료를 받지 못했으며 발병 2주에 시행한 심초음파상 거대 관상동맥류를 보였다. 이에 정맥용 면역글로불린 투여와 aspirin과 dipyridamole로 치료 중 발병 24일째 혈전증이 동반되어 urokinase와 heparin의 IV 투여를 통한 혈전용해 치료로 혈전을 완전히 제거할 수 있었다. 이후 5년간 aspirin, dipyridamole, coumadin을 투여하면서 심전도와 심초음파검사를 반복하며 외래 추적 관찰 중 점차 좌측관상동맥내 혈전이 증가하고 내경이 점차 좁아지는 소견과 함께 협착이 의심되었다. 발병 5년 후 시행한 관상동맥 혈관조영술상 관상동맥 좌측 전하지가 완전 폐색되고 우측관상동맥으로부터 발달한 측부순환에서 혈류를 공급받고 있었고, 핵의학 검사상 좌심실의 전벽과 중격, 일부 외벽에 완전 가역적 관류 결손을 보였던 가와사끼병 환아 1례를 경험하여 보고한다.

• Journal of Astronomy and Space Sciences
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• 제22권4호
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• pp.363-376
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• 2005

구상성단 NGC 7006의 거성종족에 따른 성단 중심부의 역학적 세부구조 변화를 알아보기 위해서 VGC 7006의 BV CCD영상에 대해 점광원 함수 측광을 실시하여 4개의 거성종족을 분류한 후 각 종족을 차례대로 제거하면서 ellipse맞추기를 실시하여 성단의 중심으로부터 반경에 따른 타원율과 위치각의 변화를 얻었다. 타원율과 위치각의 전체적인 변화를 살펴보면, $r_{eff}<3r_h$ 영역에서 타원율은 $0.02\~0.06$의 변화를 보이며 위치각의 경우는 $-10^{\circ}\~+90^{\circ}$의 변화를 보인다. 거성종족의 제거에 따른 NGC 7006의 중심부 타원율과 위치각의 변화를 보면, $r_{eff}인 영역에서 밝은 거성종족을 제거했을 경우 타원율과 위치각의 변화가 각각 $-0.05\~+0.05,-20^{\circ}\~+20^{\circ}$로 나타났으며, 수평계열을 제거했을 경우에는 각각 $-0.05\~+0.025,-25^{\circ}\~+20^{\circ}$로 나타나는 것으로부터 중심부에서는 밝은 거성종족과 수평계열에 의한 영향이 가장 크다는 것을 유추할 수 있다. 또한, 어두운 거성종족에 의한 중심부의 타원율과 위치각의 변화도 확인 할 수 있었던 반면 준거성종족에 의해서는 크게 영향을 받지 않는 것을 확인 할 수 있었다.

• 천문학회보
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• 제43권1호
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• pp.46.1-46.1
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• 2018

In order to investigate the origin of multiple stellar populations in the halo and bulge of the Milky Way, we have constructed chemical evolution models for the low-mass proto-Galactic subsystems such as globular clusters. Unlike previous studies, we assume that supernova blast waves undergo blowout without expelling the pre-enriched gas, while relatively slow winds of massive stars, together with the winds and ejecta from low and intermediate mass asymptotic-giant-branch stars, are all locally retained in these less massive systems. We find that the observed Na-O anti-correlations in metal-poor GCs can be reproduced when multiple episodes of starbursts are allowed to continue in these subsystems. A specific form of star formation history with decreasing time intervals between the stellar generations, however, is required to obtain this result, which is in good agreement with the parameters obtained from our stellar evolution models for the horizontal-branch. The "mass budget problem" is also much alleviated by our models without ad-hoc assumptions on star formation efficiency and initial mass function. We also applied these models to investigate the origin of super helium-rich red clump stars in the metal-rich bulge as recently suggested by Lee et al. (2015). We find that chemical enrichments by the winds of massive stars can naturally reproduce the required helium enhancement (dY/dZ = 6) for the second-generation stars. Disruption of proto-globular clusters in a hierarchical merging paradigm would have provided helium enhanced stars to the bulge field.