• Journal of The Korean Astronomical Society
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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.

• Journal of The Korean Astronomical Society
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• 제12권1호
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• pp.17-25
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• 1979

The two methods for the derivation of helium abundance are presented. The magnitude of the largest bump along the red giant branch is fainter as helium abundance decreases and metal abundance increases. Using this relation and observed bump magnitude, the He-abundances of globular clusters are estimated. The another method is to use the relation that the ratio of asymptotic giant branch and horizontal branch stars increases with decreasing He-abundance. The comparison of the He-abundances derived from the two methods with those derived from the other two methods shows that they are consistent within the dispersion less than ${\Delta}Y=0.1$.

• The Bulletin of The Korean Astronomical Society
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• 제36권2호
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• pp.67.1-67.1
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• 2011

Recent observations and modeling of globular clusters with multiple populations strongly indicate the presence of super helium-rich subpopulations in old stellar systems. Motivated by this, we have constructed new population synthesis models with and without helium-enhanced subpopulations to investigate their impact on the UV-upturn phenomenon of quiescent early-type galaxies. We find that our models with helium-enhanced subpopulations can naturally reproduce the strong UV-upturns observed in giant elliptical galaxies assuming an age similar to that of old globular clusters in the Milky Way. The major source of far-UV (FUV) flux, in this model, is relatively metal-poor and helium-enhanced hot horizontal branch stars and their progeny. The Burstein et al. (1988) relation of the FUV - V color with metallicity is also explained either by the variation of the fraction of helium-enhanced subpopulations or by the spread in mean age of stellar populations in early-type galaxies.

• Journal of Plant Biology
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• 제37권3호
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• pp.333-341
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• 1994

Effective plant regeneration from immature cotyledons of soybean [Glycine max (L.) Merr.] cultivars was achieved via somatic embryogenesis. Somatic embryogenesis was performed with the cotyledons of immature embryos 14-20 d after flowering. Immature cotyledons of cv. Whangkeum were placed abaxial or adaxial side down on modified MS medium containing 20mg/L 2,4-D. The greatest number of somatic embryos, 1.2 per cotyledon, was produced from those of 4.0-4.9 mm in length which had been placed abaxial side down. Among cvs. Pecking, Whangkeum and Baekwoon, Pecking had the highest embryo induction efficiency with 4.3 somatic embryos per cotyledon in 20mg/L 2,4-D treatment and with 1.0 embryo per cotyledon in 8mg/L NAA treatment. Germinable globular somatic embryos were induced with the highest efficiency, 27.6%, in 20mg/L 2,4-D and were proliferated efficiently on liquid medium containing 10mg/L 2,4-D. The globular somatic embryos developed into germinable mature somatic embryos on medium containing 10 $\mu$M CoCl2, 9% sucrose, and 0.5% activated charcoal. These mature somatic embryos germinated on hormone-free mediu. After transfer to the soil, regenerated plants with seeds were obtained.

• Bulletin of the Korean Space Science Society
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• 한국우주과학회 2009년도 한국우주과학회보 제18권2호
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• pp.29.3-29.3
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• 2009

Recent observations for the Galactic globular clusters (GCs) have shown that a number of GCs have characteristic features of multiple stellar populations, such as multiple main-sequences (MSs), splits in sub-giant branch (SGB), bimodal and/or extended horizontal branches (HBs). Based on the population synthesis models, we investigate star formation histories of the GCs with multiple populations, $\omega$ Cen, M54, NGC 1851, NGC 6388, NGC 6441, and NGC 2808, by comparing synthetic model color-magnitude diagrams (CMDs) with observations. We adopt most up-to-date Yonsei-Yale (Y2) stellar evolutionary tracks and isochrones from MS to post-HB, as well as improved color-temperature transformations from the recent stellar atmosphere libraries. Our models show that the observed features can be naturally explained by assuming the presence of helium-enhanced subpopulations.

• The Bulletin of The Korean Astronomical Society
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• 제41권1호
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• pp.32.2-32.2
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• 2016

The globular cluster (GC) systems in most elliptical galaxies show bimodal color distributions. This phenomenon has been generally regarded as a bimodal metallicity distribution, indicating the presence of two sub-populations in a GC system. However, since a new explanation on the bimodality was introduced where the nonlinear metallicity-to-color conversion can cause bimodal color distributions, the origin of this phenomenon has been under hot debate. In this presentation, we briefly review the ten-year debate on the origin of GC color bimodality, and present our recent pieces of evidence on the nonlinear nature of GC color-metallicity relations.

• The Bulletin of The Korean Astronomical Society
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• 제41권1호
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• pp.32.3-32.3
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• 2016

The presence of multiple stellar populations is now well established in most globular clusters in the Milky Way. Here we show that two populations of RR Lyrae stars and the double red clump observed in the Milky Way bulge are another manifestations of the same multiple population phenomenon observed in halo globular clusters. We will discuss the implications of this result on the stellar populations and formation of early-type galaxies.

• The Bulletin of The Korean Astronomical Society
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• 제41권1호
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• pp.79.2-79.2
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• 2016

The presence of multiple stellar populations is now well established in most globular clusters (GCs) in the Milky Way. The origin of this, however, is yet to be understood. In this respect, the study of NGC 2808, a GC which hosts five sub-populations differing only in light-element abundances, would help to understand the origin of this multiple stellar populations. In order to investigate CN, CH, and Ca abundances among different populations in NGC 2808, we have performed low-resolution spectroscopy for the red giant branch (RGB) stars in this GC. Here we report preliminary results from this spectroscopic analysis.

• The Bulletin of The Korean Astronomical Society
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• 제41권1호
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• pp.77.3-77.3
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• 2016

In our previous study, we showed that the peculiar globular cluster M22 contains two distinct stellar populations with different physical properties, having different chemical compositions, spatial distributions and kinematics. We proposed that M22 is most likely formed via a merger of two GCs with heterogeneous metallicities in a dwarf galaxy environment and accreted later to our Galaxy. In their recent study, Mucciarelli et al. claimed that M22 is a normal mono-metallic globular cluster without any perceptible metallicity spread among the two groups of stars, which challenges our results and those of others. We devise new strategies for the local thermodynamic equilibrium abundance analysis of red giant branch stars in globuar clusters and show there exists a spread in the iron abundance distribution in M22.

• The Bulletin of The Korean Astronomical Society
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• 제39권1호
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• pp.27-27
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• 2014

The past three decades have witnessed a renaissance in the field of extragalactic globular clusters (GCs). GC systems have now been investigated in galaxies ranging from dwarfs to giants and spanning all the morphological types. Detailed studies of GCs provide strong constraints on galaxy formation that can be obtained in the near-field. In this talk I will review some of the pivotal studies performed with the HST and large ground-based telescopes and state-of-the-art simulations. Also, I will attempt to introduce my new solution to a long-standing puzzle in this field----the origin of GC bimodality in color. I will show that the theory gives a simple, cohesive explanation for the key observations of extragalactic GCs. The implication of the results will be discussed in the context of formation of GC systems and their parent galaxies.