• The Bulletin of The Korean Astronomical Society
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• v.44 no.1
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• pp.47.4-47.4
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• 2019

Recent observations on the double red clumps in the bulge validate the close connection in stellar populations between Galactic globular clusters (GCs) and the Milky Way (MW) bulge. Intriguingly, diverse phenomena observed in early-type galaxies (ETGs) and their GC systems are also indicating the similarities with Galactic GCs with multiple populations. Here, we present the population synthesis for the Galactic bulge and ETGs using stellar populations observed in the Galactic GCs with multiple populations. Our new models well explain observations of both the MW bulge and ETGs. Also, the inclusion of GC-originated population to the population synthesis model shows substantial impacts on the age-dating of stellar populations. The implication of this result for the interpretation of the formation history and the age-dating of ETGs will be discussed in detail.

• Publications of The Korean Astronomical Society
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• v.25 no.3
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• pp.91-99
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• 2010

An increasing number of observations have confirmed the presence of multiple stellar populations in Galactic globular clusters. Multiple populations evidence come from the complex chemical pattern of stars hosted in GCs and from the split or broadening of different evolutionary sequences in the color-magnitude diagrams. Multiple stellar populations have been identified in Galactic and Magellanic Cloud clusters, as well as in external galaxies. In this paper I will summarize the observational facts.

• The Bulletin of The Korean Astronomical Society
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• v.37 no.1
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• pp.31.2-31.2
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• 2012

We present the Yonsei Evolutionary Population Synthesis (YEPS) models for spectroscopic and photometric evolutions of simple and composite stellar populations. The models are based on the most up-to-date Yonsei-Yale stellar evolutionary tracks and BaSel 3.1 flux libraries, and provide integrated spectroscopic quantities of Lick/IDS system including high-order Balmer absorption-lines. Special care has been taken to incorporate the systematic variation of horizontal branch (HB) morphology as functions of metallicity, age, alpha-element mixture, and helium abundance of simple stellar populations. Our models for normal-helium stellar populations indicate that the realistic modeling of HB and alpha-element brings about 5 Gyr and 0.1 dex differences in age and metallicity estimations, respectively, compared to those without these effects. The HB effect does not depend on the specific choice of stellar libraries and alpha-element enhancements, and this effect is non-negligible even in the metal sensitive absorption indices, such as Mg2 and Mg b. Comparison of the models to observations reveals that the HB and alpha-element effects are critical in understanding otherwise inexplicable phenomena found in globular cluster systems in the Milky Way and nearby galaxies, including the observed bimodality of the line strengths of globular clusters in massive galaxies. In addition, we found that helium-enhanced stellar populations, which are the major sources of extreme HB stars, bring about increased FUV, NUV fluxes, and thus the model colors of those filters become extremely blue. Age dating based on the YEPS model with normal-helium stellar populations reveals that the evidence for 'downsizing' of elliptical galaxies is found not only in the local field but also in Coma cluster, and that the mean age of elliptical galaxies in Coma cluster is about 1.4 Gyr younger than the mean age of those in the local field. We also 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 GCs in the Milky Way.

• The Bulletin of The Korean Astronomical Society
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• v.45 no.1
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• pp.31.3-32
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• 2020

The surface brightness fluctuation (SBF) is one of the most crucial distance indicators for unresolved stellar systems at large distances. Here, we present an evolutionary population synthesis model of the surface brightness fluctuation (SBF) for normal and He-enriched simple stellar populations (SSPs). Our SBF model for the normal-He population agrees well with other existing models, but the He-rich populations bring about a substantial change in the SBF of SSPs. Our normal-He SBF model well reproduces the observed SBFs of the Milky Way globular clusters, but the SBFs of early-type galaxies in the Virgo Cluster are placed between the normal-He and He-rich SBF models. We show that the SBF-based distance estimation would be affected by up to a 10-20% level in I- and near-IR bands at given colors. Finally, we propose that when combined with independent metallicity and age indicators such as Mg2 and H��, the UV and optical SBFs can readily detect underlying He-rich populations in unresolved stellar systems. Given the degree of the SBF variation resulting from the population difference, we suggest that the distance measurement before the proper in-depth analysis of stellar populations should be done with great caution.

• The Bulletin of The Korean Astronomical Society
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• v.44 no.1
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• pp.38.3-38.3
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• 2019

We investigate the stellar population properties of nine passive spiral galaxies in the CALIFA survey. They have NUV-r > 5 and no/weak nebular emission lines in their spectra. They lie in the redshift range of 0.001 < z < 0.021 and have stellar mass range of 10.2 < ${\log}(M{\star}/M{\odot})$ < 10.8. We analyze the stellar populations out to two effective radius, using the best-fitting model to the measured absorption line-strength indices in the Lick/IDS system. We compare the passive spirals with S0s selected in the same mass range. S0s cover a wide range in age, metallicity, and [${\alpha}/Fe$], and stellar populations of the passive spirals are encompassed in the spread of the S0 properties. However, the distribution of passive spirals are skewed toward higher values of metallicity, lower [${\alpha}/Fe$], and younger ages at all radii. These results show that passive spirals are possibly related to S0s in their stellar populations. We infer that the diversity in the stellar populations of S0s may result from different evolutionary pathways of S0 formation, and passive spirals may be one of the possible channels.

• The Bulletin of The Korean Astronomical Society
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• v.41 no.1
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• pp.43.3-43.3
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• 2016

Most globular clusters are now known to have two or more stellar populations with different chemical properties. In order to understand the origin and evolution of multiple stellar populations in these globular clusters, it is necessary to study not only the chemical property, but also the dynamical property. In our previous works (Lim et al. 2015; Han et al. 2015), we have shown that Ca narrow-band photometry can be combined with low-resolution spectroscopy to effectively study the chemical properties of globular clusters. In this talk, we will show our observations are also useful to study the radial distribution of stars in globular clusters with multiple stellar populations, and report our preliminary results.

• The Bulletin of The Korean Astronomical Society
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• v.39 no.2
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• pp.47.1-47.1
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• 2014

The calcium infrared triplet (CaT) is one of the prominent absorption features in the infrared wavelength regime. Recently, this absorption feature has been getting attention in the prediction of metallicity of stellar populations because of its strong sensitivity to the calcium abundance and metallicity of a star. However, we find that measuring metallicity directly from CaT is very dangerous because the formation mechanism of CaT is very inefficient in the cool stars which are abundant in metal-rich populations. This characteristics of CaT make the CaT-metallicity relation to converge around ~ $8{\AA}$ in the metal-rich regime. Our results suggest that, because of the converging CaT-metallicity relation in the metal-rich regime, the metallicity of simple stellar populations greater than [Fe/H]~-0.5 is unreliable when the linear conversion between CaT and metallicity is applied to derive metallicity. Based on these results, we suggest that CaT is not a good metallicity indicator for the metal-rich stellar populations.

• The Bulletin of The Korean Astronomical Society
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• v.44 no.2
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• pp.75.4-75.4
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• 2019

We present a quantitative analysis of the relationship between the gas dynamics and star formation history of DDO 210 which is an irregular dwarf galaxy in the local Universe. We perform profile analysis of an high-resolution neutral hydrogen (HI) data cube of the galaxy taken with the large Very Large Array (VLA) survey, LITTLE THINGS using newly developed algorithm based on a Bayesian Markov Chain Monte Carlo (MCMC) technique. The complex HI structure and kinematics of the galaxy are decomposed into multiple kinematic components in a quantitative way like 1) bulk motions which are most likely to follow the underlying circular rotation of the disk, 2) non-circular motions deviating from the bulk motions, and 3) kinematically cold and warm components with narrower and wider velocity dispersion. The decomposed kinematic components are then spatially correlated with the distribution of stellar populations obtained from the color-magnitude diagram (CMD) fitting method. The cold and warm gas components show negative and positive correlations between their velocity dispersions and the surface star formation rates of the populations with ages of < 40 Myr and 100~400 Myr, respectively. The cold gas is most likely to be associated with the young stellar populations. Then the stellar feedback of the young populations could influence the warm gas. The age difference between the populations which show the correlations indicates the time delay of the stellar feedback.

• Bulletin of the Korean Space Science Society
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• 2009.10a
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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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• v.44 no.1
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• pp.38.1-38.1
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• 2019

Globular clusters (GCs) are one of the excellent tools to trace the assembly history of their host galaxies. Especially, the ages and abundances of the GCs give important clues about the star formation epochs and merging progenitors. We investigate the stellar population of the GCs in M87 based on a stacking analysis using about 900 MMT/Hectospec spectra of the GCs. We measure the ages, [Z/H], and [a/Fe] from the stacked spectra of the GCs within radial bins based on Lick indices. We find clear radial gradients for [Z/H] and [a/Fe] in the GC system. In addition to the radial trends, we investigate the stellar populations of the GC subgroups divided according to colors, radial velocities, and spatial locations. We discuss the formation history of M87 based on the stellar populations of the GCs.