• Bulletin of the Korean Space Science Society
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• 2004.10b
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• pp.321-324
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• 2004

We used BV CCD images to study the dynamical substructures of three globular clusters - M5, NGC6934, NGC7006 - analyzing the radial variations of ellipticity and position angle from the point spread function stellar photometry and the ellipse surface photometry. Several populations were classified by the brightness on color-magnitude diagrams of each globular cluster. Ellipse analyses to the images, removed stars of each population from the original images of the clusters, show radial variations in ellipticity and position angle, with the amount of $0.01\~0.25$ in ellipticity and $+90\~-90$ degrees in position angle up to roughly three times of half light radius $(r_h)$. It is also apparent that there are no significant discrepancies in the dynamical substructures beyond $r_h$ among the different populations. However, dynamical substructures on the central region (i.e., inner than $\~r_h$) reflect the contributions of populations of bright red giant stars and horizontal branch stars.

• Journal of The Korean Astronomical Society
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• v.28 no.2
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• pp.153-167
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• 1995

More than 22,300 stars in NGC 6752 were measured over the region of 5' < r < 23' in B and V AAT plates. The most of these are main sequence(MS) stars and about 130 blue horizontal branch(BHB) stars were detected. The C-M diagram of all measured stars shows gaps appearing at $V{\approx}15.^{m}2$ and $16.^{m}2$ along the red giant branch(RGB) and their appearance shown by Lee & Cannon(1980) is found to be independent of measured region. The bimodal distribution of BHB stars is confirmed again and a wide gap shown by Lee & Cannon(1980) at $V{\approx}16^m$ is clearly seen for stars in the outer part (8' < r < 13') in the cluster. It is noted, however, that this gap is occupied by about a dozen of BHB staIs located in the inner region (5' < r < 8'). The number ratio of bright BHB star (V < $15^m$) to faint BHB stars (V > $15^m$) decreases with increasing radial distance from the cluster center. Three faintest BHB stars were found, and two stars ($V{\approx}18.^{m}5$) of there are located in the inner region of $r{\approx}6'$ and the other faintest one ($V{\approx}19.^{m}3$) located in the outer part of $r{\approx}13'$. Also a bluest star of (B - V) $\approx$ -0.5 at $V{\approx}17.^{m}2$ is found but it is located at the outer part of $r{\approx}13'$ in NE region. Therefore, the membership of the faintest BHB star and bluest star is suspected. The luminosity function(LF) and mass function(MF) for NGC 6752 were derived for MS stars. The LF for stars of $M_v\;<\;6^m$ in the outer part of r > 8' shows a consistency with that derived by Penny & Dickens(1986).

• Journal of Astronomy and Space Sciences
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• v.26 no.4
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• pp.417-420
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• 2009

We have used the near-infrared $JHK_S$ photometric data of resolved stars in a nearby dwarf elliptical galaxy NGC 205 to determine the magnitudes of the red giant branch tip (TRGB). By applying Savitzky-Golay filter to the observed luminosity functions (LFs) in each band, we derived the second derivatives of the LFs so as to determine the magnitudes of the TRGB. Absolute magnitudes of the TRGB in $JHK_S$ bands were measured from the Yonsei-Yale isochrones. By comparing the determined apparent magnitudes and the theoretical absolute magnitudes of the TRGB, we estimated the distance moduli of NGC 205 to be (m - M) = $24.10{\pm}0.08$, $24.08{\pm}0.12$ and $24.14{\pm}0.14$ in J, H, and $K_S$ bands, respectively.

• The Bulletin of The Korean Astronomical Society
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• v.42 no.1
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• pp.49.1-49.1
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• 2017

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 (GCs). 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 first 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 these "building blocks" in a hierarchical merging paradigm would have provided helium enhanced stars to the bulge field. Interestingly, we also find that the observed Na-O anticorrelation in metal-poor GCs can be reproduced, when multiple episodes of starbursts are allowed to continue in these subsystems. Specific star formation history with decreasing time intervals between the stellar generations, however, is required to obtain this result, as would be expected from the orbital evolution of these subsystems in a proto-Galaxy. The "mass budget problem" is also much alleviated by our models without ad-hoc assumptions on star formation efficiency and initial mass function.

• Journal of Astronomy and Space Sciences
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• v.16 no.2
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• pp.91-104
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• 1999

We use BV CCD images to investigate the reality of the color gradient within a King type globular cluster NGC 7089. Surface photometry shows that there is a strong radial color gradient in the central region of the cluster in the sense of bluer center with the amplitude of ~0.39 $\pm$ 0.07 mag/$arcsec^2$ in (B - V). In the outer region of the cluster, however, the radial color gradient shows a reverse case, i.e., redder toward the center. (B - V) color profile which was derived from resolved stars in VGC 7089 field also shows a significant color gradient in the central region of the clusters, indicating that lights from the combination of red giant stars and blue horizontal branch stars cause the radial color gradient. Color gradient of the outer region of NGC 7089 may be due to the unresolved background of the cluster. Similar color gradients in the central area of clusters have been previously observed exserved exclusively in highly concentrated systems classified as post core collapse clusters. We caution, however, to confirm the reality of the color gradient from resolved stars, we need more accurate imaging data of the cluster with exceptional seeing condition because the effect of completeness correlates with local density of stars.

• The Bulletin of The Korean Astronomical Society
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• v.41 no.1
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• pp.66.4-67
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• 2016

Our recent investigation (Lee et al. 2015) suggests that the presence of double red clump in the Milky Way bulge is another manifestation of multiple populations observed in halo globular clusters. The origin of Helium enhancement in the 2nd generation population (G2), however, is not yet fully understood. Here we investigate the origin of this super-Helium-rich population in the framework of self-enrichment scenario. We find that chemical enrichments and pollutions by asymptotic giant branch stars and winds of massive rotating stars can naturally reproduce the observed Helium enhancement. The Helium to metal enrichment ratio appears to be ${\Delta}Y/{\Delta}Z=6$ for G2, while the standard ratio, ${\Delta}Y/{\Delta}Z=2$, is appropriate for G1, which is probably enriched mostly by typeII supernovae.

• The Bulletin of The Korean Astronomical Society
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• v.41 no.1
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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.

• Journal of Astronomy and Space Sciences
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• v.25 no.3
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• pp.245-248
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• 2008

We determined values of distance modulus to nearby dwarf galaxy NGC 185 from the Tip of Red-Giant Branch (TRGB) method. Apparent magnitudes of the TRGB are estimated from the near-infrared JHK luminosity functions (LFs) of the resolved giant branch stars. Theoretical absolute magnitudes of the TRGB in near-infrared bands have been extracted from the Yonsei-Yale isochrones. The observed apparent and theoretical absolute magnitudes of the TRGB provide values of distance modulus to NGC 185 as (m - M) = $23.39{\pm}0.14$, $23.23{\pm}0.22$, and $23.27{\pm}0.26$ for J,H, and K bands, respectively. Distance modulus in bolometric magnitude is also derived as (m - M) = $23.62{\pm}0.12$.

• Journal of The Korean Astronomical Society
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• v.17 no.2
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• pp.69-103
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• 1984

Five different calibrations of metal abundances of globular clusters are examined and these are compared with metallicity ranking parameters such as $(Sp)_c$, . Q39 and IR-indices. Except for the calibration $[Fe/H]_H$ by the high dispersion echelle analysis. the other calibration scales are correlated with the morphological parameters of red giant branch. In the $[Fe/H]_H$-scale. the clusters later than ${\sim}F8$ have nearly a constant metal abundance. $[Fe/H]_H{\simeq}-1.05$, regradless of morphological characteristics of horizontal branch and red giant branch. By the two fundamental calibration scales of $[Fe/H]_L$ (derived by the low dispersion analysis) and $[Fe/H]_{{\Delta}s}$ (derived by the spectral analysis of RR Lyrae stars). the globular clusters are divided into the halo clusters with [Fe/H]<-1.0 and the disk clusters confined within the galactocentric distance ${\tau}_G=10\;kpc$ and galactic plane distance |z|=3 kpc. In this case the abundance gradient is given by d[Fe/H]/$dr_G{\approx}-0.05\;kpc^{-1}$ and d[Fe/H]/$d|z|{\simeq}-0.08\;kpc^{-1}$ within ${\tau}_G=20\;kpc$ and |z|=10 kpc, respectively. According to these characteristics of the spatial distribution of globular clusters. the chemical evolution of the galactic globular clusters can be accounted for by the two-zone (disk-halo) slow collapse model when the $[Fe/H]_L$-or $[Fe/H]_{{\Delta}s}$-scale is applied. In the case of $[Fe/H]_H$-scale, the one-zone fast collapse model is preferred for the evolution of globular clusters.