• Journal of The Korean Astronomical Society
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• v.47 no.4
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• pp.137-145
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• 2014

The astrophysical parameters of four unstudied open star cluster candidates; Ivanov 2, 7, 9, and Harvard 9; are estimated for the first time using the PPMXL database. The stellar density distributions and color-magnitude diagrams for each cluster are used to determine the geometrical structure (cluster center, limited radius, core and tidal radii, the distances from the Sun, from the Galactic center and from the Galactic plane). Also, the main photometric parameters (age, distance modulus, color excesses, membership, total mass, relaxation time, luminosity and mass functions) are estimated.

• Journal of The Korean Astronomical Society
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• v.49 no.2
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• pp.53-57
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• 2016

We present an optical UBVRI photometric analysis of the poorly studied open star cluster IC 2156 using Sloan Digital Sky Survey data in order to estimate its astrophysical properties. We compare these with results from our previous studies that relied on the 2MASS JHK near-infrared photometry. The stellar density distributions and color-magnitude diagrams of the cluster are used to determine its geometrical structure, real radius, core and tidal radii, and its distance from the Sun, the Galactic plane, and the Galactic center. We also estimate, the age, color excesses, reddening-free distance modulus, membership, total mass, luminosity function, mass function, and relaxation time of the cluster.

• Journal of Astronomy and Space Sciences
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• v.34 no.2
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• pp.83-97
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• 2017

We present extra-tidal features of spatial configuration of stars around three metal-poor globular clusters (NGC 6266, NGC 6273, NGC 6681) located in the Galactic bulge. The wide-field photometric data were obtained in BVI bands with the MOSAIC II camera at CTIO 4 m Blanco telescope. The derived color-magnitude diagrams (CMDs) contain stars in a total $71^{\prime}{\times}71^{\prime}$ area including a cluster and its surrounding field outside of the tidal radius of the cluster. Applying statistical filtering technique, we minimized the field star contaminations on the obtained cluster CMDs and extracted the cluster members. On the spatial stellar density maps around the target clusters, we found overdensity features beyond the tidal radii of the clusters. We also found that the radial density profiles of the clusters show departures from the best-fit King model for their outer regions which support the overdensity patterns.

• Bulletin of the Korean Space Science Society
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• 2009.10a
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• pp.30.1-30.1
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• 2009

We present extra-tidal features of spatial configuration of stars around three metal-poor globular clusters (NGC 6273, NGC 6266, NGC 6681) located in the Galactic bulge. The accurate wide-field photometric data were obtained in BVI bands with the MOSAICII camera at CTIO Blanco 4m telescope. The derived color-magnitude diagrams (CMDs) covered a total $71'\times71'$ area including a cluster and its surrounding field outside of the tidal radius of the cluster. Applying the statistical technique of the CMD-mask algorithm, we minimized the field star contaminations on the obtained CMDs and chose properly the cluster's member stars. On the spatial stellar density maps around the target clusters, we found overdensity features beyond the tidal radii of the clusters. We also found that the radial density profiles of the clusters show departures from the best-fit King model for the outer region of clusters. The results add further observational evidence that the observed metal-poor bulge clusters would be originated from accreted satellite systems, indicative of the merging scenario of the formation of the Galaxy.

• The Bulletin of The Korean Astronomical Society
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• v.39 no.1
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• pp.82.1-82.1
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• 2014

The presence of multiple populations is now well-established in most globular clusters (GCs) in the Milky Way. In light of this progress, here we suggest a new model explaining the origin of the Sandage period-shift and the difference in mean period of type ab RR Lyrae variables () between the two Oosterhoff groups. In our models, while matching the observed color-magnitude diagrams, the difference in is naturally reproduced as the instability strip is occupied by different subpopulations with increasing metallicity. The instability strip in the metal-poor group II clusters is populated by second generation stars (G2) with enhanced helium and CNO abundances, while the RR Lyraes in the metal-rich group I clusters are mostly produced by first generation stars (G1) without these enhancements. This population shift within the instability strip can create the observed period-shift between the two groups, since both helium and CNO abundances play a role in increasing the period of RR Lyrae variables. The presence of more metal-rich Oosterhoff group III clusters having RR Lyraes with longest can also be reproduced, if more helium-rich third generation stars (G3) are present in these GCs.

• Journal of Astronomy and Space Sciences
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• v.25 no.3
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• pp.249-254
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• 2008

We report the distance modulus of nearby dwarf irregular galaxy NGC 6822 estimated from the so-called Tip of Red-giant Branch (TRGB) method. To detect the apparent magnitudes of the TRGB we use the color-magnitude diagrams (CMDs) and luminosity functions (LFs) in the near-infrared JHK bands. Foreground stars, main-sequence stars, and supergiant stars have been classified on the (g - K, g) plane and removed on the near-infrared CMDs, from which only RGB and AGB stars are remained on the CMDs and LFs. By applying the Savitzky-Golay filter to the obtained LFs and detecting the peak in the second derivative of the observed LFs, we determined the apparent magnitudes of the TRGB. Theoretical absolute magnitudes of the TRGB are estimated from Yonsei-Yale isochrones with the age of 12Gyr and the metallicity range of -2.0 <[Fe/H]< -0.5. The derived values of distance modulus to NGC 6822 are (m - M) = $23.35{\pm}0.26$, $23.20{\pm}0.42$, and $23.27{\pm}0.50$ for J, H, and K bands, respectively. Distance modulus in bolometric magnitude is also derived as (m - M) = $23.41{\pm}0.17$. We compare the derived values of the TRGB distance modulus to NGC 6822 in the near-infrared bands with the previous results in other bands.

• Journal of The Korean Astronomical Society
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• v.49 no.5
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• pp.175-192
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• 2016

CCD photometric observations of the globular cluster (GC), M53 (NGC 5024), are performed using the 1.8 m telescope at the Bohyunsan Optical Astronomy Observatory in Korea on the same nights (2002 April and 2003 May) as the observations of the GC M92 (NGC 6341) reported by Cho and Lee using the same instrumental setup. The data for M53 is reduced using the same method as used for M92 by Cho and Lee, including preprocessing, point-spread function fitting photometry, and standardization etc. Therefore, M53 and M92 are on the same photometric system defined by Landolt, and the photometry of M53 and M92 is tied together as closely as possible. After complete photometric reduction, the V versus B − V , V versus V − I, and V versus B − I color-magnitude diagrams (CMDs) of M53 are produced to derive the relative ages of M53 and M92 and derive the various characteristics of its CMDs in future analysis. From the present analysis, the relative ages of M53 and M92 are derived using the Δ(B − V ) method reported by VandenBerg et al. The relative age of M53 is found to be 1.6 ± 0.85 Gyr younger than that of M92 if the absolute age of M92 is taken to be 14 Gyr. This relative age difference between M53 and M92 causes slight differences in the horizontal-branch morphology of these two GCs.

• Journal of The Korean Astronomical Society
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• v.44 no.5
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• pp.177-193
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• 2011

We present a photometric study of five open clusters (Czernik 5, Alessi 53, Berkeley 49, Berkeley 84, and Pfleiderer 3) in the Sloan Digital Sky Survey. The position and size of these clusters are determined using the radial number density profiles of the stars, and the member stars of the clusters are selected using the proper motion data in the literature. We estimate the reddening, distance and age of the clusters based on the isochrone fitting in the color-magnitude diagram. The foreground reddenings for these clusters are estimated to be E(B - V ) = 0.71 - 1.55 mag. The distances to these clusters are derived to be 2.0 - 4.4 kpc, and their distances from the Galactic center range from 7.57 kpc to 12.35 kpc. Their ages are in the range from 250 Myr to 1 Gyr. Berkeley 49 and Berkeley 84 are located in the Orion spur, Czernik 5 is in the Perseus arm, and Pfleiderer 3 and Alessi 53 are located beyond the Perseus arm.

• The Bulletin of The Korean Astronomical Society
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• v.39 no.1
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• pp.41.2-41.2
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• 2014

Hierarchical merging scenario indicates that galaxies go through major and minor merger events during their formation and evolution. As a result of the merging, substructural features of remnants such as stellar stream are shown around a current galaxy system. To find evidence of stellar substructures on M31 system, we used the near-infrared images of JHK filters obtained from the Wide Field Camera (WFCAM) at UKIRT 3.8m. A total sky coverage is an area of about$ 4.5^{\circ}{\times}6^{\circ}$ around M31. Indeed, M31 system which consists of several satellite systems contains stellar substructures such as giant stellar stream, loops, and spurs. By analysing stellar populations on the near-infrared color-magnitude diagrams, we selected member star candidates of each stellar substructure, from which we map out spatial distribution of stars in the vicinity of M31 system. Here, we present spatial density distribution maps of stars on each substructure over the entire field of M31 system. Also, we discuss the possible origin of the substructures and the implications on the galaxy assembly process.

• The Bulletin of The Korean Astronomical Society
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• v.38 no.2
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• pp.73.1-73.1
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• 2013

It is believed that the current census of star clusters in the Milky Way is far from being complete, because of high extinction. Taking advantage of recent wide-field infrared surveys (the WISE as well as 2MASS, UKIDSS GPS, and VVV), we search for new star clusters in the central region of the Milky Way (720 $deg^2$-wide area at |1| < $30^{\circ}$ and |b| < $6^{\circ}$). We find 1840 candidates by visual inspection of the WISE images. The spatial distribution of these candidates show a strong concentration along the Milky Way, showing that most of them belong to the Milky Way. Among them, 26 are probably star clusters, considering their morphology, color-magnitude diagrams, and the degree of central concentration of stars. Eighteen of them appear to be very young in the embedded phase, and six of them are considered to be relatively old, showing a developed red giant branch. Implications of the primary results will be discussed.