• The Bulletin of The Korean Astronomical Society
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• v.38 no.2
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• pp.45.2-45.2
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• 2013

We studied recent evolution of M31 and M33 with star-forming regions and hot massive stars. We use GALEX far-UV and near-UV imaging to detect the star-forming regions and trace the recent star formation across the entire disk of galaxies. The GALEX imaging, combining deep sensitivity and entire coverage of these galaxies, provides a complete picture of the recent star formation in M31 and M33, and its variation with environment throughout these galaxies. We also show results from recent extensive surveys in M31 and M33 with Hubble Space Telescope multi-wavelength data including UV filters, which imaged several regions at a linear resolution of less than half a pc in these galaxies. Both datasets allow us to study the hierarchical structure of star formation: the youngest stellar groups are the most compact, and are often arranged withing broader, sparser structures. The derived recent star-formation rates are rather similar for the two galaxies, when scaled for the respective areas.

• Journal of Astronomy and Space Sciences
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• v.15 no.1
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• pp.83-90
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• 1998

With a total 2780 eclipsing binary systems in the Catalogue of Approximate Photometric and Absolute Elements of Eclipsing Variable Stars by Svechnikov & Kuznetsova(1990), the empirical relations between the systemic mass and orbital argular momentum have been examined. It is found that, during the its evolution, the total orbital argular momentum of the eclipsing binary sustem is not conserved. It decreases gradually, though not at a constant rate, until the system becomes into contact from initially detached via semi-detached system.

• Journal of the Korean earth science society
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• v.36 no.5
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• pp.427-436
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• 2015

We classified Abell clusters using the magnitude differences between two or three bright member galaxies and investigated how such classification was correlated with the properties of brightest cluster galaxies (BCGs). S-type BCGs being clearly brighter than the rest of the member galaxies were likely to be red, luminous, and evolved as early type galaxies. On the other hand, T-type BCGs being not dominant at all were less luminous than early type galaxies. A small fraction of BCGs was currently forming stars, and all of the star-forming BCGs were T-type BCGs. Active galactic nuclei were most frequent for S-type BCGs. Through these quantitative analyses of the BCG properties, we discussed the possible scenario of BCG formation and the differences between S-type and T-type of BCGs.

• Journal of The Korean Astronomical Society
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• v.29 no.1
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• pp.53-62
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• 1996

31 white dwarfs in 10 open clusters are examined, and their maximum mass and the upper mass limit of their progenitors are obtained as $1.22\pm0.02M_{\bigodot}\;and\;7.2\pm0.4M_{\bigodot}$ respectively, suggesting that the upper mass limit of white dwarfs is less than 8M_{\bigodot}$ The final mass of white dwarfs shows no clear correlation with the initial mass of their progenitors, and it is found that a deficient gap of initial mass exists between $\~4\;and\;~5.2M_{\bigodot}$. This gap seems to correspond to the mass range for carbon detonation or deflagration. The total expected numbers of white dwarfs are $11\~22$ in Hyades with 7 known white dwarfs and 17 in Praesepe with 8 known white dwarfs. These known white dwrfs are all younger than the others in both clusters. But one known white dwarf in Pleiades is older one among $2\~3$ expected white dwarfs.

• Journal of The Korean Astronomical Society
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• v.23 no.2
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• pp.116-121
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• 1990

It has been suggested that there could be a large number of primordial black holes which were formed in the early universe. We analyze the growth of such a primordial black hole following two different accretion rates - the Eddington accretion rate and the Bondi accretion rate - at the center of a host star like the sun. We find that a primordial black hole with M < ${\sim}10^{17}\;g$ cannot substantially grow in any case throughout the lifetime of a host star. If M > ${\sim}10^{17}\;g$, the evolution of a host star depends entirely on the mode of accretion, but it ends as a black hole in either case. Since more stars may have primordial black holes at the center of a galaxy this may result in a cluster of such black holes, and the cluster may eventually collapse to produce a single supermassive black hole.

• The Bulletin of The Korean Astronomical Society
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• v.41 no.2
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• pp.59.1-59.1
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• 2016

Blue Straggler Stars(BSS) affect their host star cluster in various parameters like color, dynamics, etc. For this reason, it is important to know how to relate BSS frequency and evolution of their host stellar system. To statistical study about global properties of open clusters as the environments of BSS formation, we use three catalogues - (1) two galactic open clusters catalogues including BSS candidate, (2) Milky Way Star Cluster (MWSC) survey data. Then, we compare with the data of two BSS catalogues for test of the result of Marchi et al. 2006. We also investigate the radial mass distribution in open cluster, because it is possible that changing the gradient of radial mass distribution cause increasing the BSS frequency. When we group the open cluster into having BSS or not and other criteria, the groups show slight discrepancies, but we show some important results.

• Publications of The Korean Astronomical Society
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• v.34 no.3
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• pp.67-79
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• 2019

We present a semi-analytical method to calculate the global evolution of the ionized state of the intergalactic medium, on the basis of physically motivated star formation histories in the early universe. This method incorporates not only the conventional scenarios in which the star formation rate is proportional to the growth rate of the halo collapse fraction, but also the more sophisticated scenarios in which the star formation is self-regulated. We show that this variance in the star-formation model strongly impacts the resulting reionization history, which bears a prospect for observational discrimination of these models. We discuss how observations of the anisotropic polarization of the cosmic microwave background and the global 21cm signal from the high-redshift universe, most notably by Planck and EDGES, may probe the history of reionization.

• Animal Systematics, Evolution and Diversity
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• v.27 no.2
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• pp.180-182
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• 2011

Sea stars were collected with fishing nets between depths of 40-150 m from the Gangwon-do coastal region, East Sea. Specimens were identified as Evasterias echinosoma Fisher, 1926 belonging to the family Asteriidae, which is new to the Korean fauna. This species was characterized by strong external spines and a general size of more than 200 mm, thus the largest sea star identified in Korea to date. Its morphological characteristics are described here with photos. Thirty two asteroid species including E. echinosoma have been reported from the East Sea of Korea.

• The Bulletin of The Korean Astronomical Society
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• v.42 no.1
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• pp.59.1-59.1
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• 2017

Massive Population III black hole binary systems are one of the suggested candidate sources of the recently detected gravitational wave radiation (GWR). GWR detection from a black hole binary system requires a sufficiently short orbital separation at the time of their formation, such that they would undergo coalescence within the Hubble time. This condition cannot be simply fulfilled by a short initial period, because binary interactions such as mass transfer and common envelope evolution can largely change the orbital parameters and the masses of stellar components. Here, we discuss the possibility of black hole binary mergers from massive Pop III binary systems, using a new grid of Pop III binary evolutionary models with various initial primary masses ($20M_{\odot}{\leq}M{\leq}100M_{\odot}$) and initial separations, for different initial mass ratios (q = 0.5 - 0.9).

• The Bulletin of The Korean Astronomical Society
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• v.42 no.1
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• pp.43.1-43.1
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• 2017

We investigate properties of black hole (BH) binaries formed in globular clusters, by using direct N-body simulations. Comparing with previous studies which usually considered single BH masses, our models consist of two-component BH masses, or continuous BH mass function with single mass ordinary cluster stars. During the early stage of dynamical evolution, initially distributed BHs are move to the cluster center by dynamical friction, then BH-BH binaries start to be formed, and eventually be ejected from the cluster due to three body interaction. Finally we find the formation efficiency of high mass BHs are alwats larger than that of lower mass BHs, implying that a BH mass spectrum expected from GW observation should be biased to high mass. In addition, mass ratios of BHs in binaries prefer similar masses (ratio~1), while the most extreme case is less than 3. Expected merger rate from our models is about 7 BH-BH mergers per $Mpc^3$ per yr.