• 천문학회보
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• 제38권2호
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• pp.75-75
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• 2013

Recent observations have shown that some massive globular clusters (GCs) host multiple stellar populations having different heavy element abundances enriched by supernovae. They usually accompany multiple red giant branches (RGBs) in the color-magnitude diagrams (CMDs), and are distinguished from most of the other GCs which display variations only in light element abundances. In order to investigate the star formation histories of these peculiar GCs, we have constructed synthetic CMDs based on the updated versions of Yonsei.Yale ($Y^2$) isochrones and horizontal branch evolutionary tracks which include the cases of enhancements in both helium and the total CNO abundances. To estimate ages and helium abundances of subpopulations in each GC, we have compared our models with the observations on the Hess diagram by employing a ${\chi}^2$ minimization technique. In this talk, we will present our progress in the population modeling for these GCs with multiple RGBs.

• 천문학회지
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• 제26권2호
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• pp.89-98
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• 1993

Magnetostatic models of starspots of late type main sequence stars$(G5V\~K5V)$ have been constructed to investigate their physical characteristics by using the similarity law suggested by Schluter and Temesvary(1958) and later employed by Deinzer(1965) and Yun(1968). The starspots are assumed to be single, circular and in horizontal magnetostatic equilibrium. In the present study we considered only those model spots whose area covers less than $12\%$ of the entire stellar surface as suggested by observations. The computed surface field strength of our model spots ranges from $10^3$\;to\;several\;10^3$ gauss and their magnetic flux is found to be $10\~100$ times that of sunspots. The field strength is sensitive to spectral type, which increases with later spectral type. In contrast to the field strength, the area of starspots depends strongly on the total magnetic flux. Finally, it is noted that the computed field strength of model spots belonging to $G0V\~G5V$ falls below the equipartition field strength at their parent stellar surface unless the coverage is less than $2\%$. This suggests that the observed spot on $G0V\~G5V$ stars is likely to be a group of small starspots.

• 천문학회보
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• 제39권2호
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• pp.62.1-62.1
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• 2014

Metallicity distribution of globular clusters (GCs) provides an important clue for star formation history of their host galaxy. With an assumption that GCs are generally old, GC colors have been used as a proxy of GC metallicities. Bimodal GC color distributions observed in most large galaxies have, for decades, been interpreted as bimodal metallicity distributions, indicating the presence of two populations within a galaxy. However, the conventional view has been challenged by a new theory that non-linear GC color-metallicity relations (CMRs) can cause a bimodal color distribution even from a single-peaked metallicity distribution. Using the photometric and spectroscopic data of NGC 5128 GCs in combination with stellar population simulation models, we examine the effect of non-linearity in GC CMRs on the transformation of GC color distributions into metallicity distributions. Although, in some colors, offsets are present between observations and models in the CMRs, their overall shape agrees well for various colors. After the offsets are corrected, the observed spectroscopic metallicity distribution is well reproduced via modeled CMRs from various color distributions having different morphologies. On the other hand, the linearly converted metallicity distributions from GC colors show a significant discrepancy with the observed spectroscopic metallicity distribution. We discuss the implications of our results.

• 천문학회보
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• 제42권2호
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• pp.64.2-64.2
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• 2017

The Population III (Pop III) stars are thought to be massive. If massive Pop III stars form binary system and they experience mass transfer via the Roche lobe overflow, this may significantly change the properties of the system. For example, mass transfer in such system may shorten the period of the system, forming short period binary black hole (BBH) system, which is the most promising candidate for recently detected gravitational wave radiation sources. Also, there is an expectation that due to the stripped envelope of donor star by mass transfer, this system can play a significant role in the cosmic reionization by emitting more UV photons. However, this outcome highly depends on the initial properties of the system. We perform grid calculation on Pop III binary models with various initial primary masses (20 ~ 100 solar mass), initial separations, and initial mass ratios (q = 0.5 ~ 0.9). We find that 1) in most cases binary models show no increase in the number of ionizing photons and 2) formation of short period BBH system via mass transfer is highly unlikely.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2000년도 추계학술대회논문집B
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• pp.704-709
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• 2000

Laminar burning velocities of propane- and iso-octane-air mixtures have been numerically modelled over a wide range of equivalence ratio, pressure and temperature. These correlations are applicable to the modelling of stratified charged combustion like that of lean bum and GDI engine combustion. The numerical models are based on the results calculated by PREMIX code with Sloane's detailed chemical reaction mechanism for propane and FlameMaster code with Peters' for iso-octane. Laminar burning velocity for two fuels showed a pressure and temperature dependence in the following form, in the range of $0.1{\sim}4MPa$, and $300{\sim}1000K$, respectively. $S_L={\alpha}\;{\exp}[-\xi({\phi}-{\phi}_m)^2-{\exp}\{-{\xi}({\phi}-{\phi}_m)\}-{\xi}({\phi}-{\phi}_m)]$ where ${\phi}_m=1.07$, and both of ${\alpha}$ and ${\xi}$ are functions of pressure and temperature. Compared with the results of the existing models, those of the present one showed the good agreement of the recent experiment data, especially in the range of lean and rich sides. Judging from the calculated results of the stratified charged combustion by using STAR-CD, the above modelling prove to be more suitable than the other ones.

• 한국전산유체공학회지
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• 제12권2호
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• pp.8-13
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• 2007

A general purpose program NUFLEX for the analysis 3-D thermo/fluid flow and pre/post processor in complex geometry has been developed, which consists of a flow solver based on FVM and GUI based pre/post processor. The solver employs a general non-orthogonal grid system with structured grid and solves laminar and turbulent flows with standard/RNG $k-{\varepsilon}$ turbulence model. In addition, NUFLEX is incorporated with various physical models, such as interfacial tracking, cavitation, MHD, melting/solidification and spray models. For the purpose of evaluation of the program and testing the applicability, many actual problems are solved and compared with the available data. Comparison of the results with that by STAR-CD or FLUENT program has been also made for the same flow configuration and grid structure to test the validity of NUFLEX.

• 천문학회보
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• 제42권1호
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• pp.42.1-42.1
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• 2017

We investigate the evolutionary properties of red supergiant stars (RSGs), using stellar evolution model of Modules for Experiments in Stellar Astrophysics (MESA). In this study, we calculate models with mass range of 9-39M_sun and several different convection parameters (e.g. mixing length, overshooting, and semiconvection) at SMC, LMC, Milky Way, and M31 metallicities. We compare the calculated evolutionary tracks with observed RSGs in SMC, LMC, Milky Way and M31, and discuss appropriate input physical parameters in model calculation. We find that a larger mixing length parameter is necessary for M31 metallicity to fit the positions of RSGs in H-R diagram, compared to lower metallicity environments. Theoretically predicted numbers of yellow supergiant stars (YSGs) are also compared with the observed population. We find that Ledoux models with semiconvection can better explain the number of YSGs. Finally, we investigate the final radius, final star mass, and final hydrogen envelope mass of RSGs and discussed the their properties as type II-P supernova progenitors.

• 천문학회보
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• 제44권1호
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• pp.34.2-34.2
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• 2019

Several observational results show a tighter pitch angle at wavelengths of optical and near-infrared than those that are associated with star formation, which is in agreement with the prediction of the density wave theory. In my recent numerical studies, the dependence of the shock positions relative to the potential minima is due to the tendency that stronger shocks form farther downstream. This causes a systematic variation of the perpendicular Mach number, with radius and makes the pitch angle of the gaseous arms smaller than that of the stellar arms, which supports the prediction of the density-wave theory, independently. However, some observations still give controversial results which show similar pitch angles at wavelengths, and there is no statistical study comparing observations and numerical models directly. By analyzing optical image of disk galaxies in the Carnegie-Irvine Galaxy Survey (CGS), I measured the physical values of stellar and gaseous arms such as their strength, length, and pitch angles. For direct comparison with numerical results, I analyzed more than 30 additional numerical models with varying the initial parameters in model galaxies. In this talk, I will present results both of observational and numerical samples and discuss the physical properties of spiral structures based on the density-wave theory.

• 설비공학논문집
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• 제9권2호
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• pp.189-197
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• 1997

The vaporizing characteristics of LNG(liquefied natural gas) via heat exchanger with sea water are analytically studied for an open rack vaporizer(ORV). This study is intended to supply the design data for the domestic fabrication of the corrosion-resistant vaporizer tube. A computational program is developed to predict the exit temperature of LNG for various conditions. In the program, thesimple and justifiable heat transfer models are selected for fully-developed internal flow of LNG, the star-shaped finned-tube, and the external falling films of sea water, as well as the possible ice formation and the fouling on the tube walls. It is found that the enongh corrugation inside of the tube wall is the most significant in the vaporizer performance for the current operating conditions. the effects of other design parameters on the heat exchanger between LNG and sea water are quantitatively presented.

• 천문학회지
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• 제38권2호
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• pp.135-140
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• 2005

This paper summarizes the recent progress made by our group at Seoul National University on studies of the evolution and formation of distant galaxies. Various research projects are currently underway, which include: (i) the number density of distant early-type galaxies (z < 1); (ii) the optical-NIR color gradient of nearby early-type galaxies; (iii) J - K-selected Extremely Red Objects (EROs) in field (CDF-S) and the cluster environment; and (iv) the Lyman-break galaxies in the Spitzer First Look Survey (FLS) field. These works will constrain the mass evolution and the star formation history of galaxies in different environments, and the results will serve as useful contraints on galaxy formation models.