• The Bulletin of The Korean Astronomical Society
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• v.39 no.1
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• pp.81.2-81.2
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• 2014

To figure out the effect of rotation on the final mass of Pop III stars, 1D stellar evolution simulations of the evolution of mass-accreting protostars are performed, with zero metalicity and high constant mass accretion rates. The protostar reaches the Keplerian rotation very soon after the onset of mass accretion, but it may continue mass accretion via angular momentum transport induced by viscous stress or magnetic field. However, as the accreting star evolves, the envelope expands rapidly when the total mass reaches $5{\sim}6M_{\odot}$ and the corresponding Eddington factor sharply increases. Strong radiative pressure with rotation imposes different criteria for breakup at the stellar surface, and the so-called 'critical rotation (${\Omega}{\Gamma}$-limit)' is reached. As a result mass accretion rate has to be significantly lowered. This implies that characteristic masses of Pop III stars would be significantly lowered than the previous expectation.

• Journal of Astronomy and Space Sciences
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• v.12 no.2
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• pp.168-178
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• 1995

We have modeled the dust envelopes around carbon stars with close attention to the evolution of the structure of the dust shells. We use various dust density distributions to take account the effect of the superwind due to the helium shell flash by adding a density increased region. Depending on the position and quality of the density increased region, the model results are different from the results with conventional density distribution. The new results fit the observations of some carbon stars better. The IR two-color diagrams comparing the results of the super wind models and IRAS observation of 252 carbon stars have been made. The new results can explain much wider regions on the IR two-color diagrams.

• Journal of Astronomy and Space Sciences
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• v.10 no.1
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• pp.25-35
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• 1993

Stars lose their masses constantly after their birth, but the stellar mass loss is especially prominent in the last stages of their lives. It has been believed that red superginats are losing their masses at rates of 10-8∼10-4M/yr. They are known to be asymptotic giant branch stars that are at the end stages of the evolution for the stars with zero age main sequence masses of 1∼10M. Red supergiants are often characterized by the thick dust envelopes and large amplitude pulsations. According to their energy spectra, chemical composition, they are divided into three main group; M-type Miras, C-type carbon stars, and OH/IR stars. The purpose of this work is to clarify the evolutionary aspects in the physical parameters of the red supergiants mainly from the direct interpretation of their infrared spectra.

• The Bulletin of The Korean Astronomical Society
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• v.44 no.2
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• pp.75.2-75.2
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• 2019

This poster introduces the ongoing "Narrow-band Ca Photometry for Dwarf Spheroidal Galaxies" project and presents the latest results. The project aims to explain the formation and evolution of dwarf spheroidal galaxies by examining the structural properties of stellar populations as a function of metallicity. To overcome the lack of stars with known spectroscopic metallicities for dwarf spheroidal galaxies, we apply the hk index as a photometric metallicity indicator to three galaxies-Draco, Sextans, and Canes Venatici I. For all three galaxies, we found that metal-poor and metal-rich groups of red-giant-branch stars have distinct spatial distributions, in which metal-rich stars are centrally concentrated while metal-poor stars are relatively dispersed. In Sextans, we found an off-centered peak of metal-poor stars which is presumed to be a disrupting star cluster in this galaxy. We will discuss the implications of our results for the dwarf galaxy formation and possible directions on future work of this project.

• Communications for Statistical Applications and Methods
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• v.20 no.4
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• pp.271-281
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• 2013

This paper considers a problem of classification of variable stars based on functional data analysis. For a better understanding of galaxy structure and stellar evolution, various approaches for classification of variable stars have been studied. Several features that explain the characteristics of variable stars (such as color index, amplitude, period, and Fourier coefficients) were usually used to classify variable stars. Excluding other factors but focusing only on the curve shapes of variable stars, Deb and Singh (2009) proposed a classification procedure using multivariate principal component analysis. However, this approach is limited to accommodate some features of the light curve data that are unequally spaced in the phase domain and have some functional properties. In this paper, we propose a light curve estimation method that is suitable for functional data analysis, and provide a classification procedure for variable stars that combined the features of a light curve with existing functional data analysis methods. To evaluate its practical applicability, we apply the proposed classification procedure to the data sets of variable stars from the project STellar Astrophysics and Research on Exoplanets (STARE).

• The Bulletin of The Korean Astronomical Society
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• v.41 no.1
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• pp.76.1-76.1
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• 2016

During the last decade, high-resolution spectra of many very metal-poor (VMP) stars have been observed and their surface compositions have been measured. The abundance patterns of the VMP stars strongly constrain the nucleosynthesis of Pop III stars because they born from material enriched by supernovae or wind ejecta of Pop III stars. The observations show overabundances of light elements like C, N, O, Na, Mg and Al and very low $C^{12}/C^{13}$ ratios. These results indicate that mixing between the H-burning and He-burning region occurred in Pop III stars. To explain these observational results, we performed 1D stellar evolution simulations for non-rotating Pop III stars with ZAMS masses ranging from $20M_{\Box}$ to $50M_{\Box}$ and various overshooting parameters. In our grid calculation, convective mixing between helium burning layers and the hydrogen burning shell generally occurred in models with masses less than $40M_{\Box}$ without rotation and these models show an excess of light element abundances. From this result, it is expected that we could explain the observed abundance patterns with convective mixing in non-rotating massive Pop III stars and we do not necessarily have to invoke rotational mixing.

• Journal of The Korean Astronomical Society
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• v.55 no.2
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• pp.59-66
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• 2022

Stellar magnetic activity is important for formulating the evolution of the star. To represent the stellar magnetic activity, the S index is defined using the Ca II H+K flux measure from the Mount Wilson Observatory. Mg II lines are generated in a manner similar to the formation of Ca II lines, which are more sensitive to weak chromospheric activity. Mg II flux data are available from the International Ultraviolet Explorer (IUE). Thus, the main purpose of this study was to analyze the magnetic activity of stars. We used 343 high-resolution IUE spectra of 14 main-sequence G stars to obtain the Mg II continuum surface flux and Mg II line-core flux around 2,800 Å. We calculated S index using the IUE spectra and compared it with the conventional Mount Wilson S index. We found a color (B - V ) dependent association between the S index and the Mg II emission line-core flux. Furthermore, we attempted to obtain the magnetic activity cycles of these stars based on the new S index. Unfortunately, this was not successful because the IUE observation interval of approximately 17 years is too short to estimate the magnetic activity cycles of G-type stars, whose cycles may be longer than the 11 year mean activity cycle of the sun.

• The Bulletin of The Korean Astronomical Society
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• v.46 no.2
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• pp.81.1-81.1
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• 2021

Stellar kinematics is a useful tool to understand the formation and evolution of young stellar systems. Here, we present a kinematic study of the HII region, NGC 821, using the Gaia Early Data Release 3. NGC 281 contains the open cluster IC 1590. This cluster has a core and a low-stellar density halo. We detect a pattern of cluster expansion from the Gaia proper motion vectors. Most stars radially escaping from the cluster are distributed in the halo. We measure the 1-dimensional velocity dispersion of stars in the core. The velocity dispersion (1 km/s) is comparable to the expected virial velocity dispersion of this cluster, and therefore the core is at a virial state. The core has an initial mass function shallower than that of the halo, which is indicative of mass segregation. However, there is no significant correlation between stellar masses and tangential velocities. This result suggests that the mass segregation has a primordial origin. On the other hand, it has been believed that the formation of young stars in NGC 281 West was triggered by feedback from massive stars in IC 1590. We investigate the ages of stars in the two regions, but the age difference between the two regions is not comparable to the timescale of the passage of an ionization front. Also, the proper motion vectors of the NGC 281 West stars relative to IC 1590 do not show any systematic receding motion from the cluster. Our results suggest that stars in NGC 281 West might have been formed spontaneously. In conclusion, the formation of NGC 281 can be understood in the context of hierarchical star formation model.

• The Bulletin of The Korean Astronomical Society
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• v.30 no.1
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• pp.49.2-49.2
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• 2005

• The Bulletin of The Korean Astronomical Society
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• v.39 no.2
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• pp.67.2-67.2
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• 2014

The presence of multiple populations is now well-established in most globular clusters 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, the instability strip in the metal-poor group II clusters, such as M15, is populated by second generation stars (G2) with enhanced helium and CNO abundances, while the RR Lyraes in the relatively metal-rich group I clusters like M3 are mostly produced by first generation stars (G1) without these enhancements. This population shift within the instability strip with metallicity 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 clusters having Oosterhoff-intermediate characteristics, such as NGC 1851, as well as of most metal-rich clusters having RR Lyraes with longest periods (group III) can also be reproduced, as more helium-rich third and later generations of stars (G3) penetrate into the instability strip with further increase in metallicity. Therefore, although there are systems where the suggested population shift cannot be a viable explanation, for the most general cases, our models predict that the RR Lyraes are produced mostly by G1, G2, and G3, respectively, for the Oosterhoff groups I, II, and III.