• Bulletin of the Korean Space Science Society
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• 2010.04a
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• pp.32.4-32.4
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• 2010

Evolution of the Cosmos (ARCSEC). We have presented the evolutionary status of seven detached double line spectroscopic eclipsing binaries which are CD Tau, CM Lac, CW CMa, HS Hya, IT Cas, KM Hya, and ZZ Boo because the component stars in the binary systems still act as a single star. We determined the absolute dimensions of the binary systems using photometric and spectroscopic solutions by analyzing of the light curves and radial velocity curves. We chose evolutionary tracks of these binary systems. Using the luminosities, effective temperatures and masses. Finally we obtained ages and metallicity of the stars.

• The Bulletin of The Korean Astronomical Society
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• v.46 no.1
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• pp.46.2-46.2
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• 2021

Presolar silicon carbide (SiC) grains form around in the envelopes of asymptotic giant branch (AGB) stars by satisfying C/O>1 which is an optimal condition for SiC grains to condense in the stellar outflows. Ruthenium (Ru) isotopes are locked into the SiC grains during the condensation of SiC grains. We investigate the isotopic compositions of Ru in the stellar winds by using the NuGrid data, which are obtained by nucleosynthesis calculations during the stellar evolution. We compare the isotopic compositions of Ru obtained from the NuGrid data with measurements and the predictions obtained from different codes. Our results present a piece of evidence that SiC grains in the presolar system came from low-mass and low-metallicity AGB stars, also confirming that they were not from massive stars. We also suggest a new scenario in which the total stellar yields are also considered because SiC grains can condense during the collapse of molecular clouds.

• 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.

• Journal of Astronomy and Space Sciences
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• v.14 no.2
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• pp.194-201
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• 1997

We have carried out JK near-infrared photometry for the central region of the metal rich globular cluster M71, whose (K, J - K) color-magnitude diagram is presented here. Using two independent methods we derive the distance modulus to M71, 12.89 and 12.86 $pm0.12$ respectively. The former is derived using the HB luminosity $M_{Ko}$(HB) = -1.15 for globular clusters having metallicity between -1.0 to -0.3 (Kuchinski et al. 1995). The latter is derived through a comparison with IR photometry of stars in the similar metallicity cluster 47 Tuc. We also estimate the reddening to M71, E(J - K) = 0.13, based on E(B - V) = 0.04 of 47 Tuc.

• Publications of The Korean Astronomical Society
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• v.15 no.spc1
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• pp.15-30
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• 2000

A large scatter of the chemical abundances among globular cluster red giants has been observed. Especially the chemical elements C, N, O, Na, Mg, and Al vary form star to star within globular clusters. Except for $\omega$ Cen and M22, most globular clusters could be considered to be monometallic of their iron peak elements within error ranges. The variations in light elements among globuar cluster giants appear much more pronounced than in field halo giants of comparable Fe-peak metallicity. It has been found that in general the nitrogen abundance is anticorrelated with both carbon and oxygen, while it is correlated with Na and AI. These intracluster abundance inhomogeneities can be interpreted either by mixing of nucleosythesized material from the deep stellar interior during the red giant branch phase of evolution or by inhomogeneities of primordially processed material, from which the stars were formed. The simple way of distingushing between two senarios is to obtain the element abundances of main-sequence stars in globular clusters, which are too faint for high resolution spectroscopic studies until now. Both 'evolutionary' and 'primodial' origins are accepted for explanations of abundance variations among red giants and CN-CH anticorrelations among main-sequence stars in globular clusters. This paper reviews chemical abundances of light elements among globular cluster giants, with brief reviews of cannonical stellar evolution of low mass stars after main-sequence and deep mixing for abundance variations of cluster giants, and a possible connection between deep mixing and second parameter.

• The Bulletin of The Korean Astronomical Society
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• v.39 no.1
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• pp.61.2-61.2
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• 2014

We present a set of fiducial sequences of horizontal-branch stars in bright Galactic globular clusters, which have previously been observed in the Sloan Digital Sky Survey (SDSS). We derive fiducial lines on color-magnitude diagrams in multiple color indices (g - r, g - i, g - z, and u - g), after rejecting foreground and background objects as well as RR Lyrae variables utilizing these color indices. We compare our fiducial sequences with model predictions from Yonsei-Yale evolutionary tracks and BaSel spectral libraries, and find a satisfactory agreement between them in terms of their color-magnitude relations, except in u - g. We also compare theoretical models to color-magnitude diagrams of two open clusters (M67 and NGC 6791). Based on our best available cluster distance and reddening, we find that the mass of red clump (RC) stars in NGC 6791 is about a factor of two smaller than an earlier estimate from the application of asteroseismic scaling relations for solar-like oscillations. The smaller RC mass implies an enhanced mass loss along the red giant branch, which is in accordance with other compelling evidences found in this metal-rich system. Our estimated luminosity of RC stars in NGC 6791 is about 0.2 mag fainter than in earlier investigations based on solar-metallicity calibrations, and results in ~10% reduction in the RC-based distance estimation, when applied to metal-rich systems such as in the Galactic bulge.

• The Bulletin of The Korean Astronomical Society
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• v.38 no.1
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• pp.59.2-59.2
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• 2013

Wilson and Bappu found a tight correlation between the stellar absolute visual magnitude (MV) and the width of the Ca II K emission line for late-type stars in 1957. Here, we revisit the Wilson-Bappu relationship (hereafter, WBR) to claim that WBR can be an excellent indicator of stellar surface gravity of late-type stars as well as a distance indicator. We have measured the width (W) of the Ca II K emission line in high resolution spectra of 125 late-type stars, which were obtained with Bohyunsan Optical Echelle Spectrograph (BOES) and adopted from the UVES archive. Based on our measurement of the emission line width (W), we have obtained a WBR of $M_V=33.76-18.00{\log}W$. In order to extend the WBR to be a surface gravity indicator, the stellar atmospheric parameters such as effective temperature ($T_{eff}$), surface gravity (logg), metallicity ([Fe/H]), and micro-turbulence (${\xi}_{tur}$) have been derived from the self-consistent detailed analysis using the Kurucz stellar atmospheric model and the abundance analysis code, MOOG. Using these stellar parameters and logW, we found that ${\log}g=-5.85\;{\log}W+9.97\;{\log}T_{eff}-23.48$ for late-type stars.

• Publications of The Korean Astronomical Society
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• v.27 no.4
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• pp.117-122
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• 2012

The interstellar dust grains are formed and supplied to interstellar space from asymptotic giant branch (AGB) stars or supernova remnants, and become constituents of the star- and planet-formation processes that lead to the next generation of stars. Both a qualitative, and a compositional study of this cycle are essential to understanding the origin of the pre-solar grains, the missing sources of the interstellar material, and the chemical evolution of our Galaxy. The AKARI/MIR all-sky survey was performed with two mid-infrared photometric bands centered at 9 and $18{\mu}m$. These data have advantages in detecting carbonaceous and silicate circumstellar dust of AGB stars, and the interstellar polycyclic aromatic hydrocarbons separately from large grains of amorphous silicate. By using the AKARI/MIR All-Sky point source catalogue, we surveyed C-rich and O-rich AGB stars in our Galaxy, which are the dominant suppliers of carbonaceous and silicate grains, respectively. The C-rich stars are uniformly distributed across the Galactic disk, whereas O-rich stars are concentrated toward the Galactic center, following the metallicity gradient of the interstellar medium, and are presumably affected by the environment of their birth place. We will compare the distributions of the dust suppliers with the distributions of the interstellar grains themselves by using the AKARI/MIR All-Sky diffuse maps. To enable discussions on the faint diffuse interstellar radiation, we are developing an accurate AKARI/MIR All-Sky diffuse map by correcting artifacts such as the ionising radiation effects, scattered light from the moon, and stray light from bright sources.

• The Bulletin of The Korean Astronomical Society
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• v.39 no.2
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• pp.84.2-84.2
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• 2014

We report the discovery of a new Milky Way globular cluster (GC) candidate in the Galactic plane. We found this object during our WISE survey of star clusters in the Milky Way. We derived physical parameters of this object using the 2MASS JHK photometry. The color-magnitude diagram of the resolved stars shows a well-developed red giant branch (RGB). We derived its reddening, distance and metallicity. These results indicate that it is probably an old globular cluster, located behind the Galactic disk.

• Journal of The Korean Astronomical Society
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• v.27 no.2
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• pp.177-190
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• 1994

We have examined the photometric evolution of elliptical galaxies, using stellar evolutionary models covering the wide ranges of metallicity and mass, and the different IMFs (simple IMP & time-dependent bimodal IMF). The model with a time-dependent bimodal IMF can reproduce the observed integrated magnitudes and colors at all wavelengths. The computed model shows that the star formation in elliptical galaxies is still going on, although the number of newly born stars is very small. The chemical evolutionary effect is clearly seen in the C-M diagram of computed elliptical galaxies.