• The Bulletin of The Korean Astronomical Society
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• v.42 no.1
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• pp.44.2-44.2
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• 2017

Multiple stellar systems composed of triple, double+double or double+triple, etc. are very rare and interesting objects for understanding the star formation and dynamical evolution. However, only six systems have been found to be a doubly eclipsing quadruple, which consists of two eclipsing binaries, and four systems to be a triply eclipsing hierarchical triple. Recently, the 15 doubly eclipsing multiple candidates located in the Large Magellanic Cloud (LMC) have been reported by the OGLE project. In order to examine whether these candidates are real multiple systems with eclipsing features, we performed a high-cadence time-series photometry for the LMC using the KMTNet (Korea Microlensing Telescope Network) 1.6 m telescopes in three site (CTIO, SAAO, and SSO) during 2016-2017. The KMTNet data will help reveal the photometric properties of the multiple-star candidates. In this paper, we present the VI light curves and their preliminarily analyses for 12 of the 15 eclipsing systems in the LMC, based on our KMTNet observations and the OGLE-III survey data from 2001-2009.

• The Bulletin of The Korean Astronomical Society
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• v.38 no.2
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• pp.63.2-63.2
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• 2013

Unlike the conventional wisdom, observations made during the past decade have revealed that many globular clusters possess more than one stellar population. Here, we have discovered evidence for multiple red giant branches (RGBs) in the globular cluster NGC 6388 from the narrow-band Calcium and Str$\ddot{o}$mgren b & y (Caby) photometry. In order to confirm the difference in Calcium abundance, we have acquired the low resolution spectroscopy for these RGB stars. In this paper, we will present results of our photometry and hand in the preliminary results of spectroscopic observations.

• The Bulletin of The Korean Astronomical Society
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• v.37 no.2
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• pp.140.2-140.2
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• 2012

The silicate carbon star IRAS 09425-6040 shows very conspicuous crystalline silicate dust features and excessive emission at far infrared. To investigate properties of dusty envelopes around the object, we use radiative transfer models for axisymmetric and sphericallly symmetric dust distributions. We perform model calculations for various possible combinations of dust shells and disks with various dust species. We compare the model results with the observed spectral energy distributions (SEDs) including the IRAS, ISO, AKARI, MSX and 2MASS data. We find that a model with multiple disks of amorphous and crystalline silicate and multiple spherical shells of carbon dust can reproduce the observed SED fairly well. This supports the scenario for the origin of silicate carbon stars that oxygen-rich material was shed by mass loss when the primary star was an M giant and the O-rich material is stored in a circumbinary disk. Highly (about 75 %) crystallized forsterite dust in the disk can reproduce the conspicuous crystalline features of the ISO observational data. This object looks to have a detached silicate and H2O ice shell with a much higher mass-loss rate. It could be a remnant of the chemical transition phase. The last phase of stellar winds of O-rich materials looks to be a superwind.

• Journal of The Korean Astronomical Society
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• v.53 no.5
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• pp.103-115
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• 2020

We analyze the spatially resolved kinematics of gas and stars for a sample of ten hidden type 1 AGNs in order to investigate the nature of their central sources and the scaling relation with host galaxy stellar velocity dispersion. We select our sample from a large number of hidden type 1 AGNs, which are identified based on the presence of a broad (full width at half maximum ≳1000 km s^-1) component in the Hα line profile and which are frequently mis-classified as type 2 AGNs because AGN continuum and broad emission lines are weak or obscured in the optical spectral range. We used the Blue Channel Spectrograph at the 6.5-m Multiple Mirror Telescope to obtain long-slit data with a spatial scale of 0.3 arcsec pixel^-1. We detected broad Hβ lines for only two targets; however, the presence of strong broad Hα lines indicates that the AGNs we selected are all low-luminosity type 1 AGNs. We measured the velocity, velocity dispersion, and flux of stellar continuum and gas emission lines (i.e., Hβ and [O III]) as a function of distance from the center. The spatially resolved gas kinematics traced by Hβ or [O III] are generally similar to the stellar kinematics except for the inner center, where signatures of gas outflows are detected. We compare the luminosity-weighted effective stellar velocity dispersions with the black hole masses and find that our hidden type 1 AGNs, which have relatively low back hole masses, follow the same scaling relation as reverberation-mapped type 1 AGN and more massive inactive galaxies.

• The Bulletin of The Korean Astronomical Society
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• v.41 no.1
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• pp.41.1-41.1
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• 2016

We present the physical properties of KIC 6220497 exhibiting multiperiodic pulsations from the Kepler photometry. The light curve synthesis represents that the eclipsing system is a semi-detached Algol with a mass ratio of q=0.243, an orbital inclination of i=77.3 deg, and a temperature difference of ${\Delta}T=3,372K$, in which the detached primary component fills its Roche lobe by ~87% and is about 1.6 times larger than the lobe-filling secondary. To detect reliable pulsation frequencies, we analyzed separately the Kepler light curve at the interval of an orbital period. Multiple frequency analyses of the eclipse-subtracted light residuals reveal 32 frequencies in the range of $0.75-20.22d^{-1}$ with semi-amplitudes between 0.27 and 4.55 mmag. Among these, four frequencies ($f_1$, $f_2$, $f_5$, $f_7$) may be attributed to pulsation modes, while the other frequencies can be harmonic and combination terms. The pulsation constants of 0.16-0.33 d and the period ratios of $P_{pul}/P_{orb}=0.042-0.089$ indicate that the primary component is a ${\delta}$ Sct pulsating star in p modes and, thus, KIC 6220497 is an oscillating eclipsing Algol (oEA) star. The dominant pulsation period of about 0.1174 d is considerably longer than the values given by the empirical relations between the pulsational and orbital periods. The surface gravity of log $g_1=3.78$ is significantly smaller than those of the other oEA stars with similar orbital periods. The pulsation period and the surface gravity of the pulsating primary demonstrate that KIC 6220497 would be the more evolved EB, compared with normal oEA stars.

• The Bulletin of The Korean Astronomical Society
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• v.39 no.2
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• pp.86.1-86.1
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• 2014

Two substellar companion candidates with planetary mass, around a T-Tauri star in the ${\rho}$ Ophiuchi star-forming region, are discovered by results of Subaru Telescope's near-infrared imaging. Candidates are separated by 1400au and 500au. If these candidates were real companions, they are the widest-orbit and the lowest mass planetary-mass companions(PMCs) candidates. This discovery may suggest that PMCs form via extreme case of cloud core fragmentation for multiple stars. And also stellar disk are imaged by HiCIAO, hight contrast instrument for exoplanets and disks, with Subaru Telescope. This could be the first case, which imaged both of planetary mass companions and disk around same star. Even two companions candidates are not bounded around the star, they still could be one of the lowest mass objects. In this presentation, I will discuss about observations and confirmations of these objects, and the latest results about their properties.

• The Bulletin of The Korean Astronomical Society
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• v.41 no.2
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• pp.46.4-47
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• 2016

About 50% of stars reside in binary or multiple systems. However, the formation mechanism of the multiplicity is poorly understood. Theoretical studies suggest two main mechanisms for the multiplicity: turbulent fragmentation and disk fragmentation. We can testify which mechanism is more plausible by measuring the separation between companions or the alignment of stellar spins. Here we present our ALMA Cycle 2 observational results of a proto-binary system, IRAS 04191+1523, which consists of two Class I sources. We detected disks around both Class I sources, which are located in a common dense filamentary structure traced by $C^{18}O$ J=2-1. Two protostellar disks are separated by ~900 AU and their rotational axes are almost perpendicular, which strongly support that this binary system formed by the turbulent fragmentation.

• The Bulletin of The Korean Astronomical Society
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• v.36 no.2
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• pp.67.1-67.1
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• 2011

Recent observations and modeling of globular clusters with multiple populations strongly indicate the presence of super helium-rich subpopulations in old stellar systems. Motivated by this, we have constructed new population synthesis models with and without helium-enhanced subpopulations to investigate their impact on the UV-upturn phenomenon of quiescent early-type galaxies. We find that our models with helium-enhanced subpopulations can naturally reproduce the strong UV-upturns observed in giant elliptical galaxies assuming an age similar to that of old globular clusters in the Milky Way. The major source of far-UV (FUV) flux, in this model, is relatively metal-poor and helium-enhanced hot horizontal branch stars and their progeny. The Burstein et al. (1988) relation of the FUV - V color with metallicity is also explained either by the variation of the fraction of helium-enhanced subpopulations or by the spread in mean age of stellar populations in early-type galaxies.

• The Bulletin of The Korean Astronomical Society
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• v.39 no.1
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• pp.60.1-60.1
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• 2014

The star ${\zeta}$ Ophiuchi (HD 149757) is one of the brightest massive stars in the northern hemisphere and was widely studied in various wavelength domains. We report the analysis results of far ultraviolet (FUV) observations with other wavelengths for around ${\zeta}$ Ophiuchi. We study the correlation of between multi wavelength observations. We have developed a Monte Carlo code that simulates dust scattering of light including multiple encounters. The code is applied to the present Oph HII region to obtain the geometrical information of dust such as distance and thickness. Also We apply three-dimensional photoionization code to model Wisconsin $H{\alpha}$ Mapper observations of the H II region surrounding the star.

• The Bulletin of The Korean Astronomical Society
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• v.43 no.2
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• pp.39.3-40
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• 2018

The presence of double red clump (RC) in the Milky Way bulge is widely accepted as evidence for a giant X-shaped structure originated from the bar instability. We suggested, however, a drastically different interpretation based on the multiple stellar populations phenomenon as is observed in globular clusters. Our discovery of a significant difference in CN-band between two RCs strengthens our scenario. On the other hand, recent Gaia survey provides trigonometric parallax distances for more than one billion stars in our Galaxy. These distance measurements would provide the important test as to the origin of the double RC in the Milky Way bulge. In this talk, we will present our preliminary results from Gaia DR2.