• 천문학회지
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• 제50권1호
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• pp.1-5
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• 2017

Like Hipparcos, Gaia is designed to give absolute parallaxes, independent of any astrophysical reference system. And indeed, Gaia's internal zero-point error for parallaxes is likely to be smaller than any individual parallax error. Nevertheless, due in part to mechanical issues of unknown origin, there are many astrophysical questions for which the parallax zero-point error ${\sigma}({\pi}_0)$ will be the fundamentally limiting constraint. These include the distance to the Large Magellanic Cloud and the Galactic Center. We show that by using the photometric parallax estimates for RR Lyrae stars (RRL) within 8kpc, via the ultra-precise infrared period-luminosity relation, one can independently determine a hyper-precise value for ${\pi}_0$. Despite their paucity relative to bright quasars, we show that RRL are competitive due to their order-of-magnitude improved parallax precision for each individual object relative to bright quasars. We show that this method is mathematically robust and well-approximated by analytic formulae over a wide range of relevant distances.

• Journal of Astronomy and Space Sciences
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• 제21권4호
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• pp.275-282
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• 2004

2004년 6월부터 7월까지 총 13일간 레몬산천문대의 1m 망원경과 BVR 필터를 사용하여 접촉 쌍성 CK Boo의 광도곡선을 완성하고, 4개(주극심 3개, 부극심 1개)의 새로운 극심시각을 산출하였다. 1998년에 개정한 Wilson-Devinney 쌍성모델의 접촉모드를 이용하여, 우리의 BVR 광도곡선과 Rucinski & Lu(1999)의 시선속도곡선을 분석하였다. 그 결과, 우리는 CK Boo가 질량비(q=0.11)와 궤도 경사각($i=65^{\circ}$)이 작은 A형 과접촉쌍성($f=84\%$)임을 확인하였다. 우리의 측광 및 분광학적 해로 부터 이 쌍성계의 절대 물리량을 $M_1=1.42Me{\odot},\;M_2=0.15M{\odot},\;R_1=1.47R{\odot}$, 그리고 $R_2=0.59M{\odot}$으로, 거리를 129pc으로 산출하였다. 우리가 구한 거리는 Hipparcos 삼각시차에 의한 거리($157{\pm}33pc$)와 오차의 범위 내에서 일치한다.

• 한국지구과학회지
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• 제27권5호
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• pp.557-568
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• 2006

구상성단 M30의 $20'.5{\times}20'.5$ 영역에 대한 CCD UBVI 측광 관측을 수행하여 색-등급도로부터 주계열 전향점과 색지수 $V_{TO}=18.63{\pm}0.05,\;(B-V)_{TO}=0.44{\pm}0.05,\;(V-I)_{TO}=0.63{\pm}0.05$를 얻었으며, 색-색도로부터 $E(B-V)=0.05{\pm}0.01$과 중원소 함량의 지표인 UV 색 초과량 ${\delta}(U-B)=0.27{\pm}0.01$을 얻었다. 측광학적인 방법과 분광 관측 자료를 이용하여 중원소 함량 $[Fe/H]=-2.05{\pm}0.09$를 구하였다. 관측된 M30의 광도 함수는 이론적 모델에 비하여 전향점 부근에 비하여 적색 거성열의 초과 현상을 보였다. Hipparcos 위성에서 측정된 삼각 시차로부터 거리가 알려진 준왜성을 이용하여 주계열 맞추기를 하여 거리 지수 $(m-M)_o=14.75{\pm}0.12$를 구하였다. 헬륨 함량을 구하기 위하여 R과 R' 방법을 사용하여 $Y(R)=0.23{\pm}0.02,\;Y(R')=0.29{\pm}0.02$를 얻었다. 성단의 나이는 적용하는 방법과 모델에 따라서 10.7 Gyr에서 17 Gyr까지 분산을 보인다.

• 천문학회지
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• 제51권1호
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• pp.17-25
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• 2018

Detecting exoplanets around giant stars sheds light on the later-stage evolution of planetary systems. We observed the M giant HD 18438 and the K giant HD 158996 as part of a Search for Exoplanets around Northern circumpolar Stars (SENS) and obtained 38 and 24 spectra from 2010 to 2017 using the high-resolution Bohyunsan Observatory Echelle Spectrograph (BOES) at the 1.8m telescope of Bohyunsan Optical Astronomy Observatory in Korea. We obtained precise RV measurements from the spectra and found long-period radial velocity (RV) variations with period 719.0 days for HD 18438 and 820.2 days for HD 158996. We checked the chromospheric activities using Ca $\text\tiny{II}$ H and $H{\alpha}$ lines, HIPPARCOS photometry and line bisectors to identify the origin of the observed RV variations. In the case of HD 18438, we conclude that the observed RV variations with period 719.0 days are likely to be caused by the pulsations because the periods of HIPPARCOS photometric and $H{\alpha}$ EW variations for HD 18438 are similar to that of RV variations in Lomb-Scargle periodogram, and there are no correlations between bisectors and RV measurements. In the case of HD 158996, on the other hand, we did not find any similarity in the respective periodograms nor any correlation between RV variations and line bisector variations. In addition, the probability that the real rotational period can be as longer than the RV period for HD 158996 is only about 4.3%. Thus we conclude that observed RV variations with a period of 820.2 days of HD 158996 are caused by a planetary companion, which has the minimum mass of 14.0 $M_{Jup}$, the semi-major axis of 2.1 AU, and eccentricity of 0.13 assuming the stellar mass of $1.8 M_{\odot}$. HD 158996 is so far one of the brightest and largest stars to harbor an exoplanet candidate.

• Journal of Astronomy and Space Sciences
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• 제24권1호
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• pp.31-38
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• 2007

We have investigated the evolutionary status of 85 Peg within the framework of standard evolutionary theory. 85 Peg has been known to be a visual and spectroscopic binary system in the solar neighborhood. In spite of the accurate information of the total mass (${\sim}1.5M_{\odot}$) and the distance (${\sim}12pc$) from the HIPPARCOS parallax, it has been undetermined an individual mass, therefore the evolved status of the system. Moreover, the coupled uncertainties of chemical composition and age, make matters worse in predicting an evolutionary status of the system. Nevertheless, we computed the various possible models for 85 Peg, and then calibrated stellar parameters by adjusting to the recent observational data. Our modelling computation has included recently updated input physics and stellar theory such as opacity, equation of state, and chemical diffusion. Through a statistical assessment, we have derived a confident parameter set as the best solution which minimized $X^{2}$ within the observational error domain. Most of all, we found that 85 Peg is not a binary system but a triple system with an unseen companion 85 Peg $B_{b}\;{\sim}0.16M_{\odot}$. The aim of the present paper is (1) to provide a complete modelling of the stellar system based on the evolutionary theory, and (2) to constrain the physical dimensions such as mass, metallicity and age.

• 한국우주과학회:학술대회논문집(한국우주과학회보)
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• 한국우주과학회 2008년도 한국우주과학회보 제17권2호
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• pp.23.2-23.2
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• 2008

We presented the accurate absolute dimensions and distances of fifteen main sequence eclipsing binaries. The photometric and spectroscopic solutions of the binary systems were determined by analyzing light curves and radial velocity curves collected from the literature using the Wilson-Devinney computer code. The fifteen double-line spectroscopic binaries consist of nine detached systems; QX Car, AH Cep, CW Cep, ZZ Cep, XY Cet, RX Her, V451 Oph, VV Pyx and V760 Sco, six semi-detached systems; LY Aur, IU Aur, AO Cas, DM Per, V Pup and HU Tau. The temperatures of the binary systems were determined from their colors using the color-temperature calibrations. Then the temperature of each component star were determined using the temperature ratio which was adjusted from the light curves. We estimated the possible Z values and ages for the detached systems by adopting the Y2 (Yonsei-Yale) stellar evolutionary tracks. The derived distances are in good agreement of the Hipparcos distances whose error of parallax is within 10 %. Finally these well-investigated systems will be used as the standard eclipsing binaries.

• 천문학회지
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• 제48권3호
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• pp.165-175
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• 2015

We investigate physical properties of the nearby (∼ 7.5 pc) astrometric binary μ Cas in the context of standard evolutionary theory. Based on the spectroscopically determined relative abundances ([α/Fe] ≳ +0.4 dex, [Fe/H] ∼ −0.7 dex), all physical inputs such as opacities and equation of state are consistently generated. By combining recent spectroscopic analyses with the astrometric observations from the HIPPARCOS parallaxes and the CHARA array, the evolutionary model grids have been constructed. Through the statistical evaluation of the χ²-minimization among alternative models, we find a reliable evolutionary solution (M_A, M_B, t_age) = (0.74 M_⊙, 0.19 M_⊙, 11 Gyr) which excellently satisfies observational constraints. In particular, we find that the helium abundance of μ Cas is comparable with the primordial helium contents (Y_p ∼ 0.245). On the basis of the well-defined stellar parameters of the primary star, the internal structure and the p-mode frequencies have been estimated. From our seismic computation, μ Cas is expected to have a first order spacing ∆ν ∼ 169 μHz. The ultimate goal of this study is to describe physical processes inside a low-mass star through a complete modelling from the spectroscopic observation to the evolutionary computation.

• 천문학회보
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• 제37권1호
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• pp.77.1-77.1
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• 2012

We present high-resolution radial velocity measurements of K2 giant ${\varepsilon}$ CrB from February 2005 to January 2012 using the fiber-fed Bohyunsan Observatory Echelle Spectrograph at Bohyunsan Optical Astronomy Observatory. We find that the RV measurements for ${\varepsilon}$ CrB exhibit a periodic variation of 418 days with a semi-amplitude of 129 m/s. There is no correlation with RV measurements and inhomogeneous surface features by examining chromospheric activity indicator (Ca II H region), the Hipparcos photometry, and bisector velocity span. Thus, Keplerian motion is the most likely explanation, which suggests that the RV variations arise from an orbital motion. Assuming a possible stellar mass of 1.7 $M_{\odot}$, for ${\varepsilon}$ CrB, we obtain a minimum mass for the planetary companion of 6.7 $M_{Jup}$ with an orbital semi-major axis of 1.3 AU, and an eccentricity of 0.11. We support that more massive stars harbor more massive planetary companions in giant hosting planetary companions (Dollinger et al. 2009), as well as, we discuss the frequency of detected planetary companions with the metallicity distribution in giant (Pasquini et al. 2007; Quirrenbach et al. 2011).

• 천문학회보
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• 제36권2호
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• pp.140.2-140.2
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• 2011

We present high-resolution radial velocity (RV) measurements of K2 giant HD 66141 from December 2003 to January 2011 using the fiber-fed Bohyunsan Observatory Echelle Spectrograph (BOES) at Bohyunsan Optical Astronomy Observatory (BOAO). We find that the RV measurements for HD 66141 exhibit a periodic variation of 480 days with a semi-amplitude of 146 m/s. We do not find the correlation between RV variations and a chromospheric activity indicator (H line). The Hipparcos photometry as well as bisector velocity span (BVS) also do not show any obvious correlations with RV variations. Thus, Keplerian motion is the most likely explanation, which suggests that the RV variations arise from an orbital motion. Assuming a possible stellar mass of 1.5 $M{\odot}$, for HD 66141, we obtain a minimum mass for the planetary companion of 7.4 MJup with an orbital semi-major axis of 1.4 AU, and an eccentricity of 0.07. We support that planet occurrence rate around evolved stars is more than 10 % (Dollinger et al. 2009) as well as more massive stars do form significantly more massive planetary companions (Johnson et al. 2007; Lovis & Mayor 2007; Dollinger et al. 2009).

• 천문학논총
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• 제22권4호
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• pp.89-95
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• 2007

The warm ionized medium (WIM) outside classical H II regions is a fundamental gas-phase constituent of the Milky Way and other late-type spiral galaxies, and is traced by faint emission lines at optical wavelengths. We calculate the photoionization models of the WIM in the Galaxy by a stellar UV radiation with the effective temperature 35,000 K assuming not only spherical geometry but also plane parallel geometry, and compare the results with the observed emission line ratios. We also show the dependence of the emission line ratios on various gas-phase abundances. The emergent emission-line ratios are in agreement with the average-values of observed ratios of [S II] ${\lambda}6716/H{\alpha}$, [N II] ${\lambda}6583/H{\alpha}$, [O I] ${\lambda}6300/H{\alpha}$, [O III] ${\lambda}5007/H{\alpha}$, He I ${\lambda}5876/H{\alpha}$. However, their extreme values could not be explained with the photoionization models. It is also shown that the addition of all stellar radiation from the OB stars in the Hipparcos stellar catalog resembles that of an O7-O8 type star.