• 천문학회보
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• 제42권2호
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• pp.74.2-74.2
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• 2017

Faint quasars are important to test the possibility that quasars are the main contributor to the cosmic reionization. However, it has been difficult to find faint quasars due to the lack of deep, wide-field imaging data. In this poster, we present our efforts to find faint quasars in the ELAIS-N1 field through the deep data (iAB ~ 25) obtained by the Subaru Hyper Suprime-Cam (HSC) Strategic Program survey. To select reliable quasar candidate, we also use the near-infrared (NIR) data of the Infrared Medium-deep Survey (IMS) and the UKIRT Infrared Deep Sky Survey (UKIDSS) - Deep Extragalactic Survey (DXS). Using multiple-band color cuts, we select high redshift quasar candidates. To confirm them as high redshift quasars, candidates are observed by the SED camera for QUasars in EArly uNiverse (SQUEAN) instrument in several medium band filters that can sample the redshifted Lyman break efficiency. The quasar sample will be used to study the growth of BH and stellar mass, the relation between the quasar activity and the host galaxy, and their contribution to the cosmic re-ionization.

• 천문학회보
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• 제35권1호
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• pp.57-57
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• 2010

We report the discovery of a bright mid-infrared (MIR) source with prominent crystalline silicate emission using the space telescope AKARI and Spitzer. This source, IRAS 15099-5856, has a spectacular morphology with a bright central compact source (CCS) surrounded by knots, spurs, and several extended (~4') arc-like filaments. The source is seen only in infrared at ${\geq}10{\mu}m$. The Spitzer MIR spectrum of the CCS shows prominent emission features from Mg-rich crystalline silicates and strong [Ne II] 12.88 ${\mu}m$ and several other faint ionic lines. We model the MIR spectrum as thermal emission from several independent dust components and compare their properties to those of the Herbig Be star HD 100546 which shows very similar MIR spectrum. Our molecular line observations reveal two molecular clouds around the source, but no associated dense molecular cores. We discuss two possible origins for IRAS 15099-5856; a deeply embedded massive young stellar object on the other side of the Galaxy and a disrupted, protoplanetary disk being photoevaporated by the UV radiation from the nearby O star Muzzio 10.

• 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.

• 천문학회보
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• 제38권1호
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• pp.36.2-36.2
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• 2013

The SPICA (SPace Infrared Telescope for Cosmology & Astrophysics) project is a next-generation infrared space telescope optimized for mid- and far-infrared observation with a cryogenically cooled 3m-class telescope. The focal plane instruments onboard SPICA will enable us to resolve many astronomical key issues from the formation and evolution of galaxies to the planetary formation. The FPC-S (Focal Plane Camera - Sciecne) is a near-infrared instrument proposed by Korea as an international collaboration. Owing to the capability of both low-resolution imaging spectroscopy and wide-band imaging with a field of view of $5^{\prime}{\times}5^{\prime}$, it has large throughput as well as high sensitivity for diffuse light compared with JWST. In order to strengthen advantages of the FPC-S, we propose the studies of probing population III stars by the measurement of cosmic near-infrared background radiation and the star formation history at high redshift by the discoveries of active star-forming galaxies. In addition to the major scientific targets, to survey large area opens a new parameter space to investigate the deep Universe. The good survey capability in the parallel imaging mode allows us to study the rare, bright objects such as quasars, bright star-forming galaxies in the early Universe as a way to understand the formation of the first objects in the Universe, and ultra-cool brown dwarfs. Observations in the warm mission will give us a unique chance to detect high-z supernovae, ices in young stellar objects (YSOs) even with low mass, the $3.3{\mu}$ feature of shocked circumstance in supernova remnants. Here, we report the current status of SPICA/FPC project and its extragalactic sciences.

• 천문학회보
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• 제44권1호
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• pp.42.1-42.1
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• 2019

We observed 80 dense cores ($N(H_2)$ > $10^{22}cm^{-2}$) in the Orion molecular cloud complex which contains the Orion A (39 cores), B (26 cores), and ${\lambda}$ Orionis (15 cores) clouds. We investigate the behavior of the different molecular tracers and look for chemical variations of cores in the three clouds in order to systematically investigate the effects of stellar feedback. The most commonly detected molecular lines (with the detection rates higher than 50%) are $N_2H^+$, $HCO^+$, $H^{13}CO^+$, $C_2H$, HCN, and $H_2CO$. The detection rates of dense gas tracers, $N_2H^+$, $HCO^+$, $H^{13}CO^+$, and $C_2H$ show the lowest values in the ${\lambda}$ Orionis cloud. We find differences in the D/H ratio of $H_2CO$ and the $N_2H^+/HCO^+$ abundance ratios among the three clouds. Eight starless cores in the Orion A and B clouds exhibit high deuterium fractionations, larger than 0.10, while in the ${\lambda}$ Orionis cloud, no cores reveal the high ratio. These chemical properties could support that cores in the ${\lambda}$ Orionis cloud are affected by the photo-dissociation and external heating from the nearby H II region. An unexpected trend was found in the $[N_2H^+]/[HCO^+]$ ratio with a higher median value in the ${\lambda}$ Orionis cloud than in the Orion A/B clouds than; typically, the $[N_2H^+]/[HCO^+]$ ratio is lower in higher temperatures and lower column densities. This could be explained by a longer timescale in the prestellar stage in the ${\lambda}$ Orionis cloud, resulting in more abundant nitrogen-bearing molecules. In addition to these chemical differences, the kinematical difference was also found among the three clouds; the blue excess, which is an infall signature found in optically thick line profiles, is 0 in the ${\lambda}$ Orionis cloud while it is 0.11 and 0.16 in the Orion A and B clouds, respectively. This result could be another evidence of the negative feedback of active current star formation to the next generation of star formation.

• 한국지구과학회지
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• 제36권7호
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• pp.661-673
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• 2015

우즈베키스탄 Maidanak 천문대 AZT-22 1.5 m 망원경(f/7.74)과 SITe $2000{\times}800$CCD를 사용하여 3개의 ${\delta}$ Cep 변광성 CEa Cas, CEb Cas 및 CF Cas가 있는 중년 산개성단 NGC 7790의 UBVI 측광 관측을 수행하였다. 관측자료는 IRAF/DAOPHOT를 이용하여 PSF 측광을 수행하였으며, V와 I 필터에서 모두 측광된 별의 수는 총 1008개이며, 측광의 한계등급은 $V{\approx}22$등급이다. 대기소광계수 및 측광영점을 결정하기 위하여 천구적도에 있는 여러 표준별과 청색 및 적색 표준별을 다양한 대기투과량에서 관측하였다. 표준별 측광을 통해 대기소광계수 및 측광영점을 결정하고, 성단의 측광자료는 Johnson-Cousins UBVI 표준계로 변환하였다. NGC 7790의 UBVI 색-등급도와 색-색도에서 이 성단의 B와 V 필터에서의 색초과량 [$E(B-V)=0.58{\pm}0.02$], B와 V 필터에서의 선택적 소광량 [$R_V{{\equiv}}A_V/E(B-V)=3.02{\pm}0.09$] 및 거리지수($V_0-M_V=12.65{\pm}0.10$)를 얻었다. H-R도에서 세페이드 변광성의 위치를 고려하여 나이를 결정하였다. 이 과정에서 Padova 연구집단(Bressan et al., 2012)의 등연령곡선(Z=0.019)과 Geneva 연구집단($Ekstr{\ddot{o}}m$ et al., 2012)의 등연령곡선(Z=0.014)을 모두 사용하였고, 그중에서 자전을 고려한 Geneva 연구집단의 진화모형이 관측자료와 잘 일치하여 NGC 7790의 나이로 log $age=8.05{\pm}0.05$ [yr]를 얻었다. 또 NGC 7790내 세페이드 변광성의 절대등급은 세페이드 변광성의 평균 주기-광도 관계에 비해 분산 범위 내에 있기는 하지만 주어진 주기에서 평균적으로 약 0.5등급 정도 밝음을 확인하였다.

• 천문학회지
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• 제24권2호
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• pp.217-271
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• 1991

We have observed dense core around young stellar objects, DR21, S140, Orion-KL, and L1551 using four millimeter-wave transitions of $HC_3N\;J$=4-3, J=5-4, J=10-9, and J=12-11. The spatial distribution of $HC_3N$ emission closely resembles the morphology of the previous CS observations that trace high density gas. These observations reveal the existence of $HC_3N$ dense cores around central IR source, elliptical in shape and almost perpendicular to the CO bipolar outflow axis. Small differences can be explained by that $HC_3N$ molecular line is more optically thin and is seen to be more detailed structure in the neighborhood of central IR sources. In S140 and Orion-KL, massive(${\sim}10\;M_{\odot}$), slowly rotating dense cores lie near at the central IR sources of bipolar outflows. The velocity channel maps of DR21 show that the bipolar outflow gas may have a correlation with the dense core of DR21. We analyzed intensities of the four lines to derive physical conditions in dense core from two methods, LTE and LVG. The column density of $HC_3N$, $N(HC_3N)$, between LTE and LVG calculations agree well with each other. The abundances of $HC_3N$ in each observing source have been estimated using the average values of $n(H_2)$ and $N(HC_3N)$ and assuming the size of dense core. The fractional $HC_3N$ abundances in massive dense cores of DR21, S140, and Orion-KL have a range of $(2-7){\times}10^{-10}$, while that of low mass dense core, L1551, has one order of magnitude greater value of $2{\times}10^{-9}$. This should be considered good agreement with the result by Morris et al.(1976). It may be considered that dense cores of DR21, S140, and Orion-KL may have almost same stage of chemical evolution, and their abundances have a small values relative to that of L1551. The column density $N(HC_3N)$ decreases with increasing distance from the densest part of the cloud, the central infrared source, and have the relation of $N(HC_3N){\varpropto}R^{\alpha}$, where a has a range of 0.65 to 0.89. The values of $n(H_2)$ are not varied with increasing distance from the dense core, and have almost same values. Therefore, it is considered that the dense cores in these regions probably consist of dense clumps in diffuse molecular gas medium, and $n(H_2)$ of each clump is ${\sim}10^5\;cm^{-3}$. Levels in the $T_{ex}$ increases with $n(H_2)$. It is considered that the $HC_3N$ dense cores are not completely thermalized. We examine the relationships between the luminosity of central infrared sources versus mass of the dense cores, and the luminosity of central infrared sources versus molecular hydrogen column density. Luminosities of the central IR sources show good correlation with mass and hydrogen column density of the dense core. Same has been found from CS observations. However, mass and size derived from $HC_3N$ observations are one order of magnitude smaller than those from CS. It can be interpreted that we see more central part of the cloud cores in $NC_3N$ lines than CS lines.

• Journal of Astronomy and Space Sciences
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• 제26권2호
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• pp.141-156
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• 2009

근접촉쌍성 XZ CMi의 BV 측광관측을 수행하여 새로운 광도곡선과 총 7개의 극심시각을 획득하였다. 관측한 극심시각과 지금까지 발표된 극심시각을 수집하여 XZ CMi의 궤도 공전주기를 분석한 결과, 이 쌍성계의 공전주기가 지난 70년간 영년 주기감소와 더불어 규칙적으로 변화함을 확인하였다. 규칙적인 변화를 제3천체에 의한 광시간 효과로 가정하여 0.0056일의 진폭, 약 29년의 주기, 그리고 0.71의 궤도이심율의 광시간 궤도를 결정하였다. 관측된 영년 주기감소($-5.26{\times}10^{-11}d/P$)를 자기제동 항성풍의 각운동량 손실에 의한 주기감소($-8.20{\times}10^{-11}d/P$)와 질량이 작은 반성에서 주성으로 질량 이동에 의한 주기 증가($2.94{\times}10^{-11}d/P$)가 동시에 일어나는 것으로 해석하였다. 이런 관점에서 AML에 의한 주기감소율은 질량 이동에 의한 공전주기 증가율보다 그 크기가 약 3배 정도 크며, 반성에서 년간 $3.21{\times}10^{-8}M_{\odot}$의 질량이 주성으로 이동된다. 관측된 BV 광도곡선을 최근의 Wilson & Devinney 쌍성코드로 주성의 온도를 달리하는 두가지 모형(8200K와 7000K)을 상정하여 분석하였다. 두가지 모형 해 모두 XZ CMi가 반성이 로쉬 로브를 채웠으나, 주성은 아직 로쉬 로브를 채우지 않은 근접촉 쌍성임과 약 15-17%의 제3광도가 이 계에 있음을 보여준다. 그러나, 제3광도를 내는 천체가 주기연구에서 제안한 제3천체와 동일 천체가 아닌 것은 확실하다. 두 모형의 $\sum(O-C)^2$의 차이는 너무 미미하여, 현 시점에서 어느 해가 더 관측치를 잘 맞추는 지를 가릴 수는 없었다. 그간 연구자간에 불일치하였던 질량비의 다양성 문제는 아직도 풀리지 않는 숙제로 남아있다. 이를 해결하기 위해서 분광시선속도곡선과 스펙트럼의 관측과 더불어 정밀 측광관측이 필요하다.