• Journal of Astronomy and Space Sciences
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• 제29권2호
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• pp.181-189
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• 2012

To broaden the understanding of classical Cepheid structure, evolution and atmospheres, we have extended our continuing secret lives of Cepheids program by obtaining XMM/Chandra X-ray observations, and Hubble space telescope (HST) / cosmic origins spectrograph (COS) FUV-UV spectra of the bright, nearby Cepheids Polaris, ${\delta}$ Cep and ${\beta}$ Dor. Previous studies made with the international ultraviolet explorer (IUE) showed a limited number of UV emission lines in Cepheids. The well-known problem presented by scattered light contamination in IUE spectra for bright stars, along with the excellent sensitivity & resolution combination offered by HST/COS, motivated this study, and the spectra obtained were much more rich and complex than we had ever anticipated. Numerous emission lines, indicating $10^4$ K up to ${\sim}3{\times}10^5$ K plasmas, have been observed, showing Cepheids to have complex, dynamic outer atmospheres that also vary with the photospheric pulsation period. The FUV line emissions peak in the phase range ${\varphi}{\approx}0.8-1.0$ and vary by factors as large as $10{\times}$. A more complete picture of Cepheid outer atmospheres is accomplished when the HST/COS results are combined with X-ray observations that we have obtained of the same stars with XMM-Newton & Chandra. The Cepheids detected to date have X-ray luminosities of log $L_X{\approx}28.5-29.1$ ergs/sec, and plasma temperatures in the $2-8{\times}106$ K range. Given the phase-timing of the enhanced emissions, the most plausible explanation is the formation of a pulsation-induced shocks that excite (and heat) the atmospheric plasmas surrounding the photosphere. A pulsation-driven ${\alpha}^2$ equivalent dynamo mechanism is also a viable and interesting alternative. However, the tight phase-space of enhanced emission (peaking near 0.8-1.0 ${\varphi}$) favor the shock heating mechanism hypothesis.

• 천문학회보
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• 제39권1호
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• pp.53.2-53.2
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• 2014

IGRINS, the Immersion GRating INfrared Spectrometer, is a near-infrared wide-band high-resolution spectrograph jointly developed by the Korea Astronomy and Space Science Institute and the University of Texas at Austin. IGRINS employs three HAWAII-2RG focal plane array (FPA) detectors. The mechanical mounts for these detectors serves a critical function in the overall instrument design: Optically, they permit the only positional compensation in the otherwise "build to print" design. Thermally, they permit setting and control of the detector operating temperature independently of the cryostat bench. We present the design and fabrication of the mechanical mount as a single module. The detector mount includes the array housing, a housing for the SIDECAR ASIC, a field flattener lens holder, and a support base. The detector and ASIC housing will be kept at 65 K and the support base at 130 K. G10 supports thermally isolate the detector and ASIC housing from the support base. The field flattening lens holder attaches directly to the FPA array housing and holds the lens with a six-point kinematic mount. Fine adjustment features permit changes in axial position and in yaw and pitch angles. We optimized the structural stability and thermal characteristics of the mount design using computer-aided 3D modeling and finite element analysis. Based on the computer simulation, the designed detector mount meets the optical and thermal requirements very well.

• 천문학회보
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• 제44권2호
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• pp.64.2-64.2
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• 2019

HD 81817 is known as a hybrid star. Hybrid stars have both cool stellar wind properties and UV or even X-ray emission features of highly ionized atoms in their spectra. A white dwarf companion has been suggested as the source of UV or X-ray features. HD 81817 has been observed since 2004 as a part of our radial velocity (RV) survey program to search for exoplanets around K giant stars using the Bohyunsan Observatory Echelle Spectrograph (BOES) at the 1.8 m telescope of Bohyunsan Optical Astronomy Observatory (BOAO) in Korea. We obtained 84 RV measurements between 2004 and 2018 for HD 81817 and found two periodic RV variations. The obtained amplitudes of RV periods are around 200 m/s, which are significantly lower than that expected from a white dwarf companion. Furthermore, our re-analysis of the IUE spectra used by Reimers (1984) shows that the excess in UV emission can easily be explained by a pseudo-continuum of unresolved emission lines originating in the extended chromosphere of the star. We thus conclude that there are no companions of stellar mass to HD 81817. Meanwhile, we analyzed two periodicities in RV measurements and conclude that the period of 627.9 days is caused by intrinsic stellar activities based on H alpha equivalent width (EW) variations of a similar period. On the other hand, the period of 1047.8 days is likely to be caused by substellar companion which has a minimum mass of 27.6 M_JUP, a semi-major axis of 3.3 AU, and an eccentricity of 0.17 assuming the stellar mass of 4.3 M_⊙ for HD 81817. The inferred mass puts HD 81817 b in the brown dwarf desert.

• 천문학회지
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• 제56권2호
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• pp.277-286
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• 2023

It is difficult to distinguish the pure signal produced by an orbiting planetary companion around giant stars from other possible sources, such as stellar spots, pulsations, or certain activities. Since 2003, we have obtained radial (RV) data from evolved stars using the high-resolution, fiber-fed Bohyunsan Observatory Echelle Spectrograph (BOES) at the Bohyunsan Optical Astronomy Observatory (BOAO). Here, we report the results of RV variations in the binary star HD 135438. We found two significant periods: 494.98 d with eccentricity of 0.23 and 8494.1 d with eccentricity of 0.83. Considering orbital stability, it is impossible to have two companions in such close orbits with high eccentricity. To determine the nature of the changes in the RV variability, we analyzed indicators of stellar spot and stellar chromospheric activity to find that there are no signals related to the significant period of 494.98 d. However, we calculated the upper limits of rotation period of the rotational velocity and found this to be 478-536 d. One possible interpretation is that this may be closely related to the rotational modulation of an orbital inclination at 67-90 degrees. The other signal corresponding to the period of 8494.1 d is probably associated with a stellar companion orbiting the giant star. A Markov Chain Monte Carlo (MCMC) simulation considering a single companion indicates that HD 135438 system hosts a stellar companion with 0.57^+0.017 _-0.017 M_⊙ with an orbital period of 8498 d.

• 천문학회지
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• 제56권2호
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• pp.195-199
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• 2023

This paper is written as a follow-up observations to reinterpret the radial velocity (RV) of HD 36384, where the existence of planetary systems is known to be ambiguous. In giants, it is, in general, difficult to distinguish the signals of planetary companions from those of stellar activities. Thus, known exoplanetary giant hosts are relatively rare. We, for many years, have obtained RV data in evolved stars using the high-resolution, fiber-fed Bohyunsan Observatory Echelle Spectrograph (BOES) at the Bohyunsan Optical Astronomy Observatory (BOAO). Here, we report the results of RV variations in the M giant HD 36384. We have found two significant periods of 586 d and 490 d. Considering the orbital stability, it is impossible to have two planets at so close orbits. To determine the nature of the RV variability variations, we analyze the HIPPARCOS photometric data, some indicators of stellar activities, and line profiles. A significant period of 580 d was revealed in the HIPPARCOS photometry. H_α EW variations also show a meaningful period of 582 d. Thus, the period of 586 d may be closely related to the rotational modulations and/or stellar pulsations. On the other hand, the other significant period of 490 d is interpreted as the result of the orbiting companion. Our orbital fit suggests that the companion was a planetary mass of 6.6 M_J and is located at 1.3 AU from the host.

• 한국지구과학회지
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• 제38권3호
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• pp.209-221
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• 2017

가까운 은하에서 폭발적 항성생성은하의 분광 관측을 수행하여 별생성률이 높은 은하에서 별생성이 일어나는 시간 규모에 따라 방출선의 방출 기작, 별생성률, 항성질량, 금속함량 등의 물리량 혹은 물리량 상호 간의 관계가 어떻게 다른지를 살펴보았다. 관측 대상은 별생성 나이가 매우 어린 울프-레이에 은하 21개와 상대적으로 긴 시간 규모의 별생성이 진행 중인 자외선 초과복사 은하 13개로 보현산 천문대의 1.8 m 망원경과 4K CCD, 긴 슬릿 분광기를 이용해 광학영역에서의 스펙트럼을 얻었다. BPT 분석도표를 그려 관측된 은하들에서 기체를 이온화시키는 원인을 살펴보면 전체적으로는 별생성(약 50%)이 비항성적 요소인 활동은하핵(약 15%)에 비해 훨씬 높았다. 별생성과 활동은하핵이 모두 기여하는 경우도 전체의 35%였는데, 이러한 경우에 속하는 은하는 대부분 상대적으로 나이가 많을 것으로 추정되는 자외선 초과복사 은하였다. 관측된 은하의 항성질량 범위는 대부분 $10^{9-11}M_{\odot}$이고 별생성률은 $0.01-100M_{\odot}yr^{-1}$로, SDSS에서 관측된 은하들로 구성된 별생성 주계열에 위치한다. 울프-레이에 은하와 자외선 초과복사 은하들의 항성질량, 별생성률에서 큰 차이는 없었다. 또한 폭발적 항성생성은하는 질량-금속함량 관계를 보이며, 비슷한 항성질량을 가진 SDSS 은하와 비교했을 때 금속함량이 낮게 나타났다. 이는 이 은하들에서 별생성으로 인한 강한 피드백이 일어나고 있음을 보여준다.

• 한국광학회지
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• 제28권3호
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• pp.116-121
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• 2017

본 연구에서는 오염된 지표면의 화학물질을 탐지하기 위해 라만 라이다 기반의 지표면 화학물질 탐지 시스템을 설계하고 제작하였다. 화학물질에 레이저를 조사하였을 때 나오는 라만 산란광을 측정하여 측정 시료의 분광정보를 얻는 라만 라이다 시스템은 측정 시료의 라만 산란광을 유도하는 레이저를 포함한 송광부와 라만 산란광을 수광하는 수광부, 수광된 광을 파장별로 나눠주는 분광부로 구성된다. 측정거리가 변하여도 수광부에 의해 집광되는 광이 동일한 위치에 집속되도록 code V 시뮬레이터를 사용하여 광학계를 설계하였다. 총 12종의 화학물질이 유리와 콘크리트 위에 있을 때 라만 스펙트럼을 측정하여 비교한 결과 라만 스펙트럼의 특성이 유사하지만, 세기에 차이가 있는 것을 관찰하였다. 고체 시료인 PTFE (polytetra-fluoroethylene)를 사용하여 시스템과의 거리 변화에 따른 라만 스펙트럼의 세기를 비교한 결과 시뮬레이션에서 얻은 거리에 따른 수광 효율과 유사한 것을 확인하였다. 결론적으로 본 시스템을 이용하여 다양한 거리에서도 화학물질의 라만 스펙트럼 측정이 가능함을 확인하였다.

• Journal of Astronomy and Space Sciences
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• 제17권1호
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• pp.77-86
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• 2000

약 $10^{4.5}K$과 $10^{6}K$ 사이의 온도를 갖는 고온기체가 주로 복사열에 의해 냉각될 때 900 ${\AA}$에서 1 1,200 ${\AA}$사이의 원자외선 영역에서 OVI 이중선$(\lambda\lambda1032,\lambda\lambda1038)$을 가장 강하게 방출하게 된다. 은하 전체에 걸쳐 넓게 분포하는 원자외선 방출선을 검출하려는 시도가 그동안 몇 번 있어왔으나 극히 제한된 일부 시선방향으로만 OVI 방출선이 관측되었을 뿐 관측 자료가 극히 미비한 상태이다. 또한 지구 대기로부터 방출되는 여러가지 밝은 대기광들로 인해 OVI 방출선의 관측이 영향을 받게 된다. 대기광중 OVI 방출선에 가장 가까이에 위치하는 HI 1025 ${\AA}$과 01 1027 ${\AA}$ 두 line이 약 $10^{5.5}K$ photons/s/$cm^2$/sr의 강도 (intensity)로서 가장 큰 영향을 미친다. 본 연구에서 는 온테카를로 모의실험과 $\chi^2$ 검사 기법 등을 통하여 과학위성 1호에 탑재 예정인 원자외선 분광기로부터 OVI 이중선의 검출 가능성을 연구하였다. 또한 기존의 원자외선 관측 결과 및 이론으로부터 예상되는 결과와 비교하였으며 OVI 분석 결과를 실제 광학부 제작에 필요한 오차 허용 한계 (tolerance limit)를 결정짓는 중요한 변수로 사용하도록 하였다.

• Journal of Astronomy and Space Sciences
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• 제18권3호
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• pp.239-248
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• 2001

2002년에 발사 예정인 과학 위성 1호의 주 탑재체로 원자외선 분광기가 실릴 예정이다. 원자외선 분광기는 영상과 분광의 기능을 함께 수행하게 되는데, 광학 부품 중 포물 원통 만가경의 경우 천문 관측용으로는 제작된 바가 없어 제작과 이에 관련된 측정 방법을 새로이 고안하여 제작시험이 이루어 겼다. 관측 목표로부터 도출된 사양 중에서 성능과 가장 밀접한 관계가 있는 형상오차의 경우 ~ $lambda/cm$의 한계를 만족하였으며, 산란장과 연관된 표면 거칠기의 경우 새로이 고안된 비구면 pitch polishing의 방법으로 $R_{q}$의 값이 1 nm 이하로 감소하였으며, 이로써 전체 파장 영역에서 0.02, 즉 2% 미만의 빛이 산란됨을 간접적으로 확인하였다.

• 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.