• 자원환경지질
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• 제45권4호
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• pp.377-384
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• 2012

다목적위성인 아리랑 2호 (KOMPSAT-II)에 장착된 위성의 자세제어를 위한 삼축자력계(Triaxial Magnetometer, TAM)에서 측정된 지구자기장의 삼성분 자료로부터 지구 외핵에서부터 오는 주지구자기장 (main geomagnetic field)의 모델을 계산하였다. 일반적으로 지자기 표준모델이라고도 일컫는 IGRF(International Geomagnetic Reference Model)과의 비교를 통해 제작된 모델과의 상관성을 연구하였다. 선행연구에서는 KOMPSAT-I의 3일 자료만을 통해 제작한 것과 달리 2007년 11월과 12월 자료를 항공우주연구원의 협조를 받아 모델에 활용하였다. 선행 연구에서는 구면조화 함수의 차수가 5까지의 유사성을 보인 반면 이번에 얻은 모델은 차수 8-9까지 유사성을 보이며 그 이후 차수 13까지 계산에서는 국제표준모델과 많은 차이를 나타내었다. KOMPSAT-II 자료를 통한 지구자기장 모델을 제작하는 데 있어서 적절한 자료선별과정과 이와 관련된 극지방자료의 포함여부와 외부자기장성분의 적절한 제거 및 위성에 탑재한 측정자력계의 정확도가 국제지자기표준모델과의 유사성 여부에 중요한 요소로 작용하였다. 수년간의 자료입수가 가능할 경우 지구자기장의 영년변화연구에도 지상의 지자기관측소자료와 함께 KOMPSAT-II 자료의 활용이 기대된다.

• Journal of Astronomy and Space Sciences
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• 제28권3호
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• pp.163-172
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• 2011

An intensive analysis of 185 timings of WZ Cep, including our new three timings, was made to understand the dynamical picture of this active W UMa-type binary. It was found that the orbital period of the system has complexly varied in two cyclical components superposed on a secularly downward parabola over about 80y. The downward parabola, corresponding to a secular period decrease of $-9.{^d}97{\times}10^{-8}y^{-1}$, is most probably produced by the action of both angular momentum loss (AML) due to magnetic braking and mass-transfer from the massive primary component to the secondary. The period decrease rate of $-6.^{d}72{\times}10^{-8}y^{-1}$ due to AML contributes about 67% to the observed period decrease. The mass flow of about $5.16{\times}10^{-8}M_{\odot}y^{-1}$ from the primary to the secondary results the remaining 33% period decrease. Two cyclical components have an $11.^{y}8$ period with amplitude of $0.^{d}0054$ and a $41.^{y}3$ period with amplitude of $0.^{d}0178$. It is very interesting that there seems to be exactly in a commensurable 7:2 relation between their mean motions. As the possible causes, two rival interpretations (i.e., light-time effects (LTE) by additional bodies and the Applegate model) were considered. In the LTE interpretation, the minimum masses of $0.30M_{\odot}$ for the shorter period and $0.49M_{\odot}$ for the longer one were calculated. Their contributions to the total light were at most within 2%, if they were assumed to be main-sequence stars. If the LTE explanation is true for the WZ Cep system, the 7:2 relation found between their mean motions would be interpreted as a stable 7:2 orbit resonance produced by a long-term gravitational interaction between two tertiary bodies. In the Applegate model interpretation, the deduced model parameters indicate that the mechanism could work only in the primary star for both of the two period modulations, but could not in the secondary. However, we couldn't find any meaningful relation between the light variation and the period variability from the historical light curve data. At present, we prefer the interpretation of the mechanical perturbation from the third and fourth stars as the possible cause of two cycling period changes.

• Journal of Astronomy and Space Sciences
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• 제27권2호
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• pp.89-96
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• 2010

BVR CCD observations of GW Cep were made on 15 nights in November through December 2008 with a 1-m reflector at the Jincheon station of the Chungbuk National University Observatory. Nineteen new times of minimum lights for GW Cep were determined and added to a collection of all other times of minima available to us. These data were then intensively analyzed, by reference to an O-C diagram, to deduce the general form of period variation for GW Cep. It was found that the O-C diagram could be interpreted as presenting two different forms of period change: an exclusively quasi-sinusoidal change with a period of 32.6 years and an eccentricity of 0.10; and a quasi-sinusoidal change with a period of 46.2 years and an eccentricity of 0.36 superposed on an upward parabola. Although a final conclusion is somewhat premature at present, the latter seems more plausible because late-type contact binaries allow an inter-exchange of both energy and mass between the component stars. The quasi-sinusoidal characteristics were interpreted in terms of a light-time effect due to an unseen tertiary component. The minimum masses of the tertiary component for both cases were calculated to be nearly the same as the $0.23-0.26M\;{\odot}$-ranges which is hardly detectable in a light curve synthesis. The upward parabolic O-C diagram corresponding to a secular period increase of about $4.12{\times}10^{-8}\;d/yr$ was interpreted as mass being transferred from the lesser to more massive component. The transfer rate for a conservative case was calculated to be about $2.66\;{\times}\;10^{-8}\;M_{\odot}/yr$ which is compatible with other W UMa-type contact binaries.