• 천문학회보
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• 제39권1호
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• pp.58.2-58.2
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• 2014

The Arches cluster is a young (2-4 Myr), compact (~1 pc), and massive (${\sim}2{\times}10^4M_{\odot}$) star cluster located ~30 pc away from the Galactic center (GC) in projection. Being exposed to the extreme environment of the GC such as elevated temperature and turbulent velocities in the molecular clouds, strong magnetic fields, and larger tidal forces, the Arches cluster is an excellent target for understanding the effects of star-forming environment on the initial mass function (IMF) of the star cluster. However, resolving stars fainter than ~1 $M_{\odot}$ in the Arches cluster partially will have to wait until an extremely large telescope with adaptive optics in the infrared is available. Here we devise a new method to estimate the shape of the low-end mass function where the individual stars are not resolved, and apply it to the Arches cluster. This method involves histograms of pixel intensities in the observed images. We find that the initial mass function of the Arches cluster should not be too different from that for the Galactic disk such as the Kroupa IMF.

• 한국자기공명학회논문지
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• 제28권1호
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• pp.1-5
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• 2024

IscU, the iron-sulfur (Fe-S) cluster scaffold protein, is an essential protein for biogenesis of Fe-S clusters. Previous studies showed that IscU manifests a metamorphic structural feature; at least two structural states, namely the structured state (S-state) and the disordered state (D-state), interconverting in a physiological condition, was observed. Moreover, subsequent studies demonstrated that the metamorphic flexibility of IscU is important for its Fe-S cluster assembly activity as well as for an efficient interaction with various partner proteins. Although solution nuclear magnetic resonance (NMR) spectroscopy has been a useful tool to investigate this protein, the detailed molecular mechanism that sustains the structural heterogeneity of IscU is still unclear. To tackle this issue, we applied a high-pressure NMR (HP-NMR) technique to the IscU variant, IscU(I8K), which shows an increased population of the S-state. We found that the equilibrium between the S- and D-state was significantly perturbed by pressure application, and the specific regions of IscU exhibited more sensitivity to pressure than the other regions. Our results provide novel insights to appreciate the dynamic behaviors of IscU and the related versatile functionality.

• 천문학회보
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• 제39권2호
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• pp.83.2-83.2
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• 2014

Flare and rotational variabilities induced by stellar activity are important for studying the effect of magnetic fields on the evolution of red dwarf stars. The level and frequency of magnetic activity in these stars have a different aspect at every moment of the observations due to the effect of age-rotation relation. The use of both tracers is thus essential to have a relatively homogeneous set of stellar activity data for statistical studies. The archival light curves and imaging data of the open cluster M37 taken by MMT 6.5m telescope were used for this work. In order to achieve much more accurate photometric precisions and also to make the most efficient use of the data, the entire imaging database were re-analyzed with our new time-series photometry technique and carefully calibration procedures. Based on the new light curves, we study, for the first time, a variety of aspects of those two variabilities in red dwarfs and their relation to magnetic activity. In this talk, we present all observational evidences that support the idea that the strength of magnetic activity is closely connected with the rotation rate of a star and its evolutionary status (age-activity-rotation paradigm). In conclusion, we suggest future directions to improve our understanding of stellar activity in cool stars with photometric time-series data.

• 한국우주과학회:학술대회논문집(한국우주과학회보)
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• 한국우주과학회 2003년도 한국우주과학회보 제12권2호
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• pp.33-33
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• 2003

A clear bipolar (negative/positive) signature in the Ey component was observed by the Cluster satellite in the magnetotail during a sudden impulse (si) on October 11, 2001 (day 284). During the interval of the negative perturbation in Ey, the magnetic field strength in Bx, a dominant magnetic field component, was nearly constant. However, the amplitude of Bx was strongly enhanced during the positive Ey perturbation. We suggest that the observed E and B field variations are due to outward/inward plasma motions, associated with expanded and then compressed magnetopause variations. We also observed quasi-periodic geomagnetic perturbations in the Pc5 band (∼1-6 mHz) at the low-latitude ground station Kakioka (L = 1.25) following the si event. They were highly correlated with the magnetic field perturbations at Cluster in the magnetotail (Xgse = ∼12 Re). We show that the source of these perturbations is the quasi-periodic solar wind pressure variations moving tailward.

• Journal of Electrical Engineering and Technology
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• 제7권6호
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• pp.884-890
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• 2012

Hysteretic behavior model of soft magnetic material usually used in electrical machines and electronic devices is necessary for numerical solution of Maxwell equation. In this study, a new dynamic hysteresis model is presented, based on the nonlinear dynamic system identification from measured data capabilities of fuzzy clustering algorithm. The developed model is based on a Gustafson-Kessel (GK) fuzzy approach used on a normalized gathered data from measured dynamic cycles on a C core transformer made of 0.33mm laminations of cold rolled SiFe. The number of fuzzy rules is optimized by some cluster validity measures like 'partition coefficient' and 'classification entropy'. The clustering results from the GK approach show that it is not only very accurate but also provides its effectiveness and potential for dynamic magnetic hysteresis modeling.

• 한국광물학회지
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• 제20권4호
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• pp.313-325
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• 2007

본 연구에서는 점토광물 표면 클러스터의 크기와 결정학적 위상이 전자 밀도와 자기 차폐 텐서에 미치는 영향을 살펴보기 위하여 캐올리나이트 규산염 층을 대표하는 세 개의 서로 다른 위상의 모델 클러스터와 벤질 알코올과의 상호작용에 대해 다양한 수준의 양자화학 계산을 수행하였다. 모델 클러스터 1은 단순화된 7개의 규산염 고리로 이루어졌고, 모델 클러스터 2는 결정학적 위상을 가진 7개의 규산염 고리로 이루어졌으며, 모델 클러스터 3은 세 개의 규산염 고리와 팔면체 고리로 이루어져 있다. 멀리켄 전하 계산 결과 벤질 알코올과의 반응 후의 상대적인 전자 밀도 이동의 크기는 모델클러스터 3의 사면체 쪽 > 모델 클러스터 1 > 모델 클러스터 2 > 모델 클러스터 3의 팔면체 쪽의 순으로 계산되었다. 또한 벤질 알코올과 강한 수소 결합을 하는 원자들의 전자 밀도 이동이 상대적으로 크다 벤질 알코올 흡착 전에 대한 사면체 표면 원자들의 자기 차폐 텐서 결과는 결정학적 위상을 고려하지 않은 경우 표면 중심으로부터의 거리가 비슷한 산소들끼리 유사한 등방 자기 차폐 텐서 값들을 갖고, 결정학적 위상을 고려한 경우는 결정학적으로 서로 다른 산소 자리(O3, O4, O5)에 대해 각각 $228.2{\pm}3.9,\;228.9{\pm}3.4,\;222.3{\pm}3.0ppm$으로 계산되었다. 흡착 전후의 산소 원자의 화학 차폐의 차이는 알코올과 근접한 산소들에서 약 $1{\sim}5.5ppm$ 정도의 변화가 나타나며 이러한 변화는 최근의 고분해능 이차원 핵자기공명분광 분석을 이용하면 실험으로 관찰할 수 있을 것으로 예상된다. 또한 모델 클러스터 2의 화학 차폐의 변화는 모델 클러스터 1보다 상대적으로 큰 특징을 보인다. 전자밀도 이동과 화학 차폐의 변화는 약한 양의 상관관계를 가진다. 이러한 결과들은 캐올리나이트 규산염 사면체 층과 벤질 알코올이 약한 수소 결합과 벤젠 고리와 규산염 층 산소 원자들의 약한 정전기적 힘에 의해 흡착되고 있음을 보여준다. 본 연구는 점토광물과 유기물에 대한 양자 화학 계산에서 클러스터 크기와 결정학적 위상이 고려되어야 함을 제시한다.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2012년도 제42회 동계 정기 학술대회 초록집
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• pp.135-136
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• 2012

Carbon atoms near the surface of W(110) induce reconstructions such as $R(15{\times}12)$ -C/W(110) which consists of two characteristic parts, one square shaped and bright protrusion and two smaller ones. In the atomic resolution STM image, the bigger protrusion shows the periodicities of clean W(110), indicating that it is almost carbon poor region. The smaller protrusion contains hexagonal carbide surface layer of ${\alpha}$-W2C on W(110). Employing this carburized W(110) as templates, we grow Co and Fe clusters of less than ten atoms. Due to the selectivity of bonding sites, growth of larger cluster is highly unfavorable for Co and the size of clusters is very uniform. Since Co atoms prefer to sit on the bigger protrusion rather than smaller one, Co cluster can be arranged one-dimensionally in $R(15{\times}12)$-C/W(110) with quite uniform size distribution. However, Fe clusters sit on both sites without favored site, but still with uniform size distribution. On the other hand, Fe clusters can be grown with quasi one-dimensional order in $R(15{\times}3)$-C/W(110), which consists of only smaller protrusions. We investigate the magnetic properties of the ordered nano-sized clusters. Experiments using XMCD reveals little magnetic moment of Co cluster on $R(15{\times}12)$-C/W(110). This observation is consistent with the predictions of our first principles calculations that small Co clusters can be nonmagnetic or antiferromagnetic with low mean magnetic moment per atom.

• 천문학회지
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• 제49권1호
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• pp.19-23
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• 2016

Magnetic decreases are often observed in various regions of interplanetary space. Many studies are devoted to reveal the physical nature and generation mechanism of the magnetic decreases, but still we do not fully understand magnetic decreases. In this study, we investigate the structure of a magnetic decrease observed in a corotating interaction region using multi-spacecraft measurements. We use three spacecraft, ACE, Cluster, and Wind, which were widely separated in the x- and y-directions in the geocentric solar ecliptic (GSE) coordinates. The boundaries of the magnetic decrease are the same at the three locations and can be identified as tangential discontinuities. A notable feature is that the magnetic decrease has very large dimension, ≳ R_E, along the boundary, which is much larger than the size, ~ 6 R_E, along the normal direction. This suggests that the magnetic decrease has a shape of a long, thin rod or a wide slab.

• 천문학회지
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• 제37권5호
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• pp.553-556
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• 2004

We present a new model for the generation of magnetic fields on large scales occurring at the end of cosmological reionisation. The inhomogeneous radiation provided by luminous sources and the fluctuations in the matter density field are the major ingredients of the model. More specifically, differential radiation pressure acting on ions and electrons gives rise to electric currents which induce magnetic fields on large scales. We show that on protogalactic scales, this process is highly efficient, leading to magnetic field amplitudes of the order of $10^{-1l}$ Gauss. While remaining of negligible dynamical impact, those amplitudes are million times higher than those obtained in usual astrophysical magnetogenesis models. Finally, we derive the relation between the power spectrum of the generated field and the one of the matter density fluctuations. We show in particular that magnetic fields are preferably created on large (galactic or cluster) scales. Small scale magnetic fields are strongly disfavoured, which further makes the process we propose an ideal candidate to explain the origin of magnetic fields in large scale structures.

• 한국자기학회지
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• 제3권4호
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• pp.277-283
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• 1993

세개의 오스테 나이트계의 Fe-Cr-Ni 스테인레스 합금강들의 DC 자화율의 온도의존성을 4.2-300 K 온도영역에서 측정하였다. 측정결과 두개의 합금강들은 two-magnetic-phase 모델(spin glass와 초상자성)을 만족하였으며 나머지 하나는 자기적 2중 천이현상을 나타내었다. 또한 Ni/Cr의 비가 증가할수록 자화율의 최 대값이 증가하고 이 값을 나타내는 온도는 감소하였다.