• 천문학회보
• /
• 제40권1호
• /
• pp.73.2-74
• /
• 2015

The gas cloud G2 falling toward Sagittarius A* (Sgr A*), the supermassive black hole at the center of the Milky Way, is supposed to provide valuable information on the physics of accretion flows and the environment of the black hole. We observed Sgr A* with four European stations of the Global Millimeter Very Long Baseline Interferometry Array (GMVA) at 86 GHz on 1 October 2013 when parts of G2 had already passed the pericenter. We searched for possible transient asymmetric structure - such as jets or winds from hot accretion flows - around Sgr A* caused by accretion of material from G2. The interferometric closure phases (which are zero if the spatial brightness distribution of the target is symmetric, and deviate from zero otherwise) remained zero within errors during the observation time. We thus conclude that Sgr A* did not show significant asymmetric (in the observer frame) outflows in late 2013. Using simulations, we constrain the size of the outflows that we could have missed to ${\approx}2.5$ mas along the major axis, ${\approx}0.4$ mas along the minor axis of the beam, corresponding to approximately 232 and 35 Schwarzschild radii, respectively; we thus probe spatial scales on which the jets of radio galaxies are suspected to convert magnetic into kinetic energy. As probably less than 0.2 Jy of the flux from Sgr A* can be attributed to accretion from G2, one finds an effective accretion rate ${\eta}M{\leq}1.5{\times}10^9kg/s{\approx}7.7{\times}10^{-9}M_{earth}/yr$ for material from G2. Exploiting the kinetic jet power-accretion power relation of radio galaxies, one finds that the rate of accretion of matter that ends up in jets is limited to $M{\leq}10^{17}kg/s{\approx}0.5M_{Earth}/yr$ less than about 20% of the mass of G2. Accordingly, G2 appears to be largely stable against loss of angular momentum and subsequent (partial) accretion at least on time scales ${\leq}1$ year.

• 천문학논총
• /
• 제17권1호
• /
• pp.23-40
• /
• 2002

별똥에는 분산 별똥(sporadic meteors)과 별똥비(meteor shower)에 속하는 별똥이 있다. 우리는 $\ulcorner$고려사$\lrcorner$ $\ulcorner$천문지$\lrcorner$에 적혀 있는 별똥 기록을 조사하여 10세기에서 14세기까지 지구 공전 궤도 상의 별찌(meteoroids)의 분포에 대해서 연구하였다 먼저 $\ulcorner$고려사$\lrcorner$ 에 나오는 별똥 기록의 간지날짜를 점검하여 교정하고 간추려서 자료를 만들었다. 별똥이 나타난 날짜를 조사하여 고려 시대의 별똥별의 특징을 살펴보았다. 고려 시대 별똥 기록은 각 시기에 유행한 철학 사조의 영향과 전쟁과 내란 등의 영향으로 기록 수가 변하는 것으로 추정하였다. 그러나 평소에 떨어지는 분산 별똥들의 계절적 개수 변화는 매우 자연스러웠다. 우리는 일년 중 특정한 날짜에 별똥별이 많이 떨어지는 시기가 몇 개 있음을 발견하였다. 우리는 이것을 별똥비로 보고, 한국$\cdot$일본$\cdot$중국$\cdot$아랍$\cdot$유럽의 역사 기록에 나오는 별똥 소나기 기록을 모두 조사한 다음, 이것을 $\ulcorner$고려사$\lrcorner$의 별똥 기록에서 구한 별똥비와 비교하였다. 우리는 뚜렷한 별똥비 셋과 약한 별똥비 서넛을 확인할 수 있었다. 뚜렷한 세 별똥비는 사자자리 별똥비, 페르세우스 별똥비, 핼리혜성이 남기는 두 별똥비 등의 요즘 별똥비와 각각 일치함을 알아냈다 또한 나머지 약한 별똥비들도 오늘날 관측되고 있는 약한 별똥비와 대체적으로 일치함을 보았다. 결론적으로 고려시대의 별똥비의 활동성은 오늘날의 그것과 크게 다르지 않았다. 이것은 오늘날 활발한 별똥비들의 수명의 하한을 주는 결과이다. 이러한 연구를 통해 별똥 관측에 있어서 우리나라 옛 사람들이 남긴 관측 기록은 실제 자연의 모습을 그대로 읽어낼 수 있을 정도로 꾸준하고 정밀하고 정직한 것이었음을 알 수 있었다. 또한 그러한 좋은 유산을 현대 천문학에 유용한 연구 자료로서 활용할 수 있었다.

• 천문학회보
• /
• 제41권1호
• /
• pp.37.3-38
• /
• 2016

Recent studies suggest that faint active galactic nuclei may be responsible for the reionization of the universe. Confirmation of this scenario requires spectroscopic identification of faint quasars ($M_{1450}$ > -24 mag) at z > 6, but only a very small number of such quasars have been spectroscopically identified so far. Here, we report the discovery of a faint quasar IMS J220417.92+011144.8 at z ~ 6 in a $12.5deg^2$ region of the SA22 field of the Infrared Medium-deep Survey (IMS). The spectrum of the quasar shows a sharp break at ${\sim}8443{\AA}$, with emission lines redshifted to $z=5.944{\pm}0.002$ and rest-frame ultraviolet continuum magnitude $M_{1450}=-23.59{\pm}0.10$ AB mag. The discovery of IMS J220417.92+011144.8 is consistent with the expected number of quasars at z ~6 estimated from quasar luminosity functions based on previous observations of spectroscopically identified low-luminosity quasars. This suggest that the number of $M_{1450}$ ~ -23 mag quasars at z ~ 6 may not be high enough to fully account for the reionization of the universe. In addition, our study demonstrates that faint quasars in the early universe can be identified effectively with a moderately wide and deep near-infrared survey such as the IMS.

• 천문학회보
• /
• 제36권1호
• /
• pp.37.2-37.2
• /
• 2011

Our previous study on tiny pores (R < 2") observed by HINODE/Solar Optical Telescope (SOT) revealed that the plasma in the pores at the photosphere is always moving down and the pores are surrounded by the strong downward motions (highly red-shifted) of neighboring granulations. From this study, we speculated that the flow motions above the pore should be related with the motions at the photosphere, since the pore is strong magnetic field region. Meanwhile, SNU and KASI installed Fast Imaging Solar Spectrograph (FISS) in the Cude room of the 1.6 m New Solar Telescope (NST) at Big Bear Solar Observatory. FISS is a unique system that can do imaging of H-alpha and Ca II 8542 band simultaneously, which is quite suitable for studying of dynamics of chromosphere. To get some clue on the relationship between the photospheric and low-chromospheric motions at the pore region, we took a coordinate observation with NST/FISS and Hinode/SOT for new emerging active region (AR11117) on October 26, 2010. In the observed region, we could find two tiny pores and two small magnetic islands (SMIs), which have similar magnetic flux with the pores but does not look dark. Magnetic flux density and Doppler velocities at the photosphere are estimated by applying the center-of-gravity (COG) method to the HINODE/spectropolarimeter (SP) data. The line-of-sight motions above the photosphere are determined by adopting the bisector method to the wing spectra of Ha and CaII 8542 lines. As results, we found the followings. (1) There are upflow motion on the pores and downflow motion on the SMIs. (2) Towards the CaII 8542 line center, upflow motion decrease and turn to downward motion in pores, while the speed of down flow motion increases in the SMIs. (3) There is oscillating motion above pores and the SMIs, and this motion keep its pattern along the height. (4) As height increase, there is a general tendency of the speed shift to downward on pores and the SMIs. This is more clearly seen on the other regions of stronger magnetic field. In this talk, we will present preliminary understanding of the coupling of pore dynamics between the photosphere and the low-chromosphere.

• 한국전자통신학회논문지
• /
• 제12권5호
• /
• pp.817-828
• /
• 2017

단일 스트림 기준으로 10Gbps에 달하는 데이터 처리를 수행하는 시스템은 매우 제한적이고, 그에 관한 애플리케이션 역시 극소수에 불과한 실정이다. 하지만 이러한 시스템은 물리, 천문 등 과학연구 분야에서 더욱 그 중요성이 부각되고 있으며, 업그레이드 된 성능을 요구하고 있다. 이를 위해 해당 분야의 중소기업 지원을 위해 1년 전 10Gbps 수준으로 네트워크 입력되는 고속의 패킷 수신, 저장을 수행하는 초고속 스토리지가 개발되었다. 하지만 실제 연구 현장에서 사용되기 위해서는 객관화된 성능 평가가 수반되어야 하며 최적화가 이뤄져야 한다. 본 논문에서는 이에 필요한 시스템 환경을 구축한 것을 시작으로 객관화된 성능 지표를 얻기 위한 실험 과정 및 방법론에 대해 논의하였다.

• Journal of Astronomy and Space Sciences
• /
• 제37권3호
• /
• pp.157-163
• /
• 2020

High resolution spectroscopic observation of V1719 Cyg were made at 1.8 meter telescope of Bohyunsan Optical Astronomy observatory in Korea. Spectral resolving power was R=45,000, signal to noise ratio S/N>100. The abundances of 28 chemical elements from carbon to dysprosium were found with the spectrum synthesis method. The abundances of oxygen, titanium, vanadium and elements with Z>30 are overabundant by 0.2-0.9 dex with respect to the solar values. Correlations of derived abundances with condensation temperatures and second ionization potentials of these elements are discussed. The possible influence of accretion from interstellar environment is not so strong as for ρ Pup and other stars with similar temperatures. The signs of accretion are absent. The comparison of chemical composition with solar system r- & s-process abundance patterns shows the enhancement of the photosphere by s-process elements.

• 한국초전도ㆍ저온공학회논문지
• /
• 제24권4호
• /
• pp.71-77
• /
• 2022

Sub-Kelvin cooling has been generally demanded for the fields of low temperature physics, such as physical property measurements, astronomical detection, and quantum computing. The refrigeration system with a small size can be appropriately introduced when the measurement system does not require a high cooling capacity at sub-Kelvin temperature. The dilution refrigerator which is a common method to reach sub-Kelvin, however, must possess a large ³He circulation equipment at room temperature. As alternatives, a sorption refrigerator and a magnetic refrigerator can be adopted for sub-Kelvin cooling. This paper describes those coolers which have been developed by various research groups. Furthermore, a cold-cycle dilution refrigerator of which the size of the ³He circulation system is minimized, is also introduced. Subsequently, a new concept of dilution refrigerator is proposed by our group. The suggested cooler can achieve sub-Kelvin temperature with a small size since it does not require any recuperator and turbo-molecular vacuum pump. Its architecture allows the compact configuration to reach sub-Kelvin temperature by integrating the sorption pump and the magnetic refrigerators. Therefore, it may be suitably utilized in the low temperature experiments requiring low cooling capacity.

• Journal of Astronomy and Space Sciences
• /
• 제40권4호
• /
• pp.259-266
• /
• 2023

The Standard Model (SM) does not provide an information for 26% of dark matter of the universe. In the dark sector, dark matter is supposed to be linked with the hypothetical particles called dark photons that have similar role to photons in electromagnetic interaction in the SM. Besides astronomical observation, there are studies to find dark matter candidates using accelerators. In this paper, we searched for dark photons using future electron-positron colliders, including Circular Electron Positron Collider (CEPC)/CEPC, Future Circular Collider (FCC-ee)/Innovative Detector for Electron-positron Accelerator (IDEA), and International Linear Collider (ILC)/International Large Detector (ILD). Using the parameterized response of the detector simulation of Delphes, we studied the sensitivity of a double dark photon mode at each accelerator/detector. The signal mode is double dark photon decay channel, e⁺e^- → A'A', where A' (dark photon with spin 1) decaying into a muon pair. We used MadGraph5 to generate Monte Carlo (MC) events by means of a Simplified Model. We found the dark photon mass at which the cross-sections were the highest for each accelerator to obtain the maximum number of events. In this paper we show the expected number of dark photon signal events and the detector efficiency of each accelerator. The results of this study can facilitate in the dark photon search by future electron-positron accelerators.

• 대한지구과학교육학회지
• /
• 제9권3호
• /
• pp.269-275
• /
• 2016

본 연구는 우리나라의 과학과 교육과정 중 과학적 소양이 교육의 목표로 제시된 7차 교육과정부터 2015 개정 교육과정까지 '과학' 과목의 지구과학영역 내용 변화를 과학적 소양의 측면에서 알아보기 위해 실시되었다. 지구과학 영역을 천문, 지질, 기상, 해양 내용으로 구분하여 과학적 소양의 요소 중에서 STS적인 요소에 초점을 두고 내용을 분석하였다. 천문과 기상 내용 영역에서는 7차부터 2015 개정 교육과정까지 모든 시기에서 과학적 소양의 측면이 나타났으며, 지질 영역에서는 2007 개정 교육과정, 해양 영역에서는 7차 교육과정을 제외하고 모든 시기에서 과학적 소양의 측면이 나타났다. 천문 내용 영역에서는 과학과 기술의 유용성이 강조되었고, 지질과 기상 내용 영역에서는 일상생활에서의 과학의 이용이 강조되었다. 해양 내용영역에서는 자원부족과 환경오염 등 과학관련 사회적 이슈가 강조됨을 알 수 있었다. 이 연구는 우리나라 과학과 교육과정을 과학적 소양이라는 새로운 측면에서 살펴보려는 시도로 차후 지구과학의 다른 영역 및 물리, 화학, 생물 과목으로 확대하여 내용을 살펴볼 수 있을 것이며, 더 나아가 기존의 물리, 화학, 생물, 지구과학이라는 과목의 테두리에서 벗어나 통합된 학문의 관점에서 과학적 소양을 알아볼 것을 제안한다.

• 천문학회지
• /
• 제36권3호
• /
• pp.81-87
• /
• 2003

It is suggested that a flying-by star in a hot accretion disk may cool the hot accretion disk by the Comptonization of the stellar emission. Such a stellar cooling can be observed in the radio frequency regime since synchrotron luminosity depends strongly on the electron temperature of the accretion flow. If a bright star orbiting around the supermassive black hole cools the hot disk, one should expect a quasi-periodic modulation in radio, or even possible an anti-correlation of luminosities in radio and X-rays. Recently, the unprecedentedly accurate infrared imaging of the Sagittarius A$\ast$ for about ten years enables us to resolve stars around it and thus determine orbital parameters of the currently closest star S2. We explore the possibility of using such kind of observation to distinguish two quite different physical models for the central engine of the Sagittarius A$\ast$, that is, a hot accretion disk model and a jet model. We have attempted to estimate the observables using the observed parameters of the star S2. The relative difference in the electron temperature is a few parts of a thousand at the epoch when the star S2 is near at the pericenter. The relative radio luminosity difference with and without the stellar cooling is also small of order $10^{-4}$, particularly even when the star S2 is near at the pericenter. On the basis of our findings we tentatively conclude that even the currently closest pass of the star S2 is insufficiently close enough to meaningfully constrain the nature of the Sagittarius A$\ast$ and distinguish two competing models. This implies that even though Bower et al. (2002)have found no periodic radio flux variations in their data set from 1981 to 1998, which is naturally expected from the presence of a hot disk, a hot disk model cannot be conclusively ruled out. This is simply because the energy bands they have studied are too high to observe the effect of the star S2 even if it indeed interacts with the hot disk. In other words, even if there is a hot accretion disk the star like S2 has imprints in the frequency range at v $\le$ 100 MHz.