• 암석학회지
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• 제6권2호
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• pp.88-95
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• 1997

High pressure X-ray diffraction study was carried out on a polycrystalline graphite to investigate the phase transition(s) at room temperature. Energy dispersive X-ray diffraction method was employed using a Mao-Bell type diamond anvil cell with an Wiggler synchrotron Radiation at the National Synchrotron Light Source. Sodium chloride power was used as the internal pressure sensor for the high pressure determinations as well as the pressure medium for quasihydrostatic pressure environment. Graphite transforms into a hexagonal didose not agree with the previously reported observations and this phase persists when pressure is released down to 0.1 MPa. This result dose not agree with the previously reported observations and this discrepancy would be due to the kinetics in phase transition as well as the uniaxially oriented pressure field in the diamond anvil cell.

• 천문학회보
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• 제41권1호
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• pp.61.1-61.1
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• 2016

As a new way to explore the universe, astronomers are now performing time-domain astronomy by surveying the universe looking for new transient phenomena and taking movies of the universe with telescopes. Large-area, time-series survey of astronomical objects are uncovering many interesting, fast-changing objects that have now been poorly understood before, such as GRBs, tidal disruption phenomena, and new types of supernova. In order to characterize these new, exciting events, it is very critical to perform follow-up observations, and 1-2m telescopes can effectively contribute to such efforts. Since 2007, our group has been performing follow-up observations of gamma-ray bursts (GRBs) and interesting transients using BOAO and other KASI facilities. Here, we present results from several key transient studies that were done by using BOAO: (1) tidal disruption event Swift J1644+57; (2) SN 2011fe that occurred in M101; and (3) several GRB events. These study demonstrates the usefulness of BOAO as a powerful transient follow-up facility. Finally, we will discuss how BOAO research activities can possibly be bolstered in this newly emerging field of astronomy.

• 천문학회보
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• 제37권2호
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• pp.126.1-126.1
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• 2012

The middle-latitude ionization trough, the plasma density depletion in the subauroral region, has been extensively studied since its discovery in the 1960s. Our knowledge of the variability of the trough is mostly based on the observations in the northern hemisphere. Recently, the COSMIC observations enabled the investigation of the trough in both hemispheres at all local time. However, the investigation was limited to the period of the low solar activity. In this study, we investigate the variability of the trough location and morphology with local time, solar cycle, magnetic activity, and interplanetary magnetic field. For this purpose, we analyze the DMSP F15 data acquired during 2000-2010.

• 자원환경지질
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• 제32권6호
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• pp.653-660
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• 1999

Any mechanical system has a natural oscillation which can be excited, and the earth is no exception. The earth can oscillate in an indefinite number of normal modes of oscillation, rather like a giant bell. The various free modes are generally sparated into two categoridal modes and toroidal modes. Clearly the toroidal modes will produce no perturvation of the gravity field and no vertical acceleration on the surface of the earth. Hence only spheroidal modes can be detected with a gravimeter. EarthTide gravimeter was installed at AIMST in order to observe free modes of the earth. Eight major earthquakes including chinese earthquake (magnitude 7.3) with free oscillations of the earth are observed during one year (1998. 8. 1∼1999.7.31). And then the earth tides components were eilminated from earthquake records using a numerical Butterworth highpass filter. Spectral analysis of gravity readings repersent that 48 observations of shheroidal modes. The relationships between instrumental observations and theoretical predictions based on the Gutenberg earth model agree well those resulting from free oscillation in Korea.

• 천문학회보
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• 제44권1호
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• pp.58.2-58.2
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• 2019

Recent observation of the neutron star merger event, GW170817, through both gravitational wave (GW) and electromagnetic wave (EM) observations opened a new way of exploring the universe, namely, multi-messenger astronomy (MMA). One of the keys to the success of MMA is a rapid identification of EM counterpart through optical/NIR observations. We will present the strategy for prioritization of GW source host galaxy candidates to be observed with narrow-field optical telescopes. Our method relies on recent simulation results regarding plausible properties of GW source host galaxies and the low latency localization map from LIGO/Virgo. We will show the test results for both NS merger and BH merger events using previous events and possible future events and describe observing strategy with our facilities for GW events during the ongoing LIGO/Virgo O3 run.

• Journal of Astronomy and Space Sciences
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• 제39권3호
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• pp.79-86
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• 2022

Thermospheric wind observations from high to mid latitudes are compared with the newly developed Multiscale Atmosphere Geospace Environment (MAGE) model for the Nov 3-4 geomagnetic storm. The observation and simulation comparison shows a very good agreement and is better at high latitudes in general. We were able to identify a thermospheric poleward wind reduction possibly linked to a northward turning of the Interplanetary Magnetic Field (IMF) at ~22 UT on Nov 3 and an enhancement of the poleward wind to a southward turning near 10 UT on Nov 4 at high latitudes. An IMF southward turning may have led to an enhancement of equatorward winds at Boulder, Colorado near midnight. Simultaneous occurrence of aurora may be associated with an IMF By turning negative. The MAGE model wind simulations are consistent with observations in these cases. The results show the model can be a very useful tool to further study the magnetosphere and ionosphere coupling on short time scales.

• 천문학회보
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• 제44권2호
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• pp.48.3-48.3
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• 2019

In the early stages of star formation, a protostar is deeply embedded in an optically thick envelope such that it is not directly observable. Variations in the protostellar accretion rate, however, will cause luminosity changes that are reprocessed by the surrounding envelope and are observable at submillimeter wavelengths. We searched for submillimeter flux variability toward 12 Planck Galactic Cold Clumps detected by the James Clerk Maxwell Telescope (JCMT)-SCUBA-2 Continuum Observations of Pre-protostellar Evolution (SCOPE) survey. These observations were conducted at 850 ㎛ using the JCMT/SCUBA-2. Each field was observed three times over about 14 months between 2016 April and 2017 June. We applied a relative flux calibration and achieved a calibration uncertainty of ~3.6% on average. We identified 136 clumps across 12 fields and detected four sources with flux variations of ~30%. For three of these sources, the variations appear to be primarily due to large-scale contamination, leaving one plausible candidate.

• 천문학회보
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• 제45권1호
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• pp.57.4-58
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• 2020

The B-fields in Star-forming Region Observations (BISTRO) is a large program of the James Clerk Maxwell Telescope (JCMT) to study the roles of magnetic fields in molecular clouds on intermediate scales (a few thousands au or larger scales), in which a large number of researchers over the world are involved. This project was initiated in 2016 with polarimetric observations of nearby star-forming regions and has been extended toward massive and farther regions (BISTRO-2) and various evolutionary stages and environmental conditions (BISTRO-3). The current status of the BISTRO projects is reported. In addition, we discuss magnetic fields in the Serpens Main molecular cloud, which is one of the BISTRO star-forming regions. Utilizing the Histogram of Relative Orientations method, which compares polarization directions with density gradients, we show that magnetic fields are parallel to filaments in less dense filamentary structures but perpendicular to dense ones. Furthermore, the magnetic field directions with respect to density gradients vary again with density in denser core regions, which is understood by core formation and pinched fields. Note: (PI) D. Ward-Thompson, (co-PIs) P. Bastien, T. Hasegawa, W. Kwon, S. Lai, and K. Qiu

• 천문학회보
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• 제46권1호
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• pp.34.1-34.1
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• 2021

Abell 1240 is a merging galaxy cluster hosting prominent, symmetric double radio relics. To constrain its merging history, we provide the first weak-lensing analysis of the dark matter distribution of the Abell 1240 field with Subaru/Suprime-Cam observations after robustly addressing instrumental systematics. We also investigate the cluster galaxy distributions, combining our new MMT/Hectospec observations and the spectroscopic redshifts from the literature. Both weak-lensing mass reconstruction and galaxy distribution show that Abell 1240 consists of two subclusters stretched north to south between the double radio relics. We quantify the significance of the substructures and present their mass estimates. Finally, we discuss a merging stage of Abell 1240 with the current weak-lensing results and the radio relic priors.

• 천문학회보
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• 제46권2호
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• pp.41.2-41.2
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• 2021

I will start with presenting new results on the stellar populations of galaxies at a redshift of z=9-11, when the universe was only a few hundred million years old. By combining Hubble Space Telescope observations with Spitzer imaging data, I will show how challenging it is currently to measure basic physical properties of these objects such as star-formation rates, stellar masses and stellar ages. In particular, the current measurements greatly depend on the assumptions (priors) for the spectral energy distribution modeling. Finally, I will discuss how the James Webb Space Telescope (JWST) will revolutionize this field next year and allow us to probe and characterize the first generation of galaxies in much greater detail. Specifically, I will present an overview of the JWST Advanced Deep Extragalactic Survey (JADES), a joint program of the JWST/NIRCam and NIRSpec Guaranteed Time Observations (GTO) teams involving 950 hours of observation.