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1968년 ~ 2024년까지 1,211 건의 한국천문학회지를 격월간 확인하실 수 있습니다.
- The Korean Astronomical Society (The Korean Astronomical Society)
- 계간 (Quarterly)
- ISSN : 1225-4614 (ISSN : 1225-4614)
1225-4614
- DB구축현황 : 1,211건 (DB Construction : 1,211 Articles)
총 게시글 1,211건
페이지 21/122
201
- KIM, JUHAN
- Journal of the Korean astronomical society = 천문학회지
- 48, n.4
- pp.213-228
- 2015
- 원문 바로보기
The Horizon Run 4 is a cosmological N-body simulation designed for the study of coupled evolution between galaxies and large-scale structures of the Universe, and for the test of galaxy formation models. Using 6300<sup>3</sup> gravitating particles in a cubic box of L<sub>box</sub> = 3150 h<sup>−1</sup>Mpc, we build a dense forest of halo merger trees to trace the halo merger history with a halo mass resolution scale down to M<sub>s</sub> = 2.7 × 10<sup>11</sup>h<sup>−1</sup>M<sub>⊙</sub>. We build a set of particle and halo data, which can serve as testbeds for comparison of cosmological models and gravitational theories with observations. We find that the FoF halo mass function shows a substantial deviation from the universal form with tangible redshift evolution of amplitude and shape. At higher redshifts, the amplitude of the mass function is lower, and the functional form is shifted toward larger values of ln(1/σ). We also find that the baryonic acoustic oscillation feature in the two-point correlation function of mock galaxies becomes broader with a peak position moving to smaller scales and the peak amplitude decreasing for increasing directional cosine μ compared to the linear predictions. From the halo merger trees built from halo data at 75 redshifts, we measure the half-mass epoch of halos and find that less massive halos tend to reach half of their current mass at higher redshifts. Simulation outputs including snapshot data, past lightcone space data, and halo merger data are available at <uri>http://sdss.kias.re.kr/astro/Horizon-Run4</uri>.
202
- OH, JUNGHWAN
- Journal of the Korean astronomical society = 천문학회지
- 48, n.5
- pp.299-311
- 2015
- 원문 바로보기
We report first results from KVN and VERA Array (KaVA) VLBI observations obtained in the frame of our Plasma-physics of Active Galactic Nuclei (PAGaN) project. We observed eight selected AGN at 22 and 43 GHz in single polarization (LCP) between March 2014 and April 2015. Each source was observed for 6 to 8 hours per observing run to maximize the uv coverage. We obtained a total of 15 deep high-resolution images permitting the identification of individual circular Gaussian jet components and three spectral index maps of BL Lac, 3C 111 and 3C 345 from simultaneous dual-frequency observations. The spectral index maps show trends in agreement with general expectations – flat core and steep jets – while the actual value of the spectral index for jets shows indications for a dependence on AGN type. We analyzed the kinematics of jet components of BL Lac and 3C 111, detecting superluminal proper motions with maximum apparent speeds of about 5c. This constrains the lower limits of the intrinsic component velocities to ~ 0.98c and the upper limits of the angle between jet and line of sight to ~20°. In agreement with global jet expansion, jet components show systematically larger diameters d at larger core distances r, following the global relation d ≈ 0.2r, albeit within substantial scatter.
203
- Litvinenko, Yuri E.
- Journal of the Korean astronomical society = 천문학회지
- 48, n.3
- pp.187-190
- 2015
- 원문 바로보기
Data analysis and theoretical arguments support magnetic reconnection in a chromospheric current sheet as the mechanism of the observed photospheric magnetic flux cancellation on the Sun. Flux pile-up reconnection in a Sweet–Parker current sheet can explain the observed properties of canceling magnetic features, including the speeds of canceling magnetic fragments, the magnetic fluxes in the fragments, and the flux cancellation rates, inferred from the data. It is discussed how more realistic chromospheric reconnection models can be developed by relaxing the assumptions of a negligible current sheet curvature and a constant height of the reconnection site above the photosphere.
204
- Oh, Heeyoung
- Journal of the Korean astronomical society = 천문학회지
- 48, n.2
- pp.113-123
- 2015
- 원문 바로보기
We present observational results from optical long-slit spectroscopy of parsec-scale jets of DG Tau. From HH 158 and HH 702, the optical emission lines of Hα, [O i] λλ6300, 6363, [N ii] λλ6548, 6584, and [S ii] λλ6716, 6731 are obtained. The kinematics and physical properties (i.e., electron density, electron temperature, ionization fraction, and mass-loss rate) are investigated along the blueshifted jet up to 650′′ distance from the source. For HH 158, the radial velocity ranges from −50 to −250 km s<sup>−1</sup>. The proper motion of the knots is 0.′′196 − 0.′′272 yr<sup>−1</sup>. The electron density is ∼10<sup>4</sup> cm<sup>−3</sup> close to the star, and decreases to ∼10<sup>2</sup> cm<sup>−3</sup> at 14′′ away from the star. Ionization fraction indicates that the gas is almost neutral in the vicinity of the source. It increases up to over 0.4 along the distance. HH 702 is located at 650′′ from the source. It shows ∼ −80 km s<sup>−1</sup> in the radial velocity. Its line ratios are similar to those at knot C of HH 158. The mass-loss rate is estimated to be about ∼ 10<sup>−7</sup> M<sub>⊙</sub> yr<sup>−1</sup>, which is similar to values obtained from previous studies.
205
- JUNG, TAEHYUN
- Journal of the Korean astronomical society = 천문학회지
- 48, n.5
- pp.277-284
- 2015
- 원문 바로보기
We present our efforts for extending the simultaneous multi-frequency receiver system of the Korean Very Long Baseline Interferometry (VLBI) Network (KVN) to global baselines in order to measure the frequency-dependent position shifts in Active Galactic Nuclei (AGN) jets, the so called core shift effect, with an unprecedented accuracy (a few micro-arcseconds). Millimeter VLBI observations with simultaneous multi-frequency receiver systems, like those of the KVN, enable us to explore the innermost regions of AGN and high precision astrometry. Such a system is capable of locating the frequency dependent opacity changes accurately. We have conducted the feasibility test-observations with the interested partners by implementing the KVN-compatible systems. Here we describe the science case for measuring the core shift effect in the AGN jet and report progress and future plans on extending the simultaneous multi-frequency system to global baselines.
206
- CHOI, NAHYUN
- Journal of the Korean astronomical society = 천문학회지
- 48, n.3
- pp.177-185
- 2015
- 원문 바로보기
We develop a new auto-guiding system for the Camera for QUasars in the EArly uNiverse (CQUEAN). CQUEAN is an optical CCD camera system attached to the 2.1-m Otto-Struve Telescope (OST) at McDonald Observatory, USA. The new auto-guiding system differs from the original one in the following: instead of the cassegrain focus of the OST, it is attached to the finder scope; it has its own filter system for observation of bright targets; and it is controlled with the CQUEAN Auto-guiding Package, a newly developed auto-guiding program. Finder scope commands a very wide field of view at the expense of poorer light gathering power than that of the OST. Based on the star count data and the limiting magnitude of the system, we estimate there are more than 5.9 observable stars with a single FOV using the new auto-guiding CCD camera. An adapter is made to attach the system to the finder scope. The new auto-guiding system successfully guided the OST to obtain science data with CQUEAN during the test run in 2014 February. The FWHM and ellipticity distributions of stellar profiles on CQUEAN, images guided with the new auto-guiding system, indicate similar guiding capabilities with the original auto-guiding system but with slightly poorer guiding performance at longer exposures, as indicated by the position angle distribution. We conclude that the new auto-guiding system has overall similar guiding performance to the original system. The new auto-guiding system will be used for the second generation CQUEAN, but it can be used for other cassegrain instruments of the OST.
207
- AHN, SANG-HTEON
- Journal of the Korean astronomical society = 천문학회지
- 48, n.3
- pp.195-202
- 2015
- 원문 바로보기
Star forming galaxies found in the early universe exhibit asymmetric Lyα emission line that results from multiple scattering in a neutral thick medium surrounding the Lyα emission source. It is expected that emergent Lyα will be significantly polarized through a large number of resonance scattering events followed by a number of successive wing scatterings. In this study we adopt a Monte Carlo method to calculate the polarization of Lyα transferred in a very thick static slab of H<sub>I</sub>. Resonantly scattered radiation associated with transitions between <graphic></graphic> is only weakly polarized and therefore linear polarization of the emergent Lyα is mainly dependent on the number of off-resonant wing scattering events. The number of wing scattering events just before escape from the slab is determined by the product of the Doppler parameter a and the line center optical depth τ<sub>0</sub>, which, in turn, determines the behavior of the linear polarization of Lyα. This result is analogous to the study of polarized radiative transfer of Thomson scattered photons in an electron slab, where the emergent photons are polarized in the direction perpendicular to the slab when the scattering optical depth is small and polarized in the parallel direction when the slab is optically thick. Our simulated spectropolarimetry of Lyα shows that the line center is negligibly polarized, the near wing parts polarized in the direction parallel to the slab and the far wing parts are polarized in the direction perpendicular to the slab. We emphasize that the flip of polarization direction in the wing parts of Lyα naturally reflects the diffusive nature of the Lyα transfer process in thick neutral media.
208
- CHO, SE-HYUNG
- Journal of the Korean astronomical society = 천문학회지
- 48, n.6
- pp.357-364
- 2015
- 원문 바로보기
We observe ten known 22GHz H<sub>2</sub>O maser galaxies during February 19-22, 2011 using the 21 m Tamna telescope of the Korean VLBI Network and a new wide-band digital spectrometer. Simultaneously we searched for 43GHz SiO v = 1, 2, J = 1-0 maser emission. We detect H<sub>2</sub>O maser emission towards five sources (M 33, NGC 1052, NGC 1068, NGC 4258, M 82), with non-detections towards the remaining sources (UGC 3193, UGC 3789, Antennae H<sub>2</sub>O-West, M 51, NGC 6323) likely due to sensitivity. Our 22GHz spectra are consistent with earlier findings. Our simultaneous 43GHz SiO maser search produced non-detections, yielding - for the first time - upper limits on the 43GHz SiO maser emission in these sources at a 3 σ sensitivity level of 0.018K-0.033K (0.24 Jy-0.44 Jy) in a 1.75 km s<sup>−1</sup> velocity resolution. Our findings suggest that any 43GHz SiO masers in these sources (some having starburst-associated H<sub>2</sub>O kilomasers) must be faint compared to the 22GHz H<sub>2</sub>O maser emission.
209
- Lee, Sang-Sung
- Journal of the Korean astronomical society = 천문학회지
- 48, n.2
- pp.125-137
- 2015
- 원문 바로보기
We report results of the performance evaluation of a new hardware correlator in Korea, the Daejeon correlator, developed by the Korea Astronomy and Space Science Institute (KASI) and the National Astronomical Observatory of Japan (NAOJ). We conduct Very Long Baseline Interferometry (VLBI) observations at 22 GHz with the Korean VLBI Network (KVN) in Korea and the VLBI Exploration of Radio Astrometry (VERA) in Japan, and correlated the aquired data with the Daejeon correlator. For evaluating the performance of the new hardware correlator, we compare the correlation outputs from the Daejeon correlator for KVN observations with those from a software correlator, the Distributed FX (DiFX). We investigate the correlated flux densities and brightness distributions of extragalactic compact radio sources. The comparison of the two correlator outputs shows that they are consistent with each other within < 8%, which is comparable with the amplitude calibration uncertainties of KVN observations at 22 GHz. We also find that the 8% difference in flux density is caused mainly by (a) the difference in the way of fringe phase tracking between the DiFX software correlator and the Daejeon hardware correlator, and (b) an unusual pattern (a double-layer pattern) of the amplitude correlation output from the Daejeon correlator. The visibility amplitude loss by the double-layer pattern is as small as 3%. We conclude that the new hardware correlator produces reasonable correlation outputs for continuum observations, which are consistent with the outputs from the DiFX software correlator.
210
- Heo, Jeong-Eun
- Journal of the Korean astronomical society = 천문학회지
- 48, n.2
- pp.105-112
- 2015
- 원문 바로보기
The symbiotic star V1016 Cygni, a detached binary system consisting of a hot white dwarf and a mass-losing Mira variable, shows very broad emission features at around 6825 Å and 7082 Å, which are Raman scattered O vi λλ 1032, 1038 by atomic hydrogen. In the high resolution spectrum of V1016 Cyg obtained with the Bohyunsan Optical Echelle Spectrograph these broad features exhibit double peak profiles with the red peak stronger than the blue counterpart. However, their profiles differ in such a way that the blue peak of the 7082 feature is relatively weaker than the 6825 counterpart when the two Raman features are normalized to exhibit an equal red peak strength in the Doppler factor space. Assuming that an accretion flow around the white dwarf is responsible for the double peak profiles, we attribute this disparity in the profiles to the local variation of the flux ratio of O vi λλ 1032, 1038 in the accretion flow. A Monte Carlo technique is adopted to provide emissivity maps showing the local emissivity of O vi λ1032 and O vi λ1038 in the vicinity of the white dwarf. We also present a map indicating the differing flux ratios of O vi λλ 1032 and 1038. Our result shows that the flux ratio reaches its maximum of 2 in the emission region responsible for the central trough of the Raman feature and that the flux ratio in the inner red emission region is almost 1. The blue emission region and the outer red emission region exhibit an intermediate ratio around 1.5. We conclude that the disparity in the profiles of the two Raman O vi features strongly implies accretion flow around the white dwarf, which is azimuthally asymmetric.