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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 20/122
191
- LEE, ENSANG
- Journal of the Korean astronomical society = 천문학회지
- 49, n.1
- pp.19-23
- 2016
- 원문 바로보기
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<sub>E</sub>, along the boundary, which is much larger than the size, ~ 6 R<sub>E</sub>, along the normal direction. This suggests that the magnetic decrease has a shape of a long, thin rod or a wide slab.
192
- TRIPPE, SASCHA
- Journal of the Korean astronomical society = 천문학회지
- 49, n.5
- pp.193-198
- 2016
- 원문 바로보기
Early-type galaxies (ETGs) are supposed to follow the virial relation <TEX>$M=k_e{\sigma}^2R_e/G$</TEX>, with M being the mass, σ<sub>*</sub> being the stellar velocity dispersion, R<sub>e</sub> being the effective radius, G being Newton's constant, and k<sub>e</sub> being the virial factor, a geometry factor of order unity. Applying this relation to (a) the ATLAS<sup>3D</sup> sample of Cappellari et al. (2013) and (b) the sample of Saglia et al. (2016) gives ensemble-averaged factors 〈k<sub>e</sub>〉 = 5.15 ± 0.09 and 〈k<sub>e</sub>〉 = 4.01 ± 0.18, respectively, with the difference arising from different definitions of effective velocity dispersions. The two datasets reveal a statistically significant tilt of the empirical relation relative to the theoretical virial relation such that <TEX>$M{\propto}({\sigma}^2_*R_e)^{0.92}$</TEX>. This tilt disappears when replacing R<sub>e</sub> with the semi-major axis of the projected half-light ellipse, a. All best-fit scaling relations show zero intrinsic scatter, implying that the mass plane of ETGs is fully determined by the virial relation. Whenever a comparison is possible, my results are consistent with, and confirm, the results by Cappellari et al. (2013). The difference between the relations using either a or R<sub>e</sub> arises from a known lack of highly elliptical high-mass galaxies; this leads to a scaling (1 - ϵ ) ∝ M<sup>0.12</sup>, with ϵ being the ellipticity and <TEX>$R_e=a\sqrt[]{1-{\epsilon}}$</TEX>. Accordingly, a, not R<sub>e</sub>, is the correct proxy for the scale radius of ETGs. By geometry, this implies that early-type galaxies are axisymmetric and oblate in general, in agreement with published results from modeling based on kinematics and light distributions.
193
- LI, XUEBAO
- Journal of the Korean astronomical society = 천문학회지
- 49, n.4
- pp.157-162
- 2016
- 원문 바로보기
High resolution reconstruction technology is developed to help enhance the spatial resolution of observational images for ground-based solar telescopes, such as speckle masking. Near real-time reconstruction performance is achieved on a high performance cluster using the Message Passing Interface (MPI). However, much time is spent in reconstructing solar subimages in such a speckle reconstruction. We design and implement a novel parallel method for speckle masking reconstruction of solar subimage on a shared memory machine using the OpenMP. Real tests are performed to verify the correctness of our codes. We present the details of several parallel reconstruction steps. The parallel implementation between various modules shows a great speed increase as compared to single thread serial implementation, and a speedup of about 2.5 is achieved in one subimage reconstruction. The timing result for reconstructing one subimage with 256×256 pixels shows a clear advantage with greater number of threads. This novel parallel method can be valuable in real-time reconstruction of solar images, especially after porting to a high performance cluster.
194
- Lee, Dong-Joo
- Journal of the Korean astronomical society = 천문학회지
- 49, n.6
- pp.295-306
- 2016
- 원문 바로보기
We present the results of BV time-series photometry of the globular cluster NGC 288. Observations were carried out to search for variable stars using the Korea Microlensing Telescope Network (KMTNet) 1.6-m telescopes and a 4k pre-science CCD camera during a test observation from August to December, 2014. We found a new SX Phe star and confirmed twelve previously known variable stars in NGC 288. For the semi-regular variable star V1, we newly determined a period of 37.3 days from light curves spanning 137 days. The light-curve solution of the eclipsing binary V10 indicates that the system is probably a detached system. The pulsation properties of nine SX Phe stars were examined by applying multiple frequency analysis to their light curves. We derived a new Period-Luminosity (P-L) relation, <TEX>${\langle}M_V{\rangle}=-2.476({\pm}0.300){\log}P-0.354({\pm}0.385)$</TEX>, from six SX Phe stars showing the fundamental mode. Additionally, the period ratios of three SX Phe stars that probably have a double-radial mode were investigated; <TEX>$P_{FO}/P_F=0.779$</TEX> for V5, <TEX>$P_{TO}/P_{FO}=0.685$</TEX> for V9, <TEX>$P_{SO}/P_{FO}=0.811$</TEX> for V11. This paper is the first contribution in a series assessing the detections and properties of variable stars in six southern globular clusters with the KMTNet system.
195
- CHO, DONG-HWAN
- Journal of the Korean astronomical society = 천문학회지
- 48, n.6
- pp.333-341
- 2015
- 원문 바로보기
A search for hot and bright white dwarfs (WDs) in the Milky Way globular clusters M13 (NGC 6205) and M22 (NGC 6656) is carried out using the deep and homogeneous V I photometric catalog of Anderson et al. and and Sarajedini et al., based on data taken with the ACS/WFC aboard the Hubble Space Telescope (HST). V versus V − I color-magnitude diagrams (CMDs) of M13 and M22 are constructed and numerous spurious detections are rejected according to their photometric quality parameters qfit(V ) and qfit(I). In the case of M13, further radial restriction is applied to reject central stars with higher photometric errors due to central crowding. From each resultant V versus V −I CMD, sixteen and thirteen WD candidates are identified in M13 and M22, respectively. They are identified as stellar objects in the accompanying ACS/WFC images and are found to be randomly distributed across the central regions of M13 and M22. Their positions in the CMDs are in the bright part of the DA WD cooling sequences indicating that they are true WDs. In order to confirm their nature, follow-up spectroscopic observations are needed.
196
- BACH, KIEHUNN
- Journal of the Korean astronomical society = 천문학회지
- 48, n.3
- pp.165-175
- 2015
- 원문 바로보기
We investigate physical properties of the nearby (∼ 7.5 pc) astrometric binary μ Cas in the context of standard evolutionary theory. Based on the spectroscopically determined relative abundances ([α/Fe] ≳ +0.4 dex, [Fe/H] ∼ −0.7 dex), all physical inputs such as opacities and equation of state are consistently generated. By combining recent spectroscopic analyses with the astrometric observations from the HIPPARCOS parallaxes and the CHARA array, the evolutionary model grids have been constructed. Through the statistical evaluation of the χ<sup>2</sup>-minimization among alternative models, we find a reliable evolutionary solution (M<sub>A</sub>, M<sub>B</sub>, t<sub>age</sub>) = (0.74 M<sub>⊙</sub>, 0.19 M<sub>⊙</sub>, 11 Gyr) which excellently satisfies observational constraints. In particular, we find that the helium abundance of μ Cas is comparable with the primordial helium contents (Y<sub>p</sub> ∼ 0.245). On the basis of the well-defined stellar parameters of the primary star, the internal structure and the p-mode frequencies have been estimated. From our seismic computation, μ Cas is expected to have a first order spacing ∆ν ∼ 169 μHz. The ultimate goal of this study is to describe physical processes inside a low-mass star through a complete modelling from the spectroscopic observation to the evolutionary computation.
197
- IM, MYUNGSHIN
- Journal of the Korean astronomical society = 천문학회지
- 48, n.4
- pp.207-212
- 2015
- 원문 바로보기
We introduce the Lee Sang Gak Telescope (LSGT), a remotely operated, robotic 0.43-meter telescope. The telescope was installed at the Siding Spring Observatory, Australia, in 2014 October, to secure regular and exclusive access to the dark sky and excellent atmospheric conditions in the southern hemisphere from the Seoul National University (SNU) campus. Here, we describe the LSGT system and its performance, present example images from early observations, and discuss a future plan to upgrade the system. The use of the telescope includes (i) long-term monitoring observations of nearby galaxies, active galactic nuclei, and supernovae; (ii) rapid follow-up observations of transients such as gamma-ray bursts and gravitational wave sources; and (iii) observations for educational activities at SNU. Based on observations performed so far, we find that the telescope is capable of providing images to a depth of R = 21:5 mag (point source detection) at 5-σ with 15 min total integration time under good observing conditions.
198
- Park, Changbom
- Journal of the Korean astronomical society = 천문학회지
- 48, n.1
- pp.75-82
- 2015
- 원문 바로보기
We use a volume-limited sample of quasars in the Sloan Digital Sky Survey (SDSS) DR7 quasar catalog to identify quasar groups and address their statistical significance. This quasar sample has a uniform selection function on the sky and nearly a maximum possible contiguous volume that can be drawn from the DR7 catalog. Quasar groups are identified by using the Friend-of-Friend algorithm with a set of fixed comoving linking lengths. We find that the richness distribution of the richest 100 quasar groups or the size distribution of the largest 100 groups are statistically equivalent with those of randomly-distributed points with the same number density and sky coverage when groups are identified with the linking length of <TEX>$70h^{-1}Mpc$</TEX>. It is shown that the large-scale structures like the huge Large Quasar Group (U1.27) reported by Clowes et al. (2013) can be found with high probability even if quasars have no physical clustering, and does not challenge the initially homogeneous cosmological models. Our results are statistically more reliable than those of Nadathur (2013), where the test was made only for the largest quasar group. It is shown that the linking length should be smaller than <TEX>$50h^{-1}Mpc$</TEX> in order for the quasar groups identified in the DR7 catalog not to be dominated by associations of quasars grouped by chance. We present 20 richest quasar groups identified with the linking length of <TEX>$70h^{-1}Mpc$</TEX> for further analyses.
199
- KANG, SINCHEOL
- Journal of the Korean astronomical society = 천문학회지
- 48, n.5
- pp.257-265
- 2015
- 원문 바로보기
We present results of long-term multi-wavelength polarization observations of the powerful blazar 3C 279 after its γ-ray flare on 2013 December 20. We followed up this flare with single-dish polarization observations using two 21-m telescopes of the Korean VLBI Network. Observations carried out weekly from 2013 December 25 to 2015 January 11, at 22 GHz, 43 GHz, 86 GHz simultaneously, as part of the Monitoring Of GAmma-ray Bright AGN (MOGABA) program. We measured 3C 279 total flux densities of 22–34 Jy at 22 GHz, 15–28 Jy (43 GHz), and 10–21 Jy (86 GHz), showing mild variability of ≤ 50 % over the period of our observations. The spectral index between 22 GHz and 86 GHz ranged from −0.13 to −0.36. Linear polarization angles were 27°–38°, 30°–42°, and 33°–50° at 22 GHz, 43 GHz, and 86 GHz, respectively. The degree of linear polarization was in the range of 6–12 %, and slightly decreased with time at all frequencies. We investigated Faraday rotation and depolarization of the polarized emission at 22–86 GHz, and found Faraday rotation measures (RM) of −300 to −1200 rad m<sup>−2</sup> between 22 GHz and 43 GHz, and −800 to −5100 rad m<sup>−2</sup> between 43 GHz and 86 GHz. The RM values follow a power law with a mean power law index a of 2.2, implying that the polarized emission at these frequencies travels through a Faraday screen in or near the jet. We conclude that the regions emitting polarized radio emission may be different from the region responsible for the 2013 December γ-ray flare and are maintained by the dominant magnetic field perpendicular to the direction of the radio jet at milliarcsecond scales.
200
- Seon, Kwang-Il
- Journal of the Korean astronomical society = 천문학회지
- 48, n.1
- pp.57-66
- 2015
- 원문 바로보기
Monte-Carlo radiative models of the diffuse Galactic light (DGL) in our Galaxy are calculated using the dust radiative transfer code MoCafe, which is three-dimensional and takes full account of multiple scattering. The code is recently updated to use a fast voxel traversal algorithm, which has dramatically increased the computing speed. The radiative transfer models are calculated with the generally accepted dust scale-height of 0.1 kpc. The stellar scale-heights are assumed to be 0.1 or 0.35 kpc, appropriate for far-ultraviolet (FUV) and optical wavelengths, respectively. The face-on optical depth, measured perpendicular to the Galactic plane, is also varied from 0.2 to 0.6, suitable to the optical to FUV wavelengths, respectively. We find that the DGL at high Galactic latitudes is mostly due to backward or large-angle scattering of starlight originating from the local stars within a radial distance of r < 0.5 kpc from the Earth. On the other hand, the DGL measured in the Galactic plane is mostly due to stars at a distance range that corresponds to an optical depth of <TEX>$${\sim_\sim}$$</TEX> 1 measured from the Earth. Therefore, the low-latitude DGL at the FUV wavelength band would be mostly caused by the stars located at a distance of <TEX>$r{\leq}0.5$</TEX> kpc and the optical DGL near the Galactic plane mainly originates from stars within a distance range of <TEX>$1{\leq}r{\leq}2kpc$</TEX>. We also calculate the radiative transfer models in a clumpy two-phase medium. The clumpy two-phase models provide lower intensities at high Galactic latitudes compared to the uniform density models, because of the lower effective optical depth in clumpy media. However, no significant difference in the intensity at the Galactic plane is found.