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1968년 ~ 2023년까지 1,187 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 : 1,187건 (DB Construction : 1,187 Articles)
총 게시글 1,187건
페이지 15/119
141
- KIM, SEUNG-LEE
- Journal of the Korean astronomical society = 천문학회지
- 49, n.5
- pp.199-208
- 2016
- 원문 바로보기
We present photometric results of the δ Sct star V1162 Ori, which is extensively monitored for a total of 49 nights from mid-December 2014 to early-March 2015. The observations are made with three KMTNet (Korea Microlensing Telescope Network) 1.6 m telescopes installed in Chile, South Africa, and Australia. Multiple frequency analysis is applied to the data and resulted in clear detection of seven frequencies without an alias problem: five known frequencies and two new ones with small amplitudes of 1.2-1.7 mmag. The amplitudes of all but one frequency are significantly different from previous results, confirming the existence of long-term amplitude changes. We examine the variations in pulsation timings of V1162 Ori for about 30 years by using the times of maximum light obtained from our data and collected from the literatures. The O − C (Observed minus Calculated) timing diagram shows a combination of a downward parabolic variation with a period decreasing rate of (1/P)dP/dt = −4.22 × 10<sup>−6</sup> year<sup>−1</sup> and a cyclic change with a period of about 2780 days. The most probable explanation for this cyclic variation is the light-travel-time effect caused by an unknown binary companion, which has a minimum mass of 0.69 M<sub>⊙</sub>. V1162 Ori is the first δ Sct-type pulsating star of which the observed fast period decrease can be interpreted as an evolutionary effect of a pre-main sequence star, considering its membership of the Orion OB 1c association.
142
- Ann, Hong Bae
- Journal of the Korean astronomical society = 천문학회지
- 49, n.6
- pp.239-253
- 2016
- 원문 바로보기
We determined the warp parameters of 192 warped galaxies which are selected from 340 edge-on galaxies using color images as well as r-band isophotal maps. We derive the local background density (<TEX>${\Sigma}_n$</TEX>) to examine the dependence of the warp amplitudes on the galaxy environment. We find a clear trend that strongly warped galaxies are likely to be found in high density regions where tidal interactions are supposed to be frequent. However, the correlation between <TEX>${\alpha}_{\omega}$</TEX> and <TEX>${\Sigma}_n$</TEX> is too weak for weakly warped galaxies (<TEX>${\alpha}_{\omega}$</TEX> < <TEX>$4^{\circ}$</TEX>) and the cumulative distributions of weakly warped galaxies are not significantly different from those of galaxies with no detectable warps. This suggests that tidal interactions do not play a decisive role in the formation of weak warps.
143
- HODGSON, JEFFREY A.
- Journal of the Korean astronomical society = 천문학회지
- 49, n.4
- pp.137-144
- 2016
- 원문 바로보기
The calibration of Very Long Baseline Interferometry (VLBI) data has long been a time consuming process. The Korean VLBI Network (KVN) is a simple array consisting of three identical antennas. Because four frequencies are observed simultaneously, phase solutions can be transferred from lower frequencies to higher frequencies in order to improve phase coherence and hence sensitivity at higher frequencies. Due to the homogeneous nature of the array, the KVN is also well suited for automatic calibration. In this paper we describe the automatic calibration of single-polarisation KVN data using the KVN Pipeline and comparing the results against VLBI data that has been manually reduced. We find that the pipelined data using phase transfer produces better results than a manually reduced dataset not using the phase transfer. Additionally we compared the pipeline results with a manually reduced phase-transferred dataset and found the results to be identical.
144
- CHO, KYUHYOUN
- Journal of the Korean astronomical society = 천문학회지
- 49, n.1
- pp.45-51
- 2016
- 원문 바로보기
The coronagraph is an instrument that enables the investigation of faint features in the vicinity of the Sun, particularly coronal mass ejections (CMEs). So far coronagraphic observations have been mainly used to determine the geometric and kinematic parameters of CMEs. Here, we introduce a new method for the determination of CME temperature using a two filter (4025 Å and 3934 Å) coronagraph system. The thermal motion of free electrons in CMEs broadens the absorption lines in the optical spectra that are produced by the Thomson scattering of visible light originating in the photosphere, which affects the intensity ratio at two different wavelengths. Thus the CME temperature can be inferred from the intensity ratio measured by the two filter coronagraph system. We demonstrate the method by invoking the graduated cylindrical shell (GCS) model for the 3-dimensional CME density distribution and discuss its significance.
145
- Kim, Jaeheon
- Journal of the Korean astronomical society = 천문학회지
- 49, n.6
- pp.261-288
- 2016
- 원문 바로보기
We present the results of simultaneous monitoring observations of <TEX>$H_2O$</TEX> <TEX>$6_{1,6}-5_{2,3}$</TEX> (22GHz) and SiO J=1-0, 2-1, 3-2 maser lines (43, 86, 129GHz) toward five post-AGB (candidate) stars, using the 21-m single-dish telescopes of the Korean VLBI Network. Depending on the target objects, 7 - 11 epochs of data were obtained. We detected both <TEX>$H_2O$</TEX> and SiO maser lines from four sources: OH16.1-0.3, OH38.10-0.13, OH65.5+1.3, and IRAS 19312+1950. We could not detect <TEX>$H_2O$</TEX> maser emission toward OH13.1+5.1 between the late OH/IR and post-AGB stage. The detected <TEX>$H_2O$</TEX> masers show typical double-peaked line profiles. The SiO masers from four sources, except IRAS 19312+1950, show the peaks around the stellar velocity as a single peak, whereas the SiO masers from IRAS 19312+1950 occur above the red peak of the <TEX>$H_2O$</TEX> maser. We analyzed the properties of detected maser lines, and investigated their evolutionary state through comparison with the full widths at zero power. The distribution of observed target sources was also investigated in the IRAS two-color diagram in relation with the evolutionary stage of post-AGB stars. From our analyses, the evolutionary sequence of observed sources is suggested as OH65.5+1.3<TEX>${\rightarrow}$</TEX>OH13.1+5.1<TEX>${\rightarrow}$</TEX>OH16.1-0.3<TEX>${\rightarrow}$</TEX>OH38.10-0.13, except for IRAS 19312+1950. In addition, OH13.1+5.1 from which the <TEX>$H_2O$</TEX> maser has not been detected is suggested to be on the gateway toward the post-AGB stage. With respect to the enigmatic object, IRAS 19312+1950, we could not clearly figure out its nature. To properly explain the unusual phenomena of SiO and <TEX>$H_2O$</TEX> masers, it is essential to establish the relative locations and spatial distributions of two masers using VLBI technique. We also include the <TEX>$1.2-160{\mu}m$</TEX> spectral energy distribution using photometric data from the following surveys: 2MASS, WISE, MSX, IRAS, and AKARI (IRC and FIS). In addition, from the IRAS LRS spectra, we found that the depth of silicate absorption features shows significant variations depending on the evolutionary sequence, associated with the termination of AGB phase mass-loss.
146
NON-GREY RADIATIVE TRANSFER IN THE PHOTOSPHERIC CONVECTION : VALIDITY OF THE EDDINGTON APPROXIMATION
- BACH, KIEHUNN
- Journal of the Korean astronomical society = 천문학회지
- 49, n.1
- pp.1-8
- 2016
- 원문 바로보기
The aim of this study is to describe the physical processes taking place in the solar photosphere. Based on 3D hydrodynamic simulations including a detailed radiation transfer scheme, we investigate thermodynamic structures and radiation fields in solar surface convection. As a starting model, the initial stratification in the outer envelope calculated using the solar calibrations in the context of the standard stellar theory. When the numerical fluid becomes thermally relaxed, the thermodynamic structure of the steady-state turbulent flow was explicitly collected. Particularly, a non-grey radiative transfer incorporating the opacity distribution function was considered in our calculations. In addition, we evaluate the classical approximations that are usually adopted in the onedimensional stellar structure models. We numerically reconfirm that radiation fields are well represented by the asymptotic characteristics of the Eddington approximation (the diffusion limit and the streaming limit). However, this classical approximation underestimates radiation energy in the shallow layers near the surface, which implies that a reliable treatment of the non-grey line opacities is crucial for the accurate description of the photospheric convection phenomenon.
147
- KOO, BON-CHUL
- Journal of the Korean astronomical society = 천문학회지
- 49, n.3
- pp.109-122
- 2016
- 원문 바로보기
[Fe II] emission lines are prominent in the infrared (IR) and important as diagnostic tools for radiative atomic shocks. We investigate the emission characteristics of [Fe II] lines using a shock code developed by Raymond (1979) with updated atomic parameters. We first review general characteristics of the IR [Fe II] emission lines from shocked gas, and derive their fluxes as a function of shock speed and ambient density. We have compiled available IR [Fe II] line observations of interstellar shocks and compare them to the ratios predicted from our model. The sample includes both young and old supernova remnants in the Galaxy and the Large Magellanic Cloud and several Herbig-Haro objects. We find that the observed ratios of the IR [Fe II] lines generally fall on our grid of shock models, but the ratios of some mid-IR lines, e.g., [Fe II] 35.35 µm/[Fe II] 25.99 µm, [Fe II] 5.340 µm/[Fe II] 25.99 µm, and [Fe II] 5.340 µm/[Fe II] 17.94 µm, are significantly offset from our model grid. We discuss possible explanations and conclude that while uncertainties in the shock modeling and the observations certainly exist, the uncertainty in atomic rates appears to be the major source of discrepancy.
148
- ZHU, WEI
- Journal of the Korean astronomical society = 천문학회지
- 49, n.3
- pp.93-107
- 2016
- 원문 바로보기
Augmenting the Wide Field Infrared Survey Telescope (WFIRST) microlensing campaigns with intensive observations from a ground-based network of wide-field survey telescopes would have several major advantages. First, it would enable full two-dimensional (2-D) vector microlens parallax measurements for a substantial fraction of low-mass lenses as well as planetary and binary events that show caustic crossing features. For a significant fraction of the free-floating planet (FFP) events and all caustic-crossing planetary/binary events, these 2-D parallax measurements directly lead to complete solutions (mass, distance, transverse velocity) of the lens object (or lens system). For even more events, the complementary ground-based observations will yield 1-D parallax measurements. Together with the 1-D parallaxes from WFIRST alone, they can probe the entire mass range M ≳ M<sub>⊕</sub>. For luminous lenses, such 1-D parallax measurements can be promoted to complete solutions (mass, distance, transverse velocity) by high-resolution imaging. This would provide crucial information not only about the hosts of planets and other lenses, but also enable a much more precise Galactic model. Other benefits of such a survey include improved understanding of binaries (particularly with low mass primaries), and sensitivity to distant ice-giant and gas-giant companions of WFIRST lenses that cannot be detected by WFIRST itself due to its restricted observing windows. Existing ground-based microlensing surveys can be employed if WFIRST is pointed at lower-extinction fields than is currently envisaged. This would come at some cost to the event rate. Therefore the benefits of improved characterization of lenses must be weighed against these costs.
149
- 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.
150
- 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.