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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 26/119
251
- Chang, Cho-Rhong
- Journal of the Korean astronomical society = 천문학회지
- 46, n.6
- pp.203-224
- 2013
- 원문 바로보기
Wide-field <TEX>$JHK_s$</TEX> images obtained with the SIRIUS near-infrared camera of the IRSF 1.4m telescope are used to examine the tidal structures of the spatial stellar configuration around six metal-poor ([Fe/H]< -1.0) globular clusters located within 3 kpc from the Galactic center. The radial surface density profiles are obtained from the surface photometry of the cluster images and the star counting for the photometric data. For the star counting, candidates of cluster member stars are selected with an filtering algorithm in color-magnitude diagrams. We find that the six target clusters show tidal overdensity features in the radial surface density profiles. There is a break inside the tidal radius for each cluster, and the profile in the outer overdensity region is characterized by a power law. Two-dimensional density maps of all the clusters show distorted asymmetric stellar configurations in the outer region. In five out of the six target clusters, the overdensity features are likely to be associated with the effects of the Galaxy dynamical interaction and the cluster space motions. The observed tidal configurations of stars suggest that several metal-poor clusters in the Galactic bulge are possibly surviving remnants of mergers to build the old stellar system of the Galactic bulge.
252
- Park, Changbom
- Journal of the Korean astronomical society = 천문학회지
- 46, n.3
- pp.125-131
- 2013
- 원문 바로보기
We present the relation between the genus in cosmology and the Betti numbers for excursion sets of three- and two-dimensional smooth Gaussian random fields, and numerically investigate the Betti numbers as a function of threshold level. Betti numbers are topological invariants of figures that can be used to distinguish topological spaces. In the case of the excursion sets of a three-dimensional field there are three possibly non-zero Betti numbers; <TEX>${\beta}_0$</TEX> is the number of connected regions, <TEX>${\beta}_1$</TEX> is the number of circular holes (i.e., complement of solid tori), and <TEX>${\beta}_2$</TEX> is the number of three-dimensional voids (i.e., complement of three-dimensional excursion regions). Their sum with alternating signs is the genus of the surface of excursion regions. It is found that each Betti number has a dominant contribution to the genus in a specific threshold range. <TEX>${\beta}_0$</TEX> dominates the high-threshold part of the genus curve measuring the abundance of high density regions (clusters). <TEX>${\beta}_1$</TEX> dominates the genus near the median thresholds which measures the topology of negatively curved iso-density surfaces, and <TEX>${\beta}_2$</TEX> corresponds to the low-threshold part measuring the void abundance. We average the Betti number curves (the Betti numbers as a function of the threshold level) over many realizations of Gaussian fields and find that both the amplitude and shape of the Betti number curves depend on the slope of the power spectrum n in such a way that their shape becomes broader and their amplitude drops less steeply than the genus as n decreases. This behaviour contrasts with the fact that the shape of the genus curve is fixed for all Gaussian fields regardless of the power spectrum. Even though the Gaussian Betti number curves should be calculated for each given power spectrum, we propose to use the Betti numbers for better specification of the topology of large scale structures in the universe.
253
- Lim, Juhee
- Journal of the Korean astronomical society = 천문학회지
- 46, n.4
- pp.161-172
- 2013
- 원문 바로보기
A focal reducer is developed for CQUEAN (Camera for QUasars in EArly uNiverse), which is a CCD imaging system on the 2.1 m Otto Struve telescope at the McDonald observatory. It allows CQUEAN to secure a wider field of view by reducing the effective focal length by a factor of three. The optical point spread function without seeing effects is designed to be within one pixel (<TEX>$0.283^{\prime\prime}$</TEX>) over the field of view of <TEX>$4.82^{\prime}{\times}4.82^{\prime}$</TEX> in optimum wavelength ranges of 0.8-1.1 <TEX>${\mu}m$</TEX>. In this paper, we describe and discuss the characteristics of optical design, the lens and barrel fabrications and the alignment processes. The observation results show that the image quality of the focal reducer confirms the expectations from the design.
254
- Hanna, M.A.
- Journal of the Korean astronomical society = 천문학회지
- 46, n.4
- pp.151-159
- 2013
- 원문 바로보기
We discuss the orbital period changes of the Algol semi-detached eclipsing binary DI Peg by constructing the (O-C) residual diagram via using all the available precise minima times. We conclude that the period variation can be explained by a sine-like variation due to the presence of a third body orbiting the binary, together with a long-term orbital period increase (dP/dt=0.17 sec/century) that can be interpreted to be due to mass transfer from the evolved secondary component (of rate <TEX>$1.52{\times}10^{-8}M_{\odot}/yr$</TEX>) to the primary one. The detected low-mass third body (<TEX>$M_{3min.}=0.22{\pm}0.0006M_{\odot}$</TEX>) is responsible for a periodic variation of about 55 years light time effect. We have determined the orbital parameters of the third component which show a considerable eccentricity <TEX>$e_3=0.77{\pm}0.07$</TEX> together with a longitude of periastron <TEX>${\omega}_3=300^{\circ}{\pm}10^{\circ}$</TEX>.
255
- Choe, G.S.
- Journal of the Korean astronomical society = 천문학회지
- 46, n.6
- pp.261-268
- 2013
- 원문 바로보기
Magnetohydrostatic equilibria, in which the Lorentz force, the plasma pressure force and the gravitational force balance out to zero, are widely adopted as the zeroth order states of many astrophysical plasma structures. A magnetic flux-current surface is a surface, in which both magnetic field lines and current lines lie. We for the first time derive the necessary and sufficient condition for existence of magnetic flux-current surfaces in magnetohydrostatic equilibria. It is also shown that the existence of flux-current surfaces is a necessary (but not sufficient) condition for the ratio of gravity-aligned components of current density and magnetic field to be constant along each field line. However, its necessary and sufficient condition is found to be very restrictive. This finding gives a significant constraint in modeling solar coronal magnetic fields as force-free fields using photospheric magnetic field observations.
256
- Ko, Kyeong Yeon
- Journal of the Korean astronomical society = 천문학회지
- 46, n.2
- pp.75-91
- 2013
- 원문 바로보기
We estimate the power spectra of the cosmic microwave background radiation (CMB) temperature anisotropy in localized regions of the sky using the Wilkinson Microwave Anisotropy Probe (WMAP) 7-year data. We find that the north and south hat regions at high Galactic latitude (<TEX>$|b|{\geq}30^{\circ}C$</TEX>) show an anomaly in the power spectrum amplitude around the third peak, which is statistically significant up to 3. We try to explain the cause of the observed anomaly by analyzing the low Galactic latitude (<TEX>$|b|$</TEX> < <TEX>$30^{\circ}C$</TEX>) regions where the galaxy contamination is expected to be stronger, and the regions weakly or strongly dominated byWMAP instrument noise. We also consider the possible effect of unresolved radio point sources. We find another but less statistically significant anomaly in the low Galactic latitude north and south regions whose behavior is opposite to the one at high latitude. Our analysis shows that the observed north-south anomaly at high latitude becomes weaker on regions with high number of observations (weak instrument noise), suggesting that the anomaly is significant at sky regions that are dominated by the WMAP instrument noise. We have checked that the observed north-south anomaly has weak dependences on the bin-width used in the power spectrum estimation, and on the Galactic latitude cut. We also discuss the possibility that the detected anomaly may hinge on the particular choice of the multipole bin around the third peak. We anticipate that the issue of whether or not the anomaly is intrinsic one or due to WMAP instrument noise will be resolved by the forthcoming Planck data.
257
- Lee, Jung-Won
- Journal of the Korean astronomical society = 천문학회지
- 46, n.6
- pp.225-234
- 2013
- 원문 바로보기
We develop a radio receiver system operating at <TEX>${\lambda}{\sim}1.3$</TEX> mm for the 6 m telescope of Seoul Radio Astronomy Observatory. It consists of a dual polarization receiver, a couple of IF processing units, two FFT spectrometers, and associated software. By adopting sideband-separating superconductor mixers with image band terminated to waveguide load at 4.2 K, we achieve <TEX>$T_{RX}{\leq}100$</TEX> K and <TEX>$T_{sys}$</TEX> less than 150 K at best weather condition over 210-250 GHz frequency range. The intermediate frequency signal of 3.5-4.5 GHz is down converted to 0-1 GHz and fed into the FFT spectrometers. The spectrometer covers 1 GHz bandwidth with a spectral resolution of 61 KHz. Test observations are conducted toward several radio sources to evaluate the performance of the system. Aperture and beam efficiencies measured by observing planets are found to be typically 44 ~ 59% and 47 ~ 61%, respectively over the RF band, which are consistent with those measured at 3 mm band previously.
258
- Sung, Hwankyung
- Journal of the Korean astronomical society = 천문학회지
- 46, n.5
- pp.201-201
- 2013
- 원문 바로보기
259
- Kim, Eunbin
- Journal of the Korean astronomical society = 천문학회지
- 46, n.1
- pp.33-40
- 2013
- 원문 바로보기
We investigate the dependence of the extended X-ray emission from the halos of optically luminous early-type galaxies on the small-scale (the nearest neighbor distance) and large-scale (the average density inside the 20 nearest galaxies) environments. We cross-match the 3rd Data Release of the Second XMMNewton Serendipitous Source Catalog (2XMMi-DR3) to a volume-limited sample of the Sloan Digital Sky Survey (SDSS) Data Release 7 with <TEX>$M_r$</TEX> < -19.5 and 0.020 < z < 0.085, and find 20 early-type galaxies that have extended X-ray detections. The X-ray luminosity of the galaxies is found to have a tighter correlation with the optical and near infrared luminosities when the galaxy is situated in the low large-scale density region than in the high large-scale density region. Furthermore, the X-ray to optical (r-band) luminosity ratio, <TEX>$L_X/L_r$</TEX>, shows a clear correlation with the distance to the nearest neighbor and with large-scale density environment only where the galaxies in pair interact hydrodynamically with seperations of <TEX>$r_p$</TEX> < <TEX>$r_{vir}$</TEX>. These findings indicate that the galaxies in the high local density region have other mechanisms that are responsible for their halo X-ray luminosities than the current presence of a close encounter, or alternatively, in the high local density region the cooling time of the heated gas halo is longer than the typical time between the subsequent encounters.
260
- Suh, Kyung-Won
- Journal of the Korean astronomical society = 천문학회지
- 46, n.6
- pp.235-242
- 2013
- 원문 바로보기
We compare mass-loss rates of OH/IR stars obtained from radio observations with those derived from the dust radiative transfer models and IR observations. We collect radio observational data of OH maser and CO line emission sources for a sample of 1533 OH/IR stars listed in Suh & Kwon (2011). For 1259 OH maser, 76 CO(J=1-0), and 55 CO(J=2-1) emission sources, we compile data of the expansion velocity and mass-loss rate. We use a dust radiative transfer model for the dust shell to calculate the mass-loss rate as well as the IR color indices. The observed mass-loss rates are in the range predicted by the theoretical dust shell models corresponding to <TEX>$\dot{M}=10^{-8}M_{\odot}/yr-10^{-4}M_{\odot}/yr$</TEX>. We find that the dust model using a simple mixture of amorphous silicate and amorphous <TEX>$Al_2O_3$</TEX> (20% by mass) grains can explain the observations fairly well. The results indicate that the dust radiative transfer models for IR observations generally agree with the radio observations. For high mass-loss rate OH/IR stars, the mass-loss rates obtained from radio observations are underestimated compared to the mass-loss rates derived from the dust shell models. This could be because photon momentum transfer to the gas shell is not possible for the physical condition of high mass-loss rates. Alternative explanations could be the effects of different dust-to-gas ratios and/or a superwind.