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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 29/122
281
- 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.
282
- 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.
283
- 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.
284
- 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.
285
- Trippe, Sascha
- Journal of the Korean astronomical society = 천문학회지
- 46, n.3
- pp.133-140
- 2013
- 원문 바로보기
The empirical mass discrepancy-acceleration (MDA) relation of disk galaxies provides a key test for models of galactic dynamics. In terms of modified laws of gravity and/or inertia, the MDA relation quantifies the transition from Newtonian to modified dynamics at low centripetal accelerations <TEX>$a_c{\lesssim}10^{-10}ms^{-2}$</TEX>. As yet, neither dynamical models based on dark matter nor proposed modifications of the laws of gravity/inertia have predicted the functional form of the MDA relation. In this work, I revisit the MDA data and compare them to four different theoretical scaling laws. Three of these scaling laws are entirely empirical; the fourth one - the 'simple <TEX>${\mu}$</TEX>' function of Modified Newtonian Dynamics - derives from a toy model of gravity based on massive gravitons (the 'graviton picture'). All theoretical MDA relations comprise one free parameter of the dimension of an acceleration, Milgrom's constant aM. I find that the 'simple <TEX>${\mu}$</TEX>' function provides a good fit to the data free of notable systematic residuals and provides the best fit among the four scaling laws tested. The best-fit value of Milgrom's constant is <TEX>$a_M=(1.06{\pm}0.05){\times}10^{-10}ms^{-2}$</TEX>. Given the successful prediction of the functional form of the MDA relation, plus an overall agreement with the observed kinematics of stellar systems spanning eight orders of magnitude in size and 14 orders of magnitude in mass, I conclude that the 'graviton picture' is sufficient (albeit probably not a necessary nor unique approach) to describe galactic dynamics on all scales well beyond the scale of the solar system. This suggests that, at least on galactic scales, gravity behaves as if it was mediated by massive particles.
286
- Yang, Yunfei
- Journal of the Korean astronomical society = 천문학회지
- 46, n.5
- pp.191-200
- 2013
- 원문 바로보기
Full-disk solar images are provided by many solar telescopes around the world. However, the observed images show Non-Radial Variation (NRV) over the disk. In this paper, we propose algorithms for detecting distortions and restoring these images. For detecting NRV, the cross-correlation coefficients matrix of radial profiles is calculated and the minimum value in the matrix is defined as the Index of Non-radial Variation (INV). This index has been utilized to evaluate the H images of GONG, and systemic variations of different instruments are obtained. For obtaining the NRV's image, a Multi-level Morphological Filter (MMF) is designed to eliminate structures produced by solar activities over the solar surface. Comparing with the median filter, the proposed filter is a better choice. The experimental results show that the effect of our automatic detection and restoration methods is significant for getting a flat and high contrast full-disk image. For investigating the effect of our method on solar features, structural similarity (SSIM) index is utilized. The high SSIM indices (close to 1) of solar features show that the details of the structures remain after NRV restoring.
287
- Sung, Hwankyung
- Journal of the Korean astronomical society = 천문학회지
- 46, n.3
- pp.103-123
- 2013
- 원문 바로보기
Star clusters are superb astrophysical laboratories containing cospatial and coeval samples of stars with similar chemical composition. We initiate the Sejong Open cluster Survey (SOS) - a project dedicated to providing homogeneous photometry of a large number of open clusters in the SAAO Johnson-Cousins' UBV I system. To achieve our main goal, we pay much attention to the observation of standard stars in order to reproduce the SAAO standard system. Many of our targets are relatively small sparse clusters that escaped previous observations. As clusters are considered building blocks of the Galactic disk, their physical properties such as the initial mass function, the pattern of mass segregation, etc. give valuable information on the formation and evolution of the Galactic disk. The spatial distribution of young open clusters will be used to revise the local spiral arm structure of the Galaxy. In addition, the homogeneous data can also be used to test stellar evolutionary theory, especially concerning rare massive stars. In this paper we present the target selection criteria, the observational strategy for accurate photometry, and the adopted calibrations for data analysis such as color-color relations, zero-age main sequence relations, Sp - MV relations, Sp - <TEX>$T_{eff}$</TEX> relations, Sp - color relations, and <TEX>$T_{eff}$</TEX> - BC relations. Finally we provide some data analysis such as the determination of the reddening law, the membership selection criteria, and distance determination.
288
- Kwon, Young-Joo
- Journal of the Korean astronomical society = 천문학회지
- 45, n.6
- pp.139-146
- 2012
- 원문 바로보기
We investigate the properties of OH, SiO, and <TEX>$H_2O$</TEX> maser emission in O-rich AGB stars. We use a sample of 3373 objects, which is an updated version of the list of O-rich AGB stars presented in Suh & Kwon (2011). We divide the 3373 O-rich AGB stars into four different groups based on the maser emission: OH maser sources (1533), SiO sources (1627), <TEX>$H_2O$</TEX> sources (452), and sources with no maser (610). To understand the nature of the maser sources, we present various infrared two-color diagrams (2CDs) using IRAS, 2MASS, and AKARI data. For each group, we compare the positions on various infrared 2CDs with theoretical models. We find that the OH maser sources generally show larger color indices and larger dust optical depths than SiO or <TEX>$H_2O$</TEX> sources. We suggest that the differences of the color indices for different maser sources are due to different mass-loss rates and dust formation processes.
289
- Park, Jong-Ho
- Journal of the Korean astronomical society = 천문학회지
- 45, n.6
- pp.147-156
- 2012
- 원문 바로보기
We present a test of the emission statistics of active galactic nuclei (AGN), probing the connection between the red-noise temporal power spectra and multi-modal flux distributions known from observations. We simulate AGN lightcurves under the assumption of uniform stochastic emission processes for different power-law indices of their respective power spectra. For sufficiently shallow slopes (power-law indices (<TEX>${\beta}{\leq}1$</TEX>), the flux distributions (histograms) of the resulting lightcurves are approximately Gaussian. For indices corresponding to steeper slopes (<TEX>${\beta}{\geq}1$</TEX>), the flux distributions become multi-modal. This finding disagrees systematically with results of recent mm/radio observations. Accordingly, we conclude that the emission from AGN does not necessarily originate from uniform stochastic processes even if their power spectra suggest otherwise. Possible mechanisms include transitions between different activity states and/or the presence of multiple, spatially disconnected, emission regions.
290
- Choi, Chang-Su
- Journal of the Korean astronomical society = 천문학회지
- 45, n.1
- pp.7-17
- 2012
- 원문 바로보기
Y-band is a broad passband that is centered at ~1 <TEX>${\mu}m$</TEX>. It is becoming a new, popular window for extragalactic study especially for observations of red objects thanks to recent CCD technology developments. In order to better understand the general characteristics of objects in Y-band, and to investigate the promise of Y-band observations with small telescopes, we carried out imaging observations of several extragalactic fields, brown dwarfs, and high redshift quasars with Y-band filter at the Mt. Lemmon Optical Astronomy Observatory and the Maidanak observatory. From our observations, we constrain the bright end of the galaxy and the stellar number counts in Y-band. We also test the usefulness of high redshift quasar (z >6) selection via i - z - Y color-color diagram, to demonstrate that the i - z - Y color-color diagram is effective for the selection of high redshift quasars even with a conventional optical CCD camera installed at a 1-m class telescope.