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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 27/119
261
- 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.
262
- 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.
263
- 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.
264
- 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.
265
- 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.
266
- 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.
267
- Hwang, Jai-Chan
- Journal of the Korean astronomical society = 천문학회지
- 45, n.3
- pp.65-69
- 2012
- 원문 바로보기
Physical cosmology tries to understand the Universe at large with its origin and evolution. Observational and experimental situations in cosmology do not allow us to proceed purely based on the empirical means. We examine in which sense our cosmological assumptions in fact have shaped our current cosmological worldview with consequent inevitable limits. Cosmology, as other branches of science and knowledge, is a construct of human imagination reflecting the popular belief system of the era. The question at issue deserves further philosophic discussions. In Whitehead's words, 'philosophy, in one of its functions, is the critic of cosmologies'. (Whitehead 1925).
268
- Ghaderi, Kamal
- Journal of the Korean astronomical society = 천문학회지
- 45, n.1
- pp.1-6
- 2012
- 원문 바로보기
We use a Probabilistic Neural Network (PNN) technique to derive the orbital parameters of spectroscopic binary stars. Using measured radial velocity data of five double-lined spectroscopic binary systems (i.e., EQ Tau, V376 And, V776 Cas, V2377 Oph and EE Cet), we find the corresponding orbital and spectroscopic elements. Our numerical results are in good agreement with those obtained by other groups via more traditional methods.
269
- Choi, Seong-Hwan
- Journal of the Korean astronomical society = 천문학회지
- 45, n.2
- pp.31-38
- 2012
- 원문 바로보기
In this study we apply Support Vector Machine (SVM) to the prediction of geo-effective halo coronal mass ejections (CMEs). The SVM, which is one of machine learning algorithms, is used for the purpose of classification and regression analysis. We use halo and partial halo CMEs from January 1996 to April 2010 in the SOHO/LASCO CME Catalog for training and prediction. And we also use their associated X-ray flare classes to identify front-side halo CMEs (stronger than B1 class), and the Dst index to determine geo-effective halo CMEs (stronger than -50 nT). The combinations of the speed and the angular width of CMEs, and their associated X-ray classes are used for input features of the SVM. We make an attempt to find the best model by using cross-validation which is processed by changing kernel functions of the SVM and their parameters. As a result we obtain statistical parameters for the best model by using the speed of CME and its associated X-ray flare class as input features of the SVM: Accuracy=0.66, PODy=0.76, PODn=0.49, FAR=0.72, Bias=1.06, CSI=0.59, TSS=0.25. The performance of the statistical parameters by applying the SVM is much better than those from the simple classifications based on constant classifiers.
270
- Feng, Song
- Journal of the Korean astronomical society = 천문학회지
- 45, n.6
- pp.167-173
- 2012
- 원문 바로보기
Inter-granular Bright Points (igBPs) are small-scale objects in the Solar photosphere which can be seen within dark inter-granular lanes. We present a new algorithm to automatically detect and extract igBPs. Laplacian and Morphological Dilation (LMD) technique is employed by the algorithm. It involves three basic processing steps: (1) obtaining candidate 'seed' regions by Laplacian; (2) determining the boundary and size of igBPs by morphological dilation; (3) discarding brighter granules by a probability criterion. For validating our algorithm, we used the observed samples of the Dutch Open Telescope (DOT), collected on April 12, 2007. They contain 180 high-resolution images, and each has a <TEX>$85{\times}68\;arcsec^2$</TEX> field of view (FOV). Two important results are obtained: first, the identified rate of igBPs reaches 95% and is higher than previous results; second, the diameter distribution is <TEX>$220{\pm}25km$</TEX>, which is fully consistent with previously published data. We conclude that the presented algorithm can detect and extract igBPs automatically and effectively.