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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
- 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.
282
- 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.
283
- 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.
284
- Zheng, Yan-Fang
- Journal of the Korean astronomical society = 천문학회지
- 46, n.5
- pp.183-190
- 2013
- 원문 바로보기
Recurring jets, which are jets ejected from the same site, are a peculiar type among various solar jet phenomena. We report such recurring jets ejecting from the same site above an active region on January 22, 2012 with high-resolution multi-wavelength observations from Solar Dynamics Observatory(SDO). We found that the recurring jets had velocities, lengths and lifetimes, but had similar directions. The visible brightening appeared at the jet base before each jet erupted. All the plasma produced by the recurring jets could not overcome the large coronal loops. It seemed that the plasma ejecting from the jet base was confined and guided by preexisting coronal loops, but their directions were not along the paths of the loops. Two of the jets formed crossing structures with the same preexisting filament. We also examined the photospheric magnetic field at the jet base, and observed a visible flux emergence, convergence and cancellation. The four recurring jets all were associated with the impulsive cancellation between two opposite polarities occurring at the jet base during each eruption. In addition, we suggest that the fluxes, flowing out of the active region, might supply the energy for the recurring jets by examining the SDO/Helioseismic and Magnetic Imager (HMI) successive images. The observational results support the magnetic reconnection model of jets.
285
- Kang, Hyesung
- Journal of the Korean astronomical society = 천문학회지
- 46, n.1
- pp.49-63
- 2013
- 원문 바로보기
Nonthermal radiation from supernova remnants (SNRs) provides observational evidence and constraints on the diffusive shock acceleration (DSA) hypothesis for the origins of Galactic cosmic rays (CRs). Recently it has been recognized that a variety of plasma wave-particle interactions operate at astrophysical shocks and the detailed outcomes of DSA are governed by their complex and nonlinear interrelationships. Here we calculate the energy spectra of CR protons and electrons accelerated at Type Ia SNRs, using time-dependent, DSA simulations with phenomenological models for magnetic field amplification due to CR streaming instabilities, Alf<TEX>$\acute{e}$</TEX>enic drift, and free escape boundary. We show that, if scattering centers drift with the Alf<TEX>$\acute{e}$</TEX>en speed in the amplified magnetic fields, the CR energy spectrum is steepened and the acceleration efficiency is significantly reduced at strong CR modified SNR shocks. Even with fast Afv<TEX>$\acute{e}$</TEX>nic drift, DSA can still be efficient enough to develop a substantial shock precursor due to CR pressure feedback and convert about 20-30% of the SN explosion energy into CRs. Since the high energy end of the CR proton spectrum is composed of the particles that are injected in the early stages, in order to predict nonthermal emissions, especially in X-ray and <TEX>${\gamma}-ray$</TEX> bands, it is important to follow the time dependent evolution of the shock dynamics, CR injection process, magnetic field amplification, and particle escape. Thus it is crucial to understand the details of these plasma interactions associated with collisionless shocks in successful modeling of nonlinear DSA.
286
- Wanas, M.I.
- Journal of the Korean astronomical society = 천문학회지
- 46, n.3
- pp.97-102
- 2013
- 원문 바로보기
In the present work, we prove the validity of two theorems on null-paths in a version of absolute parallelismgeometry. A version of these theorems has been originally established and proved by Kermak, McCrea and Whittaker (KMW) in the context of Riemannian geometry. The importance of such theorems lies in their applications to derive a general formula for the redshift of spectral lines coming from distant objects. The formula derived in the present work can be applied to both cosmological and astrophysical redshifts. It takes into account the shifts resulting from gravitation, different motions of the source of photons, spin of the moving particle (photons) and the direction of the line of sight. It is shown that this formula cannot be derived in the context of Riemannian geometry, but it can be reduced to a formula given by KMW under certain conditions.
287
- Shin, Jihye
- Journal of the Korean astronomical society = 천문학회지
- 46, n.4
- pp.173-181
- 2013
- 원문 바로보기
We trace the dynamical evolution of dark matter (DM) content in NGC 6397, one of the native Galactic globular clusters (GCs). The relatively strong tidal field (Galactocentric radius of ~ 6 kpc) and short relaxation timescale (~0.3 Gyr) of the cluster can cause a significant amount of DM particles to evaporate from the cluster in the Hubble time. Thus, the cluster can initially contain a non-negligible amount of DM. Using the most advanced Fokker-Planck (FP) method, we calculate the dynamical evolution of GCs for numerous initial conditions to determine the maximum initial DM content in NGC 6397 that matches the present-day brightness and velocity dispersion profiles of the cluster. We find that the maximum allowed initial DM mass is slightly less than the initial stellar mass in the cluster. Our findings imply that NGC 6397 did not initially contain a significant amount of DM, and is similar to that of NGC 2419, the remotest and the most massive Galactic GC.
288
- Kim, Yong-Ha
- Journal of the Korean astronomical society = 천문학회지
- 45, n.2
- pp.39-48
- 2012
- 원문 바로보기
We investigate non-LTE effects on the <TEX>$H_3^+$</TEX> level populations to help the analysis of the observed 2 and 3.5 micron <TEX>$H_3^+$</TEX> emissions from the Jovian ionosphere. We begin by constructing a simple three-level model, in order to compute the intensity ratio of the R(3,4) line in the hot band to the Q(1,0) line in the fundamental band, which have been observed in the Jovian auroral regions. We find that non-LTE effects produce only small changes in the intensity ratios for ambient <TEX>$H_2$</TEX> densities less than or equal to <TEX>$5{\times}10^{11}cm^{-3}$</TEX>. We then construct two comprehensive models by including all the collisional and radiative transitions between pairs of more than a thousand known <TEX>$H_3^+$</TEX> rovibrational levels with energies less than 10000 <TEX>$cm^{-1}$</TEX>. By employing these models, we find that the intensity ratios of the lines in the hot and fundamental bands are affected greatly by non-LTE effects, but the details depend sensitively on the number of collisional and radiative transitions included in the models. Non-LTE effects on the rovibrational population become evident at about the same ambient <TEX>$H_2$</TEX> densities in the comprehensive models as in the three-level model. However, the models show that rotational temperatures derived from the intensities of rotational lines in the <TEX>${\nu}_2$</TEX> and <TEX>$2{\nu}_2$</TEX> bands may differ significantly from the ambie temperatures in the non-LTE regime. We find that significant non-LTE effects appear near and above the <TEX>$H_3^+$</TEX> peak, and that the kinetic temperatures in the Jovian thermospheric temperatures derived from the observed line ratios in the 2 and 3.5 micron <TEX>$H_3^+$</TEX> emissions are highly model dependent.
289
- Park, Hong-Soo
- Journal of the Korean astronomical society = 천문학회지
- 45, n.3
- pp.71-84
- 2012
- 원문 바로보기
We present a photometric study of the globular clusters (GCs) in the Virgo giant elliptical galaxy M86 based on Washington <TEX>$CT_1$</TEX> images. The colors of the GCs in M86 show a bimodal distribution with a blue peak at (<TEX>$C-T_1$</TEX>) = 1.30 and a red peak at (<TEX>$C-T_1$</TEX>) = 1.72. The spatial distribution of the red GCs is elongated similar to that of the stellar halo, while that of the blue GCs is roughly circular. The radial number density profile of the blue GCs is more extended than that of the red GCs. The radial number density profile of the red GCs is consistent with the surface brightness profile of the M86 stellar halo. The GC system has a negative radial color gradient, which is mainly due to the number ratio of the blue GCs to the red GCs increasing as galactocentric radius increases. The bright blue GCs in the outer region of M86 show a blue tilt: the brighter they are, the redder their mean colors get. These results are discussed in comparison with other Virgo giant elliptical galaxies.
290
- Minh, Young Chol
- Journal of the Korean astronomical society = 천문학회지
- 45, n.6
- pp.157-166
- 2012
- 원문 바로보기
Using the Submillimeter Array (SMA), we identified two bright hot subcores, MM1a and MM1b (size ~ 1' and mass ~ 0.5 <TEX>$M_{\odot}$</TEX>) separated by about 1.600, in the 230 GHz continuum emission toward the massive star-forming region DR21(OH). Both display typical hot core characteristics but have slightly different chemical properties. For example, highly saturated species show stronger emission toward MM1a and seem to be evaporating directly from the grain mantles. In contrast, simple sulfur-bearing species have brighter emission at MM1b. These features indicate that MM1a is at an earlier stage than MM1b, and the small-scale chemical differences between these two cores may result from the age difference of the order of <TEX>$10^4$</TEX> years.