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- The Korean Astronomical Society (The Korean Astronomical Society)
- 계간 (Quarterly)
- ISSN : 1225-4614 (ISSN : 1225-4614)
1225-4614
- DB구축현황 : 1,227건 (DB Construction : 1,227 Articles)
총 게시글 1,227건
페이지 76/123
751
- LEE SANG MIN
- Journal of the Korean astronomical society = 천문학회지
- 34, n.4
- pp.285-287
- 2001
- 원문 바로보기
We have studied the nonlinear evolution of a magnetized disk of isothermal gas, which is sustained by its self-gravity. Our objective is to investigate how the Jeans, Parker, and convective instabilities compete with each other in structuring/de-structuring large scale condensations in such disk. The Poisson equation for the self-gravity has been solved with a fourth-order accurate Fourier method along with the Green function, and the MHD part has been handled by an isothermal TVD code. When large wavelength perturbations are applied, the combined action of the Jeans and Parker instabilities suppresses the development of the convection and forms a dense core of prolate shape in the mid-plane. Peripheral structures around it are filamentary. The low density filaments connect the dense core to the diffuse upper region. On the other hand, when small wavelength perturbations are applied, the disk develops into an equilibrium state which is reminiscent of the Mouschovias's 2-D non-linear equilibrium of the classical Parker instability under an externally given gravity.
752
- LEE SANG-GAK
- Journal of the Korean astronomical society = 천문학회지
- 33, n.3
- pp.137-142
- 2000
- 원문 바로보기
CN and CH band strengths for ten new bright giants in the globular cluster M15 have been measured from archival spectra obtained with the Multiple Mirror Telescope. Using published indices for other bright M15 giants, a CN-CH band strength anticorrelation is found for bright red giants. However, stars that do not follow the CN-CH anticorrelation are also found. They seem to show a positive correlation between the two indices. Among them, all the AGB and HB stars of the sample are included. Stars I-38 and X6, which are located near the RGB fiducial line in the CMD, have low measured CH(G) indices compared with other RGB stars. Stars IV-38, S4, and S1, which are all near the RGB tip, have strong measured CH(G) indices. Therefore, most of their evolutionary states are suspected to be different from those of a normal single RGB star.
753
- HYUNG SIEK
- Journal of the Korean astronomical society = 천문학회지
- 33, n.2
- pp.81-88
- 2000
- 원문 바로보기
The international ultraviolet explorer (IUE) spectra of a low dispersion <TEX>$\~6{\AA}$</TEX>, have been investigated for two Seyfert 1 galaxies, Mrk 335 and NGC 4051, well known for the line variability. The electron densities of broad line region (BLR) of these variable Seyfert 1 galaxies have been derived, which showed a non-linear abrupt variation from <TEX>$10^8$</TEX> to <TEX>$10^{10} cm-3$</TEX> within a month. We also found the excitation (or temperature) changes in the Mrk 335 BLR from the IUE broad line profiles analysis, but no such evidence in the NGC 4051. The large amount of mass inflow activity through the bar or spiral structure of host galaxies, may trigger the density change in BLR and emission line variability for both objects. Mass of the giant black holes appear to be order of <TEX>$10^7\;M_{\bigodot}$</TEX> for both variable Seyfert l's.
754
- HAN CHEONGHO
- Journal of the Korean astronomical society = 천문학회지
- 33, n.2
- pp.89-95
- 2000
- 원문 바로보기
As an efficient method to detect blending of general gravitational microlensing events, it is proposed to measure the shift of source star image centroid caused by microlensing. The conventional method to detect blending by this method is measuring the difference between the positions of the source star image point spread function measured on the images taken before and during the event (the PSF centroid shift, <TEX>${\delta}{\theta}$</TEX>c,PSF). In this paper, we investigate the difference between the centroid positions measured on the reference and the subtracted images obtained by using the difference image analysis method (DIA centroid shift, <TEX>${\delta}{\theta}$</TEX>c.DIA), and evaluate its relative usefulness in detecting blending over the conventional method based on <TEX>${\delta}{\theta}$</TEX>c,PSF measurements. From this investigation, we find that the DIA centroid shift of an event is always larger than the PSF centroid shift. We also find that while <TEX>${\delta}{\theta}$</TEX>c,PSF becomes smaller as the event amplification decreases, <TEX>${\delta}{\theta}$</TEX>c.DIA remains constant regardless of the amplification. In addition, while <TEX>${\delta}{\theta}$</TEX>c,DIA linearly increases with the increasing value of the blended light fraction, <TEX>${\delta}{\theta}$</TEX>c,PSF peaks at a certain value of the blended light fraction and then eventually decreases as the fraction further increases. Therefore, measurements of <TEX>${\delta}{\theta}$</TEX>c,DIA instead of <TEX>${\delta}{\theta}$</TEX>c,PSF will be an even more efficient method to detect the blending effect of especially of highly blended events, for which the uncertainties in the determined time scales are high, as well as of low amplification events, for which the current method is highly inefficient.
755
- PARK SEOK JAE
- Journal of the Korean astronomical society = 천문학회지
- 33, n.1
- pp.19-28
- 2000
- 원문 바로보기
In the earlier papers we analyzed the axisymmetric, nonstationary electrodynamics of the central black hole and a surrounding thin accretion disk in an active galactic nucleus. In this paper we analyze the axisymmetric, nonstationary electrodynamics of the black hole magnetosphere in a similar way. In the earlier papers we employed the poloidal component of the plasma velocity which is confined only to the radial direction of the cylindrical coordinate system. In this paper we employ a more general poloidal velocity and get the Grad-Shafranov equation of the force-free magnetosphere of a Kerr black hole. The equation is consistent with the previous ones and is more general in many aspects as it should be. We also show in more general approaches that the angular velocity of the magnetic field lines anchored on the accreting matter tends to become close to that of the black hole at the equatorial zone of the hole.
756
- KIM CHULHEE
- Journal of the Korean astronomical society = 천문학회지
- 33, n.2
- pp.75-80
- 2000
- 원문 바로보기
In order to identify the candidates of emission-line galaxies inside the southern Hydra Void, photo-graphic objective-prism observations with the UK Schmidt Telescope were carried out using the Tech-Pan films for five fields. All observed prism plates were scanned with the APM Facility and the scanned data was processed to determine the APM plate parameters and to draw spectra. For all galaxy spectra, the emission features, the distance between emission features of H<TEX>$\beta$</TEX>4861, [OIII]<TEX>${\lambda}{\lambda}$</TEX> 4959, 5007 and the overlapping by nearby objects were investigated by eyeballing. A total of 7 candidates of emission-line galaxies inside the Hydra Void were identified.
757
- LEE HVUNSOOK
- Journal of the Korean astronomical society = 천문학회지
- 33, n.1
- pp.57-73
- 2000
- 원문 바로보기
We have done a spectroscopic study of the solar transition region using high resolution UV & EUV data obtained by SUMER(Solar Ultraviolet Measurements of Emitted Radiation) on board SOHO(Solar and Heliospheric Observatory). Optically thin and conspicuous emission lines observed at the solar limb are carefully selected to acquire average values of physical parameters for the quiet region as a function of radial distance. Our main results found from the present study can be summarized as follows. 1) Nonthermal velocities estimated from various UV lines do not decrease with height at least within one total line intensity scale height above the limb. 2) Nonthermal velocity distribution with temperature is very similar to that of the disk center, in the sense that its peak is located around <TEX>$2{\times}10^5 K$</TEX>, but the value is systematically larger than that of the disk. 3) It is found that nonthermal velocity is inversely proportional. to quadratic root of electron density up to about 10 arc seconds above the limb, i.e. <TEX>${\xi}\~N_e^{-1/4}$</TEX>, implying that the observed nonthermal broadening can be attributed to Alfven waves passing through the medium. 41 Electron density estimated from the O V 629/760 line ratio is found to range from about <TEX>$1{\times}10^{10}cm^{-3}$</TEX> to <TEX>$2{\times} 10^{12}cm^{-3}$</TEX> in the transition region.
758
- 안영숙
- Journal of the Korean astronomical society = 천문학회지
- 33, n.3
- pp.177-183
- 2000
- 원문 바로보기
We arrange Korean ancient calendar with Solar calendar during Choseon Dynasty(A.D. 1392-1910). In this period, we have one representable history books and several books, and most of information for date are found from them, ChoseonWangjosillok(조선왕조실록) and Jeungbomunheonbigo(증보문헌비고), etc. In those books many astronomical data and calendar information data are contained, so we can make chronological tables. Most of the data are arranged based on those several books, and for doubtable data are identified from eclipse, historical events and lunar phase calculations etc. We find that arranged chronological tables during Choseon Dynasty are similar to that of China and somewhat different from that of Japan. In addition we summarize all misrecorded date data in ChoseonWangjosillok
759
- ANN HONG BAE
- Journal of the Korean astronomical society = 천문학회지
- 33, n.1
- pp.1-17
- 2000
- 원문 바로보기
We have performed extensive simulations of response of gaseous disk in barred galaxies using SPH method. The gravitational potential is assumed to be generated by disk, bulge, halo, and bar. The mass of gaseous disk in SPH simulation is assumed to be negligible compared to the stellar and dark mass component, and the gravitational potential generated by other components is fixed in time. The self-gravity of the gas is not considered in most simulations, but we have made a small set of simulations including the self-gravity of the gas. Non-circular component of velocity generated by the rotating, non-axisymmetric potential causes many interesting features. In most cases, there is a strong tendency of concentration of gas toward the central parts of the galaxy. The morphology of the gas becomes quite complex, but the general behavior can be understood in terms of simple linear approximations: the locations and number of Lindblad resonances play critical role in determining the general distribution of the gas. We present our results in the form of 'atlas' of artificial galaxies. We also make a brief comment on the observational implications of our calculations. Since the gaseous component show interesting features while the stellar component behaves more smoothly, high resolution mapping using molecular emission line for barred galaxies would be desirable.
760
- 유계화
- Journal of the Korean astronomical society = 천문학회지
- 33, n.3
- pp.173-175
- 2000
- 원문 바로보기
The time-dependence of an <TEX>$\alpha$</TEX>-disk model under the influence of collisions of particles is examined. Collisions with viscosity tend to take away angular momentum. Both effects cause the disk to rotate more rapidly. The disk gradually contracts with increasing time.