모달창 닫기
- 홈으로 이동
1968년 ~ 2025년까지 1,227 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 : 1,227건 (DB Construction : 1,227 Articles)
총 게시글 1,227건
페이지 102/123
1011
- KIM KWANG TAE
- Journal of the Korean astronomical society = 천문학회지
- 28, n.1
- pp.15-30
- 1995
- 원문 바로보기
A velocity inhomogeneity, which is defined as a regional preponderence of either radial or tangential orbits, is searched with a new technique for the Coma cluster of galaxies. It is found within <TEX>$\~2h^{-1}$</TEX> Mpc from the cluster center that the Coma shows conspicuous inhomogeneities in velocity and that the inhomogeneities are real at a <TEX>$99\%$</TEX> level of confidence. Even in the central region (7' - 30' from the center), zones that are dominated by radial and tangential orbits are distinguishable. Defining the cluster's 'equator' as the direction defined by the Coma-A1367 supercluster, tangential orbits dominate the 'polar' zones in the central region. Galaxies that are located in 30'-100' also inhomogeneous in velocity in that the 'polar' zones are mostly radial while the rest is nearly homogeneous. These results indicate that the Coma galaxies are exceedingly more radial in orbit, implying that merging or infalls are either still going on or an earlier virialization is likely to have occurred preferentially near the 'equator'. Incorporating the velocity inhomogeneity into mass estimators, the most appropriate mass is turned out to be <TEX>$0.4\times10^{15}h^{-1}M_\bigodot(R\;\leq\;0.6h^{-1} Mpc),\;and\;1.0\times10^{15}h^{-1} M_\bigodot(R\;\leq\;2.1h^{-1}Mpc)$</TEX>. The corresponding mass to blue light ratio on the average is <TEX>$\~$</TEX>300h. These estimates are consistent with Merritt (1987) and Hughes (1989) and the MILE is seemed to favour the mass-follows-light models than the uniform spread of dark matter throughout the cluster.
1012
- Kim, Seung-Lee
- Journal of the Korean astronomical society = 천문학회지
- 28, n.2
- pp.197-208
- 1995
- 원문 바로보기
Real time CCD differential photometry was performed for BT Cnc in Praesepe cluster from February to March, 1994. New 885 differential V magnitudes were obtained for thirteen nights. From the frequency analysis, we have detected two distinct pulsational frequencies of <TEX>$f_1$</TEX>=9.7783c/d and <TEX>$f_2$</TEX>=7.0153c/d. The first frequency is nearly equal to the previous result(Breger 1980), but the second one is much different. Our reanalysis of the previous data obtained by Guerrero el al.(1979) indicates that the previous result of <TEX>$f_s$</TEX>=5.95c/d might be uncertain; it was not detected in the power spectrum. Also it turns out that our second frequency could not be fitted to the previous data and the reanalyzed frequency(<TEX>$f_2$</TEX>=7.8813c/d) of the previous data was poor-fitted to our data. Therefore we suggest that the second frequency which might be newly excited in the nonradial mode, has been changed over the last eighteen years.
1013
- Yoon, Tae-Sam
- Journal of the Korean astronomical society = 천문학회지
- 28, n.2
- pp.245-253
- 1995
- 원문 바로보기
We have analyzed the time series of Ca II H,K and <TEX>${\lambda}8498$</TEX> line profiles taken for a sunspot (SPO 5007) with the Echelle spectrograph attached to Vacuum Tower Telescope at Sacramento Peak Solar Observatory. Each set of spectra was taken simultaneously for 20 minutes at a time interval of 30 seconds. A total of 40 photographic films for each line was scanned by a PDS at Korea Astronomy Observatory. The central peak intensity of Ca II H (<TEX>$I_{max}$</TEX>), the intensity measured at <TEX>${\Delta}{\lambda}=-0.1{\AA}$</TEX> from the line center of <TEX>${\lambda}8498(I_{{\lambda}8489})$</TEX>, the radial velocity (<TEX>$V_r$</TEX>) and the Doppler width (<TEX>${\Delta}{\lambda}_D$</TEX>) estimated from Ca II H have been measured to study the dynamical behaviors of the sunspot chromosphere. Fourier analysis has been carried out for these measured quantities. Our main results are as follows: (1) We have confirmed the 3-minute oscillation being dominant throughout the umbra. The period of oscillations jumps from 180 sec in the umbra to 500 to 1000 sec in the penumbra. (2) The nonlinear character of the umbral oscillation is noted from the observed sawtooth shaped radial velocity fluctuations with amplitudes reaching up to <TEX>$5{\sim}6\;km/sec$</TEX>. (3) The spatial distribution of the maximum powers shows that the power of oscillations is stronger in the umbra than in the penumbra. (4) The spatial distributions of the time averaged < <TEX>$I_{max}$</TEX> > and < <TEX>$V_r$</TEX> > across the spot are found to be nearly axially symmetric, implying that the physical quantities derived from the line profiles of Ca II H and <TEX>${\lambda}8498$</TEX> are inherently associated with the geometry of the magnetic field distribution of the spot. (5) The central peaks of the CaII H emission core lead the upward motions of the umbral atmosphere by <TEX>$90^{\circ}$</TEX>, while no phase delay is found in intensities between <TEX>$I_{max}$</TEX> and <TEX>$I_{{\lambda}8498}$</TEX>, suggesting that the umbral oscillation is of standing waves.
1014
- PARK MYEONG-GU
- Journal of the Korean astronomical society = 천문학회지
- 28, n.1
- pp.97-107
- 1995
- 원문 바로보기
The stability of the geometrically thin, two-temperature hot accretion disk is studied. The general criterion for thermal instability is derived from the linear local analyses, allowing for advective cooling and dynamics in the vertical direction. Specifically, classic unsaturated Comptonization disk is analysed in detail. We find five eigen-modes: (1) Heating mode grows in thermal time scale, <TEX>$(5/3)({\alpha}{\omega})^{-1}$</TEX>, where alpha is the viscosity parameter and w the Keplerian frequency. (2) Cooling mode decays in time scale, <TEX>$(2/5)(T_e/T_i)({\alpha}{\omega})^{-1}$</TEX>, where <TEX>$T_e\;and\;T_i$</TEX> are the electron and ion temperatures, respectively. (3) Lightman-Eardley viscous mode decays in time scale, <TEX>$(4/3)(\Lambda/H)^2({\alpha}{\omega})^{-1}$</TEX>, where <TEX>$\Lambda$</TEX> is the wavelength of the perturbation and H the unperturbed disk height. (4) Two vertically oscillating modes oscillate in Keplerian time scale, <TEX>$(3/8)^{1/2}\omega^{-1}$</TEX> with growth rate <TEX>$\propto\;(H/\Lambda)^2$</TEX>. The inclusion of dynamics in the vertical direction does not affect the thermal instability, adding only the oscillatory modes which gradually grow for short wavelength modes. Also, the advective cooling is not strong enough to suppress the growth of heating modes, at least for geometrically thin disk. Non-linear development of the perturbation is followed for simple unsaturated Compton disk: depending on the initial proton temperature perturbation, the disk can evolve to decoupled state with hot protons and cool electrons, or to one-temperature state with very cool protons and electrons.
1015
- Lee, Kang-Hwan
- Journal of the Korean astronomical society = 천문학회지
- 28, n.2
- pp.153-167
- 1995
- 원문 바로보기
More than 22,300 stars in NGC 6752 were measured over the region of 5' < r < 23' in B and V AAT plates. The most of these are main sequence(MS) stars and about 130 blue horizontal branch(BHB) stars were detected. The C-M diagram of all measured stars shows gaps appearing at <TEX>$V{\approx}15.^{m}2$</TEX> and <TEX>$16.^{m}2$</TEX> along the red giant branch(RGB) and their appearance shown by Lee & Cannon(1980) is found to be independent of measured region. The bimodal distribution of BHB stars is confirmed again and a wide gap shown by Lee & Cannon(1980) at <TEX>$V{\approx}16^m$</TEX> is clearly seen for stars in the outer part (8' < r < 13') in the cluster. It is noted, however, that this gap is occupied by about a dozen of BHB staIs located in the inner region (5' < r < 8'). The number ratio of bright BHB star (V < <TEX>$15^m$</TEX>) to faint BHB stars (V > <TEX>$15^m$</TEX>) decreases with increasing radial distance from the cluster center. Three faintest BHB stars were found, and two stars (<TEX>$V{\approx}18.^{m}5$</TEX>) of there are located in the inner region of <TEX>$r{\approx}6'$</TEX> and the other faintest one (<TEX>$V{\approx}19.^{m}3$</TEX>) located in the outer part of <TEX>$r{\approx}13'$</TEX>. Also a bluest star of (B - V) <TEX>$\approx$</TEX> -0.5 at <TEX>$V{\approx}17.^{m}2$</TEX> is found but it is located at the outer part of <TEX>$r{\approx}13'$</TEX> in NE region. Therefore, the membership of the faintest BHB star and bluest star is suspected. The luminosity function(LF) and mass function(MF) for NGC 6752 were derived for MS stars. The LF for stars of <TEX>$M_v\;</TEX><TEX><</TEX><TEX>\;6^m$</TEX> in the outer part of r > 8' shows a consistency with that derived by Penny & Dickens(1986).
1016
- Sung, Hwan-Kyung
- Journal of the Korean astronomical society = 천문학회지
- 28, n.2
- pp.119-137
- 1995
- 원문 바로보기
The <TEX>$UBVI_{KC}$</TEX> CCD photometry was performed in the central region(<TEX>${\sim}20'{\times}20'$</TEX>) of the extremely young open cluster IC1805. Member stars were selected in the (B-V)-(U-B) and (V-I)-(B-V) color-color planes. Applying recent stellar evolutionary models, we derived the age, age-spread, and initial mass function(IMF) of the cluster. IC1805 was found to be an extremely young(<TEX>$t_{age}{\sim}1.5Myr$</TEX>) and has a flat IMF with the slope of <TEX>${\Gamma}=-1.0{\pm}0.2$</TEX>.
1017
- LEE SANG-WOO
- Journal of the Korean astronomical society = 천문학회지
- 28, n.1
- pp.77-87
- 1995
- 원문 바로보기
An attempt has been made to analyze time series of <TEX>$H_\alpha,\;H_\beta,\;and\; H_\gamma$</TEX> line profiles taken from a 3B/X6.1 flare which occurred on Oct. 27, 1991 in an active region, NOAA 6891. A total of 22 sets of <TEX>$H_\alpha,\;H_\beta,\;and\; H_\gamma$</TEX> taken with a low and non-uniform time resolution of 10-40 seconds were scanned by PDS with absolute intensity calibration to derive the physical characteristics of the material in the flare chromosphere. Our . results are as follows: (1) The lower Balmer lines observed during the flare activity are broadened by Stark effect. (2) At the peak of the flare activity, the electron temperature of the Balmer line emitting region reaches up to 35000K and its geometrical thickness increases to a scale of <TEX>$10^4km$</TEX>, suggesting that high energy particles penetrate deep into the photospheric level.
1018
- Ryu, Dong-Su
- Journal of the Korean astronomical society = 천문학회지
- 28, n.2
- pp.223-243
- 1995
- 원문 바로보기
We describe the implementation of a multi-dimensional numerical code to solve the equations for idea! magnetohydrodynamics (MHD) in cylindrical geometry. It is based on an explicit finite difference scheme on an Eulerian grid, called the Total Variation Diminishing (TVD) scheme, which is a second-order-accurate extension of the Roe-type upwind scheme. Multiple spatial dimensions are treated through a Strang-type operator splitting. Curvature and source terms are included in a way to insure the formal accuracy of the code to be second order. The constraint of a divergence-free magnetic field is enforced exactly by adding a correction, which involves solving a Poisson equation. The Fourier Analysis and Cyclic Reduction (FACR) method is employed to solve it. Results from a set of tests show that the code handles flows in cylindrical geometry successfully and resolves strong shocks within two to four computational cells. The advantages and limitations of the code are discussed.
1019
- LA DAILE
- Journal of the Korean astronomical society = 천문학회지
- 28, n.1
- pp.67-70
- 1995
- 원문 바로보기
In contrast to conventional belief that extended inflation ends when the Universe percolates, we find inflation may continue at least many Hubble times even after the Universe percolates. What is observed is that inflation will not stop unless the global equation of state changes from inflationary one into radiation one. Thus the energy density of shorter wavelength gravitational waves induced by bubble collision at near the end of inflation should be at least Order <TEX>$(10^2)\~O(10^3)$</TEX> times greater than previous estimation of Turner and Wilcek(TW).
1020
- Lee, Myung-Gyoon
- Journal of the Korean astronomical society = 천문학회지
- 28, n.2
- pp.177-196
- 1995
- 원문 바로보기
We present UBV CCD photometry of the double cluster NGC 1850 located at the NW edge of the bar of the Large Magellanic Cloud. The color-magnitude diagram shows that NGC 1850 has a prominent population of massive core-He burning stars which is incomparably richer than any other known star clusters. The reddening is estimated from the (U-B) - (B-V) diagram to be E(B - V) = <TEX>$0.15{\pm}0.05$</TEX>. We have estimated the ages of NGC 1850 and a very compact blue star cluster (NGC 1850A) located at <TEX>${\sim}30''$</TEX> west of NGC 1850 using isochrones based on the convective overshooting models: <TEX>$80{\sim}10$</TEX> Myrs and <TEX>$5{\sim}2$</TEX> Myra, respectively. Several evidence suggest that it is probably the compact cluster NGC 1850A that is responsible for the arc-shaped nebulosity (Henize N 103B) surrounding the east side of NGC 1850.