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- 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 101/122
1001
- 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.
1002
- 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.
1003
- 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).
1004
- 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.
1005
- Ann, Hong-Bae
- Journal of the Korean astronomical society = 천문학회지
- 28, n.2
- pp.209-221
- 1995
- 원문 바로보기
We have examined bulge morphology of 104 bright barred galaxies, using V-band surface photometry based on the Kiso Schmidt plates. By measuring the bulge ellipticity and bulge-disk misalignment, we have classified bulges into four morphological types: sphere, oblate spheroid, triaxial ellipsoid, and pseudo triaxial ellipsoid. About half of the observed galaxies are found to have triaxial bulges with mean ellipticity of 0.24. They are distributed uniformly along the Hubble sequence.
1006
- Lee, Sang-Gak
- Journal of the Korean astronomical society = 천문학회지
- 28, n.2
- pp.139-146
- 1995
- 원문 바로보기
The bright part of the halo luminosity function is derived from a sample of the 233 NLTT propermotion stars, which are selected by the 220 km/ see of cutoff velocity in transverse to rid the contamination by the disk stars and corrected for the stars omitted in the sample by the selection criterion. It is limited to the absolute magnitude range of <TEX>$M_v=4-8$</TEX>, but is based on the largest sample of halo stars up to now. This luminosity function provides a number density of <TEX>$2.3{\cdot}10^{-5}pc^{-3}$</TEX> and a mass density of <TEX>$2.3{\cdot}10^{-5}M_{o}pc^{-3}$</TEX> for 4 < <TEX>$M_v$</TEX> < 8 in the solar neighborhood. These are not sufficient for disk stability. The kinematics of the sample stars are < U > = - 7 km/sec, < V > = - 228 km/sec, and < W > = -8 km/sec with (<TEX>${\sigma_u},{\sigma_v},{\sigma_w}$</TEX>) = (192, 84, 94) km/sec. The average metallicity of them is [Fe/H] = <TEX>$- 1.7{\pm}0.8$</TEX>. These are typical values for halo stars which are selected by the high cutoff velocity. We reanalyze the luminosity function for a sample of 57 LHS proper-motion stars. The newly derived luminosity function is consistent with the one derived from the NLTT halo stars, but gives a somewhat smaller number density for the absolute magnitude range covered by the LF from NLTT stars. The luminosity function based on the LHS stars seems to have a dip in the magnitude range corresponding to the Wielen Dip, but it also seems to have some fluctuations due to a small number of sample stars.
1007
- Park, Seok-Jae
- Journal of the Korean astronomical society = 천문학회지
- 28, n.2
- pp.147-152
- 1995
- 원문 바로보기
An axisymmetric, stationary electrodynamic model of the central engine of an active galactic nucleus has been well formulated by Macdonald and Thorne. In this model the relativistic region around the central black hole must be filled by highly conducting plasma. We analyze plasma wave propagation in this region and discuss the results. We find that the ionosphere cannot exist right outside of the event horizon of the black hole. Another interesting aspect is that certain resonance phenomena can occur in this case.
1008
- KIM YONG-HA
- Journal of the Korean astronomical society = 천문학회지
- 28, n.1
- pp.89-95
- 1995
- 원문 바로보기
We examined a total of 166 images of <TEX>$3.5{\mu}m\;H_3^+$</TEX> emission in the auroral regions of Jupiter observed with the Protocam on IRTF in 1991 and 1992, and found that 30 images contain a clearly isolated small emission patch in the vicinity of the northern auroral regions. Two different time sequences of the images show the small patches at the dusk limb in the range of System III longitudes from <TEX>$270^{\circ}\;through\;0^{\circ}\;to\;90^{circ}$</TEX>. The small patches in one sequence of the images, which were taken at 10 phase between <TEX>$240^{\circ}\;and\;260^{\circ}$</TEX>, may be related to the 10 flux tube, similarly suggested by Connerney et al. (1993). However, the small patches in the other sequence are separated from Io as much as <TEX>$80^{\circ}$</TEX> in longitude. The positions of the small patches in both sequences are deviated equatorward from the 10 footprint oval by <TEX>$5^{\circ}-8^{\circ}$</TEX> latitude in the longitudinal range of <TEX>$270^{\circ}-360^{\circ}$</TEX>. A significant modification is required in current Jovian magnetic field models near the Jupiter's surface if the small patches are produced at the foot of the 10 flux tube.
1009
- Lee, Myung-Gyoon
- Journal of the Korean astronomical society = 천문학회지
- 28, n.2
- pp.169-175
- 1995
- 원문 바로보기
Deep V I CCD photometry of the Pegasus dwarf irregular galaxy shows that the tip of the red giant branch (RGB) is located at I = <TEX>$21.15{\pm}0.10$</TEX> mag and (V - I) = <TEX>$1.58{\pm}0.03$</TEX>. Using the I magnitude of the tip of the RGB (TRGB), the distance modulus of the Pegasus galaxy is estimated to be <TEX>$(m\;-\;M)_o\;=\;25.13{\pm}0.11$</TEX> mag (corresponding to a distance of d = <TEX>$1060{\pm}50$</TEX> kpc). This result is in a good agreement with the recent distance estimate based on the TRGB method by Aparicio [1994, ApJ, 437, L27],<TEX>$ (m\;-\;M)_o$</TEX> = 24.9 (d = 950 kpc). However, our distance estimate is much smaller than that based on the Cepheid variable candidates by Hoessel et al.[1990, AJ, 100, 1151], <TEX>$(m\;-\;M)_o\;=\;26.22{\pm}0.20$</TEX> (d = <TEX>$1750{\pm}160$</TEX> kpc) mag. The color-magnitude diagram illustrates that the Cepheid candidates used by Hoessel et al.are not located in the Cepheid instability strip, but in the upper part of the giant branch. This result shows that the Cepheid candidates studied by Hoessel et al.are probably not Cepheids, but other types of variable stars. Taking the average of our distance estimate and Aparicio's, the distance to the Pegasus galaxy is d= <TEX>$1000{\pm}80$</TEX> kpc. Considering the distance and velocity of the Pegasus galaxy with respect to the center of the Local Group, we conclude that the Pegasus galaxy is probably a member of the Local Group.
1010
- KIM KWANG TAE
- Journal of the Korean astronomical society = 천문학회지
- 28, n.1
- pp.1-13
- 1995
- 원문 바로보기
A list of radio sources in the field of Abell 2256 is presented at 1420 MHz. Also presented is the source count based on the list. This source count is taken with the sources above <TEX>$7\sigma$</TEX> level at 1420 MHz. The overall characteristics of the count is consistent with that of a field where no galaxy cluster presents. The excess of radio sources due to the cluster is examined in the source count but it turned out to be unnoticiable as expected.