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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 100/122
991
- KIM SANG-JOON
- Journal of the Korean astronomical society = 천문학회지
- 29, n.suppl1
- pp.347-350
- 1996
- 원문 바로보기
Spectroscopic data between 7 and 15 microns obtained in 1979 by Voyager 1 and 2 Infrared Interferometer Spectrometer (IRIS) have been revisited. Using the spectral data, Jupit.er images have been constructed at the emission bands of hydrocarbons, such as methane, ethane, and acetylene. The resultant. images show differences in emission intensities in the polar regions, suggesting inhomogeneous distributions of the hydrocarbons over the auroral regions of Jupiter.
992
- LEE SEE WOO
- Journal of the Korean astronomical society = 천문학회지
- 28, n.1
- pp.45-59
- 1995
- 원문 바로보기
In the best observed Pleiades cluster, the luminosity function(LF) and mass function(MF) for main sequence(MS) stars extended to <TEX>$Mv{\approx}15.5(V{\approx}21)$</TEX> are very similar to the initial luminosity function(ILF) and initial mass function(IMF) for field stars in the solar neighborhood showing a bump at log <TEX>$m{\simeq}-0.05$</TEX> and a dip at log <TEX>$m{\simeq}-0.12$</TEX>. This dip is equivalent to the Wielen dip appearing in the LF for the field stars. The occurence of these bump and dip is independent of adopted mass-luminosity relation(MLR) . and their characteristics could be explained by a time-dependent bimodal IMF. The model with this IMF gives a total cluster mass of <TEX>$\~700M_\bigodot,\;\~25$</TEX> brown dwarfs and <TEX>$\~3$</TEX> white dwarfs if the upper mass limit of progenitor of white dwarf is greater than <TEX>$4.5M_\bigodot$</TEX>. The cluster age on the basis of LF for brightest stars is given by <TEX>$\~8\times10^7yr$</TEX> and all stars in the cluster lie along the single age sequence in the C-M diagram without showing a large dispersion from the sequence.
993
- Park, Yong-Sun
- Journal of the Korean astronomical society = 천문학회지
- 28, n.2
- pp.255-264
- 1995
- 원문 바로보기
The HII region S140 and the associated molecular cloud L1204 have been observed with 10 molecular transitions, CO (1-0), <TEX>$^{13}CO$</TEX> (1-0), <TEX>$C^{18}O$</TEX> (1-0), CS (2-1), <TEX>$HCO^+$</TEX> (1-0), HCN (1-0), SO (<TEX>${2_2}-{1_1}$</TEX>), <TEX>$SO_2(2_{20}-3_{13})$</TEX>, OCS (8-7), and <TEX>$HNCO\;(4_{04}-3_{03})$</TEX> with <TEX>${\sim}50'$</TEX> angular resolutions. More than 7,000 spectra were obtained in total. The morphology of this region shows a massive fragment (the S140 core) and the extended envelope to the northeast. Several gas condensations have been identified in the envelope, having masses of <TEX>${\sim}10^{3}M_{\odot}$</TEX> and gas number densities of <TEX>${\lesssim}10^{4}cm^{-3}$</TEX> to <TEX>$3{\times}10^{5}cm^{-3}$</TEX> in their cores. The column densities of the observed molecular species toward the S140 core appear to be the typical warm clouds' abundances. It seems to be that the S140 core and L1204 have been swept up by an expanding shell called the Cepheus bubble. The large value of <TEX>$L_{IR}$</TEX>(embedded\;stars)/<TEX>$M_{cloud}\;{\sim}\;5\;L_{\odot}$</TEX>/<TEX>$M_{\odot}$</TEX> of the S140 core may suggest that the star formation has been stimulated by the HII region, but the shock velocity and the pressure of the region seem to give a hint of the spontaneous star formation by the self gravity.
994
- 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>.
995
- 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).
996
- 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.
997
- 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.
998
- 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.
999
- 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.
1000
- 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.