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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
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991
- HIDAYAT BAMBANG
- Journal of the Korean astronomical society = 천문학회지
- 29, n.suppl1
- pp.455-457
- 1996
- 원문 바로보기
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
- 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.
995
- 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.
996
- LEE MYUNG GYOON
- Journal of the Korean astronomical society = 천문학회지
- 28, n.1
- pp.31-43
- 1995
- 원문 바로보기
We present UBVRI CCD photometry of the Type Ie supernova SN 19941 in M51 which was discovered on April 2, 1994 (UT). UBVRI CCD photometry of SN 1994 I were obtained for the period of the first two months from April 4, 1994, using the Seoul National University Observatory 60 cm telescope. The light curves of SN 19941 show several interesting features: (a) SN 19941 reaches the maximum brightness at B-band on April 8.23 (B = 13.68 mag), at V-band on April 9.10 (V = 12.89 mag), and at I-band on April 10.32 (I = 12.48 mag); (b) The light curves around the maximum brightness are much narrower than those of other types of supernovae; (c) The light curves after the peak decline more steeply than those of other types of supernovae; and (d) The colors get redder from <TEX>$(V-R){\approx}0.2 mag ((V - I){\approx} 0.3 mag, (B - V){\approx}0.7 mag)$</TEX> on April 4 to <TEX>$(V-R){\approx}0.6 mag ((V-1){\approx}0.9 mag, (B-V){\approx}1.3 mag)$</TEX> on April 18. Afterwards (V - R) colors get bluer slightly <TEX>$(by\~0.005 mag/day)$</TEX>, while (V-I) colors stay almost constant around <TEX>$(V-1){\approx}1.0 mag$</TEX>. The color at the maximum brightness is (B-V)=0.9 mag, which is <TEX>$\~1$</TEX>mag redder than the mean color of typical Type la supernovae at the maximum brightness. The light curves of SN 1994I are similar to those of the Type Ie supernova SN 1962L in NGC 1073. Adopting the distance modulus of <TEX>$(m-M)_0 = 29.2 mag$</TEX> and the reddening of E(B - V) = 0.45 mag [Iwamoto et al. 1994, preprint for ApJ], we derive absolute magnitudes at the maximum brightness of SN 1994I, Mv(max) = -17.7 mag and MB(max) = -17.4 mag. This result shows that SN 1994I was <TEX>$\~2$</TEX>mag fainter at the maximum brightness compared with typical Type Ia supernovae. A narrower peak and faster decline after the maximum in the light curve of SN 1994I compared with other types of supernovae indicate that the progenitor of SN 1994I might be a lower mass star compared with those of other types of supernovae.
997
- 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.
998
- 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.
999
- 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).
1000
- 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>.