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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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1061
- Lee, See-Woo
- 天文學會誌 = The journal of the Korean Astronomical Society
- 24, n.1
- pp.25-53
- 1991
- 원문 바로보기
The evolution of the Galaxy is examined by the halo-disk model, using the time-dependent bimodal IMF and contraints such as cumulative metallicity distribution, differential metallicity distribution and PDMF of main sequence stars. The time scale of the Galactic halo formation is about 3Gyr during which the most of halo stars and metal abundance are formed and <TEX>${\sim}95%$</TEX> of the initial halo mass falls to the disk. The G-dwarf problem could be explained by the time-dependent bimodal IMF which is suppressed for low mass stars at the early phase (t < 1Gyr) of the disk evolution. However, the importance of this problem is much weakened by the Pagel's differential metallicity distribution which leads to less initial metal enrichment and many long-lived metal-poor stars with Z < <TEX>$1/3Z_{\odot}$</TEX> The observational distribution of abundance ratios of C, N, O elements with respect to [Fe/H] could be reproduced by the halo-disk model, including the contribution of iron product by SNIs of intermediate mass stars. The initial enrichment of elements in the disk could be explained by the halo-disk model, resulting in the slight decrease and then the increase in the slopes of the [N/Fe]- and [C/Fe]-distributions with increasing [Fe/H] in the range of [Fe/H] < -1.
1062
- Kim, K.T.
- 天文學會誌 = The journal of the Korean Astronomical Society
- 24, n.1
- pp.117-127
- 1991
- 원문 바로보기
Here I report the confirmation of a long-term modulation of a period of <TEX>$92^{+21}_{-13}$</TEX> years with the 'time-delay correlation' method on the sunspot data compiled over the last a total of 289 years. This periodicity better specifies the cycle which falls pretty well within Gleissberg cycle, and clearly contrasts with the 55 year grand cycle which Yoshimura (1979) claimed. It is argued that the period-amplitude diagram method. which Yoshimura used, ana lysed peak amplitudes only so that a large number of data were disregarded, and thus was more susceptible to a bias. The planetary tidal force on Sun as for the possible cause to the solar activity was investigated and its possibility was ruled out in view of no period correlation between them.
1063
- Moon, Yong-Jae
- 天文學會誌 = The journal of the Korean Astronomical Society
- 24, n.2
- pp.129-149
- 1991
- 원문 바로보기
An attempt has been made to examine the characteristics of acoustic and MHD waves generated in stellar convection zones(<TEX>$4000\;K\;{\leq}\;T_{eff}\;{\leq}\;7000\;K$</TEX>, <TEX>$3\;{\leq}\;\log\;g\;{\leq}\;4.5$</TEX>). With the use of wave generation theories formulated for acoustic waves by Stein (1967), for MHD body waves by Musielak and Rosner (1987, 1988) and for MHD tube waves by Musielak et al.(l989a, 1989b), the energy fluxes are calculated and their dependence on effective temperature, surface gravity and megnetic field strength are analyzed by optimization techniques. In computing magneto-convection models, the effect of magnetic fields on the efficiency of convection has been taking into account by extrapolating it from Yun's sunspot models(1968; 1970). Our study shows that acoustic wave fluxes are dominant in F and G stars, while the MHD waves dominant in K and M stars, and that the MHD wave fluxes vary as <TEX>$T_{eff}^4{\sim}T_{eff}^7$</TEX> in contrast to the acoustic fluxes, as <TEX>$T_{eff}^{10}$</TEX>. The gravity dependence, on the other hand, is found to be relatively weak; the acoustic wave fluxes <TEX>${\varpropto}\;g^{-0.5}$</TEX>, the longitudinal tube wave fluxes <TEX>${\varpropto}\;g^{0.3}$</TEX> and the transverse tube wave fluxes <TEX>${\varpropto}\;g^{0.3}$</TEX>. In the case of the MHD body waves their gravity dependence is found to be nearly negligible. Finally we assesed the computed energy fluxes by comparing them with the observed fluxes <TEX>$F_{ob}$</TEX> of CIV(<TEX>${\lambda}1549$</TEX>) lines and soft X-rays for selected main sequence stars. When we scaled the corrected wave fluxes down to <TEX>$F_{ob}$</TEX>, it is found that these slopes are almost in line with each other.
1064
- Minn, Y.K.
- 天文學會誌 = The journal of the Korean Astronomical Society
- 24, n.2
- pp.191-215
- 1991
- 원문 바로보기
We have compared the column densities of <TEX>$H_2CO$</TEX>, <TEX>$^{13}CO$</TEX>, <TEX>$C^{18}O$</TEX>, <TEX>$^{12}CO$</TEX>, CS, and <TEX>$HCO^+$</TEX> to the visual extinctions derived from star counts in eight dark clouds, L1317, B1, L1551, L1535, L1544, L134, L134N, and B335. We examined the degree of correlation between molecules and extinctions. The contours of <TEX>$^{13}CO$</TEX>, <TEX>$H_2CO$</TEX>, and CS distributions show an excellent coincidence with those of the visual extinction. Even though the plots of molecule brightness temperature or column density as a function of the visual extinction show a considerable scatter, a good correlation is found between the two quantities. The turnover of the ratios of <TEX>$^{13}CO$</TEX> and <TEX>$H_2CO$</TEX> column densities to the visual extinction at high extinctions observed in several clouds is not apparent in this work. The ratios seem to maintain constant values within the range of <TEX>$A_V$</TEX> we studied. The slopes of the relations between the molecule column density and extinction, and the threshold values of extinction for detection of molecules are comparable to the other works. The cause of the slope difference is discussed. We derived, for the first time, the relations of CS and <TEX>$HCO^+$</TEX> column densities and the visual extinction. The ratio of CS column density to extinction is at least two orders of magnitude lower than the mean value for <TEX>$^{13}CO$</TEX>, but the threshold for detection of CS is comparable to that of <TEX>$^{13}CO$</TEX>. The ratio of the <TEX>$HCO^+$</TEX> column density to extinction is one to two orders of magnitude lower than the mean ratio for <TEX>$H_2CO$</TEX>.
1065
- Lee, Hyung-Mok
- 天文學會誌 = The journal of the Korean Astronomical Society
- 23, n.2
- pp.97-105
- 1990
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The tidal radii of globular clusters reflect the tidal field of the Galaxy. The mass distribution of the Galaxy thus may be obtained if the tidal fields of clusters are well known. Although large amounts of uncertainties in the determination of tidal radii have been obstacles in utilizing this method, analysis of tidal density could give independent check for the Galactic mass distribution. Recent theoretical modeling of dynamical evolution including steady Galactic tidal field shows that the observationally determined tidal radii could be systematically larger by about a factor of 1.5 compared to the theoretical values. From the analysis of entire sample of 148 globular clusters and 7 dwarf spheroidal systems compiled by Webbink (1985), we find that such reduction from observed values would make the tidal density (the mean density within the tidal radius) distribution consistent with the flat rotation curve of our Galaxy out to large distances if the velocity distribution of clusters and dwarf spheroidals with respect to the Galactic center is isotropic.
1066
- Park, Seok-Jae
- 天文學會誌 = The journal of the Korean Astronomical Society
- 23, n.2
- pp.116-121
- 1990
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It has been suggested that there could be a large number of primordial black holes which were formed in the early universe. We analyze the growth of such a primordial black hole following two different accretion rates - the Eddington accretion rate and the Bondi accretion rate - at the center of a host star like the sun. We find that a primordial black hole with M < <TEX>${\sim}10^{17}\;g$</TEX> cannot substantially grow in any case throughout the lifetime of a host star. If M > <TEX>${\sim}10^{17}\;g$</TEX>, the evolution of a host star depends entirely on the mode of accretion, but it ends as a black hole in either case. Since more stars may have primordial black holes at the center of a galaxy this may result in a cluster of such black holes, and the cluster may eventually collapse to produce a single supermassive black hole.
1067
- Park, Seok-Jae
- 天文學會誌 = The journal of the Korean Astronomical Society
- 23, n.1
- pp.63-70
- 1990
- 원문 바로보기
The original axisymmetric, stationary electrodynamic model of the central engine in an active galactic nucleus proposed by Macdonald and Thorne consists of a supermassive black hole with magnetic field lines that pass through the region just outside the event horizon of the black hole. Each magnetic field line rotates with a constant angular velocity which will exceed the speed of light at large radii. Even though the field lines are purely mathematical entities this condition sets a stringent physical constraint on the motion of the magnetic field lines and the particles on them. In this paper we will show that we can remove this auxiliary constraint in our model by allowing nonstationary processes. As a result the magnetic field lines can be twisted and wound up in a region lying outside of the quasi-stationary magnetosphere of the black hole. We conclude that astrophysical jets are formed in that region due to the twisted and wound magnetic field lines powered by the Blandford-Znajek process and the other driving forces.
1068
- Lee, Sang-Gak
- 天文學會誌 = The journal of the Korean Astronomical Society
- 23, n.1
- pp.83-84
- 1990
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1069
- Kim, Kap-Sung
- 天文學會誌 = The journal of the Korean Astronomical Society
- 23, n.1
- pp.71-82
- 1990
- 원문 바로보기
Theoretical calculations of the combined radiative transfer and statistical equilibrium equation including the charge-particle conservations have been earned out for a multilevel hydrogen atom in quiescent prominences. Cool and dense models show the steep changes of population and radiation field in the vicinity of the surface, while these physical quantities remain unchanged for models with temperature of 7,300K, regardless of total densities. Ionization rate of hydrogen atom related with metallic line formation varies in considerable amounts from the surface to the center of model prominences cooler than 6,300K. However, such cool models cannot release enough hydrogen line emissions to explain observed intensities. Prominence models with a temperature higher than 8,000K can yield the centrally reversed Lyman line profiles confirmed by satellite EUV observations. We find that queiscent prominence with a density between <TEX>$2{\times}10^{11}$</TEX> and <TEX>$10^{12}cm^{-3}$</TEX> should be in temperature range between 6,300K and 8,300K, in order to explain consistently observed H alpha, beta line emissions and <TEX>$n_p/n_l$</TEX> ratio.
1070
- Kim, Seoung-Li
- 天文學會誌 = The journal of the Korean Astronomical Society
- 23, n.1
- pp.1-13
- 1990
- 원문 바로보기
New uvby photoelectric photometry was carried out for a <TEX>$\delta$</TEX> Scuti variable, HR1170. By applying the Fourier method and the linear least square method, three frequencies were derived: <TEX>$f_1=10.06134c/d$</TEX> (<TEX>$P_1=0.^d0994$</TEX>). <TEX>$f_2=11.91754c/d$</TEX> (<TEX>$P_2=0.^d0839$</TEX>). <TEX>$f_3=18.96776c/d$</TEX> (<TEX>$P_3=0.^d0527$</TEX>). From the observed pulsational constants and from the phase difference and amplitude ratios for color(b-y) and magnitude y. three different pulsation modes (n, l) of <TEX>$f_1(0,0)$</TEX>, <TEX>$f_2(1,2)$</TEX>, <TEX>$f_3(3,2)$</TEX> are deduced, indicating the existence of nonradial mode in HR1170. Some physical parameters indicate that HR1170 is evolving at the stage of H-shell burning.