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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
- 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.
1062
- 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>.
1063
- Chang, Kyong-Ae
- 天文學會誌 = The journal of the Korean Astronomical Society
- 24, n.1
- pp.1-12
- 1991
- 원문 바로보기
The mathematical properties of gravitational lens equations are examined in the frame work of gravitational micro-lensing effects. The caustics of the gravitational lens may be defined in terms of 'cusp' and 'folding' in general. In some cases for overfocussing, however, the critical curves (caustics) have no cusp and no folding. If the observer is in the overfocussed region, he may not see any lensed image.
1064
- Chung, H.S.
- 天文學會誌 = The journal of the Korean Astronomical Society
- 24, n.2
- pp.217-271
- 1991
- 원문 바로보기
We have observed dense core around young stellar objects, DR21, S140, Orion-KL, and L1551 using four millimeter-wave transitions of <TEX>$HC_3N\;J$</TEX>=4-3, J=5-4, J=10-9, and J=12-11. The spatial distribution of <TEX>$HC_3N$</TEX> emission closely resembles the morphology of the previous CS observations that trace high density gas. These observations reveal the existence of <TEX>$HC_3N$</TEX> dense cores around central IR source, elliptical in shape and almost perpendicular to the CO bipolar outflow axis. Small differences can be explained by that <TEX>$HC_3N$</TEX> molecular line is more optically thin and is seen to be more detailed structure in the neighborhood of central IR sources. In S140 and Orion-KL, massive(<TEX>${\sim}10\;M_{\odot}$</TEX>), slowly rotating dense cores lie near at the central IR sources of bipolar outflows. The velocity channel maps of DR21 show that the bipolar outflow gas may have a correlation with the dense core of DR21. We analyzed intensities of the four lines to derive physical conditions in dense core from two methods, LTE and LVG. The column density of <TEX>$HC_3N$</TEX>, <TEX>$N(HC_3N)$</TEX>, between LTE and LVG calculations agree well with each other. The abundances of <TEX>$HC_3N$</TEX> in each observing source have been estimated using the average values of <TEX>$n(H_2)$</TEX> and <TEX>$N(HC_3N)$</TEX> and assuming the size of dense core. The fractional <TEX>$HC_3N$</TEX> abundances in massive dense cores of DR21, S140, and Orion-KL have a range of <TEX>$(2-7){\times}10^{-10}$</TEX>, while that of low mass dense core, L1551, has one order of magnitude greater value of <TEX>$2{\times}10^{-9}$</TEX>. This should be considered good agreement with the result by Morris et al.(1976). It may be considered that dense cores of DR21, S140, and Orion-KL may have almost same stage of chemical evolution, and their abundances have a small values relative to that of L1551. The column density <TEX>$N(HC_3N)$</TEX> decreases with increasing distance from the densest part of the cloud, the central infrared source, and have the relation of <TEX>$N(HC_3N){\varpropto}R^{\alpha}$</TEX>, where a has a range of 0.65 to 0.89. The values of <TEX>$n(H_2)$</TEX> are not varied with increasing distance from the dense core, and have almost same values. Therefore, it is considered that the dense cores in these regions probably consist of dense clumps in diffuse molecular gas medium, and <TEX>$n(H_2)$</TEX> of each clump is <TEX>${\sim}10^5\;cm^{-3}$</TEX>. Levels in the <TEX>$T_{ex}$</TEX> increases with <TEX>$n(H_2)$</TEX>. It is considered that the <TEX>$HC_3N$</TEX> dense cores are not completely thermalized. We examine the relationships between the luminosity of central infrared sources versus mass of the dense cores, and the luminosity of central infrared sources versus molecular hydrogen column density. Luminosities of the central IR sources show good correlation with mass and hydrogen column density of the dense core. Same has been found from CS observations. However, mass and size derived from <TEX>$HC_3N$</TEX> observations are one order of magnitude smaller than those from CS. It can be interpreted that we see more central part of the cloud cores in <TEX>$NC_3N$</TEX> lines than CS lines.
1065
- Lee, Hae-Shim
- 天文學會誌 = The journal of the Korean Astronomical Society
- 23, n.2
- pp.112-115
- 1990
- 원문 바로보기
We test an evolution of a giant void using an N-body simulation. We find the void expansion is faster than the rest part of the universe and the shape of an isolated aspherical void becomes more spherical as it evolves.
1066
- Park, Seok-Jae
- 天文學會誌 = The journal of the Korean Astronomical Society
- 23, n.2
- pp.116-121
- 1990
- 원문 바로보기
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
- Lee, Hyung-Mok
- 天文學會誌 = The journal of the Korean Astronomical Society
- 23, n.2
- pp.97-105
- 1990
- 원문 바로보기
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.
1068
- 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.
1069
- 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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1070
- Kim, Kap-Sung
- 天文學會誌 = The journal of the Korean Astronomical Society
- 23, n.1
- pp.31-42
- 1990
- 원문 바로보기
Fine structures of a quiescent prominence are studied by analyzing high resolution H alpha filtergrams and H alpha line spectra observed at the Hida Observatory of Kyoto University. We have found two kinds of downward motions in the prominence. One of them is a movement with a constant acceleration below the solar gravity(<TEX>${\simeq}1/4g_s$</TEX>) and the other with an uniform velocity(<TEX>${\simeq}16Km/s$</TEX>). The average life time and the size of prominence knots are estimated to be about 7 minutes and 4000Km, respectively. Spatial and brightness distribution of knots are also presented in this paper. With the analytical solutions derived from magnetostatic equilibrium in the prominence, we have examined the filamentary structure based on the Kippenhahn-Schluter model. Sag angles of the magnetic fields supporting the prominence matter are predicted from the observed density profile.