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1968년 ~ 2023년까지 1,187 건의 한국천문학회지를 격월간 확인하실 수 있습니다.
- The Korean Astronomical Society (The Korean Astronomical Society)
- 계간 (Quarterly)
- ISSN : 1225-4614 (ISSN : 1225-4614)
1225-4614
- DB구축현황 : 1,187건 (DB Construction : 1,187 Articles)
총 게시글 1,187건
페이지 73/119
721
- KANG HYESUNG
- Journal of the Korean astronomical society = 천문학회지
- 33, n.2
- pp.111-121
- 2000
- 원문 바로보기
Many models of globular cluster formation assume the presence of cold dense clouds in early universe. Here we re-examine the Fall & Rees (1985) model for formation of proto-globular cluster clouds (PGCCs) via thermal instabilities in a protogalactic halo. We first argue, based on the previous study of two-dimensional numerical simulations of thermally unstable clouds in a stratified halo of galaxy clusters by Real et al. (1991), that under the protogalactic environments only nonlinear (<TEX>${\delta}{\ge}1$</TEX>) density inhomogeneities can condense into PGCCs without being disrupted by the buoyancy-driven dynamical instabilities. We then carry out numerical simulations of the collapse of overdense douds in one-dimensional spherical geometry, including self-gravity and radiative cooling down to T = <TEX>$10^4$</TEX> K. Since imprinting of Jeans mass at <TEX>$10^4$</TEX> K is essential to this model, here we focus on the cases where external UV background radiation prevents the formation of <TEX>$H_2$</TEX> molecules and so prevent the cloud from cooling below <TEX>$10^4$</TEX> K. The quantitative results from these simulations can be summarized as follows: 1) Perturbations smaller than <TEX>$M_{min}\~(10^{5.6}\;M{\bigodot})(nh/0.05cm^{-3})^{-2}$</TEX> cool isobarically, where nh is the unperturbed halo density, while perturbations larger than <TEX>$M_{min}\~(10^8\;M{\bigodot})(nh/0.05cm^{-3})^{-2}$</TEX> cool isochorically and thermal instabilities do not operate. On the other hand, intermediate size perturbations (<TEX>$M_{min} < M_{pgcc} < M_{max}$</TEX>) are compressed supersonically, accompanied by strong accretion shocks. 2) For supersonically collapsing clouds, the density compression factor after they cool to <TEX>$T_c = 10^4$</TEX> K range <TEX>$10^{2.5} - 10^6$</TEX>, while the isobaric compression factor is only <TEX>$10^{2.5}$</TEX>. 3) Isobarically collapsed clouds (<TEX>$M < M_{min}$</TEX>) are too small to be gravitationally bound. For supersonically collapsing clouds, however, the Jeans mass can be reduced to as small as <TEX>$10^{5.5}\;M_{\bigodot}(nh/0.05cm^{-3})^{-1/2}$</TEX> at the maximum compression owing to the increased density compression. 4) The density profile of simulated PGCCs can be approximated by a constant core with a halo of <TEX>$p{\infty} r^{-2}$</TEX> rather than a singular isothermal sphere.
722
- AHN SANG-HYEON
- Journal of the Korean astronomical society = 천문학회지
- 33, n.1
- pp.29-36
- 2000
- 원문 바로보기
We developed a Monte Carlo code that describes the resonant Ly<TEX>$\alpha$</TEX> line transfer in an optically thick, dusty, and static medium. The code was tested against the analytic solution derived by Neufeld (1990). We explain the line transfer mechanism by tracing histories of photons in the medium. We find that photons experiences a series of wing scatterings at the moment of thier escape from the medium, during which polarization may develop. We examined the amount of dust extinction for a wide range of dust abundances, which are compared with the analytic solution. Brief discussions on the astrophysical application of our work are presented.
723
- MOON Y.-J.
- Journal of the Korean astronomical society = 천문학회지
- 33, n.2
- pp.123-126
- 2000
- 원문 바로보기
We have developed a near real-time flare alerting system which (1) downloads the latest GOES-l0 1-8 <TEX>${\AA}$</TEX> X-ray flux 1-min data by an automated ftp program and shell scripts, (2) produces a beep sound in a simple IDL widget program when the flux is larger than a critical value, and (3) makes it possible to do a wireless alerting by a set of portable transceivers. Thanks to the system, we have made successful Ha flare observations by the Solar Flare Telescope in Bohyunsan Optical Astronomy Observatory. This system is expected to be helpful for ground-based flare observers.
724
- LEE SANG-GAK
- Journal of the Korean astronomical society = 천문학회지
- 33, n.3
- pp.137-142
- 2000
- 원문 바로보기
CN and CH band strengths for ten new bright giants in the globular cluster M15 have been measured from archival spectra obtained with the Multiple Mirror Telescope. Using published indices for other bright M15 giants, a CN-CH band strength anticorrelation is found for bright red giants. However, stars that do not follow the CN-CH anticorrelation are also found. They seem to show a positive correlation between the two indices. Among them, all the AGB and HB stars of the sample are included. Stars I-38 and X6, which are located near the RGB fiducial line in the CMD, have low measured CH(G) indices compared with other RGB stars. Stars IV-38, S4, and S1, which are all near the RGB tip, have strong measured CH(G) indices. Therefore, most of their evolutionary states are suspected to be different from those of a normal single RGB star.
725
- HYUNG SIEK
- Journal of the Korean astronomical society = 천문학회지
- 33, n.2
- pp.81-88
- 2000
- 원문 바로보기
The international ultraviolet explorer (IUE) spectra of a low dispersion <TEX>$\~6{\AA}$</TEX>, have been investigated for two Seyfert 1 galaxies, Mrk 335 and NGC 4051, well known for the line variability. The electron densities of broad line region (BLR) of these variable Seyfert 1 galaxies have been derived, which showed a non-linear abrupt variation from <TEX>$10^8$</TEX> to <TEX>$10^{10} cm-3$</TEX> within a month. We also found the excitation (or temperature) changes in the Mrk 335 BLR from the IUE broad line profiles analysis, but no such evidence in the NGC 4051. The large amount of mass inflow activity through the bar or spiral structure of host galaxies, may trigger the density change in BLR and emission line variability for both objects. Mass of the giant black holes appear to be order of <TEX>$10^7\;M_{\bigodot}$</TEX> for both variable Seyfert l's.
726
- ANN HONG BAE
- Journal of the Korean astronomical society = 천문학회지
- 33, n.1
- pp.1-17
- 2000
- 원문 바로보기
We have performed extensive simulations of response of gaseous disk in barred galaxies using SPH method. The gravitational potential is assumed to be generated by disk, bulge, halo, and bar. The mass of gaseous disk in SPH simulation is assumed to be negligible compared to the stellar and dark mass component, and the gravitational potential generated by other components is fixed in time. The self-gravity of the gas is not considered in most simulations, but we have made a small set of simulations including the self-gravity of the gas. Non-circular component of velocity generated by the rotating, non-axisymmetric potential causes many interesting features. In most cases, there is a strong tendency of concentration of gas toward the central parts of the galaxy. The morphology of the gas becomes quite complex, but the general behavior can be understood in terms of simple linear approximations: the locations and number of Lindblad resonances play critical role in determining the general distribution of the gas. We present our results in the form of 'atlas' of artificial galaxies. We also make a brief comment on the observational implications of our calculations. Since the gaseous component show interesting features while the stellar component behaves more smoothly, high resolution mapping using molecular emission line for barred galaxies would be desirable.
727
- 안영숙
- Journal of the Korean astronomical society = 천문학회지
- 33, n.3
- pp.177-183
- 2000
- 원문 바로보기
We arrange Korean ancient calendar with Solar calendar during Choseon Dynasty(A.D. 1392-1910). In this period, we have one representable history books and several books, and most of information for date are found from them, ChoseonWangjosillok(조선왕조실록) and Jeungbomunheonbigo(증보문헌비고), etc. In those books many astronomical data and calendar information data are contained, so we can make chronological tables. Most of the data are arranged based on those several books, and for doubtable data are identified from eclipse, historical events and lunar phase calculations etc. We find that arranged chronological tables during Choseon Dynasty are similar to that of China and somewhat different from that of Japan. In addition we summarize all misrecorded date data in ChoseonWangjosillok
728
- MOON Y.-J.
- Journal of the Korean astronomical society = 천문학회지
- 33, n.1
- pp.47-55
- 2000
- 원문 바로보기
Nonpotential characteristics of magnetic fields in AR 5747 are examined using Mees Solar Observatory magnetograms taken on Oct. 20, 1989 to Oct. 22, 1989. The active region showed such violent flaring activities during the observational span that strong X-ray flares took place including a 2B/X3 flare. The magnetogram data were obtained by the Haleakala Stokes Polarimeter which provides simultaneous Stokes profiles of the Fe I doublet 6301.5 and 6302.5. A nonlinear least square method was adopted to derive the magnetic field vectors from the observed Stokes profiles and a multi-step ambiguity solution method was employed to resolve the <TEX>$180^{\circ}$</TEX> ambiguity. From the ambiguity-resolved vector magnetograms, we have derived a set of physical quantities characterizing the field configuration, which are magnetic flux, vertical current density, magnetic shear angle, angular shear, magnetic free energy density, a measure of magnetic field discontinuity MAD and linear force-free coefficient. Our results show that (1) magnetic nonpotentiality is concentrated near the inversion line in the flaring sites, (2) all the physical parameters decreased with time, which may imply that the active region was in a relaxation stage of its evolution, (3) 2-D MAD has similar patterns with other nonpotential parameters, demonstrating that it can be utilized as an useful parameter of flare producing active region, and (4) the linear force-free coefficient could be a evolutionary indicator with a merit as a global nonpotential parameter.
729
- KIM HYUN-GOO
- Journal of the Korean astronomical society = 천문학회지
- 33, n.3
- pp.151-158
- 2000
- 원문 바로보기
We conducted a deep CCD observations in V band to obtain stellar density distribution and to determine the distances toward two molecular clouds with anomalous velocity in the Galactic anti-center region. Star count method based on the linear programming technique was applied to the CCD photometric data. We found two prominent peaks at distances of around 1.4 and 2.7 kpc. It is found that the first peak coincides well with stellar density enhancement of B8-A0 stars and the second one with the outer Perseus arm. The effect of the choice of the luminosity function is discussed. The stellar number density distribution is used to derive the distances to the molecular clouds and the visual extinctions caused by the clouds. We found that two molecular clouds are located almost at the same distance of about 1.1 <TEX>$\pm$</TEX> 0.1 kpc, and the peak extinctions caused by the clouds are about 2.2 <TEX>$\pm$</TEX> 0.3 mag in V band.
730
- KYEONG JAE-MANN
- Journal of the Korean astronomical society = 천문학회지
- 33, n.3
- pp.143-149
- 2000
- 원문 바로보기
We present the results of near-IR band (JHK) photometric study for the old open cluster To 2. Combined with existing optical data, our IR photometry is used to derive the reddening E(B- V)=0.24<TEX>$\pm$</TEX>0.12 and the distance (m-M)o=14.6<TEX>$\pm$</TEX>0.42. Comparison with theoretical isochrones suggests the age and metallicity of To 2 are log t<TEX>$\~$</TEX>9.3 and [Fe/H]<TEX>$\~$</TEX>-0.3, respectively.