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1968년 ~ 2024년까지 1,211 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 : 1,211건 (DB Construction : 1,211 Articles)
총 게시글 1,211건
페이지 76/122
751
- BONG SU-CHAN
- Journal of the Korean astronomical society = 천문학회지
- 33, n.3
- pp.159-163
- 2000
- 원문 바로보기
We have investigated one dimensional steady flow model of a typical magnetic flux tube in the solar transition region constrained to observed Differential Emission Measure (DEM) for the average quiet-Sun deduced by Raymond & Doyle (1981) with a flux tube geometry conforming to Doppler shifts of UV lines measured by Chae, Yun & Poland (1998). Because local heating and filling factor in the transition region are not well known, we considered two extreme cases, one characterized by the filling factor= 1 ('filled-up model') and the other set by local heating=0 ('not-heated model'). We examined how much the heating is required for the flux tube by recomputing a model through adjustment of the filling factor in such a way that 'not-heated model' accounts for the observed DEM.
752
- MINH Y. C.
- Journal of the Korean astronomical society = 천문학회지
- 33, n.1
- pp.37-45
- 2000
- 원문 바로보기
We have investigated the properties of the high-latitude cloud MBM 7 using the 3 mm transitions of CO, CS, HCN, <TEX>$HCO^+,\;C_3H_2,\;N_2H^+$</TEX>, and SiO. The molecular component of MBM 7 shows a very clumpy structure with a size of <TEX>$\le$</TEX>0.5 pc, elongated along the northwest-southeast direction, perpendicularly to an extended HI component, which could be resulted from shock formation. We have derived physical properties for two molecular cores in the central region. Their sizes are 0.1-0.3 pc and masses 1-2 M<TEX>$\bigodot$</TEX> having an average volume density <TEX>$\~2{\times}10^3 cm^{-3}$</TEX> at the peak of molecular emission. We have tested the stability of the cores using the full version of the virial theorem and found that the cores are stabilized with ambient medium, and they are expected not to be dissipated easily without external perturbations. Therefore MBM 7 does not seem to be a site for new star formation. The molecular abundances in the densest core appear to be much less (by about one order of magnitude) than the 'general' dark cloud values. If the depletions of heavy elements are not significant in the HLCs compared with those in typical dark clouds, our results may suggest different chemical evolutionary stages or different chemical environments of the HLCs compared with dense dark clouds in the Galactic plane.
753
- 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.
754
- HYUNG SIEK
- Journal of the Korean astronomical society = 천문학회지
- 33, n.2
- pp.97-110
- 2000
- 원문 바로보기
Planetary nebulae provide a direct way to probe elemental abundances, their distributions and their gradients in populations in nearby galaxies. We investigate bulge planetary nebulae in M 31 and M 32 using the strong emission lines, H<TEX>$\alpha$</TEX>, He I, [O III], [N II], [S II] and [Ne III]. From the [O III] 4363/5007 line ratio and the [O II] 3727/3729, we determine the electron temperatures and number densities. With a standard modeling procedure (Hyung, 1994), we fit the line intensities and diagnostic temperatures, and as a result, we derive the chemical abundances of individual planetary nebulae in M 31 and M 32. The derived chemical abundances are compared with those of the well-known Galactic planetary nebulae or the Sun. The chemical abundances of M 32 appear to be less enhanced compared to the Galaxy or M 31.
755
- 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.
756
- ANN HONG BAE
- Journal of the Korean astronomical society = 천문학회지
- 32, n.1
- pp.1-6
- 1999
- 원문 바로보기
We have conducted near-infrared (J- and H-band) surface photometry for two early type barred galaxies, NGC 3412 and NGC 3941. The bulges of NGC 3412 and NGC 3941 show isophotal twists which indicate that they are triaxial. NGC 3412 has a very short bar and its bulge is more centrally concentrated than that of NGC 3941. The unusually short bar and the centrally concentrated triaxial bulge of NGC 3412 might be the result of bar dissolution. The colors of the nuclear region of NGC 3941 resemble those of the blue nuclei, implying the presence of young stellar populations.
757
- SUNG HWANKYUNG
- Journal of the Korean astronomical society = 천문학회지
- 32, n.2
- pp.109-117
- 1999
- 원문 바로보기
UBVI CCD photometry has been obtained for a region around the Wolf-Rayet star WR 12. We found two young stellar associations in the observed field: the nearer one comprises the field members of Vela OBI association at d = 1.8kpc, while the farther one is the young open cluster Bochum 7 (Bo 7) at d = 4.8kpc. The stars associated with Bo 7 showed no central concentration which suggests that Bo 7 is not a young open cluster but simply a local concentration in the density of young stars belonging to the OB association (Vel OB3). These two associations have similar ages but remarkably different mass function slopes (<TEX>$\Gamma$</TEX> = -2.1 <TEX>$\pm$</TEX> 0.3 for Vel OBI and -1.0 <TEX>$\pm$</TEX> 0.3 for Bo 7). The stars in Vel OBI shows an evident age spread (<TEX>${\Delta}T\~ 9Myr$</TEX>). We also found two strong H<TEX>$\alpha$</TEX> emission stars - WR 12 and <TEX>$\sharp$</TEX>1066 - from narrow band H<TEX>$\alpha$</TEX> photometry.
758
- KIM K.-T.
- Journal of the Korean astronomical society = 천문학회지
- 32, n.2
- pp.75-82
- 1999
- 원문 바로보기
Diffuse radio emission in Abell 2256 was detected above 3 <TEX>$\sigma$</TEX> with DRAO observations at 1420 MHz. The halo size is <TEX>$\~13' {\times}10' (\~1h^{-1}_{50}\;Mpc$</TEX>) in full extent and is elongated along a position angle of about <TEX>$112^{\circ}$</TEX>. The total flux density contained in the halo is 30<TEX>$\pm$</TEX>10 mJy at 1420 MHz and its spectral index is -2.04<TEX>$\pm$</TEX>0.04, showing no evidence for steepening up to 1420 MHz. Using the size estimate, yields a more reliable equipartition magnetic field strength which is <TEX>$0.34(1 + k)^{2/7}{\mu}G$</TEX>. In addition, five new radio sources are identified.
759
- HYUNG SIEK
- Journal of the Korean astronomical society = 천문학회지
- 32, n.1
- pp.55-63
- 1999
- 원문 바로보기
NGC 6537 is an extremely high excitation bipolar planetary nebula. It exhibits a huge range of excitation from lines of [N I] to [Si VI]or [Fe VII], i.e. from neutral atoms to atoms requiring an ionization potential of <TEX>$\~$</TEX>167eV. Its kinematical structures are of special interest. We are here primarily concerned with its high resolution spectrum as revealed by the Hamilton Echelle Spectrograph at Lick Observatory (resolution <TEX>$\~0.2{\AA}$</TEX>) and supplemented by UV and near-UV data. Photoionization model reproduces the observed global spectrum of NGC 6537, the absolute H<TEX>$\beta$</TEX> flux, and the observed visual or blue magnitude fairly well. The nebulosity of NGC 6537 is likely to be the result of photo-ionization by a very hot star of <TEX>$T_{eff} \~ 180,000 K$</TEX>, although the global nebular morphology and kinematics suggest an effect by strong stellar winds and resulting shock heating. NGC 6537 can be classified as a Peimbert Type I planetary nebula. It is extremely young and it may have originated from a star of about 5 <TEX>$M_{\bigodot}$</TEX>.
760
- KIM YONG-CHEOL
- Journal of the Korean astronomical society = 천문학회지
- 32, n.2
- pp.119-126
- 1999
- 원문 바로보기
The standard stellar models for <TEX>$\alpha$</TEX> Cen A and B have been constructed without resorting to the arbitrary constraint of the Solar mixing length ratio. Assuming that the chemical compositions and the ages of the two stars are the same, series of models have been constructed. Using the observational constraints, [Z/X], we were able to constrain the number of the 'possible' models. We find that utilizing the observational constraints of [Z/X] the best models for <TEX>$\alpha$</TEX> Cen system are with the initial Z = 0.03, X = 0.66<TEX>$\~$</TEX>0.67. In particular, the primary and the secondary stars may have the same mixing length ratio 1.6<TEX>$\~$</TEX>1.7, which is the same as that of the calibrated Solar model. And, the age of the system is about 5.4 Gyr. Finally, the large spacing of the p-modes is predicted to be 104 <TEX>$\pm$</TEX> 4<TEX>$\mu$</TEX>Hz for <TEX>$\alpha$</TEX> Cen A.