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1968년 ~ 2025년까지 1,227 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 : 1,227건 (DB Construction : 1,227 Articles)
총 게시글 1,227건
페이지 77/123
761
- KIM YONG HA
- Journal of the Korean astronomical society = 천문학회지
- 33, n.2
- pp.127-135
- 2000
- 원문 바로보기
We investigate the effects of planetary rotation on the exospheres of the earth and Mars with simple collisionless models. We develope a numerical code that computes exospheric densities by integrating velocity functions at the exobase with a 10 point Gauss method. It is assumed in the model that atoms above the exobase altitude move collisionlessly on an orbit under the planet's gravity. Temperatures and densities at the exobase over the globe are adopted from MSIS-86 for the earth and from Bougher et al's MTGCM for Mars. For both the earth and Mars, the rotation affects the exospheric density distribution significantly in two ways: (1) the variation of the exospheric density distribution is shifted toward the rotational direction with respect to the variation at the exobase, (2) the exospheric densities in general increase over the non-rotating case. We find that the rotational effects are more significant for lower thermospheric temperatures. Both the enhancement of densities and shift of the exospheric distribution due to rotation have not been considered in previous models of Martian exosphere. Our non-spherical distribution with the rotational effects should contribute to refining the hot oxygen corona models of Mars which so far assume simple geometry. Our model will also help in analyzing exospheric data to be measured by the upcoming Nozomi mission to Mars.
762
- 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.
763
- 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.
764
- 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.
765
- LEE YOUNG SUN
- Journal of the Korean astronomical society = 천문학회지
- 33, n.3
- pp.165-172
- 2000
- 원문 바로보기
A baffle system for an airglow photometer, which will be on board the Korea Sounding Rocket-III(KSR-III), has been designed to suppress strong solar scattered lights from the atmosphere below the earth limb. Basic principles for designing a baffle system, such as determination of baffle dimensions, arrangement of vanes inside a baffle tube, and coating of surfaces, have been reviewed from the literature. By considering the constraints of the payload size of the KSR-III and the incident angle of solar light scattered from the earth limb, we first determined dimensions of a two-stage baffle tube for the airglow photometer. We then calculated positions and heights of vanes to prohibit diffusely reflected lights inside the baffle tube from entering into the photometer. In order to evaluate performance of the designed baffle system, we have developed a ray tracing program using a Monte Carlo method. The program computed attenuation factors of the baffle system on the order of <TEX>$10^{-6}$</TEX> for angles larger than <TEX>$10^{\circ}$</TEX>, which satisfies the requirements of the KSR-III airglow experiment. We have also measured the attenuation factors for an engineering model of the baffle system with a simple collimating beam apparatus, and confirmed the attenuation factors up to about <TEX>$10^{-4}$</TEX>. Limitation of the apparatus does not allow to make more accurate measurements of the attenuation factors.
766
- 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.
767
- 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.
768
- 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.
769
- 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.
770
- 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.