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- 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 104/123
1031
- MOON DAE-SIK
- Journal of the Korean astronomical society = 천문학회지
- 27, n.1
- pp.81-102
- 1994
- 원문 바로보기
We have decomposed the 11-cm radio continuum emission of the W51 complex into thermal and non-thermal components. The distribution of the thermal emission has been determined by analyzing HI, CO, and IRAS <TEX>$60-{\mu}m$</TEX> data. We have found a good correlation between the 11-cm thermal continuum and the 60- 11m emissions, which is used to obtain the thermal and non-thermal 11-cm continuum maps of the W51 complex. Most of the thermal continuum is emanating from the compact H II regions and their low-density ionized envelopes in W51A and W51B. All the H II regions, except G49.1-0.4 in W51B, have associated molecular clumps. The thermal radio continuum fluxes of the compact H II regions are proportional to the CO fluxes of molecular clumps. This is consistent with the previous results that the total mass of stars in an H II region is proportional to the mass of the associated molecular clump. According to our result, there are three non-thermal continuum sources in W51: G49.4-0.4 in W51A, a weak source close to G49.2-0.3 in W51B, and the shell source W51C. The non-thermal flux of G49.5-0.4 at 11-cm is <TEX>$\~28 Jy$</TEX>, which is <TEX>$\~25\%$</TEX> of its total 11-cm flux. The radio continuum spectrum between 0.15 and 300 GHz also suggests an excess emission over thermal free-free emission. We show that the excess emission can be described as a non-thermal emission with a spectral index <TEX>${\alpha}{\simeq}-1.0 (S_v{\propto}V^a)$</TEX> attenuated by thermal free-free absorptions at low-frequencies. The non-thermal source close to G49.2-0.3 is weak <TEX>$(\~9 Jy)$</TEX>. The nature of the source is not known and the reality of the non-thermal emission needs to be confirmed. The non~thermal shell source W51C has a 11-cm flux of <TEX>$\~130Jy$</TEX> and a spectral index <TEX>${\alpha}{\simeq}-0.26$</TEX>.
1032
- JUNG HEE
- Journal of the Korean astronomical society = 천문학회지
- 27, n.2
- pp.177-190
- 1994
- 원문 바로보기
We have examined the photometric evolution of elliptical galaxies, using stellar evolutionary models covering the wide ranges of metallicity and mass, and the different IMFs (simple IMP & time-dependent bimodal IMF). The model with a time-dependent bimodal IMF can reproduce the observed integrated magnitudes and colors at all wavelengths. The computed model shows that the star formation in elliptical galaxies is still going on, although the number of newly born stars is very small. The chemical evolutionary effect is clearly seen in the C-M diagram of computed elliptical galaxies.
1033
- MINH Y. C.
- Journal of the Korean astronomical society = 천문학회지
- 27, n.1
- pp.55-60
- 1994
- 원문 바로보기
The 3mm transitions to CO, <TEX>$^{13}CO,\;CS,\;HCO^+$</TEX>, and HCN have been observed toward the compact HII regions in W58 using the 14m Daeduk Radio Telescope (DRT). Some of the observed lines show high-velocity wings resulted from outflowing materials of the compact HII regions. We derive the beam averaged column densities of the observed species and compare their relative abundances. The <TEX>$HCO^+$</TEX> abundance appears to be smaller by about an order of magnitude than those of 'typical' quiet molecular clouds. CS may be a good reference molecule in comparing relative abundances in different physical conditions.
1034
- LEE YOUNGUNG
- Journal of the Korean astronomical society = 천문학회지
- 27, n.2
- pp.147-158
- 1994
- 원문 바로보기
We have obtained high angular resolution maps toward a molecular cloud associated with an HII region S287 and studied mainly kinematics of the cloud. The mapped region is 1.5 square degrees of the cloud in the transitions of <TEX>$^{12}CO\;and\;^{13}CO\;J=1-0$</TEX>. We have obtained a large range of mass, <TEX>$1.3\times10^4M_\bigodot$</TEX>, to <TEX>$7.2{\times}10^4M_{\bigodot}$</TEX> using three different techniques. The S287 molecular cloud shows a very disturbed feature: velocity field of the cloud is very complicated, and shows several arcs. It is likely that the southern part of cloud is being disrupted by the residing HII region S287 as well as external perturbing sources. In addition to an HII region, five bipolar outflows are also disturbing the molecular gas significantly. The large virial mass and the very disturbed morphology may reflect the fact that the cloud is not gravitationally bound system, as in the case of nearby giant molecular cloud (GMC) G216-2.5. The several arc structure and the filamentary features are possibly driven by external strong stellar winds, and these external perturbing sources may be driving the second generation of star-forming activities on the edges of the S287 molecular cloud.
1035
- KIM W. -T.
- Journal of the Korean astronomical society = 천문학회지
- 27, n.1
- pp.13-29
- 1994
- 원문 바로보기
We have constructed a 3-dim hydrodynamics code called BTSPH. The fluid dynamics part of the code is based on the smoothed particle hydrodynamics (SPH), and for its Poisson solver the binary tree (BT) scheme is employed. We let the smoothing length in the SPH algorithm vary with space and time, so that resolution of the calculation is considerably enhanced over the version of SPH with fixed smoothing length. The binary tree scheme calculates the gravitational force at a point by collecting the monopole forces from neighboring particles and the multipole forces from aggregates of distant particles. The BTSPH is free from geometric constraints, does not rely on grids, and needs arrays of moderate size. With the code we have run the following set of test calculations: one-dim shock tube, adiabatic collapse of an isothermal cloud, small oscillation of an equilibrium polytrope of index 3/2, and tidal encounter of the polytrope and a point mass perturber. Results of the tests confirmed the code performance.
1036
- LEE HYUN-A
- Journal of the Korean astronomical society = 천문학회지
- 27, n.2
- pp.103-117
- 1994
- 원문 바로보기
To extend the work of Gott, Park, and Lee (1989), statistical properties of gravitational lensing in a wide variety of cosmological models involving non-zero cosmological constant is investigated, using the redshifts of both lens and source and observed angular separation of images for gravitational lens systems. We assume singular isothermal sphere as lensing galaxy in homogenous and isotropic FriedmannLemaitre-Robertson- Walker universe, Schechter luminosity function, standard angular diameter distance formula and other galaxy parameters used in Fukugita and Turner (1991). To find the most adequate flat cosmological model and put a limit on the value of dimensionless cosmological constant <TEX>$\lambda_0$</TEX>, the mean value of the angular separation of images, probability distribution of angular separation and cumulative probability are calculated for given source and lens redshifts and compared with the observed values through several statistical methods. When there is no angular selection effect, models with highest value of <TEX>$\lambda_0$</TEX> is preferred generally. When the angular selection effects are considered, the preferred model depends on the shape of the selection functions and statistical methods; yet, models with large <TEX>$\lambda_0$</TEX> are preferred in general. However, the present data can not rule out any of the flat universe models with enough confidence. This approach can potentially select out best model. But at the moment, we need more data.
1037
- PYO TAE SOO
- Journal of the Korean astronomical society = 천문학회지
- 27, n.2
- pp.119-132
- 1994
- 원문 바로보기
The BVR CCD photometry was performed for the globular cluster M13 down to <TEX>$V=19^m$</TEX> over the region from the center to the west 13 arcmin and the CMD of M13 is investigated. The major photometric error in the crowded field which is due to the variation in the sky background was corrected by the median filtering method and the direct sky method. Some characteristics of the CMD of M13 obtained in the present study are as follows: Firstly, the distribution of stars on the CMD is well consistent with the mean lines of Sandage(1970) along the red giant branch(RGB), horizontal branch(HB) and asymtotic giant branch(AGB). Secondly, some gaps exists along the RGB and blue horizontal branch(BHB). Thirdly, the UV-bright stars are more concentrated at the inner region of the cluster.
1038
- CHAE JONG-CHUL
- Journal of the Korean astronomical society = 천문학회지
- 27, n.2
- pp.191-201
- 1994
- 원문 바로보기
We propose to use the entropy of power spectra defined in the frequency domain for the deconvolution of extended images. Spatial correlations requisite for extended sources may be insured by increasing the role of power entropy because the power is just a representation of spatial correlations in the frequency domain. We have derived a semi-analytical solution which is found to severely reduce computing time compared with other iteration schemes. Even though the solution is very similar to the well-known Wiener filter, the regularizingng term in the new expression is so insensitive to the noise characteristics as to assure a stable solution. Applications have been made to the IRAS <TEX>$60{\mu}m\;and\;100{\mu}m$</TEX> images of the dark cloud B34 and the optical CCD image of a solar active region containing a circular sunspot and a small pore.
1039
- CHUNG HYUN SOO
- Journal of the Korean astronomical society = 천문학회지
- 27, n.1
- pp.1-11
- 1994
- 원문 바로보기
We have observed the emission of <TEX>$HC_3N$</TEX> J=4-3, 5-4,10-9 and 12-11 transitions toward the Sgr B2 central region in an area of <TEX>$150'\times150'$</TEX> with resolutions of 16'-48'. The intensities and central velocities of line profiles show significant variations with positions. In contrast to the intensities of the low J-level transitions which gradually increase from the central source toward the outside region, the <TEX>$HC_3N$</TEX> emission of the high J-level transition become stronger toward 'the central radio continuum source MD5. Systematic change in the radial velocity of each line profile occurs along north-south direction. There are a few peaks in most line profiles, and these indicate that there are multiple velocity components along the line of sight. Distributions of excitation temperature and column density which were estimated from the excitation calculations show the existence of a small <TEX>$(1\times2pc),\;hot\;(T_{ex}>50K)$</TEX> core which contains two temperature peaks at-15' east and north of MD5. The column density of <TEX>$HC_3N\;is\;(1-3)\times10^{14}cm^{-2}$</TEX> Column density at distant position from MD5 is larger than that in the central region. We have deduced that this 'hot-core' has a mass of 105M 0, which is about an order of magnitude larger than those obtained by previous studies.
1040
- PARK SEOK JAE
- Journal of the Korean astronomical society = 천문학회지
- 27, n.1
- pp.77-80
- 1994
- 원문 바로보기
Variability of the emission-line spectra of active galactic nuclei is now a well-known phenomenon. This remains to be fully explained by a theoretical model of the central engine in an active galactic nucleus. Since the magnetic field lines are anchored on the accreting matter, they continuously fall on the event horizon of the central supermassive black hole and increase the net field strength of the hole magnetosphere. The field strength, however, cannot increase without an upper limit and, therefore, it will be decreased by some unknown processes. In this paper we discuss that these increasing and decreasing modes can be repeated periodically and explain the variability of power output, therefore, variability of active galactic nuclei.