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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
페이지 102/122
1011
- KIM YONG-HA
- Journal of the Korean astronomical society = 천문학회지
- 28, n.1
- pp.89-95
- 1995
- 원문 바로보기
We examined a total of 166 images of <TEX>$3.5{\mu}m\;H_3^+$</TEX> emission in the auroral regions of Jupiter observed with the Protocam on IRTF in 1991 and 1992, and found that 30 images contain a clearly isolated small emission patch in the vicinity of the northern auroral regions. Two different time sequences of the images show the small patches at the dusk limb in the range of System III longitudes from <TEX>$270^{\circ}\;through\;0^{\circ}\;to\;90^{circ}$</TEX>. The small patches in one sequence of the images, which were taken at 10 phase between <TEX>$240^{\circ}\;and\;260^{\circ}$</TEX>, may be related to the 10 flux tube, similarly suggested by Connerney et al. (1993). However, the small patches in the other sequence are separated from Io as much as <TEX>$80^{\circ}$</TEX> in longitude. The positions of the small patches in both sequences are deviated equatorward from the 10 footprint oval by <TEX>$5^{\circ}-8^{\circ}$</TEX> latitude in the longitudinal range of <TEX>$270^{\circ}-360^{\circ}$</TEX>. A significant modification is required in current Jovian magnetic field models near the Jupiter's surface if the small patches are produced at the foot of the 10 flux tube.
1012
- Lee, Myung-Gyoon
- Journal of the Korean astronomical society = 천문학회지
- 28, n.2
- pp.169-175
- 1995
- 원문 바로보기
Deep V I CCD photometry of the Pegasus dwarf irregular galaxy shows that the tip of the red giant branch (RGB) is located at I = <TEX>$21.15{\pm}0.10$</TEX> mag and (V - I) = <TEX>$1.58{\pm}0.03$</TEX>. Using the I magnitude of the tip of the RGB (TRGB), the distance modulus of the Pegasus galaxy is estimated to be <TEX>$(m\;-\;M)_o\;=\;25.13{\pm}0.11$</TEX> mag (corresponding to a distance of d = <TEX>$1060{\pm}50$</TEX> kpc). This result is in a good agreement with the recent distance estimate based on the TRGB method by Aparicio [1994, ApJ, 437, L27],<TEX>$ (m\;-\;M)_o$</TEX> = 24.9 (d = 950 kpc). However, our distance estimate is much smaller than that based on the Cepheid variable candidates by Hoessel et al.[1990, AJ, 100, 1151], <TEX>$(m\;-\;M)_o\;=\;26.22{\pm}0.20$</TEX> (d = <TEX>$1750{\pm}160$</TEX> kpc) mag. The color-magnitude diagram illustrates that the Cepheid candidates used by Hoessel et al.are not located in the Cepheid instability strip, but in the upper part of the giant branch. This result shows that the Cepheid candidates studied by Hoessel et al.are probably not Cepheids, but other types of variable stars. Taking the average of our distance estimate and Aparicio's, the distance to the Pegasus galaxy is d= <TEX>$1000{\pm}80$</TEX> kpc. Considering the distance and velocity of the Pegasus galaxy with respect to the center of the Local Group, we conclude that the Pegasus galaxy is probably a member of the Local Group.
1013
- KIM KWANG TAE
- Journal of the Korean astronomical society = 천문학회지
- 28, n.1
- pp.1-13
- 1995
- 원문 바로보기
A list of radio sources in the field of Abell 2256 is presented at 1420 MHz. Also presented is the source count based on the list. This source count is taken with the sources above <TEX>$7\sigma$</TEX> level at 1420 MHz. The overall characteristics of the count is consistent with that of a field where no galaxy cluster presents. The excess of radio sources due to the cluster is examined in the source count but it turned out to be unnoticiable as expected.
1014
- 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>.
1015
- 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.
1016
- 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.
1017
- 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.
1018
- 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.
1019
TIDAL EVOLUTION OF GLOBULAR CLUSTERS: THE EFFECTS OF GALACTIC TIDAL FIELD, DIFFUSION AND BLACK HOLES
- OH KAP SOO
- Journal of the Korean astronomical society = 천문학회지
- 27, n.1
- pp.61-76
- 1994
- 원문 바로보기
We investigate the dynamical evolution of globular clusters under the diffusion, the Galactic tide, and the presence of halo black holes. We compare the results with our previous work which considers the diffusion processes and the Galactic tide. We find the followings: (1) The black holes contribute the expansion of the outer part of the cluster. (2) There is no evidence for dependence on the orbital phase of the cluster as in our previous work. (3) The models of linear and Gaussian velocity distribution for the halo black holes do not show any significant differences in all cases. (4) The perturbation of black holes reduces the number of stars in lower energy regions. (5) There is a significant number of stars with retrograde orbits beyond the cutoff radius especially in the case of diffusion and the perturbation of black holes.
1020
- MINN Y. K.
- Journal of the Korean astronomical society = 천문학회지
- 27, n.1
- pp.31-44
- 1994
- 원문 바로보기
The 4.8GHz formaldehyde absorption line in the dark clouds in M17 and NGC 2024 regions has been mapped. In both nebulae, we detected two <TEX>$H_2CO$</TEX> line components. In M17, the 24km <TEX>$S^{-1}$</TEX> cloud is closely associated with the HII region located in front of the radio continuum source, and the 19km <TEX>$S^{-1}$</TEX> cloud is associated with the visual dark clouds with a larger extent which are closer to us. The 19km <TEX>$S^{-1}$</TEX> cloud has a mass motion approaching to the HII region. In both clouds, a velocity gradient from the north-east to the south-west directions is observed. The linewidth has no variation indicating no collapsing motion. In NGC 2024, the 9km <TEX>$S^{-1}$</TEX> feature is extended along the dark bar in front of the bright nebula and a weak second component at 13km <TEX>$S^{-1}$</TEX> is confined to the immediate vicinity of the radio source. Indications are that the 9km <TEX>$S^{-1}$</TEX> cloud is physically associated with the dark bar and the 13km <TEX>$S^{-1}$</TEX> cloud is located behind the radio source. The angular extent, the column density, and the total mass of the clouds are derived. The radial velocities of other molecular lines observed in these clouds are compared.