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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 64/119
631
- AHN BYUNG-HO
- Journal of the Korean astronomical society = 천문학회지
- 36, n.suppl1
- pp.93-99
- 2003
- 원문 바로보기
Various attempts have been made to explain the: pronounced seasonal and universal time (UT) variations of geomagnetic indices. As one of such attempts, we analyze the hourly-averaged auroral electroject indices obtained during the past 20 years. The AU and AL indices maximize during summer and equinoctial months, respectively. By normalizing the contribution of the solar conductivity enhancement to the AU index, or to the eastward electrojet, it is found that the AU also follows the same semiannual variation pattern of the AL index, suggesting that the electric field is the main modulator of the semiannual magnetic variation. The fact that the variation pattern of the yearly-mean AU index follows the mirror image of the AL index provides another indication that the electric field is the main modulator of magnetic disturbance. The pronounced UT variations of the auroral electrojet indices are also noted. To determine the magnetic activity dependence, the probability of recording a given activity level of AU and AL during each UT is examined. The UT variation of the AL index, thus obtained, shows a maximum at around 1200-1800 UT and a minimum around 0000-0800 UT particularly during winter. It is closely associated with the rotation of the geomagnetic pole around the rotational axis, which results in the change of the solar-originated ionospheric conductivity distribution over the polar region. On the other hand the UT variation is prominent during disturbed periods, indicating that the latitudinal mismatch between the AE stations and the auroral electrojet belt is responsible for it. Although not as prominent as the AL index, the probability distribution of the AU also shows two UT peaks. We confirm that the Dst index shows more prominent seasonal variation than the AE indices. However, the UT variation of the Dst index is only noticeable during the main phase of a magnetic storm. It is a combined result of the uneven distribution of the Dst stations and frequent developments of the partial ring current and substorm wedge current preferentially during the main phase.
632
- KIM SANG CHUL
- Journal of the Korean astronomical society = 천문학회지
- 36, n.1
- pp.13-19
- 2003
- 원문 바로보기
We present a study of the old open cluster Trumpler 5 (Tr 5), based on the CDS archival data. From the color-magnitude diagrams of Tr 5, we have found the positions of main-sequence turn-off (MSTO) and red giant clump (RGC) stars. Using the mean magnitude of the RGC stars, we have estimated the reddening toward Tr 5, E(B - V) = 0.60 <TEX>$\pm$</TEX> 0.10. Using the stars common in two data sets and the theoretical isochrones of Padova group, we have estimated the distance modulus <TEX>$V_o - M_v = 12.64 {\pm} 0.20 (d = 3.4 {\pm} 0.3 kpc)$</TEX>, the metallicity [Fe/H) = -0.30 <TEX>$\pm$</TEX> 0.10, and the age of 2.4 <TEX>$\pm$</TEX> 0.2 Gyr (log t = 9.38). These metallicity and distance values are consistent with the relation between the metallicity and the Galactocentric distance of other old open clusters, for which we obtain the slope of <TEX>${\Delta}[Fe/H]/ R_{gc} = -0.064 {\pm} 0.010\;dex\;kpc^{-1}$</TEX>.
633
- CHOI CHUL-SUNG
- Journal of the Korean astronomical society = 천문학회지
- 36, n.2
- pp.73-73
- 2003
- 원문 바로보기
Proof correction to the equation in the third paragraph of the DISCUSSION AND CONCLUSION has not been carried faithfully to the published version of the paper. The corrected equation should read <TEX>${\approx}10^{-3}\;M_8^4/^3(N_{\ast}/10^6\;pc^{-3})({\sigma}/300 km\;s^{-l})^{-l}(r/r_t)\;yr^{-1}$</TEX>, where Ms is the mass of the SMBH in units of <TEX>$10^8\;M_{\bigodot}$</TEX>, <TEX>$\sigma$</TEX> is the virial velocity of the stars, <TEX>$r_t$</TEX> is the tidal radius of the SMBH. This estimates the frequency that a star would pass within a sphere with the radius r from the SMBH, rather than the frequency of the tidal disruption event. Therefore, it increases with the mass of the SMBH. However, the loss cone effect should also be taken into account, which reduces the actual event rate. Here, we adopted a factor of one hundred to consider the deficiency from the isotrophic rate. The authors sincerely regret this error.
634
- ANN HONG BAE
- Journal of the Korean astronomical society = 천문학회지
- 36, n.3
- pp.241-248
- 2003
- 원문 바로보기
Owing to several observational evidences and theoretical predictions for morphological evolution of galaxies, it is now widely accepted that galaxies do evolve from late types to early ones along the Hubble sequence. It is also well established that non-axisymmetric potentials of bar-like or oval mass distributions can change the morphology of galaxies significantly during the Hubble time. Here, we review the observational and theoretical grounds of the secular evolution driven by bar-like potentials, and present the results of SPH simulations for the response of the gaseous disks to the imposed potentials to explore the secular evolution in the central regions of barred galaxies.
635
- AHN SANG-HYEON
- Journal of the Korean astronomical society = 천문학회지
- 36, n.3
- pp.145-148
- 2003
- 원문 바로보기
P-Cygni type Ly<TEX>$\alpha$</TEX> from starburst galaxies, either nearby galaxies or Lyman Break galaxies, are believed to be formed by galactic outflows such as galactic supershells or galactic superwinds. We develope a Monte Carlo code to calculate the Ly<TEX>$\alpha$</TEX> line transfer in a galactic supershell which is expanding and formed of uniform and dusty neutral hydrogen gas. The escape of Ly<TEX>$\alpha$</TEX> photons from the system is achieved by a number of back-scatterings. A series of emission peaks are formed by back-scatterings. When we observe P-Cygni type Ly<TEX>$\alpha$</TEX> emissions of starforming galaxies, we can usually see merely singly-peaked emission. Hence the secondary and the tertiary emission humps should be destroyed. In order to do this, dust should be spatially more extended into the inner cavity than neutral supershell. We find that the kinematic information of the expanding supershell is conserved even in dusty media. We discuss the astrophysical applications of our results.
636
- SANDERS D. B.
- Journal of the Korean astronomical society = 천문학회지
- 36, n.3
- pp.149-158
- 2003
- 원문 바로보기
Deep surveys at mid-infared through submillimeter wavelengths indicate that a substantial fraction of the total luminosity output from galaxies at high redshift (z > 1) emerges at wavelengths 30 - 300<TEX>${\mu}m$</TEX>. In addition, much of the star formation and AGN activity associated with galaxy building at these epochs appears to reside in a class of luminous infrared galaxies (LIGs), often so heavily enshrouded in dust that they appear as 'blank-fields' in deep optical/UV surveys. Here we present an update on the state of our current knowledge of the cosmic evolution of LIGs from z = 0 to z <TEX>$\~$</TEX> 4 based on the most recent data obtained from ongoing ground-based redshift surveys of sources detected in ISO and SCUBA deep fields. A scenario for the origin and evolution of LIGs in the local Universe (z < 0.3), based on results from multiwavelength observations of several large complete samples of luminous IRAS galaxies, is then discussed.
637
- PEARSON CHRIS
- Journal of the Korean astronomical society = 천문학회지
- 36, n.3
- pp.249-260
- 2003
- 원문 바로보기
ASTRO-F is the next generation Japanese infrared space mission of the Institute of Space and Astronautical Science. ASTRO-F will be dedicated to an All Sky Survey in the far-infrared in 4 bands from 50-200microns with 2 additional mid-infrared bands at 9microns and 20microns. This will be the first all sky survey in the infrared since the ground breaking IRAS mission almost 20 years ago and the first ever survey at 170microns. The All Sky Survey should detect 10's of millions of sources in the far-infrared bands most of which will be dusty luminous and ultra-luminous star forming galaxies, with as many as half lying at redshifts greater than unity. In this contribution, the ASTRO-F mission and its objectives are reviewed and many of the mission expectations are discussed.
638
- YIM KI-JEONG
- Journal of the Korean astronomical society = 천문학회지
- 35, n.2
- pp.75-85
- 2002
- 원문 바로보기
We present N-body simulations of globular clusters including gravitational field of the Galaxy, in order to study effects of tidal field systematically on the shape of outer parts of globular clusters using NBODY6. The Galaxy is assumed to be composed of central bulge and outer halo. We mvestigate the cluster of multi-mass models with a power-law initial mass function (IMF) starting with different initial masses, initial number of particles, different slopes of the IMF and different orbits of the cluster. We have examined the general evolution of the clusters, the shape of outer parts of the clusters, density profiles and the direction of tidal tails. The density profiles appear to become somewhat shallower just outside the tidal boundary consistent with some observed data. The position angle of the tidal tall depends on the location in the Galaxy as well as the direction of the motion of. clusters. We found that the clusters become more elongated at the apogalacticon than at the pengalacticon. The tidal tails may be used to trace the orbital paths of globular clusters.
639
- LEE YOUNGUNG
- Journal of the Korean astronomical society = 천문학회지
- 35, n.2
- pp.97-103
- 2002
- 원문 바로보기
We have mapped 1 <TEX>$deg^2$</TEX> region toward a high latitude cloud MBM 40 in the J = 1 - 0 transition of <TEX>$^{12}CO$</TEX> and <TEX>$^{13}CO$</TEX>, using the 3 mm SIS receiver on the 14 m telescope at Taeduk Radio Astronomy Observatory. We used a high resolution autocorrelator to resolve extremely narrow CO linewidths of the molecular gas. Though the linewidth of the molecular gas is very narrow (FWHP < 1 km <TEX>$s^{-1}$</TEX> ), it is found that there is an evident velocity difference between the middle upper part and the lower part of the cloud. Their spectra for both of <TEX>$^{12}CO$</TEX> and <TEX>$^{13}CO$</TEX> show blue wings, and the position-velocity map shows clear velocity difference of 0.4 km <TEX>$s^{-1}$</TEX> between two parts. The mean velocity of the cloud is 3.1 km <TEX>$s^{-1}$</TEX>. It is also found that the linewidths at the blueshifted region are broader than those of the rest of the cloud. We confirmed that the visual extinction is less than 3 magnitude, and the molecular gas is translucent. We discussed three mass estimates, and took a mass of 17 solar masses from CO integrated intensity using a conversion factor <TEX>$2.3 {\times} 10^{20} cm^{-2} (K\;km s^{-1})^{-1}$</TEX>. Spatial coincidence and close morphological similarity is found between the CO emission and dust far-infrared (FIR) emission. The ratio between the 100 f.Lm intensity and CO integrated intensity of MBM 40 is 0.7 (MJy/sr)/(K km <TEX>$s^{-1}$</TEX>), which is larger than those of dark clouds, but much smaller than those of GMCs. The low ratio found for MBM 40 probably results from the absence of internal heating sources, or significant nearby external heating sources.
640
- KANG HYESUNG
- Journal of the Korean astronomical society = 천문학회지
- 35, n.4
- pp.159-174
- 2002
- 원문 바로보기
Cosmological hydrodynamic simulations of large scale structure in the universe have shown that accretion shocks and merger shocks form due to flow motions associated with the gravitational collapse of nonlinear structures. Estimated speed and curvature radius of these shocks could be as large as a few 1000 km/s and several Mpc, respectively. According to the diffusive shock acceleration theory, populations of cosmic-ray particles can be injected and accelerated to very high energy by astrophysical shocks in tenuous plasmas. In order to explore the cosmic ray acceleration at the cosmic shocks, we have performed nonlinear numerical simulations of cosmic ray (CR) modified shocks with the newly developed CRASH (Cosmic Ray Amr SHock) numerical code. We adopted the Bohm diffusion model for CRs, based on the hypothesis that strong Alfven waves are self-generated by streaming CRs. The shock formation simulation includes a plasma-physics-based 'injection' model that transfers a small proportion of the thermal proton flux through the shock into low energy CRs for acceleration there. We found that, for strong accretion shocks, CRs can absorb most of shock kinetic energy and the accretion shock speed is reduced up to <TEX>$20\%$</TEX>, compared to pure gas dynamic shocks. For merger shocks with small Mach numbers, however, the energy transfer to CRs is only about <TEX>$10-20\%$</TEX> with an associated CR particle fraction of <TEX>$10^{-3}$</TEX>. Nonlinear feedback due to the CR pressure is insignificant in the latter shocks. Although detailed results depend on models for the particle diffusion and injection, these calculations show that cosmic shocks in large scale structure could provide acceleration sites of extragalactic cosmic rays of the highest energy.