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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 67/122
661
- 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.
662
- 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.
663
- 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.
664
- 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.
665
- KANG YaNG-WOO
- Journal of the Korean astronomical society = 천문학회지
- 35, n.2
- pp.87-95
- 2002
- 원문 바로보기
We have conducted VI CCD photometry of the open cluster NGC 6819 in order to understand the effects of dynamical evolution in old open clusters. Our photometry covers 18' <TEX>$\times$</TEX> 18' on the sky, centered on the cluster, which seems to cover the whole cluster field. Our photometry reaches down to V <TEX>$\approx$</TEX> 20.5, which allows us to analyze the luminosity function and spatial distribution of stars brighter than Mv <TEX>$\approx$</TEX> 8.5. There is a clear evidence for mass segregation in NGC 6819, i.e., the giants and upper main-sequence stars are concentrated in the inner regions, whereas the lower main-sequence stars distribute almost uniformly throughout the cluster. The luminosity function of the main-sequence stars of NGC 6819 is almost flat. The flat luminosity function indicates that a large number of low mass stars has escaped from the cluster unless its initial mass function is much different from the Salpeter type (<TEX>${\phi}(m){\propto} m^{-(1+x)},x = 1.35$</TEX>).
666
- YUSHCHENKO ALEXANDER
- Journal of the Korean astronomical society = 천문학회지
- 35, n.4
- pp.209-220
- 2002
- 원문 바로보기
In order to increase the completeness of the investigations of stellar abundances, we can use spectrum synthesis method, new atomic data and observation of stellar spectra with resolution comparable to solar spectral atlases. We made a brief review of main problems of these three ways. We present new results of abundance determinations in the atmospheres of four stars. The first is the implementation of new atomic data to well known Przybylski's star. We show that the number of spectral lines, which can be identificated in the spectrum of this star, can be significantly higher. The second example is the investigation of <TEX>$\zeta$</TEX> Cyg. We found the abundances of 51 elements in the atmosphere of this mild barium star. The third example is halo star HD221170. Our preliminary abundance pattern consists of 42 elements. The heaviest elements in this pattern are U and Th. The last star is the spectroscopic binary HD153720. The number of elements investigated in the spectra of components of this star is not large, but the results show that the components are Am-stars.
667
- MOON Y.-J.
- Journal of the Korean astronomical society = 천문학회지
- 35, n.3
- pp.143-149
- 2002
- 원문 바로보기
In this paper we present a methodology to derive the temporal change of the magnetic shear angle from a series of vector magnetograms, with a high time cadence. This method looks for the minimum change of the shear angle between a pair of magnetograms, free from the <TEX>$180^{\circ}$</TEX> ambiguity, and then accumulates this change over many successive pairs to derive the temporal change of magnetic shear. This methodology will work well if only the successive magnetograms occurred in an active region are well aligned and its helicity sign is reasonably determined. We have applied this methodology to a set of vector magnetograms of NOAA Active Region 9661 on October 19, 2001 by the new digital magnetograph at the Big Bear Solar Observatory (BBSO). For this work we considered well aligned magnetograms whose cross-correlation values are larger than 0.95. As a result, we have confirmed the recent report of Wang et al. that there was the abrupt shear change associated with the X1.6 flare. It is also demonstrated that the shear change map can be an useful tool to highlight the local areas that experienced the abrupt shear change. Finally, we suggest that this observation should be a direct support of the emergence of sheared magnetic fields.
668
- LEE Ho-GYU
- Journal of the Korean astronomical society = 천문학회지
- 35, n.2
- pp.105-110
- 2002
- 원문 바로보기
We present the results of VLA <TEX>$NH_3$</TEX> (1,1) and (2,2) line observations of the young-stellar object (YSO) IRAS 19550+3248. The integrated intensity map of the <TEX>$NH_3$</TEX> (1,1) line shows that there are two ammonia cores in this region; core A which is associated with the YSO, and core B which is diffuse and located at the northeast of core A. Core A is compact and elongated along the east-west direction (0.07 pc<TEX>$\times$</TEX>0.05 pc) roughly perpendicular to the molecular outflow axis. Core B is diffuse and extended (0.18 pc<TEX>$\times$</TEX>0.07 pc). <TEX>$NH_3$</TEX> (2,2) line is detected only toward core A, which indicates that it is hotter (~ 15 K), presumably due to the heating by the YSO. The <TEX>$NH_3$</TEX> (1,1) line toward core A is wide (<TEX>${\Delta}v{\ge} 3 km s^{-l}$</TEX>) and appears to have an anomalous intensity ratio of the inner satellite hyperfine lines. The large line width may be attributed to the embedded YSO, but the hyperfine anomaly is difficult to explain. We compare the results of <TEX>$NH_3$</TEX> observations with those of previous CS observations and find that the CS emission is detected only toward core A and is much more extended than the <TEX>$NH_3$</TEX> emission.
669
- AHN SANG-HYEON
- Journal of the Korean astronomical society = 천문학회지
- 35, n.4
- pp.175-185
- 2002
- 원문 바로보기
Almost half of primeval galaxies show P-Cygni type profiles in the Ly<TEX>$\alpha$</TEX> emission line. The main underlying mechanism for the profile formation in these systems is thought to be the frequency re-distribution of the line photons in expanding scattering media surrounding the emission source. A Monte Carlo code is developed to investigate the Ly<TEX>$\alpha$</TEX> line transfer in an optically thick and moving medium with a careful consideration of the scattering in the damping wings. Typical column densities and expansion velocities of neutral hydrogen investigated in this study are <TEX>$N_{H1}{\~}10^{17-20}\;cm^{-2}$</TEX> and <TEX>${\Delta}V{\~} 100 km\;s^{-1}$</TEX>. We investigate the dependence of the emergent profiles on the kinematics and on the column density. Our numerical results are applied to show that the damped Ly<TEX>$\alpha$</TEX> absorbers may possess an expanding H I supershell with bulk flow of <TEX>${\~}200 km\;s^{-l}$</TEX> and H I column density <TEX>$N_{H1}{\~}10^{19}\;cm^{-2}$</TEX>. We briefly discuss the observational implications.
670
- PARK BYEONG-GON
- Journal of the Korean astronomical society = 천문학회지
- 35, n.4
- pp.197-208
- 2002
- 원문 바로보기
Five contemporary pre-main sequence (PMS) evolution model grids are compared with the photo-metric data for a nearly complete sample of low-mass members in NGC 2264. From amongst the grids compared, the models of Baraffe et al. (1998) prove to be the most reliable in mass-age distribution. To overcome the limited mass range of the models of Baraffe et al. we derived a simple transformation relation between the mass of a PMS star from Swenson et al. (1994) and that from Baraffe et al., and applied it to the PMS stars in NGC 2264 and the Orion nebula cluster (ONC). The resulting initial mass function (IMF) of the ONC shows that the previous interpretation of the IMF is not a real feature, but an artifact caused by the evolution models adopted. The IMFs of both clusters are in a good agreement with the IMF of the field stars in the solar neighborhood. This result supports the idea proposed by Lada, Strom, & Myers (1993) that the field stars originate from the stars that are formed in clusters and spread out as a result of dynamical dissociation. Nevertheless, the IMFs of OB associations and young open clusters show diverse behavior. For the low-mass regime, the current observations suffer from difficulties in membership assignment and sample incompleteness. From this, we conclude that a more thorough study of young open clusters is necessary in order to make any definite conclusions on the existence of a universal IMF.