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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 55/123
541
- MINIATI FRANCESCO
- Journal of the Korean astronomical society = 천문학회지
- 37, n.5
- pp.465-470
- 2004
- 원문 바로보기
I address the issue of nonthermal processes in the large scale structure of the universe. After reviewing the properties of cosmic shocks and their role as particle accelerators, I discuss the main observational results, from radio to <TEX>$\gamma$</TEX>-ray and describe the processes that are thought be responsible for the observed nonthermal emissions. Finally, I emphasize the important role of <TEX>$\gamma$</TEX>-ray astronomy for the progress in the field. Non detections at these photon energies have already allowed us important conclusions. Future observations will tell us more about the physics of the intracluster medium, shocks dissipation and CR acceleration.
542
- KANG HYESUNG
- Journal of the Korean astronomical society = 천문학회지
- 37, n.5
- pp.405-412
- 2004
- 원문 바로보기
Cosmological shocks form as an inevitable consequence of gravitational collapse during the large scale structure formation and cosmic-rays (CRs) are known to be accelerated at collisionless shocks via diffusive shock acceleration (DSA). We have calculated the evolution of CR modified shocks for a wide range of shock Mach numbers and shock speeds through numerical simulations of DSA in 1D quasi-parallel plane shocks. The simulations include thermal leakage injection of seed CRs, as well as pre-existing, upstream CR populations. Bohm-like diffusion is assumed. We show that CR modified shocks evolve to time-asymptotic states by the time injected particles are accelerated to moderately relativistic energies (p/mc <TEX>$\ge$</TEX> 1), and that two shocks with the same Mach number, but with different shock speeds, evolve qualitatively similarly when the results are presented in terms of a characteristic diffusion length and diffusion time. We find that <TEX>$10^{-4} - 10^{-3}$</TEX> of the particles passed through the shock are accelerated to form the CR population, and the injection rate is higher for shocks with higher Mach number. The CR acceleration efficiency increases with shock Mach number, but it asymptotes to <TEX>${\~}50\%$</TEX> in high Mach number shocks, regardless of the injection rate and upstream CR pressure. On the other hand, in moderate strength shocks (<TEX>$M_s {\le} 5$</TEX>), the pre-existing CRs increase the overall CR energy. We conclude that the CR acceleration at cosmological shocks is efficient enough to lead to significant nonlinear modifications to the shock structures.
543
- MINH Y. C.
- Journal of the Korean astronomical society = 천문학회지
- 37, n.4
- pp.131-135
- 2004
- 원문 바로보기
The <TEX>$H_2S\;(2_{2,0} - 2_{1,1})$</TEX> line emission is observed to be strongly localized toward Sgr B2(M), and emissions from other positions in the more extended SgrB2 region are almost negligible. <TEX>$H_2S$</TEX> is thought to form effectively by the passage of the C-type shocks but to be quickly transformed to <TEX>$SO_2$</TEX> or other sulfur species (Pineau des Forets et al. 1993). Such a shock may have enhanced the <TEX>$H_2S$</TEX> abundance in Sgr B2(M), where massive star formation is taking place. But the negligible emission of <TEX>$H_2S$</TEX> from other observed positions may indicate that these positions have not been affected by shocks enough to produce <TEX>$H_2S$</TEX>, or if they have experienced shocks, <TEX>$H_2S$</TEX> may have transformed already to other sulfur-containing species. The <TEX>$SO_2\;22_{2,20} - 22_{1,21}$</TEX> line was also observed to be detectable only toward the (M) position. The line intensity ratios of these two molecules appear to be very similar at Sgr B2(M) and IRAS 16239-2422, where the latter is a region of low-mass star formation. This may suggest that the shock environment in these two star-forming regions is similar and that the shock chemistry also proceeds in a similar fashion in these two different regions, if we accept shock formation of these two species.
544
- MEDVEDEV MIKHAIL V.
- Journal of the Korean astronomical society = 천문학회지
- 37, n.5
- pp.533-541
- 2004
- 원문 바로보기
The origin of magnetic fields in the universe remains an outstanding problem in cosmology. We propose that these fields are produced by shocks during the large-scale structure formation. We discuss the mechanism of the field generation via the counter-streaming (Weibel) instability. We also show that these Weibel-generated fields are long-lived and weakly coupled to dissipation. Subsequent field amplification by the intra-cluster turbulence may also take place, thus maintaining the magnetic energy density close to equipartition.
545
- RYU DONGSU
- Journal of the Korean astronomical society = 천문학회지
- 37, n.5
- pp.477-482
- 2004
- 원문 바로보기
Shock waves form in the intergalactic space as an ubiquitous consequence of cosmic structure formation. Using N-body/hydrodynamic simulation data of a ACDM universe, we examined the properties of cosmological shock waves including their morphological distribution. Adopting a diffusive shock acceleration model, we then calculated the amount of cosmic ray energy as well as that of gas thermal energy dissipated at the shocks. Finally, the dynamical consequence of those cosmic rays on cluster properties is discussed.
546
- FUJITA YUTAKA
- Journal of the Korean astronomical society = 천문학회지
- 37, n.5
- pp.571-574
- 2004
- 원문 바로보기
Clusters of galaxies are filled with X-ray emitted hot gas with the temperature of T <TEX>${\~}$</TEX>2-10 keV. Recent X-ray observations have been revealing unexpectedly that many cluster cores have complicated, peculiar X-ray structures, which imply dynamical motion of the hot gas. Moreover, X-ray spectra indicate that radiative cooling of the cool gas is suppressed by unknown heating mechanisms (the 'cooling flow problem'). Here we propose a novel mechanism reproducing both the inhomogeneous structures and dynamics of the hot gas in the cluster cores, based on state-of-the-art hydrodynamic simulations. We showed that acoustic-gravity waves, which are naturally expected during the process of hierarchical structure formation of the universe, surge in the X-ray hot gas, causing a serous impact on the core. This reminds us of tsunamis on the ocean surging into an distant island. We found that the waves create fully-developed, stable turbulence, which reproduces the complicated structures in the core. Moreover, if the wave amplitude is large enough, they can suppress the cooling of the core. The turbulence could be detected in near-future space X-ray missions such as ASTRO-E2.
547
- SOHN J,
- Journal of the Korean astronomical society = 천문학회지
- 37, n.4
- pp.261-263
- 2004
- 원문 바로보기
We present a progress report on HCN(1-0) line observations toward starless cores to probe inward motions. We have made a single pointing survey toward the central regions of 85 starless cores and performed mapping observations of 6 infall candidate starless cores. The distributions of the velocity difference between HCN(1-0) hyperfine lines and the optically thin tracer <TEX>$N_2H^+$</TEX>(1-0) are significantly skewed to the blue, meaning that HCN(1-0) frequently detects inward motions. Their skewness to the blue is even greater than that of CS(2-1) Lee et al., possibly implying more infall occurrence than CS(1-0). We identify 19 infall candidates by using several characteristics illustrating spectral infall asymmetry seen in HCN(1-0) hyperfine lines, CS(3-2), CS(2-1), <TEX>$DCO^+(2-1)$</TEX> and <TEX>$N_2H^+$</TEX> observations. The HCN(1-0) F(O-l) with the least optical depth usually shows a similar intensity distribution to that of <TEX>$N_2H^+$</TEX> which closely traces the density distribution of the cores, indicating that HCN(1-0) is less chemically affected and so believed to reflect kinematics occurring in rather inner regions of the cores. Detailed radiative transfer model fits of the spectra are underway to analyze central infall kinematics in starless cores.
548
- KURTZ S.
- Journal of the Korean astronomical society = 천문학회지
- 37, n.4
- pp.265-268
- 2004
- 원문 바로보기
Molecular clouds present many levels of structure, including clumps and cores of varying size and density. We present a brief summary of these cores, describing their observed physical properties and their place in the star formation process. We conclude with some speculation about pre-proto-stellar stages of molecular cores and the observational challenges in their observation.
549
- MAZZOTTA PASQUALE
- Journal of the Korean astronomical society = 천문학회지
- 37, n.5
- pp.381-385
- 2004
- 원문 바로보기
Similarly to other cluster of galaxies previously classified as cooling flow systems, the Chandra observation of MKW 3s reveals that this object has a complex X-ray structure hosting both a X-ray cavity and a X-ray filament. Unlike the other clusters, however, the temperature map of the core of MKW 3s shows the presence of extended regions of gas heated above the radially averaged gas temperature at any radius. As the cluster does not show evidences for ongoing major mergers Mazzotta et al. suggest a connection between the heated gas and the activity of the central AGN. Nevertheless, due to the lack of high quality radio maps, this interpretation was controversial. In this paper we present the results of two new radio observations of MKW 3s at 1.28 GHz and 604 MHz obtained at the GMRT. Together with the Chandra observation and a separate VLA observation at 327 MHz from Young, we show unequivocal evidences for a close connection between the heated gas region and the AGN activity and we briefly summarize possible implications.
550
- HENRY J. PATRICK
- Journal of the Korean astronomical society = 천문학회지
- 37, n.5
- pp.371-374
- 2004
- 원문 바로보기
We review the observational evidence for the existence of a warm-hot intergalactic medium (WHIM). We expect that the morphology of this material is similar to that of cosmic rays and magnetic fields in large-scale structure, i.e., filaments connecting clusters of galaxies. Direct evidence for the WHIM, either in emission or absorption, is weak.