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1968년 ~ 2024년까지 1,211 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 : 1,211건 (DB Construction : 1,211 Articles)
총 게시글 1,211건
페이지 58/122
571
- 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.
572
- 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.
573
- KWON S. M.
- Journal of the Korean astronomical society = 천문학회지
- 37, n.4
- pp.179-184
- 2004
- 원문 바로보기
We have developed a wide-field imaging camera system, called WICZO, to monitor light of the night sky over extended period. Such monitoring is necessary for studying the morphology of interplanetary dust cloud and also the time and spatial variations of airglow emission. The system consists of an electric cooler a CCD camera with <TEX>$60\%$</TEX> quantum efficiency at 500nm, and a fish-eye lens with <TEX>$180^{\circ}$</TEX> field of view. Wide field imaging is highly desired in light of the night sky observations in general, because the zodiacal light and the airglow emission extend over the entire sky. This paper illustrates the design of WICZO, reports the result of its laboratory performance test, and presents the first night sky image, which was taken, under collaboration with Byulmaro Observatory, on top of Mt. Bongrae at Yongweol in January, 2004.
574
- KOO BON-CHUL
- Journal of the Korean astronomical society = 천문학회지
- 37, n.2
- pp.61-77
- 2004
- 원문 바로보기
We have searched for HI 21 cm line emission from shocked atomic gas associated with southern supernova remnants (SNRs) using data from the Southern Galactic Plane Survey. Among the 97 sources studied, we have detected 10 SNRs with high-velocity HI emission confined to the SNR. The large velocity and the spatial confinement suggest that the emission is likely from the gas accelerated by the SN blast wave. We also detected 22 SNRs which show HI emission significantly brighter than the surrounding regions over a wide (<TEX>$>10 km\;s^{-1}$</TEX>) velocity interval. The association with these SNRs is less certain. We present the parameters and maps of the excess emission in these SNRs. We discuss in some detail the ten individual SNRs with associated high-velocity HI emission.
575
- 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.
576
- 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.
577
- KUWABARA TAKUHITO
- Journal of the Korean astronomical society = 천문학회지
- 37, n.5
- pp.601-603
- 2004
- 원문 바로보기
We present the results of the linear analysis for the Parker-Jeans instability in the magnetized gas disks including the effect of cosmic-ray diffusion along the magnetic field lines. We adopted an uni-formly rotating two temperature layered disk with a horizontal magnetic fields and solved the perturbed equations numerically. Fragmentation of gases takes place and filamentary structures are formed by the growth of the instability. Nagai et al. (1998) showed that the direction of filaments being formed by the Parker-Jeans instability depends on the strength of pressure outside the unperturbed gas disk. We found that at some range of external pressures the direction of filaments is also governed by the value of the diffusion coefficient of CR along the magnetic field lines k.
578
- LEE CHANG WON
- Journal of the Korean astronomical society = 천문학회지
- 37, n.4
- pp.257-259
- 2004
- 원문 바로보기
We compare the results of the surveys of starless cores performed with CS (2-1) and (3-2) lines to study inward motions in the cores. The velocity shifts of the CS(3-2) and (2-1) lines with respect to <TEX>$N_2H^+$</TEX> are found to correlate well with each other and to have similar number distributions, implying that, in many cores, systematic inward motions of gaseous material may occur over a range of density of at least a factor <TEX>${\~}$</TEX>4. Fits of the CS spectra to a 2-layer radiative transfer model in ten infall candidates suggest that the median effective line-of-sight speed of the inward-moving gas is <TEX>${\~}0.07 km\;s^{-l}$</TEX> for CS (3-2) and <TEX>${\~} 0.04 km\;s^{-l}$</TEX> for CS(2-1). Considering that the optical depth obtained from the fits is usually smaller in CS(3-2) than in (2-1) line, this may indicate that CS(3-2) usually traces inner, denser gas with greater inward motions than CS(2-1) implying that many of the infall candidates have faster infall toward the center. However, this conclusion may not be representative of all starless core infall candidates, due to the statistically small number analyzed here. Further line observations will be useful to test this conclusion.
579
- SANTILLAN ALFREDO
- Journal of the Korean astronomical society = 천문학회지
- 37, n.4
- pp.233-235
- 2004
- 원문 바로보기
High-velocity clouds are flows of neutral hydrogen, located at high galactic latitudes, with large velocities (<TEX>$[VLSR]{\ge} 100 km/s$</TEX>) that do not match a simple model of circular rotation for our Galaxy. Numerical simulations have been performed for the last 20 years to study the details of their evolution, and their possible interaction with the Galactic disk. Here we present a brief review of the models that have been already published, and describe newly performed three-dimensional magnetohydrodynamic simulations.
580
- PFROMMER C.
- Journal of the Korean astronomical society = 천문학회지
- 37, n.5
- pp.455-460
- 2004
- 원문 바로보기
There have been many speculations about the presence of cosmic ray protons (CRps) in galaxy clusters over the past two decades. However, no direct evidence such as the characteristic <TEX>$\gamma$</TEX>-ray signature of decaying pions has been found so far. These pions would be a direct tracer of hadronic CRp interactions with the ambient thermal gas also yielding observable synchrotron and inverse Compton emission by additionally produced secondary electrons. The obvious question concerns the type of galaxy clusters most likely to yield a signal: Particularly suited sites should be cluster cooling cores due to their high gas and magnetic energy densities. We studied a nearby sample of clusters evincing cooling cores in order to place stringent limits on the cluster CRp population by using non-detections of EGRET. In this context, we examined the possibility of a hadronic origin of Coma-sized radio halos as well as radio mini-halos. Especially for mini-halos, strong clues are provided by the very plausible small amount of required CRp energy density and a matching radio profile. Introducing the hadronic minimum energy criterion, we show that the energetically favored CRp energy density is constrained to <TEX>$2\%{\pm}1\%$</TEX> of the thermal energy density in Perseus. We also studied the CRp population within the cooling core region of Virgo using the TeV <TEX>$\gamma$</TEX>-ray detection of M 87 by HEGRA. Both the expected radial <TEX>$\gamma$</TEX>-ray profile and the required amount of CRp support this hadronic scenario.