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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 61/122
601
- BOWYER STUART
- Journal of the Korean astronomical society = 천문학회지
- 37, n.5
- pp.579-581
- 2004
- 원문 바로보기
Recently, claims have been made of the detection of 'warm-hot' gas in the intergalactic medium. Kaastra et al. (2003) claimed detection of <TEX>${\~} 10^6$</TEX> K material in the Coma Cluster but studies by Arnaud et al. (2001), and our analysis of the Chandra observations of Coma (Vikhlinin et al. 2001), find no evidence for a <TEX>$10^6$</TEX> K gas in the cluster. Finoguenov et al. (2003) claimed the detection of <TEX>$3 {\times} 10^6$</TEX> gas slightly off-center from the Coma Cluster. However, our analysis of ROSAT data from this region shows no excess in this region. We propose an alternative explanation which resolves all these conflicting reports. A number of studies (e.g. Robertson et al., 2001) have shown that the local interstellar medium undergoes charge exchange with the solar wind. The resulting recombination spectrum shows lines of O VII and O VIII (Wargelin et al. 2004). Robertson & Cravens (2003) have .shown that as much as <TEX>$25\%$</TEX> of the Galactic polar flux is heliospheric recombination radiation and that this component is highly variable. Sporadic heliospheric emission could account for all the claims of detections of 'warm-hot' gas and explain the conflicts cited above.
602
- KIM JONGSOO
- Journal of the Korean astronomical society = 천문학회지
- 37, n.4
- pp.237-241
- 2004
- 원문 바로보기
We perform numerical experiments on supernova-driven turbulent flows in order to see whether or not supernovae playa major role in driving turbulence in the interstellar medium. In a <TEX>$(200pc)^3$</TEX> computational box, we set up, as initial conditions, uniformly magnetized gas distributions with different pairs of hydrogen number densities and magnetic field strengths, which cover the observed values in the Galactic midplane. We then explode supernovae at randomly chosen positions at a Galactic explosion rate and follow up the evolution of the supernova-driven turbulent flows by integrating numerically the ideal MHD equations with cooling and heating terms. From the numerical experiments we find that the density-weighted velocity dispersions of the flows are in the range of 5-10 km <TEX>$s^{-l}$</TEX>, which are consistent with the observed velocity dispersions of cold and warm neutral media. Additionally, we find that strong compressible flows driven by supernova explosions quickly change into solenoidal flows.
603
- INOUE SUSUMU
- Journal of the Korean astronomical society = 천문학회지
- 37, n.5
- pp.447-454
- 2004
- 원문 바로보기
During the hierarchical formation of large scale structure in the universe, the progressive collapse and merging of dark matter should inevitably drive shocks into the gas, with nonthermal particle acceleration as a natural consequence. Two topics in this regard are discussed, emphasizing what important things nonthermal phenomena may tell us about the structure formation (SF) process itself. 1. Inverse Compton gamma-rays from large scale SF shocks and non-gravitational effects, and the implications for probing the warm-hot intergalactic medium. We utilize a semi-analytic approach based on Monte Carlo merger trees that treats both merger and accretion shocks self-consistently. 2. Production of <TEX>$^6Li$</TEX> by cosmic rays from SF shocks in the early Galaxy, and the implications for probing Galaxy formation and uncertain physics on sub-Galactic scales. Our new observations of metal-poor halo stars with the Subaru High Dispersion Spectrograph are highlighted.
604
- LEE S. M.
- Journal of the Korean astronomical society = 천문학회지
- 37, n.4
- pp.249-255
- 2004
- 원문 바로보기
Here we present a linear stability analysis and an MHD 2D model for the Parker-Jeans instability in the Galactic gaseous disk. The magnetic field is assumed parallel to a Galactic spiral arm, and the gaseous disk is modelled as a multi-component, magnetized, and isothermal gas layer. The model employs the observed vertical stratifications for the gas density and the gravitational acceleration in the Solar neighborhood, and the self-gravity of the gas is also included. By solving Poisson's equation for the gas density stratification, we determine the vertical acceleration due to self-gravity as a function of z. Subtracting it from the observed gravitational acceleration, we separate the total acceleration into self and external gravities. The linear stability analysis provides the corresponding dispersion relations. The time and length scales of the fastest growing mode of the Parker-Jeans instability are about 40 Myr and 3.3 kpc, respectively. In order to confirm the linear stability analysis, we have performed two-dimensional MHD simulations. These show that the Parker-Jeans instability under the self and external gravities evolves into a quasi-equilibrium state, creating condensations on the northern and southern sides of the plane, in an alternate manner.
605
- HONG S. S.
- Journal of the Korean astronomical society = 천문학회지
- 37, n.4
- pp.159-169
- 2004
- 원문 바로보기
This is a proposal to probe local part of the interplanetary dust (IPD) cloud complex and retrieve mean volume emissivity of the local IPDs at mid-infrared wavelengths. This will be done by monitoring, with Infrared Camera (IRC) aboard the ASTRO-F, the annual modulation of the zodiacal emission. In pointing mode of the ASTRO-F mission the spacecraft can make attitude maneuvering over approximately <TEX>${\pm}1^{\circ}$</TEX> range centered at solar elongation <TEX>$90^{\circ}$</TEX> in the ecliptic plane. The attitude maneuvering combined with high sensitivity of the IRC will provide us with a unique opportunity observationally to take derivatives of the zodiacal emission brightness with respect to the solar elongation. From the resulting differential of the brightness over the <TEX>${\pm}1^{\circ}$</TEX> range, one can directly determine the mean volume emissivity of the local IPDs with a sufficient accuracy to de-modulate the annual emissivity variations due to the Earth's elliptical motion and the dis-alignment of the maximum IPD density plane with respect to the ecliptic. The non-zero eccentricity (<TEX>$e_{\oplus}$</TEX>= 0.0167) of the Earth's orbit combined with the sensitive temperature dependence of the Planck function would bring modulations of amplitude at least <TEX>$3.34\%$</TEX> to the zodiacal emission brightness at mid-infrared wavelengths, with which one may determine the IPD temperature T(r) and mean number density n(r) as functions of heliocentric distance r. This will in turn fix the power-law exponent <TEX>$\delta$</TEX> in the relation <TEX>$T(r) = T_o(r/r_o)^{-\delta}$</TEX> for the dust temperature and v in <TEX>$n(r) = n_o(r/r_o)^-v$</TEX> for the density. We discuss how one may de-couple the notorious degeneracy of cross-section, density, reference temperature <TEX>$T_o$</TEX> and exponent <TEX>$\delta$</TEX>.
606
- KUO PING-HUNG
- Journal of the Korean astronomical society = 천문학회지
- 37, n.5
- pp.597-600
- 2004
- 원문 바로보기
A substantial number of processes have been suggested as possible contributors to the extragalactic <TEX>$\gamma$</TEX>-ray background (EGRB). Yet another contribution to this background will be emission produced in hadronic interactions of cosmic-ray protons with the cluster thermal gas; this class of cosmic rays (CRs) has been shown to be responsible for the EUV emission in the Coma Cluster of galaxies. In this paper we assume the CRs in the Coma Cluster is prototypic of all clusters and derive the contribution to the EGRB from all clusters over time. We examine two different possibilities for the scaling of the CR flux with cluster size: the number density of the CRs scale with the number density of the thermal plasma, and alternatively, the energy density of the CRs scale with the energy density of the plasma. We find that in all scenarios the EGRB produced by this process is sufficiently low that it will not be observable in comparison with other mechanisms that are likely to produce an EGRB.
607
- DOLAG KLAUS
- Journal of the Korean astronomical society = 천문학회지
- 37, n.5
- pp.427-431
- 2004
- 원문 바로보기
We use simulations of large-scale structure formation to study the build-up of magnetic fields (MFs) in the intergalactic medium. Our basic assumption is that cosmological MFs grow in a magnetohy-drodynamical (MHD) amplification process driven by structure formation out of a magnetic seed field present at high redshift. This approach is motivated by previous simulations of the MFs in galaxy clusters which, under the same hypothesis that we adopt here, succeeded in reproducing Faraday rotation measurements (RMs) in clusters of galaxies. Our ACDM initial conditions for the dark matter density fluctuations have been statistically constrained by the observed large-scale density field within a sphere of 110 Mpc around the Milky Way, based on the IRAS 1.2-Jy all-sky redshift survey. As a result, the positions and masses of prominent galaxy clusters in our simulation coincide closely with their real counterparts in the Local Universe. We find excellent agreement between RMs of our simulated galaxy clusters and observational data. The improved numerical resolution of our simulations compared to previous work also allows us to study the MF in large-scale filaments, sheets and voids. By tracing the propagation of ultra high energy (UHE) protons in the simulated MF we construct full-sky maps of expected deflection angles of protons with arrival energies <TEX>$E = 10^{20}\;eV$</TEX> and <TEX>$4 {\times} 10^{19}\;eV$</TEX>, respectively. Accounting only for the structures within 110 Mpc, we find that strong deflections are only produced if UHE protons cross galaxy clusters. The total area on the sky covered by these structures is however very small. Over still larger distances, multiple crossings of sheets and filaments may give rise to noticeable deflections over a significant fraction of the sky; the exact amount and angular distribution depends on the model adopted for the magnetic seed field. Based on our results we argue that over a large fraction of the sky the deflections are likely to remain smaller than the present experimental angular sensitivity. Therefore, we conclude that forthcoming air shower experiments should be able to locate sources of UHE protons and shed more light on the nature of cosmological MFs.
608
- RUDNICK LAWRENCE
- Journal of the Korean astronomical society = 천문학회지
- 37, n.5
- pp.329-335
- 2004
- 원문 바로보기
Observations of magnetic fields on scales up to several Mpc are important for understanding cluster and large-scale structure evolution. Our current census of such structures is heavily biased - towards fields of several <TEX>$\mu$</TEX>G, towards fields in deep potential wells, and towards high inferred field strengths m cooling flow and other clusters from improper analysis of rotation measure data. After reviewing these biases, I show some recent results on two relics that are powered in very different ways. I describe new investigations that are now uncovering weak diffuse fields in the outskirts of clusters and other low density environments, and the good prospects for further progress.
609
- JONES T. W.
- Journal of the Korean astronomical society = 천문학회지
- 37, n.5
- pp.421-426
- 2004
- 원문 바로보기
I briefly review the current theoretical status of the origins of ultrahigh energy cosmic rays with special emphasis on models associated with galaxy clusters. Some basic constraints on models are laid out, including those that apply both to so-called 'top-down' and 'bottom-up' models. The origins of these UHECRs remain an enigma; no model stands out as a clear favorite. Large scale structure formation shocks, while very attractive conceptually in this context, are unlikely to be able to accelerate particles to energies much above <TEX>$10^{18}eV$</TEX>. Terminal shocks in relativistic AGN jets seem to be more viable candidates physically, but suffer from their rarity in the local universe. Several other, representative, models are outlined for comparison.
610
- DRURY LUKE O'C
- Journal of the Korean astronomical society = 천문학회지
- 37, n.5
- pp.393-398
- 2004
- 원문 바로보기
This paper describes some recent developments in our understanding of particle acceleration by shocks. It is pointed out that while good agreement now exists as to steady nonlinear modifications to the shock structure, there is. also growing evidence that the mesoscopic scales may not in fact be steady and that siginficant instabilties associated with magnetic field amplification may be a feature of strong collisionless plasma shocks.