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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 56/122
551
- KIM S.-L.
- Journal of the Korean astronomical society = 천문학회지
- 37, n.4
- pp.143-149
- 2004
- 원문 바로보기
We present photometric results for four new variable stars discovered in the vicinity of the ZZ Ceti-type pulsating white dwarf BR Cam. Observations were performed on 5 nights in November 2003 using the 1.8m telescope at Bohyunsan Optical Astronomy Observatory with no filter, on 3 nights in December 2003 using the 0.61m telescope at Sobaeksan Optical Astronomy Observatory with V, I filters, and on 3 nights in October 2004 using the 1.0m telescope at Mt. Lemmon Optical Astronomy Observatory with V, I filters. We estimated their periods from the phase-match technique for one eclipsing binary and the multiple frequency analysis for three pulsating stars. By considering the light curve shape, period and amplitude difference between two passbands, we classified the objects by their variability types as follows: V1 (USNO-A2.0 1425-05691757) is a W UMa-type eclipsing binary with an orbital period of <TEX>$0^d.4641$</TEX>; V2 (USNO-A2.0 1425-05703335) is a multi-periodic <TEX>$\delta$</TEX> Set-type pulsating star with a dominant period of <TEX>$0^d.0649$</TEX>; V3 (USNO-A2.0 1425-05699659) is also a <TEX>$\delta$</TEX> Set-type pulsating star with a period of <TEX>$0^d.1408$</TEX>; and V 4 (USNO-A2.0 1425-05707705) is a RR Lyr-type pulsating star with a period of <TEX>$0^d.2643$</TEX>.
552
- SHEEN YUN-KYEONG
- Journal of the Korean astronomical society = 천문학회지
- 37, n.2
- pp.87-90
- 2004
- 원문 바로보기
Spectrophotometry of the night sky over Mount Bohyun is presented for the nearly entire visible wavelengths of <TEX>$3600{\~}$8600{\AA}$</TEX>. The data was obtained under moonless clear sky in February 2004 with the 1.8-m telescope and the long slit spectrograph. The sky spectrum shows a number of strong emission lines originated from light pollution, especially due to high pressure sodium lamps. When compared to the night sky of Kitt Peak, our sky continuum is 1 to 2 magnitude brighter at all wavelengths, the worst being around the broad emission region near 6000<TEX>${\AA}$</TEX>. The night sky spectrum presented here with almost complete line identifications is a useful reference for arc-independent wavelength calibrations to check the gravity flexure of the spectrograph and the wavelength shift between FeNeArHe arc frames and science frames.
553
- CHO JUNGYEON
- Journal of the Korean astronomical society = 천문학회지
- 37, n.5
- pp.557-562
- 2004
- 원문 바로보기
We discuss diffusion of particles in turbulent flows. In hydrodynamic turbulence, it is well known that distance between two particles imbedded in a turbulent flow exhibits a random walk behavior. The corresponding diffusion coefficient is <TEX>${\~}$</TEX> <TEX>${\upsilon}_{inj}{\iota}_{turb}$</TEX>, where <TEX>${\upsilon}_{inj}$</TEX> is the amplitude of the turbulent velocity and <TEX>${\iota}_{turb}$</TEX> is the scale of the turbulent motions. It Is not clear whether or not we can use a similar expression for magnetohydrodynamic turbulence. However, numerical simulations show that mixing motions perpendicular to the local magnetic field are, up to high degree, hydrodynamical. This suggests that turbulent heat transport in magnetized turbulent fluid should be similar to that in non-magnetized one, which should have a diffusion coefficient <TEX>${\upsilon}_{inj}{\iota}_{turb}$</TEX>. We review numerical simulations that support this conclusion. The application of this idea to thermal conductivity in clusters of galaxies shows that this mechanism may dominate the diffusion of heat and may be efficient enough to prevent cooling flow formation when turbulence is vigorous.
554
- MOON Y.-J.
- Journal of the Korean astronomical society = 천문학회지
- 37, n.1
- pp.41-53
- 2004
- 원문 바로보기
It has been a big mystery what drives filament eruptions and flares. We have studied in detail an X1.8 flare and its associated filament eruption that occurred in NOAA Active Region 9236 on November 24,2000. For this work we have analyzed high temporal (about 1 minute) and spatial (about 1 arcsec) resolution images taken by Michelson Doppler Imager (MDI) onboard the Solar and Heliospheric Observatory, Hoc centerline and blue wing (<TEX>$-0.6{\AA}$</TEX>) images from Big Bear Solar Observatory, and 1600 <TEX>${\AA}$</TEX> UV images by the Transition Region and Corona Explorer (TRACE). We have found that there were several transient brightenings seen in H<TEX>$\alpha$</TEX> and, more noticeably in TRACE 1600 <TEX>${\AA}$</TEX> images around the preflare phase. A closer look at the UV brightenings in 1600 <TEX>${\AA}$</TEX> images reveals that they took place near one end of the erupting filament, and are a kind of jets supplying mass into the transient loops seen in 1600 <TEX>${\AA}$</TEX>. These brightenings were also associated with canceling magnetic features (CMFs) as seen in the MDI magnetograms. The flux variations of these CMFs suggest that the flux cancellation may have been driven by the emergence of the new flux. For this event, we have estimated the ejection speeds of the filament ranging from 10 to 160 km <TEX>$s^{-1}$</TEX> for the first twenty minutes. It is noted that the initiation of the filament eruption (as defined by the rise speed less than 20 km <TEX>$s^{-1}$</TEX>) coincided with the preflare activity characterized by UV brightenings and CMFs. The speed of the associated LASCO CME can be well extrapolated from the observed filament speed and its direction is consistent with those of the disturbed UV loops associated with the preflare activity. Supposing the H<TEX>$\alpha$</TEX>/UV transient brightenings and the canceling magnetic features are due to magnetic reconnect ion in the low atmosphere, our results may be strong observational evidence supporting that the initiation of the filament eruption and the preflare phase of the associated flare may be physically related to low-atmosphere magnetic reconnection.
555
- 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.
556
- 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.
557
- 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.
558
- 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.
559
- 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>.
560
- 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.