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- 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 61/123
601
- BRUNETTI GIANFRANCO
- Journal of the Korean astronomical society = 천문학회지
- 37, n.5
- pp.493-500
- 2004
- 원문 바로보기
The existence and extent of non-thermal phenomena in galaxy clusters is now well established. A key question in our understanding of these phenomena is the origin of the relativistic electrons which may be constrained by the modelling of the fine radio properties of radio halos and of their statistics. In this paper we argue that present data favour a scenario in which the emitting electrons in the intracluster medium (ICM) are reaccelerated in situ on their way out. An overview of turbulent-particle acceleration models is given focussing on recent time-dependent calculations which include a full coupling between particles and MHD waves.
602
- KIM WOONG-TAE
- Journal of the Korean astronomical society = 천문학회지
- 37, n.4
- pp.243-248
- 2004
- 원문 바로보기
Disk galaxies abound with intermediate-scale structures such as OB star complexes, giant clouds, and dust spurs in a close geometrical association with spiral arms. Various mechanisms have been proposed as candidates for their origin, but a comprehensive theory should encompass fundamental physical agents such as self-gravity, magnetic fields, galactic differential rotation, and spiral arms, all of which are known to exist in disk galaxies. Recent numerical simulations incorporating all these physical processes show that magneto-Jeans instability (MJI), in which magnetic tension resists the stabilizing Coriolis force of galaxy rotation, is much more powerful than swing-amplification or the Parker instability in forming self-gravitating intermediate-scale structures. The MJI occurring in shearing and expanding flows off spiral arms rapidly forms structures elongated along the direction perpendicular to the arms, remarkably similar to dust spurs seen in HST images of spiral galaxies. In highly nonlinear stages, these spurs fragment to form bound clumps, possibly evolving into bright arm and interarm H II regions, suggesting that all these intermediate-scale structures in spiral galaxies probably share a common dynamical origin.
603
- KAASTRA JELLE S.
- Journal of the Korean astronomical society = 천문학회지
- 37, n.5
- pp.375-379
- 2004
- 원문 바로보기
In this paper I give an overview of the detection of emission from the warm-hot intergalactic medium (WHIM) in the outer parts of clusters of galaxies. The evidence for the presence of soft excess X-ray emission in 7 out of 21 clusters is summarized, and it is demonstrated that several of these clusters show the signatures of thermal emission in the outer parts. A strong signature is the presence of redshifted O VII emission at 0.57 keV. In the central parts, several clusters show also a soft excess, but m this case the observations cannot well discriminate between a thermal or non-thermal origin of the soft X-ray excess.
604
- 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.
605
- OH SU YEON
- Journal of the Korean astronomical society = 천문학회지
- 37, n.4
- pp.151-157
- 2004
- 원문 바로보기
It is investigated quantitative relations between the magnetic storm magnitude and the solar wind parameters such as the Interplanetary Magnetic Field (hereinafter, IMF) magnitude (B), the southward component of IMF (Bz), and the dynamic pressure during the main phase of the magnetic storm with focus on the role of the interplanetary shock (hereinafter, IPS) in order to build the space weather fore-casting model in the future capable to predict the occurrence of the magnetic storm and its magnitude quantitatively. Total 113 moderate and intense magnetic storms and 189 forward IPSs are selected for four years from 1998 to 2001. The results agree with the general consensus that solar wind parameter, especially, Bz component in the shocked gas region plays the most important role in generating storms (Tsurutani and Gonzales, 1997). However, we found that the correlations between the solar wind parameters and the magnetic storm magnitude are higher in case the storm happens after the IPS passing than in case the storm occurs without any IPS influence. The correlation coefficients of B and <TEX>$BZ_(min)$</TEX> are specially over 0.8 while the magnetic storms are driven by IPSs. Even though recently a Dst prediction model based on the real time solar wind data (Temerin and Li, 2002) is made, our correlation test results would be supplementary in estimating the prediction error of such kind of model and in improving the model by using the different fitting parameters in cases associated with IPS or not associated with IPS rather than single fitting parameter in the current model.
606
- 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.
607
- MARTOS MARCO
- Journal of the Korean astronomical society = 천문학회지
- 37, n.4
- pp.199-203
- 2004
- 원문 바로보기
The gas response to a proposed spiral stellar pattern for our Galaxy is presented here as calculated via 2D hydrodynamic calculations utilizing the ZEUS code in the disk plane. The locus is that found by Drimmel (2000) from emission profiles in the K band and at 240 <TEX>${\mu}m$</TEX>. The self-consistency of the stellar spiral pattern was studied in previous work (see Martos et al. 2004). It is a sensitive function of the pattern rotation speed, <TEX>$\Omega$</TEX>p, among other parameters which include the mass in the spiral and its pitch angle. Here we further discuss the complex gaseous response found there for plausible values of <TEX>$\Omega$</TEX>p in our Galaxy, and argue that its value must be close to <TEX>$20 km s^{-l}\;kpc^{-1}$</TEX> from the strong self-consistency criterion and other recent, independent studies which depend on such parameter. However, other values of <TEX>$\Omega$</TEX>p that have been used in the literature are explored to study the gas response to the stellar (K band) 2-armed pattern. For our best fit values, the gaseous response to the 2-armed pattern displayed in the K band is a four-armed pattern with complex features in the interarm regions. This response resembles the optical arms observed in the Milky Way and other galaxies with the smooth underlying two-armed pattern of the old stellar disk populations in our interpretation. The complex gaseous response appears to be related to resonances in stellar orbits. Among them, the 4:1 resonance is paramount for the axisymmetric Galactic model employed, and the set of parameters explored. In the regime seemingly proper to our Galaxy, the spiral forcing appears to be marginally strong in the sense that the 4:1 resonance terminates the stellar pattern, despite its relatively low amplitude. In current work underway, the response for low values of <TEX>$\Omega$</TEX>p tends to remove most of the rich structure found for the optimal self-consistent model and the gaseous pattern is ring-like. For higher values than the optimal, more features and a multi-arm structure appears.
608
- MOCHIZUKI KENJI
- Journal of the Korean astronomical society = 천문학회지
- 37, n.4
- pp.193-197
- 2004
- 원문 바로보기
Anomalies in the far-infrared [C II] 158 <TEX>${\mu}m$</TEX> line emission observed in the central one-kiloparsec regions of spiral galaxies are reviewed. Low far-infrared intensity ratios of the [C II] line to the continuum were observed in the center of the Milky Way, because the heating ratio of the gas to the dust is reduced by the soft interstellar radiation field due to late-type stars in the Galactic bulge. In contrast, such low line-to-continuum ratios were not obtained in the center of the nearby spiral M31, in spite of its bright bulge. A comparison with numerical simulations showed that a typical column density of the neutral interstellar medium between illuminating sources at <TEX>$hv {\~} 1 eV $</TEX> is <TEX>$N_H {\le}10^{21}\;cm^{-2}$</TEX> in the region; the medium is translucent for photons sufficiently energetic to heat the grains but not sufficiently energetic to heat the gas. This interpretation is consistent with the combination of the extremely high [C Il]/CO J = 1-0 line intensity ratios and the low recent star-forming activity in the region; the neutral interstellar medium is not sufficiently opaque to protect the species even against the moderately intense incident UV radiation. The above results were unexpected from classical views of the [C II] emission, which was generally considered to trace intense interstellar UV radiation enhanced by active star formation.
609
- HYUNG SIEK
- Journal of the Korean astronomical society = 천문학회지
- 37, n.4
- pp.273-279
- 2004
- 원문 바로보기
Chemical compositions of planetary nebulae are of interest for a study of the late stage of stellar evolution and for elemental contributions to the interstellar medium of reprocessed elements since possibly a large fraction of stars in 0.8 - 8 <TEX>$M_{\bigodot}$</TEX> range go through this stage. One of the methods for getting chemical composition is a construction of theoretical photoionization models, which involves geometrical complexities and a variety of physical processes. With modelling effort, one can analyze the high dispersion and find the elemental abundances for a number of planetary nebulae. The model also gives the physical parameter of planetary nebula and its central star physical parameter along with the knowledge of its evolutionary status. Two planetary nebulae, NGC 7026 and Hu 1-2, which could have evolved from about one solar mass progenitor stars, showed radically different chemical abundances: the former has high chemical abundances in most elements, while the latter has extremely low abundances. We discuss their significance in the light of the evolution of our Galaxy.
610
- BOHIGAS JOAQUIN
- Journal of the Korean astronomical society = 천문학회지
- 37, n.4
- pp.281-284
- 2004
- 원문 바로보기
Optical imaging and spectroscopy of G353.2+0.9, the brightest part of the giant H II region NGC 6357, shows that this H II region is optically thin, contains <TEX>${\~}300\;M_{\bigodot}$</TEX> of ionized gas and is probably expanding into the surrounding medium. Its chemical composition is similar to that found in other H II regions at similar galactocentric distances if temperature fluctuations are significant. The inner regions are probably made of thin shells and filaments, whereas extended slabs of material, maybe shells seen edge-on, are found in the periphery. The radio continuum and H<TEX>$\alpha$</TEX> emission maps are very similar, indicating that most of the optical nebula is not embedded in the denser regions traced by molecular gas and the presence of IR sources. About <TEX>$10^{50}$</TEX> UV photons per second are required to produce the H<TEX>$\beta$</TEX> flux from the 1l.3'<TEX>${\times}$</TEX>10' region surrounding the Pis 24 cluster that is south of G353.2+0.9. Most of the energy powering this region is produced by the 03-7 stars in Pis 24. Most of the 2MASS sources in the field with large infrared excesses are within G353.2+0.9, indicating that the most recent star forming process occured within it. The formation of Pis 24 preceded and caused the formation of this new generation of stars and may be responsible for the present-day morphology of the entire NGC 6357 region.