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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 60/122
591
- 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.
592
- 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}$</TEX> in the region; the medium is translucent for photons sufficiently energetic to heat the grains but not sufficiently energetic to heat the gas. This interpretation is consistent with the combination of the extremely high [C Il]/CO J = 1-0 line intensity ratios and the low recent star-forming activity in the region; the neutral interstellar medium is not sufficiently opaque to protect the species even against the moderately intense incident UV radiation. The above results were unexpected from classical views of the [C II] emission, which was generally considered to trace intense interstellar UV radiation enhanced by active star formation.
593
- HYUNG SIEK
- Journal of the Korean astronomical society = 천문학회지
- 37, n.4
- pp.273-279
- 2004
- 원문 바로보기
Chemical compositions of planetary nebulae are of interest for a study of the late stage of stellar evolution and for elemental contributions to the interstellar medium of reprocessed elements since possibly a large fraction of stars in 0.8 - 8 <TEX>$M_{\bigodot}$</TEX> range go through this stage. One of the methods for getting chemical composition is a construction of theoretical photoionization models, which involves geometrical complexities and a variety of physical processes. With modelling effort, one can analyze the high dispersion and find the elemental abundances for a number of planetary nebulae. The model also gives the physical parameter of planetary nebula and its central star physical parameter along with the knowledge of its evolutionary status. Two planetary nebulae, NGC 7026 and Hu 1-2, which could have evolved from about one solar mass progenitor stars, showed radically different chemical abundances: the former has high chemical abundances in most elements, while the latter has extremely low abundances. We discuss their significance in the light of the evolution of our Galaxy.
594
- BOHIGAS JOAQUIN
- Journal of the Korean astronomical society = 천문학회지
- 37, n.4
- pp.281-284
- 2004
- 원문 바로보기
Optical imaging and spectroscopy of G353.2+0.9, the brightest part of the giant H II region NGC 6357, shows that this H II region is optically thin, contains <TEX>${\~}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.
595
- 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.
596
- 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>.
597
- BOHIGAS JOAQUIN
- Journal of the Korean astronomical society = 천문학회지
- 37, n.4
- pp.285-288
- 2004
- 원문 바로보기
Near-infrared imaging photometry supplemented by optical spectroscopy and narrow-band imaging of the H II region Sh 2-128 and its environment are presented. This region contains a developed H II region and the neighboring compact H II region S 128N associated with a pair of water maser sources. Midway between these, the core of a CO cloud is located. The principal ionizing source of Sh 2-128 is an 07 star close to its center. A new spectroscopic distance of 9.4 kpc is derived, very similar to the kinematic distance to the nebula. This implies a galactocentric distance of 13.5 kpc and z = 550 pc. The region is optically thin with abundances close to those predicted by galactocentric gradients. The <TEX>$JHK_s$</TEX> images show that S 128N contains several infrared point sources and nebular emission knots with large near-infrared excesses. One of the three red Ks knots coincides with the compact H II region. A few of the infrared-excess objects are close to known mid- and far-infrared emission peaks. Star counts in J and <TEX>$K_s$</TEX> show the presence of a small cluster of B-type stars, mainly associated with S 128N. The <TEX>$JHK_s$</TEX> photometric properties together with the characteristics of the other objects in the vicinity suggest that Sh 2-128 and S 128N constitute a single complex formed from the same molecular cloud, with ages <TEX>${\~}10^6$</TEX> and < <TEX>$3 {\times} 10^5$</TEX> years respectively. No molecular hydrogen emission was detected at 2.12 <TEX>${\mu}m$</TEX>. The origin of this remote star forming region is an open problem.
598
- SUH KYUNG- WON
- Journal of the Korean astronomical society = 천문학회지
- 37, n.4
- pp.289-294
- 2004
- 원문 바로보기
The main sources of interstellar dust are believed to be dust envelopes around AGB stars. The outflowing envelopes around the long period pulsating variables are very suitable place for massive dust formation. Oxygen-rich silicate dust grains or carbon-rich dust grains form in the envelopes around AGB stars depending on the chemical composition of the stellar surface. The dust grains expelled from AGB stars get mixed up and go through some physical and chemical changes in interstellar medium. There are similarities and differences between interstellar dust and dust grains in AGB stars. The mass cycle in the Galaxy may be best manifested by the fact that the dust grains at various regions have many similarities and understandable differences.
599
- KIM SUNGEUN
- Journal of the Korean astronomical society = 천문학회지
- 37, n.4
- pp.211-216
- 2004
- 원문 바로보기
We present the results of an H I aperture synthesis mosaic of the Large Magellanic Cloud (LMC), made by combining data from 1344 separate pointing centers using the Australia Telescope Compact' Array (ATCA) and the Parkes multibeam receiver. The resolution of the mosaiced images is 50' (<15 pc, using a distance to the LMC of 55kpc). This mosaic, with a spatial resolution .15 times higher than that which had been previously obtained, emphasises the turbulent and fractal structure of the ISM on the small scale, resulting from the dynamical feedback of the star formation processes with the ISM. We also have done a widefield panoramic survey of H<TEX>$\alpha$</TEX> emission from the Magellanic Clouds with an imager mounted on the 16-inch telescope at Siding Spring Observatory. This survey produced H<TEX>$\alpha$</TEX> images which are equal to the ATCA survey in area coverage and resolution. This survey allows us to produce a continuum-subtracted image of the entire LMC. In contrast with its appearance in the H<TEX>$\alpha$</TEX> image, the LMC is remarkably symmetric in H I on the largest scales, with the bulk of the H I residing in a disk of diameter 8. <TEX>$^{\circ}4$</TEX> (7.3 kpc) and a spiral structure is clearly seen. The structure of the neutral atomic ISM in the LMC is dominated by H I filaments combined with numerous shells and holes.
600
- KRONBER PHILIPP P.
- Journal of the Korean astronomical society = 천문학회지
- 37, n.5
- pp.501-507
- 2004
- 원문 바로보기
The uniquely large dimensions of Giant radio galaxies (GRGs) make it possible to probe for stringent limits on total energy content, Faraday rotation, Alfven speeds, particle transport and radiation loss times. All of these quantities are more stringently limited or specified for GRG's than in more 'normal' FRII radio sources. I discuss how both global and detailed analyses of GRG's lead to constraints on the CR electron acceleration mechanisms in GRG's and by extension in all FRII radio sources. The properties of GRG's appear to rule out large scale Fermi-type shock acceleration. The plasma parameters in these systems set up conditions that are favorable for magnetic reconnection, or some other very efficient process of conversion of magnetic to particle energy. We conclude that whatever mechanism operates in GRG's is probably the primary extragalactic CR acceleration mechanism in the Universe.