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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 42/123
411
- Migenes, Victor
- Journal of the Korean astronomical society = 천문학회지
- 40, n.4
- pp.131-132
- 2007
- 원문 바로보기
The research of OH maser emission sources with high angular resolution is complicated by the effects of interstellar scattering: more over, most of the OH maser sources are located in the galactic plane where the scattering is largest. However, the data available from pulsar studies on the spatial distribution of the amount of scattering indicate that there is a strong non-uniformity in the distribution of the amount of scattering material. There are directions in the galactic plane where the scattering is an order of magnitude higher than the average, as well as directions where the scattering is much lower. The latter provide an opportunity to investigate OH masers with the full angular resolution offered by very long baseline interferometry instruments, like the VLBA, and measure their true angular size, shape and brightness temperature. We have observed approximately 100 OH maser sources, distributed all over the northern hemisphere, with the VLBA in order to study the scattering properties of the interstellar medium.
412
- Minh, Y.C.
- Journal of the Korean astronomical society = 천문학회지
- 40, n.3
- pp.61-65
- 2007
- 원문 바로보기
The 2-1 and 5-4 transitions of SiO have been observed toward the Sgr B2 region, including the Principal Cloud(the GMC containing Sgr B2(M)) and its surroundings. The morphology and velocity structure of the SiO emission show a close resemblance with the HNCO Ring feature, identified by Minh & Irvine(2006), of about 10 pc in diameter, which may be expanding and colliding with the Principal Cloud. Three SiO clumps have been found around the Ring, with total column densities <TEX>$N_{SiO}{\sim}1{\times}10^{14}cm^{-2}$</TEX> at the peak positions of these clumps. The fractional SiO abundance relative to <TEX>$H_2$</TEX> has been estimated to be <TEX>${\sim}(0.5-1){\times}10^{-9}$</TEX>, which is about two orders of magnitude larger than the quiet dense cloud values. Our SiO observational result supports the existence of an expanding ring, which may be triggering active star formations in the Principal Cloud.
413
- Chae, Jong-Chul
- Journal of the Korean astronomical society = 천문학회지
- 40, n.3
- pp.67-82
- 2007
- 원문 바로보기
The basic building block of solar filaments/prominences is thin threads of cool plasma. We have studied the spectral properties of velocity threads, clusters of thinner density threads moving together, by analyzing a sequence of <TEX>$H{\alpha}$</TEX> images of a quiescent filament. The images were taken at Big Bear Solar Observatory with the Lyot filter being successively tuned to wavelengths of -0.6, -0.3, 0.0, +0.3, and +0.6 <TEX>${\AA}$</TEX> from the centerline. The spectra of contrast constructed from the image data at each spatial point were analyzed using cloud models with a single velocity component, or three velocity components. As a result, we have identified a couple of velocity threads that are characterized by a narrow Doppler width(<TEX>$\Delta\lambda_D=0.27{\AA}$</TEX>), a moderate value of optical thickness at the <TEX>$H{\alpha}$</TEX> absorption peak(<TEX>$\tau_0=0.3$</TEX>), and a spatial width(FWHM) of about 1'. It has also been inferred that there exist 4-6 velocity threads along the line of sight at each spatial resolution element inside the filament. In about half of the threads, matter moves fast with a line-of-sight speed of <TEX>$15{\pm}3km\;s^{-1}$</TEX>, but in the other half it is either at rest or slowly moving with a line-of-sight velocity of <TEX>$0{\pm}3km\;s^{-1}$</TEX>. It is found that a statistical balance approximately holds between the numbers of blue-shifted threads and red-shifted threads, and any imbalance between the two numbers is responsible for the non-zero line-of-sight velocity determined using a single-component model fit. Our results support the existence not only of high speed counter-streaming flows, but also of a significant amount of cool matter either being at rest or moving slowly inside the filament.
414
- Kurtz, Stan
- Journal of the Korean astronomical society = 천문학회지
- 40, n.4
- pp.137-140
- 2007
- 원문 바로보기
Triggered star formation is not a new idea - it's been around for at least the last 30 years. Although it has never disappeared from the scene, it seems to be enjoying something of a renaissance in recent years, in both observational and theoretical studies. Here we give a brief discussion of the background of triggered star formation, describe some of our own recent observational efforts in this regard, and briefly mention some initial conclusions that may be drawn.
415
- Park, Geum-Sook
- Journal of the Korean astronomical society = 천문학회지
- 40, n.4
- pp.123-125
- 2007
- 원문 바로보기
NGC 1333 is a nearby star forming region, and IRAS 4A and IRAS 4BI are low-mass Class 0 protostars. IRAS 4A is a protobinary system. The NGC 1333 IRAS 4 region was observed in the 22 GHz water maser with a high resolution (0.08') using the Very Large Array. Two groups of masers were detected: one near A2 and the other near BI. Most of the masers associated with A2 are located very close (< 100 AU) to the radio continuum source. They may be associated with the circumstellar disk. Since no maser was detected near AI, the A2 disk is relatively more active than the Al disk. Most of the masers in the BI region are distributed along a straight line, and they are probably related with the outflow. As in many other water maser sources, the IRAS 4 water masers seem to trace selectively either the disk or the outflow. Considering the outflow lifetimes, the disk-outflow dichotomy is probably unrelated with the evolutionary stage of protostars. A possible explanation may be that both the outflow-maser and the disk-maser are rare phenomena and that detecting both kinds of maser around a single protostar may be even rarer.
416
- Ann, H.B.
- Journal of the Korean astronomical society = 천문학회지
- 40, n.1
- pp.9-16
- 2007
- 원문 바로보기
We examine the morphology and luminosity distribution of a strongly warped spiral galaxy PGC 20348 by conducting a detailed BVI CCD surface photometry using BOAO 1.8m telescope. The radial surface brightness shows a break at warp radius <TEX>$(r_{\omega})$</TEX> with a shallow gradient in the inner disk and a steeper gradient in the outer disk. The luminosity of east side of the disk is <TEX>${\sim}0.5$</TEX> mag fainter than the west side at r > <TEX>$r_{\omega}$</TEX>. The reason for the asymmetric luminosity distribution is thought to be the asymmetric flarings that result in the formation of a large diffuse region at the edge of the east disk and a smaller diffuse region at the west disk. The vertical luminosity profiles show a thick disk component whose scale heights increase with increasing galactocentric distances. The warp of PGC 20348 seems to be made by the tidal interactions with the two massive companion galaxies since the flarings and radial increase of disk scale heights are thought to be general properties of tidally perturbed disks. According to the colors of the two clumps inside the diffuse region at the edge of the east disk, they seem to be sites of active star formation triggered by tidal forces from the companion galaxies.
417
- Richer, Michael G.
- Journal of the Korean astronomical society = 천문학회지
- 40, n.4
- pp.183-186
- 2007
- 원문 바로보기
The spectroscopic properties of bright extragalactic planetary nebulae are reviewed, considering primarily their chemical abundances and their internal kinematics. Low-resolution spectroscopy is used to investigate how the precursor stars of bright planetary nebulae modify their original composition through nucleosynthesis and dredge up. At present, the evidence indicates that oxygen and neon abundances usually remain unchanged, helium abundances are typically enhanced by less than 50%, while nitrogen enhancements span a very wide range. Interpreting these changes in terms of the masses of their progenitor stars implies that the progenitor stars typically have masses or order <TEX>$1.5M_{\bigodot}$</TEX> or less, though no models satisfactorily explain the nitrogen enrichment. High-resolution spectroscopy is used to study the internal kinematics of bright planetary nebulae in Local Group galaxies. At first sight, the expansion velocities are remarkably uniform, with a typical expansion velocity of 18 km/s and a range of 8-28 km/s, independent of the progenitor stellar population. Upon closer examination, bright planetary nebulae in the bulge of M31 expand slightly faster than their counterparts in M31's disk, a result that may extend generally to the planetary nebulae arising from old and young stellar populations. There are no very strong correlations between expansion velocity and global nebular properties, except that there are no large expansion velocities at the highest <TEX>$H{\beta}$</TEX> luminosities (i.e., the youngest objects never expand rapidly). These results independently suggest that bright planetary nebulae arise from a similar mass range in all galaxies. Nonetheless, there are good reasons to believe that bright planetary nebulae do not arise from identical progenitor stars in all galaxies.
418
- Lee, Jeong-Eun
- Journal of the Korean astronomical society = 천문학회지
- 40, n.4
- pp.83-89
- 2007
- 원문 바로보기
A new type of object called 'Very Low Luminosity Objects (VeLLOs)' has been discovered by the Spitzer Space Telescope. VeLLOs might be substellar objects forming by accretion. However, some VeLLOs are associated with strong outflows, indicating the previous existence of massive accretion. The thermal history, which significantly affects the chemistry, between substellar objects with a continuous low accretion rate and objects in a quiescent phase after massive accretion (outburst) must be greatly different. In this study, the chemical evolution has been calculated in an episodic accretion model to show that CO and <TEX>$N_2H^+$</TEX> have a relation different from starless cores or Class 0/I objects. Furthermore, the <TEX>$CO_2$</TEX> ice feature at <TEX>$15.2{\mu}m$</TEX> will be a good tracer of the thermal process in VeLLOs.
419
- Migenes, V.
- Journal of the Korean astronomical society = 천문학회지
- 40, n.4
- pp.127-129
- 2007
- 원문 바로보기
It is well known that water vapor maser emission at 22.2 GHz is associated with the earliest stages of both low- and high-mass star formation and it can be considered a reliable diagnostic of their evolutionary state. Bright Rimmed Clouds (BRCs) are clouds that have been compressed by an external ionization-shock front which focuses the neutral gas into compact globules. The boundary layer between the neutral gas and the gas ionized by the incident photons is often called 'bright rim' but the clumps are sometimes classified also as speck globules or cometary globules depending on their appearance. Small globules with bright rims have been considered to be potential sites of star formation and have been studied in several individual regions. We present results from high resolution VLA observations searching for new candidates of recent star formation in bright-rimmed clouds/globules associated with IRAS point sources.
420
- Shin, Ji-Hye
- Journal of the Korean astronomical society = 천문학회지
- 40, n.4
- pp.91-97
- 2007
- 원문 바로보기
The Fokker-Planck (FP) model is one of the commonly used methods for studies of the dynamical evolution of dense spherical stellar systems such as globular clusters and galactic nuclei. The FP model is numerically stable in most cases, but we find that it encounters numerical difficulties rather often when the effects of tidal shocks are included in two-dimensional (energy and angular momentum space) version of the FP model or when the initial condition is extreme (e.g., a very large cluster mass and a small cluster radius). To avoid such a problem, we have developed a new integration scheme for a two-dimensional FP equation by adopting an Alternating Direction Implicit (ADI) method given in the Douglas-Rachford split form. We find that our ADI method reduces the computing time by a factor of <TEX>${\sim}2$</TEX> compared to the fully implicit method, and resolves problems of numerical instability.