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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 41/123
401
- Ahn, Kwang-Su
- Journal of the Korean astronomical society = 천문학회지
- 41, n.2
- pp.39-47
- 2008
- 원문 바로보기
Spectral line profiles of filaments/prominences to be observed by the Fast Imaging Solar Spectrograph (FISS) are studied. The main spectral lines of interests are <TEX>$H{\alpha}$</TEX>, Ca II 8542, and Ca II K. FISS has a high spectral resolving power of <TEX>$2{\times}10^5$</TEX>, and supports simultaneous dual-band recording. This instrument will be installed at the 1.6m New Solar Telescope (NST) of Big Bear Solar Observatory, which has a high spatial resolution of 0.065' at 500nm. Adopting the cloud model of radiative transfer and using the model parameters inferred from pre-existing observations, we have simulated a set of spectral profiles of the lines that are emitted by a filament on the disk or a prominence at the limb. Taking into account the parameters of the instrument, we have estimated the photon count to be recorded by the CCD cameras, the signal-to-noise ratios, and so on. We have also found that FISS is suitable for the study of multi-velocity threads in filaments if the spectral profiles of Ca II lines are recorded together with <TEX>$H{\alpha}$</TEX> lines.
402
- Minh, Y.C.
- Journal of the Korean astronomical society = 천문학회지
- 41, n.5
- pp.139-145
- 2008
- 원문 바로보기
The molecular cloud, embedding AFGL 2591, has a 'head-and-tail' structure with a total mass of <TEX>${\sim}\;1800\;M_{\odot}$</TEX>, about half of the mass (<TEX>${\sim}\;900\;M_{\odot}$</TEX>) in the head (size <TEX>${\sim}\;1.2\;pc$</TEX> in diameter), and another half in the envelope (<TEX>${\sim}\;3.5\;pc$</TEX> in the east-west direction). We found a new cloud in the direction toward north-east from AFGL 2591 (projected distance <TEX>${\sim}\;2.4\;pc$</TEX>), which is probably associated with the AFGL 2591 cloud. The <TEX>$^{12}CO$</TEX> spectrum clearly shows a blue-shifted high-velocity wing at around the velocity <TEX>$-20\;{\sim}\;-10\;km\;s^{-1}$</TEX>, but it is not clear whether this high-velocity component has a bipolar nature in our observations. The observed CN spectra also show blue-shifted wing component but the existence of the red-shifted component is not clear, either. In some CN and HCN spectra, the highvelocity components appear as a different velocity component, not a broad line-wing component. The dense cores, traced by CN and HCN, exist in the 'head' of the AFGL 2591 cloud with an elongated morphology roughly in the north-south direction with a size of about 0.5 pc. The abundance ratio between CN and HCN is found to be about 2 - 3 within the observed region, which may suggest a possibility that this core is being affected by the embedded YSOs or by possible shocks from outside.
403
- Kim, Hyouk
- Journal of the Korean astronomical society = 천문학회지
- 41, n.2
- pp.23-37
- 2008
- 원문 바로보기
The high-resolution optical region spectroscopic data of the symbiotic nova AG Peg secured with the Hamilton Echelle Spectrograph at the Lick Observatory, have been analyzed along with the International Ultraviolet Explorer UV archive data. We measure about 700 line intensities in the wavelengths of 3859 to <TEX>$9230{\AA}$</TEX> and identify about 300 lines. We construct pure photoionization models that represent the observed lines and the physical condition for this symbiotic nova. The spectral energy distribution of the ionizing radiation is adopted from stellar model atmospheres. Based on photoionization models, we derive the elemental abundances; C & N appear to be similar to be smaller than the Galactic planetary nebular value while O is enhanced. Our result is compared with the Contini (1997, 2003) who analyzed the UV region spectral data with the shock + ionization model. The Fe abundance appears to be enhanced than that of normal planetary nebulae, which suggests that AG Peg may have formed in the Galactic disk. The models indicate that the temperature of the central star which excite the shell gas may have fluctuated to an unexpected extent during the years 1998 - 2002.
404
- Kim, Chul-Hee
- Journal of the Korean astronomical society = 천문학회지
- 41, n.2
- pp.17-22
- 2008
- 원문 바로보기
We present an analysis of the soft X-ray emission of MRK 841 to investigate its long-term variation. The light variation of MRK 841 for three different energy bands of soft, medium, and hard values were studied. The maximum variability with a factor of 5 for about two years was confirmed at all three different bands. The light curves exhibit a gradual variation of brightness. In addition to a gradual variation, the short- term or micro variation was also confirmed with a factor of about two for all three different bands. The light variation of each band did not exhibit a correlation between them, but the flare event is strongest in the soft band. The hardness ratio for hard and soft bands shows irregular variation but there was no correlation between them. It was confirmed that there is a gradual decrease of the photon index. Results of our analysis are discussed within the framework of the accretion disk phenomenon.
405
- Sim, Chae-Kyung
- Journal of the Korean astronomical society = 천문학회지
- 41, n.3
- pp.65-76
- 2008
- 원문 바로보기
We have investigated the optical properties of the global haze on Titan from spectra recorded between 7100 and <TEX>$9200{\AA}$</TEX>, where <TEX>$CH_4$</TEX> absorption bands of various intensities occur. The Titan spectra were obtained on Feb. 23, 2005 (UT), near the times of the Cassini T3 flyby and Huygens probe, using an optical echelle spectrograph (BOES) on the 1.8-m telescope at Bohyunsan Observatory in Korea. In order to derive the optical properties of the haze as a function of altitude, we developed an inversion radiative-transfer program using an atmospheric model of Titan and laboratory <TEX>$CH_4$</TEX> absorption coefficients available from the literature. The derived extinction coefficients of the haze increase toward the surface, and the coefficients at shorter wavelengths are greater than those at longer wavelengths for the 30 - 120 km altitude range, indicating that the Titanian haze becomes optically thin toward the longer wavelength range. Total optical depths of the haze are estimated to be 1.4 and 1.2 for the 7270 - <TEX>$7360{\AA}$</TEX> and 8940 - <TEX>$9150{\AA}$</TEX> ranges, respectively. Based on the Huygens/DISR data set, Tomasko et al. (2005) reported total optical depths of 2.5 - 3.5 at <TEX>$8290{\AA}$</TEX>, depending on the assumed fractal aggregate particle model. The total optical depths based on our results are smaller than those of Tomasko et al., but they partially overlap with their results if we consider a large uncertainty from possible variations of the <TEX>$CH_4$</TEX> mixing ratio over Titan's disk. We also derived the single scattering albedo of the haze particles as a function of altitude: it is less than 0.5 at altitudes higher than <TEX>${\sim}150\;km$</TEX>, and approaches 1.0 toward the surface. This behavior suggests that, at altitudes above <TEX>${\sim}150\;km$</TEX>, the average particle radius is smaller than the wavelengths, whereas near the surface, it becomes comparable or greater.
406
- Seon, Kwang-Il
- Journal of the Korean astronomical society = 천문학회지
- 41, n.4
- pp.99-107
- 2008
- 원문 바로보기
When a new counting experiment is proposed, it is crucial to predict whether the desired source signal will be detected, or how much observation time is required in order to detect the signal at a certain significance level. The concept of the a priori prediction of the detection limit in a newly proposed experiment should be distinguished from the a posteriori claim or decision whether a source signal was detected in an experiment already performed, and the calculation of statistical significance of a measured source signal. We formulate precise definitions of these concepts based on the statistical theory of hypothesis testing, and derive an approximate formula to estimate quickly the a priori detection limit of expected Poissonian source signals. A more accurate algorithm for calculating the detection limits in a counting experiment is also proposed. The formula and the proposed algorithm may be used for the estimation of required integration or observation time in proposals of new experiments. Applications include the calculation of integration time required for the detection of faint emission lines in a newly proposed spectroscopic observation, and the detection of faint sources in a new imaging observation. We apply the results to the calculation of observation time required to claim the detection of the surface thermal emission from neutron stars with two virtual instruments.
407
- Sanchez-Salcedo, F.J.
- Journal of the Korean astronomical society = 천문학회지
- 40, n.4
- pp.171-177
- 2007
- 원문 바로보기
In external galaxies, the velocity dispersion of the atomic hydrogen gas shows a remarkably flat distribution with the galactocentric radius. This has been a long-standing puzzle because if the gas velocity dispersion is due to turbulence caused by supernova explosions, it should decline with radius. After a discussion on the role of spiral arms and ram pressure in driving interstellar turbulence in the outer parts of galactic disks, we argue that the constant bombardment by tiny high-velocity halo clouds can be a significant source of random motions in the outer disk gas. Recent observations of the flaring of H I in the Galaxy are difficult to explain if the dark halo is nearly spherical as the survival of the streams of tidal debris of Sagittarius dwarf spheroidal galaxy suggests. The radial enhancement of the gas velocity dispersion (at R > 25 kpc) due to accretion of cloudy gas might naturally explain the observed flaring in the Milky Way. Other motivations and implications of this scenario have been highlighted.
408
- Elias, Federico
- Journal of the Korean astronomical society = 천문학회지
- 40, n.4
- pp.141-145
- 2007
- 원문 바로보기
Here we analyze if the ionized shells associated with giant HII regions represent the progenitors of the larger neutral hydrogen supershells detected in the Milky Way and other spiral and dwarf irregular galaxies. We calculate the evolutionary tracks that 12 HII shells found by Relano et al. (2005, 2007) would have if they expanded into the interstellar medium because of multiple supernovae explosions occurring inside the cavity. We find, contrary to Relano et al. (2007), that the evolutionary tracks of these HII shells are inconsistent with the observed parameters of the largest and most massive neutral hydrogen supershells. Thus, an additional energy source to the multiple supernovae explosions is required in order to explain the origin of the most massive neutral hydrogen shells.
409
- 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.
410
- Lee, Young-Ung
- Journal of the Korean astronomical society = 천문학회지
- 40, n.4
- pp.133-135
- 2007
- 원문 바로보기
A survey project of TRAO with the fifteen beam array receiver system is presented. A multibeam array receiver system has been purchased from FCRAO, and is being installed on TRAO 14m telescope. The target region of the survey is from <TEX>${\iota}=120^{\circ}{\sim}137^{\circ},\;b=-1^{\circ}{\sim}+ 1^{\circ}$</TEX>, and velocity resolution would be 1 km/sec after smoothing from the original resolution of <TEX>$0.64km\;s^{-1}$</TEX> in the transition of J = 1-0 of <TEX>$^{13}CO$</TEX>. The survey region is a part of the <TEX>$^{12}CO$</TEX> Outer Galaxy Survey(OGS), and would be an extension of the Bell Laboratories <TEX>$^{13}CO$</TEX> Galactic Plane Survey. By combining with the existing <TEX>$^{12}CO$</TEX> database of the Outer Galaxy Survey, we will derive physical properties of identified molecular clouds and will conduct and statistical analysis of the Outer Galalxy molecular clouds. Reduction process and analysis methods will be introduced.