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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 38/122
371
- Son, Dong-Hoon
- Journal of the Korean astronomical society = 천문학회지
- 42, n.5
- pp.125-134
- 2009
- 원문 바로보기
We investigated the circumnuclear region of the Seyfert 2 galaxy NGC 5728, using the CFHT 3.6 m OASIS <TEX>$[S_{II}]$</TEX>, <TEX>$[O_{III}]$</TEX> & <TEX>$H\beta$</TEX> spectral images complemented with the IUE spectra. The physical condition of the circumnuclear zone has been derived: the gas density (indicated by <TEX>$[S_{II}]$</TEX>6716/31 ratio) around the C core is generally similar to that around the NW core, i.e., <TEX>$\sim500cm^{-3}$</TEX>. However, there appears to be evidence of a higher density shell in front of the NW core, <TEX>$\sim10^4cm^{-3}$</TEX> at -250 km <TEX>$s^-1$</TEX>. The IUE <TEX>$Si_{III}$</TEX>]1892/<TEX>$C_{III}$</TEX>]1909 ratio implies a possible presence of a broad emission region of gas densities of <TEX>$\sim10^{10}cm^{-3}$</TEX>. The SE cone and surrounding area show several prominent features, while the NW cone does not show any particular structure: we identified three prominent blobs in the SE cone and one possible candidate in the NW cone. The outflow activities exist within the relatively large conic opening angle. We discussed the possibility of inflow or outflow activities of blobs found in the circumnuclear region of NGC 5728. The gas around two cores, two cones, and several blobs, is likely to be excited by the AGN hot source(s).
372
- Lee, Jong-Chul
- Journal of the Korean astronomical society = 천문학회지
- 41, n.5
- pp.109-119
- 2008
- 원문 바로보기
We present an optical-infrared photometric study of galaxies in six nearby clusters of galaxies at <TEX>$z=0.041{\sim}0.098$</TEX> (A1436, A1773, A1809, A2048, A2142, and A2152). Using BV I photometry obtained at the Bohyunsan Optical Astronomical observatory and <TEX>$JHK_S$</TEX> photometry extracted from the 2-Micron All-Sky Survey catalog, we investigate the colors of galaxies in the clusters. Using the (B - V) versus (<TEX>$I\;-\;K_S$</TEX>) color-color diagrams in comparison with the simple stellar population model, we estimate the ages and metallicities of bright early-type member galaxies. Early-type galaxies in each cluster show the color-magnitude relation. Ages and metallicities of early-type members show little dependence on their velocity dispersions. Mean ages of early-types in the clusters range from 3 Gyr to 20 Gyr, showing a large dispersion, and mean metallicities range from Z = 0.03 to 0.05 above the solar value, showing a negligible dispersion.
373
- Sung, Hwan-Kyung
- Journal of the Korean astronomical society = 천문학회지
- 41, n.1
- pp.1-6
- 2008
- 원문 바로보기
The X-ray spectra of late type stars can generally be well fitted by a two temperature component model of the corona. We find that the temperatures of both components are strong functions of stellar age, although the temperature of the hotter plasma in the corona shows a larger scatter and is probably affected by the activity of stars, such as flares. We confirm the power-law decay of the temperature of the hot plasma, but the temperature of the cool plasma component decays linearly with log(age).
374
- Kim, Hye- Rim
- Journal of the Korean astronomical society = 천문학회지
- 41, n.6
- pp.181-186
- 2008
- 원문 바로보기
Recently, Choe & Cheng (2002) have demonstrated that multiple magnetic flux systems with closed configurations can have more magnetic energy than the corresponding open magnetic fields. In relation to this issue, we have addressed two questions: (1) how much fraction of eruptive solar active regions shows multiple flux system features, and (2) what winding angle could be an eruption threshold. For this investigation, we have taken a sample of 105 front-side halo CMEs, which occurred from 1996 to 2001, and whose source regions were located near the disk center, for which magnetic polarities in SOHO/MDI magnetograms are clearly discernible. Examining their soft X-ray images taken by Yohkoh SXT in pre-eruption stages, we have classified these events into two groups: multiple flux system events and single flux system events. It is found that 74% (78/105) of the sample events show multiple flux system features. Comparing the field configuration of an active region with a numerical model, we have also found that the winding angle of the eruptive flux system is slightly above <TEX>$1.5{\pi}$</TEX>.
375
- Kang, Eun-Ha
- Journal of the Korean astronomical society = 천문학회지
- 41, n.3
- pp.49-58
- 2008
- 원문 바로보기
Resonance doublets including O VI 1032, 1038, NV 1239, 1243 and C IV 1548, 1551 constitute prominent emission lines in symbiotic stars and planetary nebulae. Spectroscopic studies of symbiotic stars and planetary nebulae from UV space telescopes show various line ratios of these doublets deviating from the theoretical ratio of 2:1. Using a Monte Carlo technique, we investigate the collisional de-excitation effect in these emission nebulae. We consider an emission nebula around the hot component of a symbiotic star characterized by the collisional de-excitation probability <TEX>$p_{coll}\;{\sim}\;10^{-3}\;-\;10^{-4}$</TEX> per each resonance scattering, and the line center optical depths for major resonance doublets in the range <TEX>${\tau}_0\;{\sim}\;10^2\;-\;10^5$</TEX>. We find that various line ratios are obtained when the product <TEX>$p_{coll}{\tau}_0$</TEX> is of order unity. Our Monte Carlo calculations show that the flux ratio can be approximately fitted by a linear function of <TEX>${\log}{\tau}_0$</TEX> when <TEX>${\tau}_0p_{coll}\;{\sim}\;1$</TEX>. It is briefly discussed that this corresponds to the range relevant to the emission nebulae of symbiotic stars.
376
- Park, Yong-Sun
- Journal of the Korean astronomical society = 천문학회지
- 41, n.3
- pp.77-81
- 2008
- 원문 바로보기
As a continuation of a previous work by Park et al. (2006), we have developed a two-element radio interferometer that can measure both the phase and amplitude of a visibility function. Two small radio telescopes with diameters of 2.3 m are used as before, but this time an external reference oscillator is shared by the two telescopes so that the local oscillator frequencies are identical. We do not use a hardware correlator; instead we record signals from the two telescopes onto a PC and then perform software correlation. Complex visibilities are obtained toward the sun at <TEX>${\lambda}\;=\;21\;cm$</TEX>, for 24 baselines with the use of the earth rotation and positional changes of one element, where the maximum baseline length projected onto UV plane is <TEX>${\sim}\;90{\lambda}$</TEX>. As expected, the visibility amplitude decreases with the baseline length, while the phase is almost constant. The image obtained by the Fourier transformation of the visibility function nicely delineates the sun, which is barely resolved due to the limited baseline length. The experiment demonstrates that this system can be used as a 'toy' interferometer at least for the education of (under)graduate students.
377
- 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.
378
- Takeda, Yoichi
- Journal of the Korean astronomical society = 천문학회지
- 41, n.4
- pp.83-98
- 2008
- 원문 바로보기
In an attempt of clarifying the connection between the photospheric abundance anomalies and the stellar rotation as well as of exploring the nature of 'normal A' stars, the abundances of seven elements (C, O, Si, Ca, Ti, Fe, and Ba) and the projected rotational velocity for 46 A-type field stars were determined by applying the spectrum-fitting method to the high-dispersion spectral data obtained with BOES at BOAO. We found that the peculiarities(underabundances of C, O, and Ca; an overabundance of Ba) seen in slow rotators efficiently decrease with an increase of rotation, which almost disappear at <TEX>$v_esin\;i{\gtrsim}100km\;s^{-1}$</TEX>. This further suggests that stars with sufficiently large rotational velocity may retain the original composition at the surface without being altered. Considering the subsolar tendency(by several tenths dex below) exhibited by the elemental abundances of such rapidly-rotating (supposedly normal) A stars, we suspect that the gas metallicity may have decreased since our Sun was born, contrary to the common picture of galactic chemical evolution.
379
- 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.
380
- 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.