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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 40/123
391
- Choi, Youn-Su
- Journal of the Korean astronomical society = 천문학회지
- 41, n.6
- pp.163-172
- 2008
- 원문 바로보기
Using the spectral data in the 3700 to <TEX>$10050{\AA}$</TEX> wavelength range secured with the Hamilton Echelle Spectrograph (HES) at the Lick observatory, we have investigated the expansion velocities and the physical conditions of the elliptical planetary nebula NGC 6803. Various forbidden and permitted lines, e.g. HI, HeI, HeII, [OIII], [NII], [ArIII], and [SII], indicate complicated but systematic physical conditions variation: electron temperatures <TEX>$T_{\varepsilon}\;{\sim}\;9000$</TEX> - 11000 K and electron number densities <TEX>$N_{\varepsilon}\;{\sim}\;2000$</TEX> - <TEX>$9000\;cm^{-3}$</TEX>. The line profile analysis of these ions also indicates the systematic change or the acceleration of the expansion velocities in the range of 10 - <TEX>$22\;km\;s^{-1}$</TEX>. We show that the velocity gradient and physical condition found in various ions are closely related to the prolate ellipsoidal structure of NGC 6803. The expansion velocity and the ionic abundance of <TEX>$O^{2+}$</TEX> were derived based on the OII and [OIII] lines. In spite of the discrepancy of ionic abundances derived by the two cases and their line profiles, the expansion velocities of them agree well. We find that the ratios of the red to blue line component of the HeII & OII lines are different from those of the [OIII] or other forbidden lines that indicates a possible involvement of emission of HeII & OII lines. This subtle difference and the different physical condition of the lines are likely to be caused by the elongated geometry and the latitude dependence of the emission zone.
392
- 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).
393
- 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.
394
- 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.
395
- 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.
396
- 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>.
397
- Ran, In-Woo
- Journal of the Korean astronomical society = 천문학회지
- 41, n.3
- pp.59-64
- 2008
- 원문 바로보기
To detect exoplanets and study pulsation of K giant stars, we have observed precise RV (radial velocity) of about 55 early K giant (K0 - K4) stars brighter than V = 5 magnitude since 2003 by using BOES, a high resolution Echelle spectrograph attached to the 1.8 m telescope at BOAO (Bohyunsan Optical Astronomy Observatory). We detected periodic RV variation of KO III star <TEX>$\beta$</TEX> Gem (HD 62509) with a period <TEX>$P\;=\;596.6\;{\pm}\;2.3$</TEX> days and a semi-amplitude <TEX>$K\;=\;44.8\;{\pm}\;0.7\;ms^{-1}$</TEX>. If we adopt 1.7 <TEX>$M_{\odot}$</TEX> for the mass of <TEX>$\beta$</TEX> Gem, this yields the minimum mass of the companion m sin i = 2.64 <TEX>$M_{Jupiter}$</TEX>. Our results agree well with Hatzes et al. (2006) and Reffert et al. (2006), and confirm their discovery of a planetary object around <TEX>$\beta$</TEX> Gem. We also confirmed about 192 minutes short period stellar oscillation found by Hatzes and Zechmeister (2007). This is the first report of exoplanet detection using BOES and demonstrates that the RV observation using BOES is accurate and stable enough to detect exoplanets around bright K giant stars.
398
- Moon, Hong-Kyu
- Journal of the Korean astronomical society = 천문학회지
- 41, n.1
- pp.7-15
- 2008
- 원문 바로보기
We carried out a set of simulations to reproduce the performance of wide-field NEO surveys based on the revised population model of Near Earth Objects (NEOs) constructed by Morbidelli (2006). This is the first time where the new model is carefully compared with discovery statistics, and with the exception of population model, the simulation is identical to the procedure described in Moon et al. (2008). Our simulations show rather large discrepancy between the number of NEO discoveries made by the actual and the simulated surveys. First of all, unlike Bottke et al. (2002)'s, Morbidelli (2006)'s population model overestimates the number of NEOs. However, the latter reproduces orbit distributions of the actual population better. Our analysis suggests that both models significantly underestimate Amors, while overestimating the number of Apollos. Our simulation result implies that substantial modifications of both models are needed for more accurate reproduction of survey observations. We also identify Hungaria region (HU) to be one of the most convincing candidates that supply a large fraction of asteroids to the inner Solar System.
399
- Sung, Eon-Chang
- Journal of the Korean astronomical society = 천문학회지
- 41, n.5
- pp.121-137
- 2008
- 원문 바로보기
We present optical and near-infrared imaging and long-slit spectroscopy for the blue compact dwarf galaxy (BCD) Mrk 49 in the Virgo Cluster. The surface brightness distribution analysis shows that Mrk 49 consists of an off-centered blue bright compact core of r = 10' and a red faint outer exponential envelope. The <TEX>$H_{\alpha}$</TEX> image and color difference suggest that these two components have different stellar populations: a high surface brightness population of massive young stars and an underlying low surface brightness population of older stars. The redder near-infrared colors of the inner most region suggest that the near-infrared flux of Mrk 49 originates from evolved massive stars associated with the current star-forming activity. The total apparent magnitude is <TEX>$B_T\;=\;14.32$</TEX> mag and the mean effective surface brightness is <TEX>${\mu}_{eff}(B)\;=\;21.56$</TEX> mag <TEX>$arcsec^{-2}$</TEX>. Long-slit spectroscopy shows that Mrk 49 rotates apparently as a solid body within r = 10' in a plane at position angle 55 degrees with an amplitude of about <TEX>$20\;km\;sec^{-1}$</TEX>. The measured radial velocity of Mrk 49 was derived as <TEX>$1,535\;km\;sec^{-1}$</TEX>; and the total mass of stars and gases is in the range of 3 to <TEX>$6\;{\times}\;10^9\;M_{\odot}$</TEX>. The mass-to-light ratios for the central region of Mrk 49 in I and B band are estimated 1.0 and 0.5, respectively. The upper limit of the dark matter to visible matter ratio seems to be < 5. The oxygen abundance is <TEX>$12\;+\;\log(O/H)\;=\;8.21\;{\pm}\; 0.1$</TEX> which is about one quarter of the solar value while the relative helium abundance appears to be similar to that of the sun.
400
- 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.