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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
- Chae, Jong-Chul
- Journal of the Korean astronomical society = 천문학회지
- 42, n.3
- pp.61-69
- 2009
- 원문 바로보기
Transverse velocity vectors can be determined from a pair of images successively taken with a time interval using an optical flow technique. We have tested the performance of the new technique called NAVE (non-linear affine velocity estimator) recently implemented by Chae & Sakurai using real image data taken by the Narrowband Filter Imager (NFI) of the Solar Optical Telescope (SOT) aboard the Hinode satellite. We have developed two methods of estimating the errors in the determination of velocity vectors, one resulting from the non-linear fitting <TEX>${\sigma}_{\upsilon}$</TEX> and the other <TEX>${\epsilon}_u$</TEX> resulting from the statistics of the determined velocity vectors. The real error is expected to be somewhere between <TEX>${\sigma}_{\upsilon}$</TEX> and <TEX>${\epsilo</TEX><TEX>n}_u$</TEX>. We have investigated the dependence of the determined velocity vectors and their errors on the different parameters such as the critical speed for the subsonic filtering, the width of the localizing window, the time interval between two successive images, and the signal-to-noise ratio of the feature. With the choice of <TEX>$v_{crit}$</TEX> = 2 pixel/step for the subsonic filtering, and the window FWHM of 16 pixels, and the time interval of one step (2 minutes), we find that the errors of velocity vectors determined using the NAVE range from around 0.04 pixel/step in high signal-to-noise ratio features (S/N <TEX>$\sim$</TEX> 10), to 0.1 pixel/step in low signa-to-noise ratio features (S/N <TEX>$\sim$</TEX> 3) with the mean of about 0.06 pixel/step where 1 pixel/step corresponds roughly to 1 km/s in our case.
372
- Kyeong, Jae-Mann
- Journal of the Korean astronomical society = 천문학회지
- 41, n.6
- pp.147-155
- 2008
- 원문 바로보기
We present UBV I photometry of the old open cluster NGC 1193. Color-magnitude diagrams (CMDs) of this cluster show a well defined main sequence and a sparse red giant branch. For the inner region of r < 50', three blue straggler candidates are newly found in addition to the objects Kaluzny (1988) already found. The color-color diagrams show that the reddening value toward NGC 1193 is E(B - V ) = <TEX>$0.19{\pm}0.04$</TEX>. From the ultraviolet excess measurement, we derived the metallicity to be [Fe/H]= <TEX>$-0.45{\pm}0.12$</TEX>. A distance modulus of <TEX>${(m\;-\;M)}_0$</TEX> = <TEX>$13.3{\pm}0.15$</TEX> is obtained from zero age main sequence fitting with the empirically calibrated Hyades isochrone of Pinsonneault et al. (2004). CMD comparison with the Padova isochrones by Bertelli et al. (1994) gives an age of log t = <TEX>$9.7{\pm}0.1$</TEX>.
373
- Kim, Yeon-Han
- Journal of the Korean astronomical society = 천문학회지
- 41, n.6
- pp.173-180
- 2008
- 원문 바로보기
Using the MHD coronal seismology technique, we estimated the magnetic field for three spicules observed in 2008 June. For this study, we used the high resolution Ca II H line (<TEX>$3968.5\;{\AA}$</TEX>) images observed by the Hinode SOT and considered a vertical thin flux tube as a spicule model. To our knowledge, this is the first attempt to estimate the spicule magnetic field using the Hinode observation. From the observed oscillation properties, we determined the periods, amplitudes, minimum wavelengths, and wave speeds. We interpreted the observed oscillations as MHD kink waves propagating through a vertical thin flux tube embedded in a uniform field environment. Then we estimated spicule magnetic field assuming spicule densities. Major results from this study are as follows : (1) we observed three oscillating spicules having durations of 5-7 minutes, oscillating periods of 2-3 minutes, and transverse displacements of 700-1000 km. (2) The estimated magnetic field in spicules is about 10-18 G for lower density limit and about 43-76 G for upper density limit. (3) In this analysis, we can estimate the minimum wavelength of the oscillations, such as 60000 km, 56000 km, and 45000 km. This may be due to the much longer wavelength comparing with the height of spicules. (4) In the first event occurred on 2008 June 03, the oscillation existed during limited time (about 250 s). This means that the oscillation may be triggered by an impulsive mechanism (like low atmospheric reconnection), not continuous. Being compared with the ground-based observations of spicule oscillations, our observation indicates quite different one, i.e., more than one order longer in wavelength, a factor of 3-4 larger in wave speed, and 2-3 times longer in period.
374
- Lee, Young-Ung
- Journal of the Korean astronomical society = 천문학회지
- 41, n.6
- pp.157-161
- 2008
- 원문 바로보기
We have estimated the fractal dimension of the molecular clouds associated with the Hii region Sh 156 in the Outer Galaxy. We selected the <TEX>$^{12}CO$</TEX> cube data from the FCRAO CO Survey of the Outer Galaxy. Using a developed code within IRAF, we identified slice-clouds (2-dimensional clouds in velocity-channel maps) with two threshold temperatures to estimate the fractal dimension. With the threshold temperatures of 1.8 K, and 3 K, we identified 317 slice-clouds and 217 slice-clouds, respectively. There seems to be a turn-over location in fractional dimension slope around NP (area; number of pixel) = 40. The fractal dimensions was estimated to be D = <TEX>$1.5\;{\sim}\;1.53$</TEX> for <TEX>$NP\;{\geq}\;40$</TEX>, where <TEX>$P\;{\propto}\;A^{D/2}$</TEX> (P is perimeter and A is area), which is slightly larger than other results. The sampling rate (spatial resolution) of observed data must be an important parameter when estimating fractal dimension. Fractal dimension is apparently invariant when varying the threshold temperatures applied to slice-clouds identification.
375
- 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.
376
- 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.
377
- 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).
378
- 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>.
379
- 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.
380
- 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.