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1968년 ~ 2023년까지 1,187 건의 한국천문학회지를 격월간 확인하실 수 있습니다.
- The Korean Astronomical Society (The Korean Astronomical Society)
- 계간 (Quarterly)
- ISSN : 1225-4614 (ISSN : 1225-4614)
1225-4614
- DB구축현황 : 1,187건 (DB Construction : 1,187 Articles)
총 게시글 1,187건
페이지 36/119
351
- 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>.
352
- 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.
353
- 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.
354
- 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.
355
- 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.
356
- 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.
357
- 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.
358
- 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>.
359
- 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.
360
- 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.