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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 39/123
381
- 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).
382
- Lim, Beomdu
- Journal of the Korean astronomical society = 천문학회지
- 42, n.6
- pp.161-174
- 2009
- 원문 바로보기
Observation of standard stars is of crucial importance in stellar photometry. We have studied the standard transformation relations of the UBV RI CCD photometric system at the Maidanak Astronomical Observatory in Uzbekistan. All observations were made with the AZT-22 1.5m telescope, SITe 2k CCD or Fairchild 486 CCD, and standard Bessell UBV RI filters from 2003 August to 2007 September. We observed many standard stars around the celestial equator observed by SAAO astronomers. The atmospheric extinction coefficients, photometric zero points, and time variation of photometric zero points of each night were determined. Secondary extinction coefficients and photometric zero points were very stable, while primary extinction coefficients showed a distinct seasonal variation. We also determined the transformation coefficients for each filter. For B, V, R, and I filters, the transformation to the SAAO standard system could be achieved with a straight line or a combination of two straight lines. However, in the case of the U filter and Fairchild 486 CCD combination, a significant non-linear correction term - related to the size of Balmer jump or the strength of the Balmer lines - of up to 0:08 mags was required. We found that our data matched well the SAAO photometry in V, B - V, V - I, and R - I. But in U - B, the difference in zero point was about 3.6 mmag and the scatter was about 0.02 mag. We attribute the relatively large scatter in U -B to the larger error in U of the SAAO photometry. We confirm the mostly small differences between the SAAO standard UBV RI system and the Landolt standard system. We also attempted to interpret the seasonal variation of the atmospheric extinction coefficients in the context of scattering sources in the earth's atmosphere.
383
- Bong, Su-Chan
- Journal of the Korean astronomical society = 천문학회지
- 42, n.1
- pp.1-7
- 2009
- 원문 바로보기
The e-CALLISTO is a global network of frequency-agile solar radio spectrometers that was constructed in a collaboration between Swiss Federal Institute of Technology Zurich (ETH Zurich) and local host institutes. It is intended to monitor solar radio bursts 24 hours a day in frequency range between 45 MHz and 870 MHz. One of e-CALLISTO spectrometer was installed at Korea Astronomy and Space Science Institute (KASI) in 2007 October. The spectrometer gets signals from a horizontally polarized log-periodic antenna mounted on an automatic Sun-tracking system. Tracking status and data are monitored in Space Weather Monitoring Laboratory (SWML) of KASI in real time, and flare time data are transferred to ETH Zurich data archive daily. Using this spectrometer we obtained a couple of type II solar radio bursts on 2007 December 31, and found that these bursts are associated with a CME which occurred on the east limb.
384
- Oh, Seung-Kyung
- Journal of the Korean astronomical society = 천문학회지
- 42, n.2
- pp.17-26
- 2009
- 원문 바로보기
We estimate the enclosed mass profile in the central 10 pc of the Milky Way by analyzing the infrared photometry and the velocity observations of dynamically relaxed stellar population in the Galactic center. HST/NICMOS and Gemini Adaptive Optics images in the archive are used to obtain the number density profile, and proper motion and radial velocity data were compiled from the literature to find the velocity dispersion profile assuming a spherical symmetry and velocity isotropy. From these data, we calculate the the enclosed mass and density profiles in the central 10 pc of the Galaxy using the Jeans equation. Our improved estimates can better describe the exact evolution of the molecular clouds and star clusters falling down to the Galactic center, and constrain the star formation history of the inner part of the Galaxy.
385
- Nouh, M.I.
- Journal of the Korean astronomical society = 천문학회지
- 42, n.3
- pp.47-54
- 2009
- 원문 바로보기
In this paper we present an independent FORTRAN code for calculating LTE-plane-parallel model atmospheres. The transfer equation has been solved using Avrett and Loeser method. It is shown that, using an approximate non-gray temperature distribution together with the iteration factors method (Simonneau and Crivellari) for correcting the temperature distribution reduce the number of iteration required to achieve the condition of radiative equilibrium. Preliminary results for pure helium model atmospheres are presented.
386
- Suh, Kyung-Won
- Journal of the Korean astronomical society = 천문학회지
- 42, n.4
- pp.81-91
- 2009
- 원문 바로보기
We make a new catalog of AGB stars in our Galaxy from the sources listed in the Infrared Astronomical Satellite (IRAS) point source catalog (PSC) compiling the lists of previous works with verifying processes. We verify the class identification of AGB stars into oxygen-rich and carbon-rich stars using the information from recent investigations. For the large sample of AGB stars, we present infrared two-color diagrams from the observations at near infrared bands and IRAS PSC. On the two-color diagrams, we plot the tracks of theoretical radiative transfer model results with increasing dust shell optical depths. Comparing the observations with the theoretical tracks, we discuss the meaning of the infrared two-color diagrams.
387
- 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.
388
- 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.
389
- 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>.
390
- 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.