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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 78/123
771
- MOON Y.-J.
- Journal of the Korean astronomical society = 천문학회지
- 33, n.2
- pp.123-126
- 2000
- 원문 바로보기
We have developed a near real-time flare alerting system which (1) downloads the latest GOES-l0 1-8 <TEX>${\AA}$</TEX> X-ray flux 1-min data by an automated ftp program and shell scripts, (2) produces a beep sound in a simple IDL widget program when the flux is larger than a critical value, and (3) makes it possible to do a wireless alerting by a set of portable transceivers. Thanks to the system, we have made successful Ha flare observations by the Solar Flare Telescope in Bohyunsan Optical Astronomy Observatory. This system is expected to be helpful for ground-based flare observers.
772
- ANN HONG BAE
- Journal of the Korean astronomical society = 천문학회지
- 32, n.1
- pp.1-6
- 1999
- 원문 바로보기
We have conducted near-infrared (J- and H-band) surface photometry for two early type barred galaxies, NGC 3412 and NGC 3941. The bulges of NGC 3412 and NGC 3941 show isophotal twists which indicate that they are triaxial. NGC 3412 has a very short bar and its bulge is more centrally concentrated than that of NGC 3941. The unusually short bar and the centrally concentrated triaxial bulge of NGC 3412 might be the result of bar dissolution. The colors of the nuclear region of NGC 3941 resemble those of the blue nuclei, implying the presence of young stellar populations.
773
- SUNG HWANKYUNG
- Journal of the Korean astronomical society = 천문학회지
- 32, n.2
- pp.109-117
- 1999
- 원문 바로보기
UBVI CCD photometry has been obtained for a region around the Wolf-Rayet star WR 12. We found two young stellar associations in the observed field: the nearer one comprises the field members of Vela OBI association at d = 1.8kpc, while the farther one is the young open cluster Bochum 7 (Bo 7) at d = 4.8kpc. The stars associated with Bo 7 showed no central concentration which suggests that Bo 7 is not a young open cluster but simply a local concentration in the density of young stars belonging to the OB association (Vel OB3). These two associations have similar ages but remarkably different mass function slopes (<TEX>$\Gamma$</TEX> = -2.1 <TEX>$\pm$</TEX> 0.3 for Vel OBI and -1.0 <TEX>$\pm$</TEX> 0.3 for Bo 7). The stars in Vel OBI shows an evident age spread (<TEX>${\Delta}T\~ 9Myr$</TEX>). We also found two strong H<TEX>$\alpha$</TEX> emission stars - WR 12 and <TEX>$\sharp$</TEX>1066 - from narrow band H<TEX>$\alpha$</TEX> photometry.
774
- KIM K.-T.
- Journal of the Korean astronomical society = 천문학회지
- 32, n.2
- pp.75-82
- 1999
- 원문 바로보기
Diffuse radio emission in Abell 2256 was detected above 3 <TEX>$\sigma$</TEX> with DRAO observations at 1420 MHz. The halo size is <TEX>$\~13' {\times}10' (\~1h^{-1}_{50}\;Mpc$</TEX>) in full extent and is elongated along a position angle of about <TEX>$112^{\circ}$</TEX>. The total flux density contained in the halo is 30<TEX>$\pm$</TEX>10 mJy at 1420 MHz and its spectral index is -2.04<TEX>$\pm$</TEX>0.04, showing no evidence for steepening up to 1420 MHz. Using the size estimate, yields a more reliable equipartition magnetic field strength which is <TEX>$0.34(1 + k)^{2/7}{\mu}G$</TEX>. In addition, five new radio sources are identified.
775
- HYUNG SIEK
- Journal of the Korean astronomical society = 천문학회지
- 32, n.1
- pp.55-63
- 1999
- 원문 바로보기
NGC 6537 is an extremely high excitation bipolar planetary nebula. It exhibits a huge range of excitation from lines of [N I] to [Si VI]or [Fe VII], i.e. from neutral atoms to atoms requiring an ionization potential of <TEX>$\~$</TEX>167eV. Its kinematical structures are of special interest. We are here primarily concerned with its high resolution spectrum as revealed by the Hamilton Echelle Spectrograph at Lick Observatory (resolution <TEX>$\~0.2{\AA}$</TEX>) and supplemented by UV and near-UV data. Photoionization model reproduces the observed global spectrum of NGC 6537, the absolute H<TEX>$\beta$</TEX> flux, and the observed visual or blue magnitude fairly well. The nebulosity of NGC 6537 is likely to be the result of photo-ionization by a very hot star of <TEX>$T_{eff} \~ 180,000 K$</TEX>, although the global nebular morphology and kinematics suggest an effect by strong stellar winds and resulting shock heating. NGC 6537 can be classified as a Peimbert Type I planetary nebula. It is extremely young and it may have originated from a star of about 5 <TEX>$M_{\bigodot}$</TEX>.
776
- KIM YONG-CHEOL
- Journal of the Korean astronomical society = 천문학회지
- 32, n.2
- pp.119-126
- 1999
- 원문 바로보기
The standard stellar models for <TEX>$\alpha$</TEX> Cen A and B have been constructed without resorting to the arbitrary constraint of the Solar mixing length ratio. Assuming that the chemical compositions and the ages of the two stars are the same, series of models have been constructed. Using the observational constraints, [Z/X], we were able to constrain the number of the 'possible' models. We find that utilizing the observational constraints of [Z/X] the best models for <TEX>$\alpha$</TEX> Cen system are with the initial Z = 0.03, X = 0.66<TEX>$\~$</TEX>0.67. In particular, the primary and the secondary stars may have the same mixing length ratio 1.6<TEX>$\~$</TEX>1.7, which is the same as that of the calibrated Solar model. And, the age of the system is about 5.4 Gyr. Finally, the large spacing of the p-modes is predicted to be 104 <TEX>$\pm$</TEX> 4<TEX>$\mu$</TEX>Hz for <TEX>$\alpha$</TEX> Cen A.
777
- ANN H. B.
- Journal of the Korean astronomical society = 천문학회지
- 32, n.1
- pp.7-16
- 1999
- 원문 바로보기
Open clusters are useful tools to investigate the structure and evolution of the Galactic disk. We have started a long-term project to obtain UBVI CCD photometry of open clusters which were little studied before, using the Doyak 1.8 m telescope of Bohyunsan Optical Astronomy Observatory in Korea. The primary goals of this project are (1) to make a catalog of UBVI photometry of open clusters, (2) to make an atlas of open clusters, and (3) to survey and monitor variable stars in open clusters. Here we describe this project and report the first results based on preliminary analysis of the data on four open clusters in the survey sample: Be 14, Cr 74, Biu 9, and NGC 2355. Isochrone fitting of the color-magnitude diagrams of the clusters shows that all of them are intermediate age to old (0.3-1.6 Gyrs) open clusters with moderate metallicity.
778
- MOON YONG-JAE
- Journal of the Korean astronomical society = 천문학회지
- 32, n.1
- pp.65-73
- 1999
- 원문 바로보기
In this study we present a new improved nonlinear calibration method for vector magnetograms made by the Solar Flare Telescope of BOAO. To identify Fe I 6302.5 line, we have scanned monochromatic images of the line integrated over filter passband, changing the location of the central transmission wavelength of a Lyot filter. Then we obtained a filter-convolved line profile, which is in good agreement with spectral atlas data provided by the Sacramento Peak Solar Observatory. The line profile has been used to derive calibration coefficients of longitudinal and transverse fields, employing the conventional line slope method under the weak field approximation. Our improved nonlinear calibration method has also been used to calculate theoretical Stokes polarization signals with various angles of inclination of magnetic fields. For its numerical test, we have compared input magnetic fields with the calibrated ones, which have been derived from the new improved non-linear method and the conventional method respectively. The numerical test shows that the calibrated fields obtained from the improved method are consistent with the input fields, but not with those from the conventional method. Finally, we applied our new improved method to a dipole model which characterizes a typical field configuration of a single, round sunspot. It is noted that the conventional method remarkably underestimates the transverse field component near the inner penumbra.
779
- LEE JUNG-DEOK
- Journal of the Korean astronomical society = 천문학회지
- 32, n.2
- pp.91-107
- 1999
- 원문 바로보기
We present BVRI CCD photometry and near-infrared K photometry of the intermediate-aged open cluster NGC 7790. The reddening, E(B - V) = 0.54 <TEX>$\pm$</TEX> 0.05 and the distance modulus, (m - M)o = 12.45 <TEX>$\pm$</TEX> 0.10 for the cluster were determined by zero-age-main-sequence fitting and theoretical isochrone fitting using not only (V, B - V), (V, V - 1), (V, V - R) but also (V, V - K) color-magnitude diagrams. The reddening corresponded approximately to the average value derived from previous studies, while the distance modulus was found to be almost midway between the CCD photometric results of Romeo et al. (1989) and those of Mateo & Madore (1988). We have used four colors to distinguish members from field stars. The expected colors were calculated using the derived distance modulus, and were then were compared with the observed colors (B - V), (V - 1), (V - R), and (V - K). Thus, a color excess E(B - V) for each star was determined which could give the minimum difference between the calculated and observed colors. Single and binary members of the cluster were determined on the basis of the E(B - V) distribution of stars.
780
- CHAE JONGCHUL
- Journal of the Korean astronomical society = 천문학회지
- 32, n.2
- pp.137-147
- 1999
- 원문 바로보기
Recent studies show the importance of understanding three-dimensional magnetic reconnect ion on the solar surface. For this purpose, I consider non-coplanar magnetic reconnection, a simple case of three-dimensional reconnect ion driven by a collision of two straight flux tubes which are not on the same plane initially. The relative angle e between the two tubes characterizes such reconnection, and can be regarded as a measure of magnetic shear. The observable characteristics of non-coplanar reconnection are compared between the two cases of small and large angles. An important feature of the non-coplanar reconnect ion is that magnetic twist can be produced via the re-ordering of field lines. This is a consequence of the conversion of mutual helicity into self helicities by reconnection. It is shown that the principle of energy conservation when combined with the production of magnetic twist puts a low limit on the relative angle between two flux tubes for reconnect ion to occur. I provide several observations supporting the magnetic twist generation by reconnection, and discuss its physical implications for the origin of magnetic twist on the solar surface and the problem of coronal heating.