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1968년 ~ 2025년까지 1,227 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 : 1,227건 (DB Construction : 1,227 Articles)
총 게시글 1,227건
페이지 19/123
181
- SUH, KYUNG-WON
- Journal of the Korean astronomical society = 천문학회지
- 49, n.4
- pp.127-136
- 2016
- 원문 바로보기
We investigate optical properties of amorphous alumina (Al2O3) dust grains in the envelopes around O-rich asymptotic giant branch (AGB) stars using laboratory measured optical data. We derive the optical constants of amorphous alumina over a wide wavelength range that satisfy the Kramers-Kronig relation and reproduce the laboratory data. Using the amorphous alumina and silicate dust, we compare the radiative transfer model results with the observed spectral energy distributions. Comparing the theoretical models with observations on various IR two-color diagrams for a large sample of O-rich AGB stars, we find that the amorphous alumina dust (about 10-40%) mixed with amorphous silicate better models the observed points for the O-rich AGB stars with thin dust envelopes.
182
- Lee, Yongung
- Journal of the Korean astronomical society = 천문학회지
- 49, n.6
- pp.255-259
- 2016
- 원문 바로보기
We estimate the fractal dimension of the <TEX>${\rho}$</TEX> Ophiuchus Molecular Cloud Complex, associated with star forming regions. We selected a cube (<TEX>${\upsilon}$</TEX>, l, b) database, obtained with J = 1-0 transition lines of <TEX>$^{12}CO$</TEX> and <TEX>$^{13}CO$</TEX> at a resolution of 22' using a multibeam receiver system on the 14-m telescope of the Five College Radio Astronomy Observatory. Using a code developed within IRAF, we identified slice-clouds with two threshold temperatures to estimate the fractal dimension. With threshold temperatures of 2.25 K (<TEX>$3{\sigma}$</TEX>) and 3.75 K (<TEX>$5{\sigma}$</TEX>), the fractal dimension of the target cloud is estimated to be D = 1.52-1.54, where <TEX>$P{\propto}A^{D/2}$</TEX>, which is larger than previous results. We suggest that the sampling rate (spatial resolution) of observed data must be an important parameter when estimating the fractal dimension, and that narrower or wider dispersion around an arbitrary fit line and the intercepts at NP = 100 should be checked whether they relate to firms noise level or characteristic structure of the target cloud. This issue could be investigated by analysing several high resolution databases with different quality (low or moderate sensitivity).
183
- Lee, Jae-Woo
- Journal of the Korean astronomical society = 천문학회지
- 49, n.6
- pp.289-293
- 2016
- 원문 바로보기
We investigate the long-term spatial drift of the center and the temporal variation of the shutter delay time map of Y4KCam mounted on the CTIO 1.0 m telescope. We have collected shutter delay time maps over eight years as a part of our long-term survey program. We find that the center of the shutter delay time map can drift up to <TEX>$450{\mu}m$</TEX>, equivalent to <TEX>${\approx}30pixels$</TEX>, on the CCD. This effect can result in a small amount of error in integration time without the proper shutter delay time correction, but it does not appear to cause any significant problems in photometric measurements. We obtain a mean shutter delay time of <TEX>$69.1{\pm}0.8$</TEX> ms and find no temporal variation of the shutter delay time of Y4KCam over eight years, indicative of the mechanical stability of the shutter. We suggest that using a master shutter delay time correction frame would be sufficient to achieve high precision photometry, which does not exceed photometric errors <TEX>${\approx}1.7mmag$</TEX> across the CCD frame for exposure times longer than 1 s.
184
- KANG, HYESUNG
- Journal of the Korean astronomical society = 천문학회지
- 49, n.3
- pp.83-92
- 2016
- 원문 바로보기
The Toothbrush radio relic associated with the merging cluster 1RXS J060303.3 is presumed to be produced by relativistic electrons accelerated at merger-driven shocks. Since the shock Mach number inferred from the observed radio spectral index, M<sub>radio</sub> ≈ 2.8, is larger than that estimated from X-ray observations, M<sub>X</sub> ≲ 1.5, we consider the re-acceleration model in which a weak shock of M<sub>s</sub> ≈ 1.2 - 1.5 sweeps through the intracluster plasma with a preshock population of relativistic electrons. We find the models with a power-law momentum spectrum with the slope, s ≈ 4.6, and the cutoff Lorentz factor, γ<sub>e,c</sub> ≈ 7-8×10<sup>4</sup> can reproduce reasonably well the observed profiles of radio uxes and integrated radio spectrum of the head portion of the Toothbrush relic. This study confirms the strong connection between the ubiquitous presence of fossil relativistic plasma originated from AGNs and the shock-acceleration model of radio relics in the intracluster medium.
185
- KIM, SEUNG-LEE
- Journal of the Korean astronomical society = 천문학회지
- 49, n.5
- pp.199-208
- 2016
- 원문 바로보기
We present photometric results of the δ Sct star V1162 Ori, which is extensively monitored for a total of 49 nights from mid-December 2014 to early-March 2015. The observations are made with three KMTNet (Korea Microlensing Telescope Network) 1.6 m telescopes installed in Chile, South Africa, and Australia. Multiple frequency analysis is applied to the data and resulted in clear detection of seven frequencies without an alias problem: five known frequencies and two new ones with small amplitudes of 1.2-1.7 mmag. The amplitudes of all but one frequency are significantly different from previous results, confirming the existence of long-term amplitude changes. We examine the variations in pulsation timings of V1162 Ori for about 30 years by using the times of maximum light obtained from our data and collected from the literatures. The O − C (Observed minus Calculated) timing diagram shows a combination of a downward parabolic variation with a period decreasing rate of (1/P)dP/dt = −4.22 × 10<sup>−6</sup> year<sup>−1</sup> and a cyclic change with a period of about 2780 days. The most probable explanation for this cyclic variation is the light-travel-time effect caused by an unknown binary companion, which has a minimum mass of 0.69 M<sub>⊙</sub>. V1162 Ori is the first δ Sct-type pulsating star of which the observed fast period decrease can be interpreted as an evolutionary effect of a pre-main sequence star, considering its membership of the Orion OB 1c association.
186
- Ann, Hong Bae
- Journal of the Korean astronomical society = 천문학회지
- 49, n.6
- pp.239-253
- 2016
- 원문 바로보기
We determined the warp parameters of 192 warped galaxies which are selected from 340 edge-on galaxies using color images as well as r-band isophotal maps. We derive the local background density (<TEX>${\Sigma}_n$</TEX>) to examine the dependence of the warp amplitudes on the galaxy environment. We find a clear trend that strongly warped galaxies are likely to be found in high density regions where tidal interactions are supposed to be frequent. However, the correlation between <TEX>${\alpha}_{\omega}$</TEX> and <TEX>${\Sigma}_n$</TEX> is too weak for weakly warped galaxies (<TEX>${\alpha}_{\omega}$</TEX> < <TEX>$4^{\circ}$</TEX>) and the cumulative distributions of weakly warped galaxies are not significantly different from those of galaxies with no detectable warps. This suggests that tidal interactions do not play a decisive role in the formation of weak warps.
187
- LEE, ENSANG
- Journal of the Korean astronomical society = 천문학회지
- 49, n.2
- pp.59-64
- 2016
- 원문 바로보기
In this study, we investigate the kinetic properties of magnetic decreases observed in the solar wind at ~1 AU using the Cluster observations. We study two different magnetic decreases: one with a short observation duration of ~2.5 minutes and stable structure and the other with a longer observation duration of ~40 minutes and some fluctuations and substructures. Despite the contrast in durations and magnetic structures, the velocity space distributions of ions are similar in both events. The velocity space distribution becomes more anisotropic along the direction parallel to the magnetic field, which differs from observations obtained at high heliographic latitudes. On the other hand, electrons show different features from the ions. The core component of the electrons shows similar anisotropy to the ions, though the anisotropy is much weaker. However, while ions are heated in the magnetic decreases, the core electrons are slightly cooled, especially in the perpendicular direction. The halo component does not change much in the magnetic decreases from the ambient solar wind. The strahl component is observed only in one of the magnetic decreases. The results imply that the ions and electrons in the magnetic decreases can behave differently, which should be considered for the formation mechanism of the magnetic decreases.
188
- KIM, SEUNG-LEE
- Journal of the Korean astronomical society = 천문학회지
- 49, n.1
- pp.37-44
- 2016
- 원문 바로보기
The Korea Microlensing Telescope Network (KMTNet) is a wide-field photometric system installed by the Korea Astronomy and Space Science Institute (KASI). Here, we present the overall technical specifications of the KMTNet observation system, test observation results, data transfer and image processing procedure, and finally, the KMTNet science programs. The system consists of three 1.6 m wide-field optical telescopes equipped with mosaic CCD cameras of 18k by 18k pixels. Each telescope provides a 2.0 by 2.0 square degree field of view. We have finished installing all three telescopes and cameras sequentially at the Cerro-Tololo Inter-American Observatory (CTIO) in Chile, the South African Astronomical Observatory (SAAO) in South Africa, and the Siding Spring Observatory (SSO) in Australia. This network of telescopes, which is spread over three different continents at a similar latitude of about -30 degrees, enables 24-hour continuous monitoring of targets observable in the Southern Hemisphere. The test observations showed good image quality that meets the seeing requirement of less than 1.0 arcsec in I-band. All of the observation data are transferred to the KMTNet data center at KASI via the international network communication and are processed with the KMTNet data pipeline. The primary scientific goal of the KMTNet is to discover numerous extrasolar planets toward the Galactic bulge by using the gravitational microlensing technique, especially earth-mass planets in the habitable zone. During the non-bulge season, the system is used for wide-field photometric survey science on supernovae, asteroids, and external galaxies.
189
- KANG, HYESUNG
- Journal of the Korean astronomical society = 천문학회지
- 49, n.4
- pp.145-155
- 2016
- 원문 바로보기
The Sausage radio relic is the arc-like radio structure in the cluster CIZA J2242.8+5301, whose observed properties can be best understood by synchrotron emission from relativistic electrons accelerated at a merger-driven shock. However, there remain a few puzzles that cannot be explained by the shock acceleration model with only in-situ injection. In particular, the Mach number inferred from the observed radio spectral index, M<sub>radio</sub> ≈ 4.6, while the Mach number estimated from X-ray observations, M<sub>X−ray</sub> ≈ 2.7. In an attempt to resolve such a discrepancy, here we consider the re-acceleration model in which a shock of M<sub>s</sub> ≈ 3 sweeps through the intracluster gas with a pre-existing population of relativistic electrons. We find that observed brightness profiles at multi frequencies provide strong constraints on the spectral shape of pre-existing electrons. The models with a power-law momentum spectrum with the slope, s ≈ 4.1, and the cutoff Lorentz factor, γ<sub>e,c</sub> ≈ 3−5×10<sup>4</sup>, can reproduce reasonably well the observed spatial profiles of radio fluxes and integrated radio spectrum of the Sausage relic. The possible origins of such relativistic electrons in the intracluster medium remain to be investigated further.
190
- GOULD, ANDREW
- Journal of the Korean astronomical society = 천문학회지
- 49, n.4
- pp.123-126
- 2016
- 원문 바로보기
Microlensing is generally thought to probe planetary systems only out to a few Einstein radii. Microlensing events generated by bound planets beyond about 10 Einstein radii generally do not yield any trace of their hosts, and so would be classified as free floating planets (FFPs). I show that it is already possible, using adaptive optics (AO), to constrain the presence of potential hosts to FFP candidates at separations comparable to the Oort Cloud. With next-generation telescopes, planets at Kuiper-Belt separations can be probed. Next generation telescopes will also permit routine vetting for all FFP candidates, simply by obtaining second epochs 4-8 years after the event. At present, the search for such hosts is restricted to within the 'confusion limit' of θ<sub>confus</sub> ∼ 0.25′′, but future WFIRST (Wide Field Infrared Survey Telescope) observations will allow one to probe beyond this confusion limit as well.