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1968년 ~ 2024년까지 1,211 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 : 1,211건 (DB Construction : 1,211 Articles)
총 게시글 1,211건
페이지 15/122
141
- Chang, Seok-Jun
- Journal of the Korean astronomical society = 천문학회지
- 51, n.1
- pp.5-16
- 2018
- 원문 바로보기
We investigate the escape of <TEX>$Ly{\beta}$</TEX> from emission nebulae with a significant population of excited hydrogen atoms in the level n = 2, rendering them optically thick in <TEX>$H{\alpha}$</TEX>. The transfer of <TEX>$Ly{\beta}$</TEX> line photons in these optically thick regions is complicated by the presence of another scattering channel leading to re-emission of <TEX>$H{\alpha}$</TEX>, alternating their identities between <TEX>$Ly{\beta}$</TEX> and <TEX>$H{\alpha}$</TEX>. In this work, we develop a Monte Carlo code to simulate the transfer of <TEX>$Ly{\beta}$</TEX> line photons incorporating the scattering channel into <TEX>$H{\alpha}$</TEX>. Both <TEX>$H{\alpha}$</TEX> and <TEX>$Ly{\beta}$</TEX> lines are formed through diffusion in frequency space, where a line photon enters the wing regime after a fairly large number of resonance scatterings with hydrogen atoms. Various line profiles of <TEX>$H{\alpha}$</TEX> and <TEX>$Ly{\beta}$</TEX> emergent from our model nebulae are presented. It is argued that the electron temperature is a critical parameter which controls the flux ratio of emergent <TEX>$Ly{\beta}$</TEX> and <TEX>$H{\alpha}$</TEX>. Specifically for <TEX>$T\;=\;3{\times}10^4\;K$</TEX> and <TEX>$H{\alpha}$</TEX> line center optical depth <TEX>$\tau{\alpha}\;=\;10$</TEX>, the number flux ratio of emergent <TEX>$Ly{\beta}$</TEX> and <TEX>$H{\alpha}$</TEX> is ~ 49 percent, which is quite significant. We propose that the leaking <TEX>$Ly{\beta}$</TEX> can be an interesting source for the formation of <TEX>$H{\alpha}$</TEX> wings observed in many symbiotic stars and active galactic nuclei. Similar broad <TEX>$H{\alpha}$</TEX> wings are also expected in <TEX>$Ly{\alpha}$</TEX> emitting halos found in the early universe, which can be potentially probed by the James Webb Telescope in the future.
142
- Bang, Tae-Yang
- Journal of the Korean astronomical society = 천문학회지
- 51, n.1
- pp.17-25
- 2018
- 원문 바로보기
Detecting exoplanets around giant stars sheds light on the later-stage evolution of planetary systems. We observed the M giant HD 18438 and the K giant HD 158996 as part of a Search for Exoplanets around Northern circumpolar Stars (SENS) and obtained 38 and 24 spectra from 2010 to 2017 using the high-resolution Bohyunsan Observatory Echelle Spectrograph (BOES) at the 1.8m telescope of Bohyunsan Optical Astronomy Observatory in Korea. We obtained precise RV measurements from the spectra and found long-period radial velocity (RV) variations with period 719.0 days for HD 18438 and 820.2 days for HD 158996. We checked the chromospheric activities using Ca <TEX>$\text\tiny{II}$</TEX> H and <TEX>$H{\alpha}$</TEX> lines, HIPPARCOS photometry and line bisectors to identify the origin of the observed RV variations. In the case of HD 18438, we conclude that the observed RV variations with period 719.0 days are likely to be caused by the pulsations because the periods of HIPPARCOS photometric and <TEX>$H{\alpha}$</TEX> EW variations for HD 18438 are similar to that of RV variations in Lomb-Scargle periodogram, and there are no correlations between bisectors and RV measurements. In the case of HD 158996, on the other hand, we did not find any similarity in the respective periodograms nor any correlation between RV variations and line bisector variations. In addition, the probability that the real rotational period can be as longer than the RV period for HD 158996 is only about 4.3%. Thus we conclude that observed RV variations with a period of 820.2 days of HD 158996 are caused by a planetary companion, which has the minimum mass of 14.0 <TEX>$M_{Jup}$</TEX>, the semi-major axis of 2.1 AU, and eccentricity of 0.13 assuming the stellar mass of <TEX>$1.8 M_{\odot}$</TEX>. HD 158996 is so far one of the brightest and largest stars to harbor an exoplanet candidate.
143
- Yang, Heesu
- Journal of the Korean astronomical society = 천문학회지
- 51, n.2
- pp.27-36
- 2018
- 원문 바로보기
In a solar coronagraph, the most important component is an occulter to block the direct light from the disk of the sun. Because the intensity of the solar outer corona is <TEX>$10^{-6}$</TEX> to <TEX>$10^{-10}$</TEX> times of that of the solar disk (<TEX>$I_{\odot}$</TEX>), it is necessary to minimize scattering at the optical elements and diffraction at the occulter. Using a Fourier optic simulation and a stray light test, we investigated the performance of a compact coronagraph that uses an external truncated-cone occulter without an internal occulter and Lyot stop. In the simulation, the diffracted light was minimized to the order of <TEX>$7.6{\times}10^{-10}I_{\odot}$</TEX> when the cone angle <TEX>${\theta}_c$</TEX> was about <TEX>$0.39^{\circ}$</TEX>. The performance of the cone occulter was then tested by experiment. The level of the diffracted light reached the order of <TEX>$6{\times}10^{-9}I_{\odot}$</TEX> at <TEX>${\theta}_c=0.40^{\circ}$</TEX>. This is sufficient to observe the outer corona without additional optical elements such as a Lyot stop or inner occulter. We also found the manufacturing tolerance of the cone angle to be <TEX>$0.05^{\circ}$</TEX>, the lateral alignment tolerance was <TEX>$45{\mu}m$</TEX>, and the angular alignment tolerance was <TEX>$0.043^{\circ}$</TEX>. Our results suggest that the physical size of coronagraphs can be shortened significantly by using a cone occulter.
144
- Cho, K.S.
- Journal of the Korean astronomical society = 천문학회지
- 50, n.5
- pp.139-149
- 2017
- 원문 바로보기
The Korea Astronomy and Space Science Institute plans to develop a coronagraph in collaboration with National Aeronautics and Space Administration (NASA) and to install it on the International Space Station (ISS). The coronagraph is an externally occulted one-stage coronagraph with a field of view from 3 to 15 solar radii. The observation wavelength is approximately 400 nm, where strong Fraunhofer absorption lines from the photosphere experience thermal broadening and Doppler shift through scattering by coronal electrons. Photometric filter observations around this band enable the estimation of 2D electron temperature and electron velocity distribution in the corona. Together with a high time cadence (<12 min) of corona images used to determine the geometric and kinematic parameters of coronal mass ejections, the coronagraph will yield the spatial distribution of electron density by measuring the polarized brightness. For the purpose of technical demonstration, we intend to observe the total solar eclipse in August 2017 with the filter system and to perform a stratospheric balloon experiment in 2019 with the engineering model of the coronagraph. The coronagraph is planned to be installed on the ISS in 2021 for addressing a number of questions (e.g., coronal heating and solar wind acceleration) that are both fundamental and practically important in the physics of the solar corona and of the heliosphere.
145
- Seo, Hyunjong
- Journal of the Korean astronomical society = 천문학회지
- 50, n.1
- pp.7-20
- 2017
- 원문 바로보기
We carry out the study of <TEX>$850{\mu}m$</TEX> sources in a part of the XMM-LSS field. The <TEX>$850{\mu}m$</TEX> imaging data were obtained by the SCUBA-2 on the James Clerk Maxwell Telescope (JCMT) for three days in July 2015 with an integration time of 6.1 hours, covering a circular area with a radius of 15'. We choose the central area up to a radius of 9'.15 for the study, where the noise distribution is relatively uniform. The root mean square (rms) noise at the center is 2.7 mJy. We identify 17 sources with S/N > 3.5. Differential number count is estimated in flux range between 3.5 and 9.0 mJy after applying various corrections derived by imaging simulations, which is consistent with previous studies. For detailed study on the individual sources, we select three sources with more reliable measurements (S/N > 4.5), and construct their spectral energy distributions (SEDs) from optical to far-infrared band. Redshift distribution of the sources ranges from 0.36 to 3.28, and their physical parameters are extracted using MAGPHYS model, which yield infrared luminosity <TEX>$L_{IR}=10^{11.3}-10^{13.4}L_{\odot}$</TEX>, star formation rate <TEX>$SFR=10^{1.3}-10^{3.2}M_{\odot}yr^{-1}$</TEX> and dust temperature <TEX>$T_D=30-53K$</TEX>. We investigate the correlation between <TEX>$L_{IR}$</TEX> and <TEX>$T_D$</TEX>, which appears to be consistent with previous studies.
146
- Lee, Hee-Jae
- Journal of the Korean astronomical society = 천문학회지
- 50, n.3
- pp.41-49
- 2017
- 원문 바로보기
We conduct BVRI and R band photometric observations of asteroid (5247) Krylov from January 2016 to April 2016 for 51 nights using the Korea Microlensing Telescope Network (KMTNet). The color indices of (5247) Krylov at the light curve maxima are determined as <TEX>$B-V=0.841{\pm}0.035$</TEX>, <TEX>$V-R=0.418{\pm}0.031$</TEX>, and <TEX>$V-I=0.871{\pm}0.031$</TEX> where the phase angle is <TEX>$14.1^{\circ}$</TEX>. They are acquired after the standardization of BVRI instrumental measurements using the ensemble normalization technique. Based on the color indices, (5247) Krylov is classified as a S-type asteroid. Double periods, that is, a primary period <TEX>$P_1=82.188{\pm}0.013h$</TEX> and a secondary period <TEX>$P_2=67.13{\pm}0.20h$</TEX> are identified from period searches of its R band light curve. The light curve phases with <TEX>$P_1$</TEX> and this indicate that it is a typical Non-Principal Axis (NPA) asteroid. We discuss the possible causes of its NPA rotation.
147
- Lee, Hyun-Uk
- Journal of the Korean astronomical society = 천문학회지
- 50, n.3
- pp.51-59
- 2017
- 원문 바로보기
The presence of blue stragglers pose challenges to standard stellar evolution theory, in the sense that explaining their presence demands a complex interplay between stellar evolution and cluster dynamics. In the meantime, mass transfer in binary systems and stellar collisions are widely studied as a blue straggler formation channel. We explore properties of the Galactic open clusters where blue stragglers are found, in attempting to estimate the relative importance of these two favored processes, by comparing them with those resulting from open clusters in which blue stragglers are absent as of now. Unlike previous studies which require a sophisticated process in understanding the implication of the results, this approach is straightforward and has resulted in a supplementary supporting evidence for the current view on the blue straggler formation mechanism. Our main findings are as follows: (1) Open clusters in which blue stragglers are present have a broader distribution with respect to the Z-axis pointing towards the North Galactic Pole than those in which blue stragglers are absent. The probability that two distributions with respect to the Z-axis are drawn from the same distribution is 0.2%. (2) Average values of <TEX>$log_10(t)$</TEX> of the clusters with blue stragglers and those without blue stragglers are <TEX>$8.58{\pm}0.232$</TEX> and <TEX>$7.52{\pm}0.285$</TEX>, respectively. (3) The clusters with blue stragglers tend to be relatively redder than the others, and are distributed broader in colors. (4) The clusters with blue stragglers are likely brighter than those without blue stragglers. (5) Finally, blue stragglers seem to form in condensed clusters rather than simply dense clusters. Hence, we conclude that mass transfer in binaries seems to be a relatively important physical mechanism of the generation of blue stragglers in open clusters, provided they are sufficiently old.
148
- Chae, Jongchul
- Journal of the Korean astronomical society = 천문학회지
- 50, n.2
- pp.21-27
- 2017
- 원문 바로보기
The autoregressive method provides a univariate procedure to predict the future sunspot number (SSN) based on past record. The strength of this method lies in the possibility that from past data it yields the SSN in the future as a function of time. On the other hand, its major limitation comes from the intrinsic complexity of solar magnetic activity that may deviate from the linear stationary process assumption that is the basis of the autoregressive model. By analyzing the residual errors produced by the method, we have obtained the following conclusions: (1) the optimal duration of the past time for the forecast is found to be 8.5 years; (2) the standard error increases with prediction horizon and the errors are mostly systematic ones resulting from the incompleteness of the autoregressive model; (3) there is a tendency that the predicted value is underestimated in the activity rising phase, while it is overestimated in the declining phase; (5) the model prediction of a new Solar Cycle is fairly good when it is similar to the previous one, but is bad when the new cycle is much different from the previous one; (6) a reasonably good prediction of a new cycle can be made using the AR model 1.5 years after the start of the cycle. In addition, we predict the next cycle (Solar Cycle 25) will reach the peak in 2024 at the activity level similar to the current cycle.
149
- Kim, Sujin
- Journal of the Korean astronomical society = 천문학회지
- 50, n.4
- pp.125-129
- 2017
- 원문 바로보기
We investigate the solar cycle variation of microwave and extreme ultraviolet (EUV) intensity in latitude to compare microwave polar brightening (MPB) with the EUV polar coronal hole (CH). For this study, we used the full-sun images observed in 17 GHz of the Nobeyama Radioheliograph from 1992 July to 2016 November and in two EUV channels of the Atmospheric Imaging Assembly (AIA) <TEX>$193{\AA}$</TEX> and <TEX>$171{\AA}$</TEX> on the Solar Dynamics Observatory (SDO) from 2011 January to 2016 November. As a result, we found that the polar intensity in EUV is anti-correlated with the polar intensity in microwave. Since the depression of EUV intensity in the pole is mostly owing to the CH appearance and continuation there, the anti-correlation in the intensity implies the intimate association between the polar CH and the MPB. Considering the report of Gopalswamy et al. (1999) that the enhanced microwave brightness in the CH is seen above the enhanced photospheric magnetic field, we suggest that the pole area during the solar minimum has a stronger magnetic field than the quiet sun level and such a strong field in the pole results in the formation of the polar CH. The emission mechanism of the MPB and the physical link with the polar CH are not still fully understood. It is necessary to investigate the MPB using high resolution microwave imaging data, which can be obtained by the high performance large-array radio observatories such as the ALMA project.
150
- Choi, Changsu
- Journal of the Korean astronomical society = 천문학회지
- 50, n.3
- pp.71-78
- 2017
- 원문 바로보기
We present the characteristics and the performance of the new CCD camera system, SNUCAM-II (Seoul National University CAMera system II) that was installed on the Lee Sang Gak Telescope (LSGT) at the Siding Spring Observatory in 2016. SNUCAM-II consists of a deep depletion chip covering a wide wavelength from <TEX>$0.3{\mu}m$</TEX> to <TEX>$1.1{\mu}m$</TEX> with high sensitivity (QE at > 80% over 0.4 to <TEX>$0.9{\mu}m$</TEX>). It is equipped with the SDSS ugriz filters and 13 medium band width (50 nm) filters, enabling us to study spectral energy distributions (SEDs) of diverse objects from extragalactic sources to solar system objects. On LSGT, SNUCAM-II offers <TEX>$15.7{\times}15.7$</TEX> arcmin field-of-view (FOV) at a pixel scale of 0.92 arcsec and a limiting magnitude of g = 19.91 AB mag and z=18.20 AB mag at <TEX>$5{\sigma}$</TEX> with 180 sec exposure time for point source detection.