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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 12/119
111
- Chang, Seo-Won
- Journal of the Korean astronomical society = 천문학회지
- 51, n.5
- pp.129-142
- 2018
- 원문 바로보기
The DEEP-South (the Deep Ecliptic Patrol of the Southern Sky) photometric census of small Solar System bodies produces massive time-series data of variable, transient or moving objects as a by-product. To fully investigate unexplored variable phenomena, we present an application of multi-aperture photometry and FastBit indexing techniques for faster access to a portion of the DEEP-South year-one data. Our new pipeline is designed to perform automated point source detection, robust high-precision photometry and calibration of non-crowded fields which have overlap with previously surveyed areas. In this paper, we show some examples of catalog-based variability searches to find new variable stars and to recover targeted asteroids. We discover 21 new periodic variables with period ranging between 0.1 and 31 days, including four eclipsing binary systems (detached, over-contact, and ellipsoidal variables), one white dwarf/M dwarf pair candidate, and rotating variable stars. We also recover astrometry (< <TEX>${\pm}1-2$</TEX> arcsec level accuracy) and photometry of two targeted near-earth asteroids, 2006 DZ169 and 1996 SK, along with the small- (~0.12 mag) and relatively large-amplitude (~0.5 mag) variations of their dominant rotational signals in R-band.
112
- 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.
113
- 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.
114
- 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.
115
- Kim, Chunglee
- Journal of the Korean astronomical society = 천문학회지
- 51, n.5
- pp.165-170
- 2018
- 원문 바로보기
The Galactic Center is one of the most dense stellar environments in the Galaxy and is considered to be a plausible place to harbor many neutron stars. In this brief review, we summarize observational efforts in search of neutron stars within a few degrees about the Galactic Center. Up to 10% of Galactic neutron stars may reside in this central region and it is possible that more than a thousand neutron stars are located within only ~ 2500 (<TEX>${\leq}1pc$</TEX>) about the Galactic Center. Based on observations, we discuss prospects of detecting neutron stars in the Galactic Center via gravitational waves as well as electromagnetic waves.
116
- Hui, Chung-Yue
- Journal of the Korean astronomical society = 천문학회지
- 51, n.6
- pp.171-183
- 2018
- 원문 바로보기
To celebrate the tenth anniversary since the launch of Fermi Gamma-ray Space Telescope, we take a retrospect to a series of breakthroughs Fermi has contributed to pulsar astronomy in the last decade. Apart from significantly enlarging the population of <TEX>${\gamma}$</TEX>-ray pulsars, observations with the Large Area Telescope onboard Fermi also show the population is not homogeneous. Instead, many classes and sub-classes have been revealed. In this paper, we will review the properties of different types of <TEX>${\gamma}$</TEX>-ray pulsars, including radio-quiet <TEX>${\gamma}$</TEX>-ray pulsars, millisecond pulsars, <TEX>${\gamma}$</TEX>-ray binaries. Also, we will discuss the prospects of pulsar astronomy in the high energy regime.
117
- Seo, Hyunjong
- Journal of the Korean astronomical society = 천문학회지
- 51, n.3
- pp.49-63
- 2018
- 원문 바로보기
We carry out a study of Sub-Millimeter Galaxies (SMGs) in the AKARI NEP-Deep field using the James Clerk Maxwell Telescope (JCMT) SCUBA-2 <TEX>$850{\mu}m$</TEX> source catalog, released as part of the SCUBA-2 Cosmology Legacy Survey (S2CLS) program. The SCUBA-2 <TEX>$850{\mu}m$</TEX> map has a root mean square (rms) noise of <TEX>$1.2mJy\;beam^{-1}$</TEX> and covers an area of <TEX>$0.60degree^2$</TEX>. We find four SMGs which have counterparts to Herschel sources with spectroscopic redshifts in the literature. In addition, three dust obscured galaxies (DOGs) detected in Herschel bands are selected as a comparison sample. We derive IR luminosities of SMGs using the CIGALE code, which are similar to those of high redshift SMGs from previous studies. The contribution of AGN to the total IR luminosity in SMGs (2%-11%) is smaller than the lower limit for the one in DOGs (19%-35%), which is consistent with the expectation from the evolutionary scenario of massive galaxies. We search for SMGs in overdense regions as protocluster candidates and investigate four regions, including candidates around three DOGs. Finally, we argue that follow-up spectroscopic observation for the NEP-Deep field will provide crucial information to understand the role of SMGs in the evolution of massive galaxies.
118
- Yoo, Sung-Moon
- Journal of the Korean astronomical society = 천문학회지
- 51, n.5
- pp.143-153
- 2018
- 원문 바로보기
We present the first results of the invariant point (IVP) coordinates of the KVN Ulsan and Tamna radio telescopes. To determine the IVP coordinates in the geocentric frame (ITRF2014), a coordinate transformation method from the local frame, in which it is possible to survey using the optical instrument, to the geocentric frame was adopted. The least-square circles are fitted in three dimensions using the Gauss-Newton method to determine the azimuth and elevation axes in the local frame. The IVP in the local frame is defined as the mean value of the intersection points of the azimuth axis and the orthogonal vector between the azimuth and elevation axes. The geocentric coordinates of the IVP are determined by obtaining the seven transformation parameters between the local frame and the east-north-up (ENU) geodetic frame. The axis-offset between the azimuth and elevation axes is also estimated. To validate the results, the variation of coordinates of the GNSS station installed at KVN Ulsan was compared to the movement of the IVP coordinates over 9 months, showing good agreement in both magnitude and direction. This result will provide an important basis for geodetic and astrometric applications.
119
- Hwang, K.H.
- Journal of the Korean astronomical society = 천문학회지
- 51, n.6
- pp.197-206
- 2018
- 원문 바로보기
We present the analysis of KMT-2016-BLG-0212, a low flux-variation (<TEX>$I_{flux-var}{\sim}20mag$</TEX>) microlensing event, which is in a high-cadence (<TEX>${\Gamma}=4hr^{-1}$</TEX>) field of the three-telescope Korea Microlensing Telescope Network (KMTNet) survey. The event shows a short anomaly that is incompletely covered due to the brief visibility intervals that characterize the early microlensing season when the anomaly occurred. We show that the data are consistent with two classes of solutions, characterized respectively by low-mass brown-dwarf (q = 0.037) and sub-Neptune (q < <TEX>$10^{-4}$</TEX>) companions. Future high-resolution imaging should easily distinguish between these solutions.
120
- 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.