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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 14/122
131
- Kim, Joonho
- Journal of the Korean astronomical society = 천문학회지
- 51, n.4
- pp.89-110
- 2018
- 원문 바로보기
Active Galactic Nucleus (AGN) variability can be used to study the physics of the region in the vicinity of the central black hole. In this paper, we investigated intra-night optical variability of AGN in the COSMOS field in order to understand the AGN instability at the smallest scale. Observations were performed using the KMTNet on three separate nights for 2.5 to 5 hours at a cadence of 20 to 30 min. We find that the observation enables the detection of short-term variability as small as ~ 0.02 and 0.1 mag for R ~ 18 and 20 mag sources, respectively. Using four selection methods (X-rays, mid-infrared, radio, and matching with SDSS quasars), 394 AGN are detected in the <TEX>$4deg^2$</TEX> field of view. After differential photometry and <TEX>${\chi}^2$</TEX>-test, we classify intra-night variable AGN. The fraction of variable AGN (0-8%) is statistically consistent with a null result. Eight out of 394 AGN are found to be intra-night variable in two filters or two nights with a variability level of 0.1 mag, suggesting that they are strong candidates for intra-night variable AGN. Still they represent a small population (2%). There is no sub-category of AGN that shows a statistically significant intra-night variability.
132
- Seo, Jeongbhin
- Journal of the Korean astronomical society = 천문학회지
- 51, n.2
- pp.37-48
- 2018
- 원문 바로보기
Massive stars blow powerful stellar winds throughout their evolutionary stages from the main sequence to Wolf-Rayet phases. The amount of mechanical energy deposited in the interstellar medium by the wind from a massive star can be comparable to the explosion energy of a core-collapse supernova that detonates at the end of its life. In this study, we estimate the kinetic energy deposition by massive stars in our Galaxy by considering the integrated Galactic initial mass function and modeling the stellar wind luminosity. The mass loss rate and terminal velocity of stellar winds during the main sequence, red supergiant, and Wolf-Rayet stages are estimated by adopting theoretical calculations and observational data published in the literature. We find that the total stellar wind luminosity due to all massive stars in the Galaxy is about <TEX>${\mathcal{L}}_w{\approx}1.1{\times}10^{41}erg\;s^{-1}$</TEX>, which is about 1/4 of the power of supernova explosions, <TEX>${\mathcal{L}}_{SN}{\approx}4.8{\times}10^{41}erg\;s^{-1}$</TEX>. If we assume that ~ 1 - 10 % of the wind luminosity could be converted to Galactic cosmic rays (GCRs) through collisonless shocks such as termination shocks in stellar bubbles and superbubbles, colliding-wind shocks in binaries, and bow-shocks of massive runaway stars, stellar winds might be expected to make a significant contribution to GCR production, though lower than that of supernova remnants.
133
- 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.
134
- Kim, Jung-Hee
- Journal of the Korean astronomical society = 천문학회지
- 51, n.4
- pp.119-127
- 2018
- 원문 바로보기
We examine whether the solar eclipse effect is dependent on the geographic conditions under which the geomagnetic field variations are recorded. We concentrate our attention on the dependence of the solar eclipse effect on a number of factors, including, the magnitude of a solar eclipse (defined as the fraction of the angular diameter of the Sun being eclipsed), the magnetic latitude of the observatory, the duration of the observed solar eclipse at the given geomagnetic observatory, and the location of the geomagnetic observatory in the path of the Moon's shadow. We analyze an average of the 207 geomagnetic field variation data sets observed by 100 INTERMAGNET geomagnetic nodes, during the period from 1991 to 2016. As a result, it is demonstrated that (1) the solar eclipse effect on the geomagnetic field, i.e., an increase in the Y component and decreases in the X, Z and F componenets, becomes more distinct as the magnitude of solar eclipse increases, (2) the solar eclipse effect is most conspicuous when the modulus of the magnetic latitude is between <TEX>$30^{\circ}$</TEX> and <TEX>$50^{\circ}$</TEX>, (3) the more slowly Moon's shadow passes the geomagnetic observatory, the more clear the solar eclipse effect, (4) the geomagnetic observatory located in the latter half of the path of Moon's shadow with respect to the position of the greatest eclipse is likely to observe a more clear signal. Finally, we conclude by stressing the importance of our findings.
135
- Pak, Sungmin
- Journal of the Korean astronomical society = 천문학회지
- 51, n.4
- pp.111-117
- 2018
- 원문 바로보기
A numerical method is proposed to calculate the response of detectors measuring particle energies from incident isotropic fluxes of electrons and positive ions. The isotropic flux is generated by injecting particles moving radially inward on a hypothetical, spherical surface encompassing the detectors. A geometric projection of the field-of-view from the detectors onto the spherical surface allows for the identification of initial positions and momenta corresponding to the clear field-of-view of the detectors. The contamination of detector responses by particles penetrating through, or scattering off, the structure is also similarly identified by tracing the initial positions and momenta of the detected particles. The relative contribution from the contaminating particles is calculated using GEANT4 to obtain the geometric factor of the instrument as a function of the energy. This calculation clearly shows that the geometric factor is a strong function of incident particle energies. The current investigation provides a simple and decisive method to analyze the instrument geometric factor, which is a complicated function of contributions from the anticipated field-of-view particles, together with penetrating or scattered particles.
136
- 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.
137
- 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.
138
- 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.
139
- 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.
140
- 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.