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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
- Ann, Hong Bae
- Journal of the Korean astronomical society = 천문학회지
- 51, n.4
- pp.73-88
- 2018
- 원문 바로보기
We present a sample of 54 disk galaxies which have well developed extraplanar structures. We selected them using visual inspections from the color images of the Sloan Digital Sky Survey. Since the sizes of the extraplanar structures are comparable to the disks, they are considered as prominent stellar halos rather than large bulges. A single <TEX>$S{\acute{e}}rsic$</TEX> profile fitted to the surface brightness along the minor-axis of the disk shows a luminosity excess in the central regions for the majority of sample galaxies. This central excess is considered to be caused by the central bulge component. The mean <TEX>$S{\acute{e}}rsic$</TEX> index of the single component model is <TEX>$1.1{\pm}0.9$</TEX>. A double <TEX>$S{\acute{e}}rsic$</TEX> profile model that employs n = 1 for the inner region, and varying n for the outer region, provides a better fit than the single <TEX>$S{\acute{e}}rsic$</TEX> profile model. For a small fraction of galaxies, a <TEX>$S{\acute{e}}rsic$</TEX> profile fitted with n = 4 for the inner region gives similar results. There is a weak tendency of increasing n with increasing luminosity and central velocity dispersion, but there is no dependence on the local background density.
132
- Zheng, Yanfang
- Journal of the Korean astronomical society = 천문학회지
- 51, n.3
- pp.65-71
- 2018
- 원문 바로보기
The near real-time speckle masking reconstruction technique has been developed to accelerate the processing of solar images to achieve high resolutions for ground-based solar telescopes. However, the reconstruction of solar subimages in such a speckle reconstruction is very time-consuming. We design and implement a new parallel speckle masking reconstruction algorithm based on the Compute Unified Device Architecture (CUDA) on General Purpose Graphics Processing Units (GPGPU). Tests are performed to validate the correctness of our program on NVIDIA GPGPU. Details of several parallel reconstruction steps are presented, and the parallel implementation between various modules shows a significant speed increase compared to the previous serial implementations. In addition, we present a comparison of runtimes across serial programs, the OpenMP-based method, and the new parallel method. The new parallel method shows a clear advantage for large scale data processing, and a speedup of around 9 to 10 is achieved in reconstructing one solar subimage of <TEX>$256{\times}256pixels$</TEX>. The speedup performance of the new parallel method exceeds that of OpenMP-based method overall. We conclude that the new parallel method would be of value, and contribute to real-time reconstruction of an entire solar image.
133
- Suh, Kyung-Won
- Journal of the Korean astronomical society = 천문학회지
- 51, n.5
- pp.155-164
- 2018
- 원문 바로보기
We present various infrared two-color diagrams (2CDs) using WISE data for asymptotic giant branch (AGB) stars and Planetary Nebulae (PNe) and investigate possible evolutionary tracks. We use the sample of 5036 AGB stars, 660 post-AGB stars, and 2748 PNe in our Galaxy. For each object, we cross-identify the IRAS, AKARI, WISE, and 2MASS counterparts. To investigate the spectral evolution from AGB stars to PNe, we compare the theoretical model tracks of AGB stars and post-AGB stars with the observations on the IR 2CDs. We find that the theoretical dust shell model tracks can roughly explain the observations of AGB stars, post-AGB stars, and PNe on the various IR 2CDs. WISE data are useful in studying the evolution of AGB stars and PNe, especially for dim objects. We find that most observed color indices generally increase during the evolution from AGB stars to PNe. We also find that <TEX>$Fe_{0.9}Mg_{0.1}O$</TEX> dust is useful to fit the observed WISE W3-W4 colors for O-rich AGB stars with thin dust shells.
134
- 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.
135
- 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.
136
- 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.
137
- 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.
138
- 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.
139
- 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.
140
- 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.