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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 10/119
91
- Algaba, Juan-Carlos
- Journal of the Korean astronomical society = 천문학회지
- 52, n.2
- pp.31-40
- 2019
- 원문 바로보기
A long standing problem in the study of Active Galactic Nuclei (AGNs) is that the observed VLBI core is in fact a blending of the actual AGN core (classically defined by the <TEX>${\tau}=1$</TEX> surface) and the upstream regions of the jet or optically thin flows. This blending may cause some biases in the observables of the core, such as its flux density, size or brightness temperature, which may lead to misleading interpretation of the derived quantities and physics. We study the effects of such blending under the view of the Korean VLBI Network (KVN) for a sample of AGNs at 43 GHz by comparing their observed properties with observations obtained using the Very Large Baseline Array (VLBA). Our results suggest that the observed core sizes are a factor ~ 11 larger than these of VLBA, which is similar to the factor expected by considering the different resolutions of the two facilities. We suggest the use of this factor to consider blending effects in KVN measurements. Other parameters, such as flux density or brightness temperature, seem to possess a more complicated dependence.
92
- Kim, Seungjong
- Journal of the Korean astronomical society = 천문학회지
- 52, n.5
- pp.173-180
- 2019
- 원문 바로보기
We present IR flux density measurements, models of the broadband SED, and results of SED modeling for the Pulsar Wind Nebula (PWN) 3C 58. We find that the Herschel flux density seems to be slightly lower than suggested by interpolation of previous measurements in nearby wavebands, implying that there may be multiple electron populations in 3C 58. We model the SED using a simple stationary one-zone and a more realistic time-evolving multi-zone scenario. The latter includes variations of flow properties in the PWN (injected energy, magnetic field, and bulk speed), radiative energy losses, adiabatic expansion, and diffusion, similar to previous PWN models. From the modeling, we find that a PWN age of 2900-5400 yrs is preferred and that there may be excess emission at <TEX>${\sim}10^{11}Hz$</TEX>. The latter may imply multiple populations of electrons in the PWN.
93
- Li, Yongchun
- Journal of the Korean astronomical society = 천문학회지
- 52, n.2
- pp.49-55
- 2019
- 원문 바로보기
The New Vacuum Solar Telescope (NVST) is the largest solar telescope in China. When using CCDs for imaging, equal-thickness fringes caused by thin-film interference can occur. Such fringes reduce the quality of NVST data but cannot be removed using standard flat fielding. In this paper, a correction method based on multi-scale decomposition and adaptive partitioning is proposed. The original image is decomposed into several sub-scales by multi-scale decomposition. The region containing fringes is found and divided by an adaptive partitioning method. The interference fringes are then filtered by a frequency-domain Gaussian filter on every partitioned image. Our analysis shows that this method can effectively remove the interference fringes from a solar image while preserving useful information.
94
- Sim, Gyuheon
- Journal of the Korean astronomical society = 천문학회지
- 52, n.5
- pp.145-158
- 2019
- 원문 바로보기
We conducted a survey of open clusters within 1 kpc from the Sun using the astrometric and photometric data of the Gaia Data Release 2. We found 655 cluster candidates by visual inspection of the stellar distributions in proper motion space and spatial distributions in l - b space. All of the 655 cluster candidates have a well defined main-sequence except for two candidates if we consider that the main sequence of very young clusters is somewhat broad due to differential extinction. Cross-matching of our 653 open clusters with known open clusters in various catalogs resulted in 207 new open clusters. We present the physical properties of the newly discovered open clusters. The majority of the newly discovered open clusters are of young to intermediate age and have less than ~50 member stars.
95
- Minh, Young Chol
- Journal of the Korean astronomical society = 천문학회지
- 52, n.3
- pp.83-88
- 2019
- 원문 바로보기
Using ALMA observations of the <TEX>$^{13}CN$</TEX> and DCN lines in the massive star-forming region G33.92+0.11A, we investigate the CN/HCN abundance ratio, which serves as a tracer of photodissociation chemistry, over the whole observed region. Even considering the uncertainties in calculating the abundance ratio, we find high ratios (<TEX>${\gg}1$</TEX>) in large parts of the source, especially in the outer regions of star-forming clumps A1, A2, and A5. Regions with high CN/HCN ratios coincide with the inflows of accreted gas suggested by Liu et al. (2015). We conclude that we found strong evidence for interaction between the dense gas clumps and the accreted ambient gas which may have sequentially triggered the star formation in these clumps.
96
- Kim, Minjun
- Journal of the Korean astronomical society = 천문학회지
- 52, n.2
- pp.41-47
- 2019
- 원문 바로보기
We report on the timing properties of the 'Crab twin' pulsar PSR B0540-69 measured with X-ray data taken with the Swift telescope over a period of 1100 days. The braking index of the pulsar was estimated to be <TEX>$n=0.03{\pm}0.013$</TEX> in a previous study performed in 2015 with 500-day Swift data. This small value of n is unusual for pulsars, and a comparison with an old estimate of <TEX>$n{\approx}2.1$</TEX> for the same target determined ~10 years earlier suggests a dramatic change in the braking index. To confirm the small value and therefore the large change of n, we used 1100-day Swift observations including the data used in the earlier determination of n = 0.03. In this study we find that the braking index of PSR B0540-69 is <TEX>$n=0.163{\pm}0.001$</TEX>, somewhat larger than 0.03. Since the measured value of n is still much smaller than 2.1, we can confirm the dramatic change in the braking index for this pulsar.
97
- Im, Myungshin
- Journal of the Korean astronomical society = 천문학회지
- 52, n.1
- pp.11-21
- 2019
- 원문 바로보기
Intensive Monitoring Survey of Nearby Galaxies (IMSNG) is a high cadence observation program monitoring nearby galaxies with high probabilities of hosting supernovae (SNe). IMSNG aims to constrain the SN explosion mechanism by inferring sizes of SN progenitor systems through the detection of the shock-heated emission that lasts less than a few days after the SN explosion. To catch the signal, IMSNG utilizes a network of 0.5-m to 1-m class telescopes around the world and monitors the images of 60 nearby galaxies at distances D < 50 Mpc to a cadence as short as a few hours. The target galaxies are bright in near-ultraviolet (NUV) with <TEX>$M_{NUV}$</TEX> < -18.4 AB mag and have high probabilities of hosting SNe (<TEX>$0.06SN\;yr^{-1}$</TEX> per galaxy). With this strategy, we expect to detect the early light curves of 3.4 SNe per year to a depth of R ~ 19.5 mag, enabling us to detect the shock-heated emission from a progenitor star with a radius as small as <TEX>$0.1R_{\odot}$</TEX>. The accumulated data will be also useful for studying faint features around the target galaxies and other science projects. So far, 18 SNe have occurred in our target fields (16 in IMSNG galaxies) over 5 years, confirming our SN rate estimate of <TEX>$0.06SN\;yr^{-1}$</TEX> per galaxy.
98
- Bach, Yoonsoo P.
- Journal of the Korean astronomical society = 천문학회지
- 52, n.3
- pp.71-82
- 2019
- 원문 바로보기
We derive the geometric albedo of a near-Earth asteroid, (4179) Toutatis, to investigate its surface physical conditions. The asteroid has been studied rigorously not only via ground-based photometric, spectrometric, polarimetric, and radar observations but also via in situ observation by the Chinese Chang'e-2 space probe; however, its geometric albedo is not well understood. We conducted V-band photometric observations when the asteroid was at opposition in April 2018 using the three telescopes in the southern hemisphere that compose the Korea Microlensing Telescope Network (KMTNet). The observed time-variable cross section was corrected using the radar shape model. We find that Toutatis has a geometric albedo <TEX>$p_V=0.185^{+0.045}_{-0.039}$</TEX>, which is typical of S-type asteroids. We compare the geometric albedo with archival polarimetric data and further find that the polarimetric slope-albedo law provides a reliable estimate for the albedo of this S-type asteroid. The thermal infrared observation also produced similar results if the size of the asteroid is updated to match the results from Chang'e-2. We conjecture that the surface of Toutatis is covered with grains smaller than that of the near-Sun asteroids including (1566) Icarus and (3200) Phaethon.
99
- Jeong, Il-Gyo
- Journal of the Korean astronomical society = 천문학회지
- 52, n.6
- pp.227-233
- 2019
- 원문 바로보기
We report the performance of the 13.7-meter Taeduk Radio Astronomy Observatory (TRAO) radio telescope. The telescope has been equipped with a new receiver, SEQUOIA-TRAO, a new backend system, FFT2G, and a new VxWorks operating system. The receiver system features a 16-pixel focal plane array using high-performance MMIC preamplifiers; it shows very low system noise levels, with system noise temperatures from 150 K to 450 K at frequencies from 86 to 115 GHz. With the new backend system, we can simultaneously obtain 32 spectra, each with a velocity coverage of 163 km s<sup>-1</sup> and a resolution of 0.04 km s<sup>-1</sup> at 115 GHz. The new operating system, VxWorks, has successfully handled the LMTMC-TRAO observing software. The main observing method is the on-the-fly (OTF) mapping mode; a position-switching mode is available for small-area observations. Remote observing is provided. The antenna surface has been newly adjusted using digital photogrammetry, achieving a rms surface accuracy better than 130 ㎛. The pointing uncertainty is found to be less than 5' over the entire sky. We tested the new receiver system with multi-frequency observations in OTF mode. The aperture efficiencies are 43±1%, 42±1%, 37±1%, and 33±1%, the beam efficiencies are 45±2%, 48±2%, 46±2%, and 41±2% at 86, 98, 110, and 115 GHz, respectively.
100
- Shim, Hyunjin
- Journal of the Korean astronomical society = 천문학회지
- 52, n.6
- pp.235-244
- 2019
- 원문 바로보기
We present the results of near-infrared imaging observations of the galaxy overdensity around the z = 1.44 radio-loud active galactic nucleus (AGN) 6CE1100+3505, which was carried out with the purpose of sampling the redshifted Hα emission from the actively star-forming galaxies that could constitute the overdensity. The existence of the structure around this AGN was spectroscopically confirmed by previous grism observations which are however limited to the central region. Using the CH4Off narrow/medium-band and H broad band filters in the Wide Infrared Camera (WIRCam) on the Canada-France-Hawaii Telescope (CFHT), we constructed a sample of objects that show a flux excess in the CH4Off band due to line emission. The emission line flux is ~ 4.9 × 10<sup>-16</sup> erg s<sup>-1</sup> cm<sup>-2</sup>, corresponding to a star formation rate (SFR) of ~ 50 M<sub>⊙</sub> yr<sup>-1</sup> for galaxies at redshifts z ~ 1.4. None of the galaxies with medium-band flux excess is located within 1 Mpc from the central AGN, and there is no evidence that the selected galaxies are associated with the proposed cluster. Along with the star formation quenching near the center that was found from the previous grism observations, the lack of extreme starbursts in the structure suggests that at z ~ 1.4, overdense regions are no longer favorable locations for vigorous star formation.