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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 12/122
111
- 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.
112
- Lim, Daye
- Journal of the Korean astronomical society = 천문학회지
- 52, n.4
- pp.133-144
- 2019
- 원문 바로보기
We develop forecast models of daily probabilities of major flares (M- and X-class) based on empirical relationships between photospheric magnetic parameters and daily flaring rates from May 2010 to April 2018. In this study, we consider ten magnetic parameters characterizing size, distribution, and non-potentiality of vector magnetic fields from Solar Dynamics Observatory (SDO)/Helioseismic and Magnetic Imager (HMI) and Geostationary Operational Environmental Satellites (GOES) X-ray flare data. The magnetic parameters are classified into three types: the total unsigned parameters, the total signed parameters, and the mean parameters. We divide the data into two sets chronologically: 70% for training and 30% for testing. The empirical relationships between the parameters and flaring rates are used to predict flare occurrence probabilities for a given magnetic parameter value. Major results of this study are as follows. First, major flare occurrence rates are well correlated with ten parameters having correlation coefficients above 0.85. Second, logarithmic values of flaring rates are well approximated by linear equations. Third, using total unsigned and signed parameters achieved better performance for predicting flares than the mean parameters in terms of verification measures of probabilistic and converted binary forecasts. We conclude that the total quantity of non-potentiality of magnetic fields is crucial for flare forecasting among the magnetic parameters considered in this study. When this model is applied for operational use, it can be used using the data of 21:00 TAI with a slight underestimation of 2-6.3%.
113
- Magara, Tetsuya
- Journal of the Korean astronomical society = 천문학회지
- 52, n.4
- pp.89-97
- 2019
- 원문 바로보기
We demonstrate the subsurface origin of the observed evolution of the solar active region 10930 (AR10930) associated with merging and breakup of magnetic polarity regions at the solar surface. We performed a magnetohydrodynamic simulation of an emerging magnetic flux tube whose field-line twist is asymmetrically distributed along its axis, which is a key to merging and fragmentation in this active region. While emerging into the surface, the flux tube is subjected to partial splitting of its weakly twisted portion, forming separate polarity regions at the solar surface. As emergence proceeds, these separate polarity regions start to merge and then break up, while in the corona sigmoidal structures form and a solar eruption occurs. We discuss what physical processes could be involved in the characteristic evolution of an active region magnetic field that leads to the formation of a sunspot surrounded by satellite polarity regions.
114
- Ahn, Sang-Hyeon
- Journal of the Korean astronomical society = 천문학회지
- 52, n.5
- pp.159-172
- 2019
- 원문 바로보기
It is known that the number of astronomers of a country registered to the International Astronomical Union (IAU) is correlated with that country's gross domestic product (GDP). However, the robustness of this relationship could be doubted, as the fraction of astronomers joining the IAU differs from country to country. Here we revisit this correlation by using more recent data, updated as of 2017. We find a similar correlation by using the total number of astronomers and astrophysicists with PhD degrees that are working in each country, instead of adopting the number of IAU members. We confirm the existence of the correlation. We also confirm the existence of two subgroups within this correlation. One group consists of advanced European countries having a long history of modern astronomy, while the other group consists of countries having experienced recent rapid economic development. In order to determine the cause for the correlation, we obtained the long-term variations of the number of astronomers, population, and the GDP for a number of countries. We find that the number of astronomers per capita for recently developing countries has increased more rapidly as GDP per capita increased, than that for fully developed countries. We collected demographic data of the Korean astronomical community and find that it has experienced recent rapid growth. From these findings we estimate the proper size of the Korean astronomical community by considering Korea economic power and population. The current number of PhD astronomers working in Korea is approximately 310, but it should be 550 in order for it to be comparable and competitive to the sizes of the Spanish, Canadian, and Japanese astronomical communities. If current trends continue, this number will be reached by 2030. In order to be comparable to the German, French, and Italian communities, there should be 800 PhD astronomers in Korea. We discuss ways to overcome the vulnerability of the Korean astronomical community, based on the statistics of national R&D expenditure structure in comparison with that of other major advanced countries.
115
- Kim, Minjun
- Journal of the Korean astronomical society = 천문학회지
- 52, n.2
- pp.41-47
- 2019
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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.
116
- Im, Myungshin
- Journal of the Korean astronomical society = 천문학회지
- 52, n.1
- pp.11-21
- 2019
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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.
117
- 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.
118
- Zhao, Guang-Yao
- Journal of the Korean astronomical society = 천문학회지
- 52, n.1
- pp.23-30
- 2019
- 원문 바로보기
The KVN(Korean VLBI Network)-style simultaneous multi-frequency receiving mode is demonstrated to be promising for mm-VLBI observations. Recently, other Very long baseline interferometry (VLBI) facilities all over the globe start to implement compatible optics systems. Simultaneous dual/multi-frequency VLBI observations at mm wavelengths with international baselines are thus possible. In this paper, we present the results from the first successful simultaneous 22/43 GHz dual-frequency observation with KaVA(KVN and VERA array), including images and astrometric results. Our analysis shows that the newly implemented simultaneous receiving system has brought a significant extension of the coherence time of the 43 GHz visibility phases along the international baselines. The astrometric results obtained with KaVA are consistent with those obtained with the independent analysis of the KVN data. Our results thus confirm the good performance of the simultaneous receiving systems for the nonKVN stations. Future simultaneous observations with more global stations bring even higher sensitivity and micro-arcsecond level astrometric measurements of the targets.
119
- Castro Tirado, Miguel Angel
- Journal of the Korean astronomical society = 천문학회지
- 52, n.4
- pp.99-108
- 2019
- 원문 바로보기
This work addresses the development of the astronomical observatory all through history, from an architectural point of view, as a building in relation to the observing instruments and their functioning as a heterogeneous work center. We focused on 32 observatories (in the period 1259-2007) and carefully analyzed the architectures. Considering the impact of the construction itself or its facilities on the results of the research (thermal or structural stability, poor weather protection, turbulence, etc.), there is little attention paid to theories or studies of the architectural or construction aspects of the observatories. Therefore, this work aims to present a theoretical-critical contribution that, at least, invites the reflection of those involved in the development of astronomical observatories in the future.
120
- Singha, Akram Chandrajit
- Journal of the Korean astronomical society = 천문학회지
- 52, n.1
- pp.1-9
- 2019
- 원문 바로보기
We present the results of the spectral and temporal analysis of eight X-ray point sources in five nearby (distance < 20 Mpc) galaxies observed with Chandra. For spectral analysis, an absorbed powerlaw and an absorbed diskblackbody were used as empirical models. Six sources were found to be equally fitted by both the models while two sources were better fitted by the powerlaw model. Based on model parameters, we estimate the X-ray luminosity of these sources in the energy range 0.3 - 10.0 keV, to be of the order of <TEX>${\sim}10^{39}ergs\;s^{-1}$</TEX> except for one source (X-8) with <TEX>$L_X</TEX><TEX>></TEX><TEX>10^{40}ergs\;s^{-1}$</TEX>. Five of these maybe classified as Ultraluminous X-ray sources (ULXs) with powerlaw photon index within the range, <TEX>${\Gamma}{\sim}1.63-2.63$</TEX> while the inner disk temperature, kT ~ 0.68 - 1.93 keV, when fitted with the disk blackbody model. The black hole masses harboured by the X-ray point sources were estimated using the disk blackbody model to be in the stellar mass range, however, the black hole mass of one source (X-6) lies within the range <TEX>$68.37M_{\odot}{\leq}M_{BH}{\leq}176.32M_{\odot}$</TEX>, which at the upper limit comes under the Intermediate mass black hole range. But if the emission is considered to be beamed by a factor ~ 5, the black hole mass reduces to <TEX>${\sim}75M_{\odot}$</TEX>. The timing analysis of these sources does not show the presence of any short term variations in the kiloseconds timescales.