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통합검색

통합검색

한국천문학회지

1968년 ~ 2018년까지 1,081 건한국천문학회지를 격월간 확인하실 수 있습니다.

  • The Korean Astronomical Society (The Korean Astronomical Society)
  • 계간 (Quarterly)
  • ISSN : 1225-4614 (ISSN : 1225-4614)
  • DB구축현황 : 1,081건 (DB Construction : 1,081 Articles)
안내사항
총 게시글 1,081 페이지 3/109
21
  • Sudou, Hiroshi
  • Journal of the Korean astronomical society = 천문학회지
  • 50, n.6
  • pp.157-165
  • 2017
  • 원문 바로보기
$H_2O$ maser emission at 22 GHz in the circumstellar envelope is one of the good tracers of detailed physics and kinematics in the mass loss process of asymptotic giant branch stars. Long-term monitoring of an $H_2O$ maser spectrum with high time resolution enables us to clarify acceleration processes of the expanding shell in the stellar atmosphere. We monitored the $H_2O$ maser emission of the semi-regular variable R Crt with the Kagoshima 6-m telescope, and obtained a large data set of over 180 maser spectra over a period of 1.3 years with an observational span of a few days. Using an automatic peak detection method based on least-squares fitting, we exhaustively detected peaks as significant velocity components with the radial velocity on a $0.1kms^{-1}$ scale. This analysis result shows that the radial velocity of red-shifted and blue-shifted components exhibits a change between acceleration and deceleration on the time scale of a few hundred days. These velocity variations are likely to correlate with intensity variations, in particular during flaring state of $H_2O$ masers. It seems reasonable to consider that the velocity variation of the maser source is caused by shock propagation in the envelope due to stellar pulsation. However, it is difficult to explain the relationship between the velocity variation and the intensity variation only from shock propagation effects. We found that a time delay of the integrated maser intensity with respect to the optical light curve is about 150 days.
22
  • Han, Junghwan
  • Journal of the Korean astronomical society = 천문학회지
  • 50, n.6
  • pp.185-190
  • 2017
  • 원문 바로보기
We present observations of $HCO^+$ 1-0 absorption lines toward two extragalactic compact radio sources, NRAO 150 and BL Lac with the Korean VLBI Network in order to investigate their time variation over 20 years by Galactic foreground clouds. It is found that the line shape of $-17kms^{-1}$ component changed marginally during 1993-1998 period and has remained unaltered thereafter for NRAO 150. Its behavior is different from that of $H_2CO$ $1_{10}-1_{11}$ , suggesting chemical differentiation on ~ 20 AU scale, the smallest ever seen. On the other hand, BL Lac exhibits little temporal variation for the $HCO^+$ and $H_2CO$ lines. Our observation also suggests that Korea VLBI Network performs reliably in the spectrum mode in that the shapes of the new $HCO^+$ 1-0 spectra are in good agreement with the previous ones to an accuracy of a few percent except the time varying component toward NRAO 150.
23
  • Magara, Tetsuya
  • Journal of the Korean astronomical society = 천문학회지
  • 50, n.6
  • pp.179-184
  • 2017
  • 원문 바로보기
We present a new method for solving an inverse problem of flux emergence which transports subsurface magnetic flux from an inaccessible interior to the surface where magnetic structures may be observed to form, such as solar active regions. To make a quantitative evaluation of magnetic structures having various characteristics, we derive physical properties of subsurface magnetic field that characterize those structures formed through flux emergence. The derivation is performed by inversion from an evolutionary relation between two observables obtained at the surface, emerged magnetic flux and injected magnetic helicity, the former of which provides scale information while the latter represents the configuration of magnetic field.
24
  • Lee, Ki-Won
  • Journal of the Korean astronomical society = 천문학회지
  • 50, n.6
  • pp.191-200
  • 2017
  • 원문 바로보기
We analyze the time data recorded in Korean astronomical almanacs for the years from 1913 to 1945, which belong to the period in which Japan occupied Korea (1910-1945). These almanacs, published by Japanese scholars, differ from previous almanacs in terms of organization, content, and calendrical methods. In this study, we first extract twelve kinds of time data from the almanacs at the following times: solar terms, rising and setting of the Sun and Moon, transit of the Sun, phases of the Moon (i.e., new Moon, first quarter Moon, full Moon, and last quarter Moon), and eclipses of the Sun and Moon. Then, we compare the time data with that obtained from modern calculations. Even though all time data in the almanacs are tabulated in units of minutes, we calculate the data in units of seconds and determine the root mean square (RMS) deviation values for each kind of time data to estimate the accuracy of the data. Our findings are as follows: First, the kind and tabulation method of time data changes several times. For instance, solar transit time is listed only for six years from 1937 to 1942. Second, the times of two equinoxes and those of a new Moon are considerably close to midnight. Third, there are some typographical errors in the almanacs, particularly in the times of moonrise and moonset. Fourth, the contact times for lunar eclipses represent the times of the umbra and not of the penumbra, which is different from the times for solar eclipses. Finally, the RMS deviation values are approximately 0.5 min on average in all kinds of time data, even though they show slightly large differences in the times related to the Moon. In conclusion, we believe that this study is useful for investigating the time data in the almanacs of other East Asian countries that were published during the same period, such as China, Japan, and Manchuria.
25
  • Seo, Hyunjong
  • Journal of the Korean astronomical society = 천문학회지
  • 50, n.1
  • pp.7-20
  • 2017
  • 원문 바로보기
We carry out the study of $850{\mu}m$ sources in a part of the XMM-LSS field. The $850{\mu}m$ imaging data were obtained by the SCUBA-2 on the James Clerk Maxwell Telescope (JCMT) for three days in July 2015 with an integration time of 6.1 hours, covering a circular area with a radius of 15'. We choose the central area up to a radius of 9'.15 for the study, where the noise distribution is relatively uniform. The root mean square (rms) noise at the center is 2.7 mJy. We identify 17 sources with S/N > 3.5. Differential number count is estimated in flux range between 3.5 and 9.0 mJy after applying various corrections derived by imaging simulations, which is consistent with previous studies. For detailed study on the individual sources, we select three sources with more reliable measurements (S/N > 4.5), and construct their spectral energy distributions (SEDs) from optical to far-infrared band. Redshift distribution of the sources ranges from 0.36 to 3.28, and their physical parameters are extracted using MAGPHYS model, which yield infrared luminosity $L_{IR}=10^{11.3}-10^{13.4}L_{\odot}$ , star formation rate $SFR=10^{1.3}-10^{3.2}M_{\odot}yr^{-1}$ and dust temperature $T_D=30-53K$ . We investigate the correlation between $L_{IR}$ and $T_D$ , which appears to be consistent with previous studies.
26
  • Ann, Hong Bae
  • Journal of the Korean astronomical society = 천문학회지
  • 50, n.4
  • pp.111-124
  • 2017
  • 원문 바로보기
The environmental dependence of the morphology of dwarf galaxies in isolated satellite systems is analyzed to understand the origin of the dwarf galaxy morphology using the visually classified morphological types of 5836 local galaxies with $z{\leq}0.01$ . We consider six sub-types of dwarf galaxies, dS0, dE, $dE_{bc}$ , dSph, $dE_{blue}$ , and dI, of which the first four sub-types are considered as early-type and the last two as late-type. The environmental parameters we consider are the projected distance from the host galaxy ( $r_p$ ), local and global background densities, and the host morphology. The spatial distributions of dwarf satellites of early-type galaxies are much different from those of dwarf satellites of late-type galaxies, suggesting the host morphology combined with $r_p$ plays a decisive role on the morphology of the dwarf satellite galaxies. The local and global background densities play no significant role on the morphology of dwarfs in the satellite systems hosted by early-type galaxies. However, in the satellite system hosted by late-type galaxies, the global background densities of dE and dSph satellites are significantly different from those of $dE_{bc}$ , $dE_{blue}$ , and dI satellites. The blue-cored dwarf satellites ( $dE_{bc}$ ) of early-type galaxies are likely to be located at $r_p$ > 0.3 Mpc to keep their cold gas from the ram pressure stripping by the hot corona of early-type galaxies. The spatial distribution of $dE_{bc}$ satellites of early-type galaxies and their global background densities suggest that their cold gas is intergalactic material accreted before they fall into the satellite systems.
27
  • Kim, Sujin
  • Journal of the Korean astronomical society = 천문학회지
  • 50, n.4
  • pp.125-129
  • 2017
  • 원문 바로보기
We investigate the solar cycle variation of microwave and extreme ultraviolet (EUV) intensity in latitude to compare microwave polar brightening (MPB) with the EUV polar coronal hole (CH). For this study, we used the full-sun images observed in 17 GHz of the Nobeyama Radioheliograph from 1992 July to 2016 November and in two EUV channels of the Atmospheric Imaging Assembly (AIA) $193{\AA}$ and $171{\AA}$ on the Solar Dynamics Observatory (SDO) from 2011 January to 2016 November. As a result, we found that the polar intensity in EUV is anti-correlated with the polar intensity in microwave. Since the depression of EUV intensity in the pole is mostly owing to the CH appearance and continuation there, the anti-correlation in the intensity implies the intimate association between the polar CH and the MPB. Considering the report of Gopalswamy et al. (1999) that the enhanced microwave brightness in the CH is seen above the enhanced photospheric magnetic field, we suggest that the pole area during the solar minimum has a stronger magnetic field than the quiet sun level and such a strong field in the pole results in the formation of the polar CH. The emission mechanism of the MPB and the physical link with the polar CH are not still fully understood. It is necessary to investigate the MPB using high resolution microwave imaging data, which can be obtained by the high performance large-array radio observatories such as the ALMA project.
28
  • Kang, Hyesung
  • Journal of the Korean astronomical society = 천문학회지
  • 50, n.4
  • pp.93-103
  • 2017
  • 원문 바로보기
We explore the shock acceleration model for giant radio relics, in which relativistic electrons are accelerated via diffusive shock acceleration (DSA) by merger-driven shocks in the outskirts of galaxy clusters. In addition to DSA, turbulent acceleration by compressive MHD modes downstream of the shock are included as well as energy losses of postshock electrons due to Coulomb scattering, synchrotron emission, and inverse Compton scattering off the cosmic background radiation. Considering that only a small fraction of merging clusters host radio relics, we favor a reacceleration scenario in which radio relics are generated preferentially by shocks encountering the regions containing low-energy ( ${\gamma}_e{\leq}300$ ) cosmic ray electrons (CRe). We perform time-dependent DSA simulations of spherically expanding shocks with physical parameters relevant for the Sausage radio relic, and calculate the radio synchrotron emission from the accelerated CRe. We find that significant level of postshock turbulent acceleration is required in order to reproduce broad profiles of the observed radio flux densities of the Sausage relic. Moreover, the spectral curvature in the observed integrated radio spectrum can be explained, if the putative shock should have swept up and exited out of the preshock region of fossil CRe about 10 Myr ago.
29
  • Park, Songyoun
  • Journal of the Korean astronomical society = 천문학회지
  • 50, n.5
  • pp.151-155
  • 2017
  • 원문 바로보기
We investigate the radio properties of the dwarf galaxy SDSS J133245.62+263449.3 which shows optical signatures of black hole activity. Dwarf galaxies are known to host intermediate mass black holes (IMBHs) with masses $M_{BH}{\sim}10^{4-6}M_{\odot}$ , some of them being radio loud. Recently, Reines et al. (2013) found dwarf galaxy candidates which show signatures of being black hole hosts based on optical spectral lines. SDSS J133245.62+263449.3 is one of them; it shows a flux density of ~ 20 mJy at 1.4 GHz, which corresponds to $L_{1.4GHz}{\sim}10^{23}W\;Hz^{-1}$ . This is much brighter than other black hole host dwarf galaxies. However, star formation activity can contribute to radio continuum emission as well. To understand the nature of the radio emission from SDSS J133245.62+263449.3, we imaged this radio loud dwarf galaxy at low frequencies (325 MHz and 610 MHz) using the Giant Metrewave Radio Telescope (GMRT). We present here the high resolution images from our GMRT observations. While we detect no obvious extended emission from radio jets from the central AGN, we do find the emission to be moderately extended and unlikely to be dominated by disk star formation. VLBI observations using the Korean VLBI Network (KVN) are now being planned to understand the emission morphology and radiation mechanism.
30
  • Lee, Hyun-Uk
  • Journal of the Korean astronomical society = 천문학회지
  • 50, n.3
  • pp.51-59
  • 2017
  • 원문 바로보기
The presence of blue stragglers pose challenges to standard stellar evolution theory, in the sense that explaining their presence demands a complex interplay between stellar evolution and cluster dynamics. In the meantime, mass transfer in binary systems and stellar collisions are widely studied as a blue straggler formation channel. We explore properties of the Galactic open clusters where blue stragglers are found, in attempting to estimate the relative importance of these two favored processes, by comparing them with those resulting from open clusters in which blue stragglers are absent as of now. Unlike previous studies which require a sophisticated process in understanding the implication of the results, this approach is straightforward and has resulted in a supplementary supporting evidence for the current view on the blue straggler formation mechanism. Our main findings are as follows: (1) Open clusters in which blue stragglers are present have a broader distribution with respect to the Z-axis pointing towards the North Galactic Pole than those in which blue stragglers are absent. The probability that two distributions with respect to the Z-axis are drawn from the same distribution is 0.2%. (2) Average values of $log_10(t)$ of the clusters with blue stragglers and those without blue stragglers are $8.58{\pm}0.232$ and $7.52{\pm}0.285$ , respectively. (3) The clusters with blue stragglers tend to be relatively redder than the others, and are distributed broader in colors. (4) The clusters with blue stragglers are likely brighter than those without blue stragglers. (5) Finally, blue stragglers seem to form in condensed clusters rather than simply dense clusters. Hence, we conclude that mass transfer in binaries seems to be a relatively important physical mechanism of the generation of blue stragglers in open clusters, provided they are sufficiently old.