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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 16/122
151
- Cho, K.S.
- Journal of the Korean astronomical society = 천문학회지
- 50, n.2
- pp.29-39
- 2017
- 원문 바로보기
We investigate two abnormal CME-Storm pairs that occurred on 2014 September 10 - 12 and 2015 March 15 - 17, respectively. The first one was a moderate geomagnetic storm (<TEX>$Dst_{min}{\sim}-75nT$</TEX>) driven by the X1.6 high speed flare-associated CME (<TEX>$1267km\;s^{-1}$</TEX>) in AR 12158 (N14E02) near solar disk center. The other was a very intense geomagnetic storm (<TEX>$Dst_{min}{\sim}-223nT$</TEX>) caused by a CME with moderate speed (<TEX>$719km\;s^{-1}$</TEX>) and associated with a filament eruption accompanied by a weak flare (C9.1) in AR 12297 (S17W38). Both CMEs have large direction parameters facing the Earth and southward magnetic field orientation in their solar source region. In this study, we inspect the structure of Interplanetary Flux Ropes (IFRs) at the Earth estimated by using the torus fitting technique assuming self-similar expansion. As results, we find that the moderate storm on 2014 September 12 was caused by small-scale southward magnetic fields in the sheath region ahead of the IFR. The Earth traversed the portion of the IFR where only the northward fields are observed. Meanwhile, in case of the 2015 March 17 storm, our IFR analysis revealed that the Earth passed the very portion where only the southward magnetic fields are observed throughout the passage. The resultant southward magnetic field with long-duration is the main cause of the intense storm. We suggest that 3D magnetic field geometry of an IFR at the IFR-Earth encounter is important and the strength of a geomagnetic storm is strongly affected by the relative location of the Earth with respect to the IFR structure.
152
- 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.
153
- Kim, Sang Chul
- Journal of the Korean astronomical society = 천문학회지
- 50, n.3
- pp.79-92
- 2017
- 원문 바로보기
We present a BV I optical photometric study of the old open cluster Ruprecht 6 using the data obtained with the SMARTS 1.0 m telescope at the CTIO, Chile. Its color-magnitude diagrams show the clear existence of the main-sequence stars, whose turn-off point is located around <TEX>$V{\approx}18.45mag$</TEX> and <TEX>$B-V{\approx}0.85mag$</TEX>. Three red clump (RC) stars are identified at V = 16.00 mag, I = 14.41 mag and B - V = 1.35 mag. From the mean <TEX>$K_s-band$</TEX> magnitude of RC stars (<TEX>$K_s=12.39{\pm}0.21mag$</TEX>) in Ruprecht 6 from 2MASS photometry and the known absolute magnitudes of the RC stars (<TEX>$M_{K_S}=-1.595{\pm}0.025mag$</TEX>), we obtain the distance modulus to Ruprecht 6 of <TEX>$(m-M)_0=13.84{\pm}0.21mag$</TEX> (<TEX>$d=5.86{\pm}0.60kpc$</TEX>). From the (<TEX>$J-K_s$</TEX>) and (B - V ) colors of the RC stars, comparison of the (B - V ) and (V - I) colors of the bright stars in Ruprecht 6 with those of the intrinsic colors of dwarf and giant stars, and the PARSEC isochrone fittings, we derive the reddening values of E(B - V ) = 0.42 mag and E(V - I) = 0.60 mag. Using the PARSEC isochrone fittings onto the color-magnitude diagrams, we estimate the age and metallicity to be: <TEX>$log(t)=9.50{\pm}0.10(t=3.16{\pm}0.82Gyr)$</TEX> and <TEX>$[Fe/H]=-0.42{\pm}0.04dex$</TEX>. We present the Galactocentric radial metallicity gradient analysis for old (age > 1 Gyr) open clusters of the Dias et al. catalog, which likely follow a single relation of <TEX>$[Fe/H]=(-0.034{\pm}0.007)R_{GC}+(0.190{\pm}0.080)$</TEX> (rms = 0.201) for the whole radial range or a dual relation of <TEX>$[Fe/H]=(-0.077{\pm}0.017)R_{GC}+(0.609{\pm}0.161)$</TEX> (rms = 0.152) and constant ([Fe/H] ~ -0.3 dex) value, inside and outside of RGC ~ 12 kpc, respectively. The metallicity and Galactocentric radius (<TEX>$13.28{\pm}0.54kpc$</TEX>) of Ruprecht 6 obtained in this study seem to be consistent with both of the relations.
154
- Han, Junghwan
- Journal of the Korean astronomical society = 천문학회지
- 50, n.6
- pp.185-190
- 2017
- 원문 바로보기
We present observations of <TEX>$HCO^+$</TEX> 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 <TEX>$-17kms^{-1}$</TEX> component changed marginally during 1993-1998 period and has remained unaltered thereafter for NRAO 150. Its behavior is different from that of <TEX>$H_2CO$</TEX> <TEX>$1_{10}-1_{11}$</TEX>, suggesting chemical differentiation on ~ 20 AU scale, the smallest ever seen. On the other hand, BL Lac exhibits little temporal variation for the <TEX>$HCO^+$</TEX> and <TEX>$H_2CO$</TEX> lines. Our observation also suggests that Korea VLBI Network performs reliably in the spectrum mode in that the shapes of the new <TEX>$HCO^+$</TEX> 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.
155
- 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 <TEX>$z{\leq}0.01$</TEX>. We consider six sub-types of dwarf galaxies, dS0, dE, <TEX>$dE_{bc}$</TEX>, dSph, <TEX>$dE_{blue}$</TEX>, 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 (<TEX>$r_p$</TEX>), 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 <TEX>$r_p$</TEX> 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 <TEX>$dE_{bc}$</TEX>, <TEX>$dE_{blue}$</TEX>, and dI satellites. The blue-cored dwarf satellites (<TEX>$dE_{bc}$</TEX>) of early-type galaxies are likely to be located at <TEX>$r_p$</TEX> > 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 <TEX>$dE_{bc}$</TEX> 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.
156
- Shin, Jihey
- Journal of the Korean astronomical society = 천문학회지
- 50, n.3
- pp.61-70
- 2017
- 원문 바로보기
We study the angular correlation function of bright (<TEX>$K_s{\leq}19.5$</TEX>) Extremely Red Objects (EROs) selected in the Subaru GTO 2<TEX>$deg^2$</TEX> field. By applying the color selection criteria of <TEX>$R-K_s$</TEX> > 5.0, 5.5, and 6.0, we identify 9055, 4270, and 1777 EROs, respectively. The number density is consistent with similar studies on the optical - NIR color selected red galaxies. The angular correlation functions are derived for EROs with different limiting magnitude and different <TEX>$R-K_s$</TEX> color cut. When we assume that the angular correlation function <TEX>$w({\theta})$</TEX> follows a form of a power-law (i.e., <TEX>$w({\theta})=A{\theta}^{-{\delta}}$</TEX>), the value of the amplitude A was larger for brighter EROs compared to the fainter EROs. The result suggests that the brighter, thus more massive high-redshift galaxies, are clustered more strongly compared to the less massive galaxies. Assuming that EROs have redshift distribution centered at <z> ~ 1.1 with <TEX>${\sigma}_z=0.15$</TEX>, the spatial correlation length <TEX>$r_0$</TEX> of the EROs estimated from the observed angular correlation function ranges <TEX>${\sim}6-10h^{-1}Mpc$</TEX>. A comparison with the clustering of dark matter halos in numerical simulation suggests that the EROs are located in most massive dark matter halos and could be progenitors of <TEX>$L_{\ast}$</TEX> elliptical galaxies.
157
- Kim, Dae-Won
- Journal of the Korean astronomical society = 천문학회지
- 50, n.6
- pp.167-178
- 2017
- 원문 바로보기
We present a study of the inexplicit connection between radio jet activity and <TEX>${\gamma}$</TEX>-ray emission of BL Lacertae (BL Lac; 2200+420). We analyze the long-term millimeter activity of BL Lac via interferometric observations with the Korean VLBI Network (KVN) obtained at 22, 43, 86, and 129 GHz simultaneously over three years (from January 2013 to March 2016); during this time, two <TEX>${\gamma}$</TEX>-ray outbursts (in November 2013 and March 2015) can be seen in <TEX>${\gamma}$</TEX>-ray light curves obtained from Fermi observations. The KVN radio core is optically thick at least up to 86 GHz; there is indication that it might be optically thin at higher frequencies. To first order, the radio light curves decay exponentially over the time span covered by our observations, with decay timescales of <TEX>$411{\pm}85$</TEX> days, <TEX>$352{\pm}79$</TEX> days, <TEX>$310{\pm}57$</TEX> days, and <TEX>$283{\pm}55$</TEX> days at 22, 43, 86, and 129 GHz, respectively. Assuming synchrotron cooling, a cooling time of around one year is consistent with magnetic field strengths <TEX>$B{\sim}2{\mu}T$</TEX> and electron Lorentz factors <TEX>${\gamma}$</TEX> ~ 10 000. Taking into account that our formal measurement errors include intrinsic variability and thus over-estimate the statistical uncertainties, we find that the decay timescale <TEX>${\tau}$</TEX> scales with frequency <TEX>${\nu}$</TEX> like <TEX>${\tau}{\propto}{\nu}^{-0.2}$</TEX>. This relation is much shallower than the one expected from opacity effects (core shift), but in agreement with the (sub-)mm radio core being a standing recollimation shock. We do not find convincing radio flux counterparts to the <TEX>${\gamma}$</TEX>-ray outbursts. The spectral evolution is consistent with the 'generalized shock model' of Valtaoja et al. (1992). A temporary increase in the core opacity and the emergence of a knot around the time of the second <TEX>${\gamma}$</TEX>-ray event indicate that this <TEX>${\gamma}$</TEX>-ray outburst might be an 'orphan' flare powered by the 'ring of fire' mechanism.
158
- Suh, Kyung-Won
- Journal of the Korean astronomical society = 천문학회지
- 50, n.4
- pp.131-138
- 2017
- 원문 바로보기
We present a new catalog of AGB stars based on infrared two-color diagrams (2CDs) and known properties of the pulsations and spectra. We exclude some misclassified objects from previous catalogs. We identify color areas in two IR 2CDs where most O-rich and C-rich objects listed in previous catalogs of AGB stars are found. By collecting new objects in these color selection areas in the two IR 2CDs, we find candidate objects for AGB stars. By using the color selection method, we identify 3996 new objects in the O-rich areas, 1487 new objects in the C-rich areas, and 295 new objects in the overlap areas of the two 2CDs simultaneously. We have found that 470 O-rich and 9 C-rich objects are Mira variables with positive spectral identification and they are newly identified AGB stars. We present a new catalog of 3828 O-rich AGB stars and 1168 C-rich AGB stars excluding misclassified objects and adding newly identified objects.
159
- 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.
160
- Jeong, Dong-Gwon
- Journal of the Korean astronomical society = 천문학회지
- 50, n.4
- pp.105-109
- 2017
- 원문 바로보기
The well-known solar cycle controls almost the entire appearance of the solar photosphere. We therefore presume that the continuous emission of visible light from the solar surface follows the solar cyclic variation. In this study, we examine the solar cyclic variation of photospheric brightness in the visible range using solar images taken by the Solar and Heliospheric Observatory (SOHO)/Michelson Doppler Imager (MDI). The photospheric brightness in the visible range is quantified via the relative intensity acquired from in the raw solar images. In contrast to total solar irradiance, the relative intensity is out of phase with the solar cycle. During the solar minimum of solar cycles 23-24, the relative intensity shows enhanced heliolatitudinal asymmetry due to a positive asymmetry of the sunspot number. This result can be explained by the strength of the solar magnetic field that controls the strength of convection, implying that the emission in the visible range is controlled by the strength of convection. This agrees with the photospheric brightness increasing during a period of long spotless days.