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통합검색

통합검색

한국천문학회지

1968년 ~ 2020년까지 1,115 건한국천문학회지를 격월간 확인하실 수 있습니다.

  • The Korean Astronomical Society (The Korean Astronomical Society)
  • 계간 (Quarterly)
  • ISSN : 1225-4614 (ISSN : 1225-4614)
  • DB구축현황 : 1,115건 (DB Construction : 1,115 Articles)
안내사항
총 게시글 1,115 페이지 6/112
51
  • 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 $V{\approx}18.45mag$ and $B-V{\approx}0.85mag$ . 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 $K_s-band$ magnitude of RC stars ( $K_s=12.39{\pm}0.21mag$ ) in Ruprecht 6 from 2MASS photometry and the known absolute magnitudes of the RC stars ( $M_{K_S}=-1.595{\pm}0.025mag$ ), we obtain the distance modulus to Ruprecht 6 of $(m-M)_0=13.84{\pm}0.21mag$ ( $d=5.86{\pm}0.60kpc$ ). From the ( $J-K_s$ ) 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: $log(t)=9.50{\pm}0.10(t=3.16{\pm}0.82Gyr)$ and $[Fe/H]=-0.42{\pm}0.04dex$ . 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 $[Fe/H]=(-0.034{\pm}0.007)R_{GC}+(0.190{\pm}0.080)$ (rms = 0.201) for the whole radial range or a dual relation of $[Fe/H]=(-0.077{\pm}0.017)R_{GC}+(0.609{\pm}0.161)$ (rms = 0.152) and constant ([Fe/H] ~ -0.3 dex) value, inside and outside of RGC ~ 12 kpc, respectively. The metallicity and Galactocentric radius ( $13.28{\pm}0.54kpc$ ) of Ruprecht 6 obtained in this study seem to be consistent with both of the relations.
52
  • 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 ( $Dst_{min}{\sim}-75nT$ ) driven by the X1.6 high speed flare-associated CME ( $1267km\;s^{-1}$ ) in AR 12158 (N14E02) near solar disk center. The other was a very intense geomagnetic storm ( $Dst_{min}{\sim}-223nT$ ) caused by a CME with moderate speed ( $719km\;s^{-1}$ ) 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.
53
  • Chae, Jongchul
  • Journal of the Korean astronomical society = 천문학회지
  • 50, n.2
  • pp.21-27
  • 2017
  • 원문 바로보기
The autoregressive method provides a univariate procedure to predict the future sunspot number (SSN) based on past record. The strength of this method lies in the possibility that from past data it yields the SSN in the future as a function of time. On the other hand, its major limitation comes from the intrinsic complexity of solar magnetic activity that may deviate from the linear stationary process assumption that is the basis of the autoregressive model. By analyzing the residual errors produced by the method, we have obtained the following conclusions: (1) the optimal duration of the past time for the forecast is found to be 8.5 years; (2) the standard error increases with prediction horizon and the errors are mostly systematic ones resulting from the incompleteness of the autoregressive model; (3) there is a tendency that the predicted value is underestimated in the activity rising phase, while it is overestimated in the declining phase; (5) the model prediction of a new Solar Cycle is fairly good when it is similar to the previous one, but is bad when the new cycle is much different from the previous one; (6) a reasonably good prediction of a new cycle can be made using the AR model 1.5 years after the start of the cycle. In addition, we predict the next cycle (Solar Cycle 25) will reach the peak in 2024 at the activity level similar to the current cycle.
54
  • 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.
55
  • 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.
56
  • Choi, Changsu
  • Journal of the Korean astronomical society = 천문학회지
  • 50, n.3
  • pp.71-78
  • 2017
  • 원문 바로보기
We present the characteristics and the performance of the new CCD camera system, SNUCAM-II (Seoul National University CAMera system II) that was installed on the Lee Sang Gak Telescope (LSGT) at the Siding Spring Observatory in 2016. SNUCAM-II consists of a deep depletion chip covering a wide wavelength from $0.3{\mu}m$ to $1.1{\mu}m$ with high sensitivity (QE at > 80% over 0.4 to $0.9{\mu}m$ ). It is equipped with the SDSS ugriz filters and 13 medium band width (50 nm) filters, enabling us to study spectral energy distributions (SEDs) of diverse objects from extragalactic sources to solar system objects. On LSGT, SNUCAM-II offers $15.7{\times}15.7$ arcmin field-of-view (FOV) at a pixel scale of 0.92 arcsec and a limiting magnitude of g = 19.91 AB mag and z=18.20 AB mag at $5{\sigma}$ with 180 sec exposure time for point source detection.
57
  • 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.
58
  • 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.
59
  • 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.
60
  • 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.