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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 14/119
131
- 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 <TEX>$log_10(t)$</TEX> of the clusters with blue stragglers and those without blue stragglers are <TEX>$8.58{\pm}0.232$</TEX> and <TEX>$7.52{\pm}0.285$</TEX>, 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.
132
- Cho, K.S.
- Journal of the Korean astronomical society = 천문학회지
- 50, n.5
- pp.139-149
- 2017
- 원문 바로보기
The Korea Astronomy and Space Science Institute plans to develop a coronagraph in collaboration with National Aeronautics and Space Administration (NASA) and to install it on the International Space Station (ISS). The coronagraph is an externally occulted one-stage coronagraph with a field of view from 3 to 15 solar radii. The observation wavelength is approximately 400 nm, where strong Fraunhofer absorption lines from the photosphere experience thermal broadening and Doppler shift through scattering by coronal electrons. Photometric filter observations around this band enable the estimation of 2D electron temperature and electron velocity distribution in the corona. Together with a high time cadence (<12 min) of corona images used to determine the geometric and kinematic parameters of coronal mass ejections, the coronagraph will yield the spatial distribution of electron density by measuring the polarized brightness. For the purpose of technical demonstration, we intend to observe the total solar eclipse in August 2017 with the filter system and to perform a stratospheric balloon experiment in 2019 with the engineering model of the coronagraph. The coronagraph is planned to be installed on the ISS in 2021 for addressing a number of questions (e.g., coronal heating and solar wind acceleration) that are both fundamental and practically important in the physics of the solar corona and of the heliosphere.
133
- Lee, Hee-Jae
- Journal of the Korean astronomical society = 천문학회지
- 50, n.3
- pp.41-49
- 2017
- 원문 바로보기
We conduct BVRI and R band photometric observations of asteroid (5247) Krylov from January 2016 to April 2016 for 51 nights using the Korea Microlensing Telescope Network (KMTNet). The color indices of (5247) Krylov at the light curve maxima are determined as <TEX>$B-V=0.841{\pm}0.035$</TEX>, <TEX>$V-R=0.418{\pm}0.031$</TEX>, and <TEX>$V-I=0.871{\pm}0.031$</TEX> where the phase angle is <TEX>$14.1^{\circ}$</TEX>. They are acquired after the standardization of BVRI instrumental measurements using the ensemble normalization technique. Based on the color indices, (5247) Krylov is classified as a S-type asteroid. Double periods, that is, a primary period <TEX>$P_1=82.188{\pm}0.013h$</TEX> and a secondary period <TEX>$P_2=67.13{\pm}0.20h$</TEX> are identified from period searches of its R band light curve. The light curve phases with <TEX>$P_1$</TEX> and this indicate that it is a typical Non-Principal Axis (NPA) asteroid. We discuss the possible causes of its NPA rotation.
134
- 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 <TEX>$0.3{\mu}m$</TEX> to <TEX>$1.1{\mu}m$</TEX> with high sensitivity (QE at > 80% over 0.4 to <TEX>$0.9{\mu}m$</TEX>). 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 <TEX>$15.7{\times}15.7$</TEX> 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 <TEX>$5{\sigma}$</TEX> with 180 sec exposure time for point source detection.
135
- 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.
136
- 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.
137
- 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.
138
- 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.
139
- 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.
140
- 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 <TEX>$M_{BH}{\sim}10^{4-6}M_{\odot}$</TEX>, 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 <TEX>$L_{1.4GHz}{\sim}10^{23}W\;Hz^{-1}$</TEX>. 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.