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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
- 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 (<TEX>${\gamma}_e{\leq}300$</TEX>) 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.
152
- Sudou, Hiroshi
- Journal of the Korean astronomical society = 천문학회지
- 50, n.6
- pp.157-165
- 2017
- 원문 바로보기
<TEX>$H_2O$</TEX> 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 <TEX>$H_2O$</TEX> maser spectrum with high time resolution enables us to clarify acceleration processes of the expanding shell in the stellar atmosphere. We monitored the <TEX>$H_2O$</TEX> 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 <TEX>$0.1kms^{-1}$</TEX> 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 <TEX>$H_2O$</TEX> 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.
153
- Gould, Andrew
- Journal of the Korean astronomical society = 천문학회지
- 50, n.1
- pp.1-5
- 2017
- 원문 바로보기
Like Hipparcos, Gaia is designed to give absolute parallaxes, independent of any astrophysical reference system. And indeed, Gaia's internal zero-point error for parallaxes is likely to be smaller than any individual parallax error. Nevertheless, due in part to mechanical issues of unknown origin, there are many astrophysical questions for which the parallax zero-point error <TEX>${\sigma}({\pi}_0)$</TEX> will be the fundamentally limiting constraint. These include the distance to the Large Magellanic Cloud and the Galactic Center. We show that by using the photometric parallax estimates for RR Lyrae stars (RRL) within 8kpc, via the ultra-precise infrared period-luminosity relation, one can independently determine a hyper-precise value for <TEX>${\pi}_0$</TEX>. Despite their paucity relative to bright quasars, we show that RRL are competitive due to their order-of-magnitude improved parallax precision for each individual object relative to bright quasars. We show that this method is mathematically robust and well-approximated by analytic formulae over a wide range of relevant distances.
154
- 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.
155
- Seo, Hyunjong
- Journal of the Korean astronomical society = 천문학회지
- 50, n.1
- pp.7-20
- 2017
- 원문 바로보기
We carry out the study of <TEX>$850{\mu}m$</TEX> sources in a part of the XMM-LSS field. The <TEX>$850{\mu}m$</TEX> 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 <TEX>$L_{IR}=10^{11.3}-10^{13.4}L_{\odot}$</TEX>, star formation rate <TEX>$SFR=10^{1.3}-10^{3.2}M_{\odot}yr^{-1}$</TEX> and dust temperature <TEX>$T_D=30-53K$</TEX>. We investigate the correlation between <TEX>$L_{IR}$</TEX> and <TEX>$T_D$</TEX>, which appears to be consistent with previous studies.
156
- 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.
157
- 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.
158
- 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.
159
- 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.
160
- 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.