모달창 닫기
- 홈으로 이동
1968년 ~ 2025년까지 1,227 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 : 1,227건 (DB Construction : 1,227 Articles)
총 게시글 1,227건
페이지 18/123
171
- 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.
172
- 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.
173
- 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.
174
- 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.
175
- 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.
176
- 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.
177
- 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.
178
- 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.
179
- 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.
180
- 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.