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- 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 17/122
161
- 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.
162
- 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.
163
- 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.
164
- 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.
165
- KIM, SEUNG-LEE
- Journal of the Korean astronomical society = 천문학회지
- 49, n.5
- pp.199-208
- 2016
- 원문 바로보기
We present photometric results of the δ Sct star V1162 Ori, which is extensively monitored for a total of 49 nights from mid-December 2014 to early-March 2015. The observations are made with three KMTNet (Korea Microlensing Telescope Network) 1.6 m telescopes installed in Chile, South Africa, and Australia. Multiple frequency analysis is applied to the data and resulted in clear detection of seven frequencies without an alias problem: five known frequencies and two new ones with small amplitudes of 1.2-1.7 mmag. The amplitudes of all but one frequency are significantly different from previous results, confirming the existence of long-term amplitude changes. We examine the variations in pulsation timings of V1162 Ori for about 30 years by using the times of maximum light obtained from our data and collected from the literatures. The O − C (Observed minus Calculated) timing diagram shows a combination of a downward parabolic variation with a period decreasing rate of (1/P)dP/dt = −4.22 × 10<sup>−6</sup> year<sup>−1</sup> and a cyclic change with a period of about 2780 days. The most probable explanation for this cyclic variation is the light-travel-time effect caused by an unknown binary companion, which has a minimum mass of 0.69 M<sub>⊙</sub>. V1162 Ori is the first δ Sct-type pulsating star of which the observed fast period decrease can be interpreted as an evolutionary effect of a pre-main sequence star, considering its membership of the Orion OB 1c association.
166
- Ann, Hong Bae
- Journal of the Korean astronomical society = 천문학회지
- 49, n.6
- pp.239-253
- 2016
- 원문 바로보기
We determined the warp parameters of 192 warped galaxies which are selected from 340 edge-on galaxies using color images as well as r-band isophotal maps. We derive the local background density (<TEX>${\Sigma}_n$</TEX>) to examine the dependence of the warp amplitudes on the galaxy environment. We find a clear trend that strongly warped galaxies are likely to be found in high density regions where tidal interactions are supposed to be frequent. However, the correlation between <TEX>${\alpha}_{\omega}$</TEX> and <TEX>${\Sigma}_n$</TEX> is too weak for weakly warped galaxies (<TEX>${\alpha}_{\omega}$</TEX> < <TEX>$4^{\circ}$</TEX>) and the cumulative distributions of weakly warped galaxies are not significantly different from those of galaxies with no detectable warps. This suggests that tidal interactions do not play a decisive role in the formation of weak warps.
167
- Kim, Jaeheon
- Journal of the Korean astronomical society = 천문학회지
- 49, n.6
- pp.261-288
- 2016
- 원문 바로보기
We present the results of simultaneous monitoring observations of <TEX>$H_2O$</TEX> <TEX>$6_{1,6}-5_{2,3}$</TEX> (22GHz) and SiO J=1-0, 2-1, 3-2 maser lines (43, 86, 129GHz) toward five post-AGB (candidate) stars, using the 21-m single-dish telescopes of the Korean VLBI Network. Depending on the target objects, 7 - 11 epochs of data were obtained. We detected both <TEX>$H_2O$</TEX> and SiO maser lines from four sources: OH16.1-0.3, OH38.10-0.13, OH65.5+1.3, and IRAS 19312+1950. We could not detect <TEX>$H_2O$</TEX> maser emission toward OH13.1+5.1 between the late OH/IR and post-AGB stage. The detected <TEX>$H_2O$</TEX> masers show typical double-peaked line profiles. The SiO masers from four sources, except IRAS 19312+1950, show the peaks around the stellar velocity as a single peak, whereas the SiO masers from IRAS 19312+1950 occur above the red peak of the <TEX>$H_2O$</TEX> maser. We analyzed the properties of detected maser lines, and investigated their evolutionary state through comparison with the full widths at zero power. The distribution of observed target sources was also investigated in the IRAS two-color diagram in relation with the evolutionary stage of post-AGB stars. From our analyses, the evolutionary sequence of observed sources is suggested as OH65.5+1.3<TEX>${\rightarrow}$</TEX>OH13.1+5.1<TEX>${\rightarrow}$</TEX>OH16.1-0.3<TEX>${\rightarrow}$</TEX>OH38.10-0.13, except for IRAS 19312+1950. In addition, OH13.1+5.1 from which the <TEX>$H_2O$</TEX> maser has not been detected is suggested to be on the gateway toward the post-AGB stage. With respect to the enigmatic object, IRAS 19312+1950, we could not clearly figure out its nature. To properly explain the unusual phenomena of SiO and <TEX>$H_2O$</TEX> masers, it is essential to establish the relative locations and spatial distributions of two masers using VLBI technique. We also include the <TEX>$1.2-160{\mu}m$</TEX> spectral energy distribution using photometric data from the following surveys: 2MASS, WISE, MSX, IRAS, and AKARI (IRC and FIS). In addition, from the IRAS LRS spectra, we found that the depth of silicate absorption features shows significant variations depending on the evolutionary sequence, associated with the termination of AGB phase mass-loss.
168
- HODGSON, JEFFREY A.
- Journal of the Korean astronomical society = 천문학회지
- 49, n.4
- pp.137-144
- 2016
- 원문 바로보기
The calibration of Very Long Baseline Interferometry (VLBI) data has long been a time consuming process. The Korean VLBI Network (KVN) is a simple array consisting of three identical antennas. Because four frequencies are observed simultaneously, phase solutions can be transferred from lower frequencies to higher frequencies in order to improve phase coherence and hence sensitivity at higher frequencies. Due to the homogeneous nature of the array, the KVN is also well suited for automatic calibration. In this paper we describe the automatic calibration of single-polarisation KVN data using the KVN Pipeline and comparing the results against VLBI data that has been manually reduced. We find that the pipelined data using phase transfer produces better results than a manually reduced dataset not using the phase transfer. Additionally we compared the pipeline results with a manually reduced phase-transferred dataset and found the results to be identical.
169
- CHO, KYUHYOUN
- Journal of the Korean astronomical society = 천문학회지
- 49, n.1
- pp.45-51
- 2016
- 원문 바로보기
The coronagraph is an instrument that enables the investigation of faint features in the vicinity of the Sun, particularly coronal mass ejections (CMEs). So far coronagraphic observations have been mainly used to determine the geometric and kinematic parameters of CMEs. Here, we introduce a new method for the determination of CME temperature using a two filter (4025 Å and 3934 Å) coronagraph system. The thermal motion of free electrons in CMEs broadens the absorption lines in the optical spectra that are produced by the Thomson scattering of visible light originating in the photosphere, which affects the intensity ratio at two different wavelengths. Thus the CME temperature can be inferred from the intensity ratio measured by the two filter coronagraph system. We demonstrate the method by invoking the graduated cylindrical shell (GCS) model for the 3-dimensional CME density distribution and discuss its significance.
170
- ZHU, WEI
- Journal of the Korean astronomical society = 천문학회지
- 49, n.3
- pp.93-107
- 2016
- 원문 바로보기
Augmenting the Wide Field Infrared Survey Telescope (WFIRST) microlensing campaigns with intensive observations from a ground-based network of wide-field survey telescopes would have several major advantages. First, it would enable full two-dimensional (2-D) vector microlens parallax measurements for a substantial fraction of low-mass lenses as well as planetary and binary events that show caustic crossing features. For a significant fraction of the free-floating planet (FFP) events and all caustic-crossing planetary/binary events, these 2-D parallax measurements directly lead to complete solutions (mass, distance, transverse velocity) of the lens object (or lens system). For even more events, the complementary ground-based observations will yield 1-D parallax measurements. Together with the 1-D parallaxes from WFIRST alone, they can probe the entire mass range M ≳ M<sub>⊕</sub>. For luminous lenses, such 1-D parallax measurements can be promoted to complete solutions (mass, distance, transverse velocity) by high-resolution imaging. This would provide crucial information not only about the hosts of planets and other lenses, but also enable a much more precise Galactic model. Other benefits of such a survey include improved understanding of binaries (particularly with low mass primaries), and sensitivity to distant ice-giant and gas-giant companions of WFIRST lenses that cannot be detected by WFIRST itself due to its restricted observing windows. Existing ground-based microlensing surveys can be employed if WFIRST is pointed at lower-extinction fields than is currently envisaged. This would come at some cost to the event rate. Therefore the benefits of improved characterization of lenses must be weighed against these costs.