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- 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 28/123
271
- Kim, Jae-Young
- Journal of the Korean astronomical society = 천문학회지
- 47, n.5
- pp.195-199
- 2014
- 원문 바로보기
We present a GUI-based interactive Python program, VIMAP, which generates radio spectral index maps of active galactic nuclei (AGN) from Very Long Baseline Interferometry (VLBI) maps obtained at different frequencies. VIMAP is a handy tool for the spectral analysis of synchrotron emission from AGN jets, specifically of spectral index distributions, turn-over frequencies, and core-shifts. In general, the required accurate image alignment is difficult to achieve because of a loss of absolute spatial coordinate information during VLBI data reduction (self-calibration) and/or intrinsic variations of source structure as function of frequency. These issues are overcome by VIMAP which in turn is based on the two-dimensional cross-correlation algorithm of Croke & Gabuzda (2008). In this paper, we briefly review the problem of aligning VLBI AGN maps, describe the workflow of VIMAP, and present an analysis of archival VLBI maps of the active nucleus 3C 120.
272
- Nwaffiah, J.U.
- Journal of the Korean astronomical society = 천문학회지
- 47, n.4
- pp.147-152
- 2014
- 원문 바로보기
We present measurements of the Fe <TEX>$K{\alpha}$</TEX> emission line of the intermediate polar V1223 Sagittarii observed with the Suzaku satellite. The spectrum is modeled with an absorbed thermal bremsstrahlung spectrum and three Gaussians for the three components of the Fe <TEX>$K{\alpha}$</TEX> lines. We resolve the neutral or low-ionized (6.41keV), He-like (6.70keV), and H-like (7.00keV) iron lines. We also obtain a thermal continuum temperature of 25 keV, which supports a thermal origin of the hard X-rays observed from the shock heated layers of gas between the white dwarf and the shock front. Hence, we believe that the He-like and H-like lines are from the collisional plasma. On the origin of the Fe <TEX>$K{\alpha}$</TEX> fluorescence line, we find that it could be partly from reflections of hard X-rays from the white dwarf surface and the <TEX>$N_H$</TEX> absorption columns. We also discuss the Fe <TEX>$K{\alpha}$</TEX> emission line as veritable tool for the probe of some astrophysical sites.
273
- Lee, Byeong-Cheol
- Journal of the Korean astronomical society = 천문학회지
- 47, n.2
- pp.69-76
- 2014
- 원문 바로보기
We report the detection of an exoplanet candidate in orbit around <TEX>${\sigma}$</TEX> Persei from a radial velocity (RV) survey. The system exhibits periodic RV variations of <TEX>$579.8{\pm}2.4$</TEX> days. The purpose of the survey is to search for low-amplitude and long-period RV variations in giants and examine the origin of the variations using the fiber-fed Bohyunsan Observatory Echelle Spectrograph installed at the 1.8-m telescope of Bohyunsan Optical Astronomy Observatory in Korea. We present high-accuracy RV measurements of <TEX>${\sigma}$</TEX> Per made from December 2003 to January 2014. We argue that the RV variations are not related to the surface inhomogeneities but instead a Keplerian motion of the planetary companion is the most likely explanation. Assuming a stellar mass of <TEX>$2.25{\pm}0.5$</TEX> <TEX>$M_{\odot}$</TEX>, we obtain a minimum planetary companion mass of <TEX>$6.5{\pm}1.0$</TEX> <TEX>$M_{Jup}$</TEX>, with an orbital semi-major axis of <TEX>$1.8{\pm}0.1$</TEX> AU, and an eccentricity of <TEX>$0.3{\pm}0.1$</TEX> around <TEX>${\sigma}$</TEX> Per.
274
- Kwon, Young-Joo
- Journal of the Korean astronomical society = 천문학회지
- 47, n.4
- pp.123-135
- 2014
- 원문 바로보기
A silicate carbon star is a carbon star which shows circumstellar silicate dust features. We collect a sample of 44 silicate carbon stars from the literature and investigate the validity of the classification. For some objects, it is uncertain whether the central star is a carbon star. We confirm that 29 objects are verified silicate carbon stars. We classify the confirmed objects into three subclasses based on the evolution phase of the central star. To investigate the effect of the chemical transition phase from O to C, we use the radiative transfer models for the detached silicate dust shells. The spectral energy distributions and the infrared two-color diagrams of the silicate carbon stars are compared with the theoretical model results. For the chemical transition model without considering the effect of a disk, we find that the life time of the silicate feature is about 50 to 400 years depending on the initial dust optical depth.
275
- Yang, Dan
- Journal of the Korean astronomical society = 천문학회지
- 47, n.3
- pp.105-113
- 2014
- 원문 바로보기
We apply differential affine velocity estimator (DAVE) to the Solar Dynamics Observatory (SDO)/Helioseismic and Magnetic Imager (HMI) 12-min line-of-sight magnetograms, and separately calculate the injected magnetic helicity for the leading and the following polarities of nine emerging bipolar active regions (ARs). Comparing magnetic helicity flux of the leading polarity with the following polarity, we find that six ARs studied in this paper have the following polarity that injected more magnetic helicity flux than that of the leading polarity. We also measure the mean area of each polarity in all the nine ARs, and find that the compact polarity tend to possess more magnetic helicity flux than the fragmented one. Our results confirm the previous studies on asymmetry of magnetic helicity that emerging bipolar ARs have a polarity preference in injecting magnetic helicity. Based on the changes of unsigned magnetic flux, we divide the emergence process into two evolutionary stages: (1) an increasing stage before the peak flux and (2) a constant or decreasing stage after the peak flux. Obvious changes on magnetic helicity flux can be seen during transition from one stage to another. Seven ARs have one or both polarity that changed the sign of magnetic helicity flux. Additionally, the prevailing polarity of the two ARs, which injects more magnetic helicity, changes form the following polarity to the leading one.
276
- Yun, Youngjoo
- Journal of the Korean astronomical society = 천문학회지
- 47, n.6
- pp.201-207
- 2014
- 원문 바로보기
We report the development of solar flux receivers operating at 2.8 GHz to monitor solar radio activity. Radio waves from the sun are amplified, filtered, and then transmitted to a power meter sensor without frequency down-conversion. To measure solar flux, a calibration scheme is designed with a noise source, an ambient load, and a hot load at <TEX>$100^{\circ}C$</TEX>. The receiver is attached to a 1.8 m parabolic antenna in Icheon, owned by National Radio Research Agency, and observation is being conducted during day time on a daily basis. We compare the solar fluxes measured for last seven months with solar fluxes obtained by DRAO in Penticton, Canada, and by the Hiraiso solar observatory in Japan, and finally establish equations to convert observed flux to the so-called Penticton flux with an accuracy better than 3.2 sfu.
277
- Gould, Andrew
- Journal of the Korean astronomical society = 천문학회지
- 47, n.4
- pp.153-158
- 2014
- 원문 바로보기
I show that the standard microlensing technique to measure the angular radius of a star using color/surface-brightness relations can be inverted, via late-time proper motion measurements, to calibrate these relations. The method is especially useful for very metal-rich stars because such stars are in short supply in the solar neighborhood where other methods are most effective, but very abundant in Galactic bulge microlensing fields. I provide a list of eight spectroscopically identified high-metallicity bulge stars with the requisite finite-source effects, seven of which will be suitable calibrators when the Giant Magellan Telescope comes on line. Many more such sources can be extracted from current and future microlensing surveys.
278
PPMXL PHOTOMETRIC STUDY OF FOUR OPEN CLUSTER CANDIDATES (IVANOV 2, IVANOV 7, IVANOV 9 AND HARVARD 9)
- Tadross, A.L.
- Journal of the Korean astronomical society = 천문학회지
- 47, n.4
- pp.137-145
- 2014
- 원문 바로보기
The astrophysical parameters of four unstudied open star cluster candidates; Ivanov 2, 7, 9, and Harvard 9; are estimated for the first time using the PPMXL database. The stellar density distributions and color-magnitude diagrams for each cluster are used to determine the geometrical structure (cluster center, limited radius, core and tidal radii, the distances from the Sun, from the Galactic center and from the Galactic plane). Also, the main photometric parameters (age, distance modulus, color excesses, membership, total mass, relaxation time, luminosity and mass functions) are estimated.
279
- Trippe, Sascha
- Journal of the Korean astronomical society = 천문학회지
- 47, n.4
- pp.159-161
- 2014
- 원문 바로보기
The jet production efficiency of radio galaxies can be quantified by comparison of their kinetic jet powers <TEX>$P_{jet}$</TEX> and Bondi accretion powers <TEX>$P_B$</TEX>. These two parameters are known to be related linearly, with the jet power resulting from the Bondi power by multiplication with an efficiency factor of order 1%. Using a recently published (Nemmen & Tchekhovskoy 2014) high-quality sample of 27 radio galaxies, I construct a <TEX>$P_B$</TEX> - <TEX>$P_{jet}$</TEX> diagram that includes information on optical AGN types as far as available. This diagram indicates that the jet production efficiency is a function of AGN type: Seyfert 2 galaxies seem to be systematically (with a false alarm probability of <TEX>$4.3{\times}10^{-4}$</TEX>) less efficient, by about one order of magnitude, in powering jets than Seyfert 1 galaxies, LINERs, or the remaining radio galaxies. This suggests an evolutionary sequence from Sy 2s to Sy 1s and LINERs, controlled by an interplay of jets on the one hand and dust and gas in galactic nuclei on the other hand. When taking this effect into account, the <TEX>$P_B$</TEX> - <TEX>$P_{jet}$</TEX> relation is probably much tighter intrinsically than currently assumed.
280
- Park, Changbom
- Journal of the Korean astronomical society = 천문학회지
- 46, n.3
- pp.125-131
- 2013
- 원문 바로보기
We present the relation between the genus in cosmology and the Betti numbers for excursion sets of three- and two-dimensional smooth Gaussian random fields, and numerically investigate the Betti numbers as a function of threshold level. Betti numbers are topological invariants of figures that can be used to distinguish topological spaces. In the case of the excursion sets of a three-dimensional field there are three possibly non-zero Betti numbers; <TEX>${\beta}_0$</TEX> is the number of connected regions, <TEX>${\beta}_1$</TEX> is the number of circular holes (i.e., complement of solid tori), and <TEX>${\beta}_2$</TEX> is the number of three-dimensional voids (i.e., complement of three-dimensional excursion regions). Their sum with alternating signs is the genus of the surface of excursion regions. It is found that each Betti number has a dominant contribution to the genus in a specific threshold range. <TEX>${\beta}_0$</TEX> dominates the high-threshold part of the genus curve measuring the abundance of high density regions (clusters). <TEX>${\beta}_1$</TEX> dominates the genus near the median thresholds which measures the topology of negatively curved iso-density surfaces, and <TEX>${\beta}_2$</TEX> corresponds to the low-threshold part measuring the void abundance. We average the Betti number curves (the Betti numbers as a function of the threshold level) over many realizations of Gaussian fields and find that both the amplitude and shape of the Betti number curves depend on the slope of the power spectrum n in such a way that their shape becomes broader and their amplitude drops less steeply than the genus as n decreases. This behaviour contrasts with the fact that the shape of the genus curve is fixed for all Gaussian fields regardless of the power spectrum. Even though the Gaussian Betti number curves should be calculated for each given power spectrum, we propose to use the Betti numbers for better specification of the topology of large scale structures in the universe.