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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 25/122
241
- 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.
242
- Shin, Jihye
- Journal of the Korean astronomical society = 천문학회지
- 47, n.3
- pp.87-98
- 2014
- 원문 바로보기
We develop a parallel cosmological hydrodynamic simulation code designed for the study of formation and evolution of cosmological structures. The gravitational force is calculated using the TreePM method and the hydrodynamics is implemented based on the smoothed particle hydrodynamics. The initial displacement and velocity of simulation particles are calculated according to second-order Lagrangian perturbation theory using the power spectra of dark matter and baryonic matter. The initial background temperature is given by Recfast and the temperature uctuations at the initial particle position are assigned according to the adiabatic model. We use a time-limiter scheme over the individual time steps to capture shock-fronts and to ease the time-step tension between the shock and preshock particles. We also include the astrophysical gas processes of radiative heating/cooling, star formation, metal enrichment, and supernova feedback. We test the code in several standard cases such as one-dimensional Riemann problems, Kelvin-Helmholtz, and Sedov blast wave instability. Star formation on the galactic disk is investigated to check whether the Schmidt-Kennicutt relation is properly recovered. We also study global star formation history at different simulation resolutions and compare them with observations.
243
- 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.
244
- Woo, Jong-Hak
- Journal of the Korean astronomical society = 천문학회지
- 47, n.5
- pp.167-178
- 2014
- 원문 바로보기
We search for misclassified type 1 AGNs among type 2 AGNs identified with emission line flux ratios, and investigate the properties of the sample. Using 4 113 local type 2 AGNs at 0.02 < z < 0.05 selected from Sloan Digital Sky Survey Data Release 7, we detected a broad component of the <TEX>$H{\alpha}$</TEX> line with a Full-Width at Half-Maximum (FWHM) ranging from 1 700 to <TEX>$19090km\;s^{-1}$</TEX> for 142 objects, based on the spectral decomposition and visual inspection. The fraction of the misclassified type 1 AGNs among type 2 AGN sample is ~3.5%, implying that a large number of missing type 1 AGN population may exist. The misclassified type 1 AGNs have relatively low luminosity with a mean broad <TEX>$H{\alpha}$</TEX> luminosity, log <TEX>$L_{H\alpha}=40.50{\pm}0.35\;erg\;s^{-1}$</TEX>, while black hole mass of the sample is comparable to that of the local black hole population, with a mean black hole mass, log <TEX>$M_{BH}=6.94{\pm}0.51\;M_{\odot}$</TEX>. The mean Eddington ratio of the sample is log <TEX>$L_{bol}/L_{Edd}=-2.00{\pm}0.40$</TEX>, indicating that black hole activity is relatively weak, hence, AGN continuum is too weak to change the host galaxy color. We find that the O III lines show significant velocity offsets, presumably due to outflows in the narrow-line region, while the velocity offset of the narrow component of the <TEX>$H{\alpha}$</TEX> line is not prominent, consistent with the ionized gas kinematics of general type 1 AGN population.
245
- 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.
246
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.
247
- 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.
248
- Jeong, Eui-Jun
- Journal of the Korean astronomical society = 천문학회지
- 47, n.6
- pp.209-214
- 2014
- 원문 바로보기
In this study we develop a set of solar proton event (SPE) forecast models with NOAA scales by Multi Layer Perceptron (MLP), one of neural network methods, using GOES solar X-ray flare data from 1976 to 2011. Our MLP models are the first attempt to forecast the SPE scales by the neural network method. The combinations of X-ray flare class, impulsive time, and location are used for input data. For this study we make a number of trials by changing the number of layers and nodes as well as combinations of the input data. To find the best model, we use the summation of F-scores weighted by SPE scales, where F-score is the harmonic mean of PODy (recall) and precision (positive predictive value), in order to minimize both misses and false alarms. We find that the MLP models are much better than the multiple linear regression model and one layer MLP model gives the best result.
249
- Ann, Hong Bae
- Journal of the Korean astronomical society = 천문학회지
- 47, n.1
- pp.1-13
- 2014
- 원문 바로보기
We analyze the dependence of disk morphology (arm class, Hubble type, bar type) of nearby spiral galaxies on the galaxy environment by using local background density (<TEX>${\Sigma}_n$</TEX>), projected distance (<TEX>$r_p$</TEX>), and tidal index (T I) as measures of the environment. There is a strong dependence of arm class and Hubble type on the galaxy environment, while the bar type exhibits a weak dependence with a high frequency of SB galaxies in high density regions. Grand design fractions and early-type fractions increase with increasing <TEX>${\Sigma}_n$</TEX>, <TEX>$1/r_p$</TEX>, and T I, while fractions of flocculent spirals and late-type spirals decrease. Multiple-arm and intermediate-type spirals exhibit nearly constant fractions with weak trends similar to grand design and early-type spirals. While bar types show only a marginal dependence on <TEX>${\Sigma}_n$</TEX>, they show a fairly clear dependence on <TEX>$r_p$</TEX> with a high frequency of SB galaxies at small <TEX>$r_p$</TEX>. The arm class also exhibits a stronger correlation with <TEX>$r_p$</TEX> than <TEX>${\Sigma}_n$</TEX> and T I, whereas the Hubble type exhibits similar correlations with <TEX>${\Sigma}_n$</TEX> and <TEX>$r_p$</TEX>. This suggests that the arm class is mostly affected by the nearest neighbor while the Hubble type is affected by the local densities contributed by neighboring galaxies as well as the nearest neighbor.
250
- Gould, Andrew
- Journal of the Korean astronomical society = 천문학회지
- 47, n.6
- pp.215-218
- 2014
- 원문 바로보기
One-dimensional (1-D) microlens parallaxes can be combined with heliocentric lens-source relative proper motion measurements to derive the lens mass and distance, as suggested by Ghosh et al. (2004). Here I present the first mathematical anlysis of this procedure, which I show can be represented as a quadratic equation. Hence, it is formally subject to a two-fold degeneracy. I show that this degeneracy can be broken in many cases using the relatively crude 2-D parallax information that is often available for microlensing events. I also develop an explicit formula for the region of parameter space where it is more difficult to break this degeneracy. Although no mass/distance measurements have yet been made using this technique, it is likely to become quite common over the next decade.