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1968년 ~ 2023년까지 1,187 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 : 1,187건 (DB Construction : 1,187 Articles)
총 게시글 1,187건
페이지 23/119
221
- 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.
222
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.
223
- 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.
224
- 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.
225
- 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.
226
- Hong, Sungwook E.
- Journal of the Korean astronomical society = 천문학회지
- 47, n.2
- pp.49-67
- 2014
- 원문 바로보기
A novel method to characterize the topology of the early-universe intergalactic medium during the epoch of cosmic reionization is presented. The 21-cm radiation background from high redshift is analyzed through calculation of the 2-dimensional (2D) genus. The radiative transfer of hydrogen- ionizing photons and ionization-rate equations are calculated in a suite of numerical simulations under various input parameters. The 2D genus is calculated from the mock 21-cm images of high-redshift universe. We construct the 2D genus curve by varying the threshold differential brightness temperature, and compare this to the 2D genus curve of the underlying density field. We find that (1) the 2D genus curve reflects the evolutionary track of cosmic reionization and (2) the 2D genus curve can discriminate between certain reionization scenarios and thus indirectly probe the properties of radiation-sources. Choosing the right beam shape of a radio antenna is found crucial for this analysis. Square Kilometre Array (SKA) is found to be a suitable apparatus for this analysis in terms of sensitivity, even though some deterioration of the data for this purpose is unavoidable under the planned size of the antenna core.
227
- Ahn, Jieun
- Journal of the Korean astronomical society = 천문학회지
- 47, n.2
- pp.77-86
- 2014
- 원문 바로보기
Using a cosmological <TEX>${\Lambda}CDM$</TEX> simulation, we analyze the differences between the widely-used spin parameters suggested by Peebles and Bullock. The dimensionless spin parameter <TEX>${\lambda}$</TEX> proposed by Peebles is theoretically well-justified but includes an annoying term, the potential energy, which cannot be directly obtained from observations and is computationally expensive to calculate in numerical simulations. The Bullock's spin parameter <TEX>${\lambda}^{\prime}$</TEX> avoids this problem assuming the isothermal density profile of a virialized halo in the Newtonian potential model. However, we find that there exists a substantial discrepancy between <TEX>${\lambda}$</TEX> and <TEX>${\lambda}^{\prime}$</TEX> depending on the adopted potential model (Newtonian or Plummer) to calculate the halo total energy and that their redshift evolutions differ to each other significantly. Therefore, we introduce a new spin parameter, <TEX>${\lambda}^{\prime\prime}$</TEX>, which is simply designed to roughly recover the value of <TEX>${\lambda}$</TEX> but to use the same halo quantities as used in <TEX>${\lambda}^{\prime}$</TEX>. If the Plummer potential is adopted, the <TEX>${\lambda}^{\prime\prime}$</TEX> is related to the Bullock's definition as <TEX>${\lambda}^{\prime\prime}=0.80{\times}(1+z)^{-1/12}{\lambda}^{\prime}$</TEX>. Hence, the new spin parameter <TEX>${\lambda}^{\prime\prime}$</TEX> distribution becomes consistent with a log-normal distribution frequently seen for the <TEX>${\lambda}^{\prime}$</TEX> while its mean value is much closer to that of <TEX>${\lambda}$</TEX>. On the other hand, in case of the Newtonian potential model, we obtain the relation of <TEX>${\lambda}^{\prime\prime}=(1+z)^{-1/8}{\lambda}^{\prime}$</TEX>; there is no significant difference at z = 0 as found by others but <TEX>${\lambda}^{\prime}$</TEX> becomes more overestimated than <TEX>${\lambda}$</TEX> or <TEX>${\lambda}^{\prime\prime}$</TEX> at higher redshifts. We also investigate the dependence of halo spin parameters on halo mass and redshift. We clearly show that although the <TEX>${\lambda}^{\prime}$</TEX> for small-mass halos with <TEX>$M_h$</TEX> < <TEX>$2{\times}10^{12}M_{\odot}$</TEX> seems redshift independent after z = 1, all the spin parameters explored, on the whole, show a stronger correlation with the increasing halo mass at higher redshifts.
228
- Trippe, Sascha
- Journal of the Korean astronomical society = 천문학회지
- 47, n.1
- pp.15-39
- 2014
- 원문 바로보기
Polarization is a basic property of light and is fundamentally linked to the internal geometry of a source of radiation. Polarimetry complements photometric, spectroscopic, and imaging analyses of sources of radiation and has made possible multiple astrophysical discoveries. In this article I review (i) the physical basics of polarization: electromagnetic waves, photons, and parameterizations; (ii) astrophysical sources of polarization: scattering, synchrotron radiation, active media, and the Zeeman, Goldreich-Kylafis, and Hanle effects, as well as interactions between polarization and matter (like birefringence, Faraday rotation, or the Chandrasekhar-Fermi effect); (iii) observational methodology: on-sky geometry, influence of atmosphere and instrumental polarization, polarization statistics, and observational techniques for radio, optical, and <TEX>$X/{\gamma}$</TEX> wavelengths; and (iv) science cases for astronomical polarimetry: solar and stellar physics, planetary system bodies, interstellar matter, astrobiology, astronomical masers, pulsars, galactic magnetic fields, gamma-ray bursts, active galactic nuclei, and cosmic microwave background radiation.
229
- Kim, Chulhee
- Journal of the Korean astronomical society = 천문학회지
- 47, n.5
- pp.163-166
- 2014
- 원문 바로보기
In order to identify peculiar X-ray sources, we select 442 sources with no counterparts in other wavelength bands (as of the year 1999) from the ROSAT All-Sky Survey Bright Source Catalog. We cross-correlate this initial list with the NASA/IPAC Extragalactic Database, the USNO and WISE catalogs, and the HEASARC XRAY Master Catalog. Eventually, we are left with four unidentified sources with no counterparts in other wavelength bands. We present these four sources and their X-ray properties in this paper.
230
- Kang, Ji-Hyun
- Journal of the Korean astronomical society = 천문학회지
- 47, n.6
- pp.259-277
- 2014
- 원문 바로보기
G192.8-1.1 has been known as one of the faintest supernova remnants (SNRs) in the Galax until the radio continuum of G192.8-1.1 is proved to be thermal by Gao et al. (2011). Yet, the nature of G192.8-1.1 has not been fully investigated. Here, we report the possible discovery of faint non-thermal radio continuum components with a spectral index <TEX>${\alpha}{\sim}0.56(S_{\nu}{\propto}{\nu}^{-{\alpha}})$</TEX> around G192.8-1.1, while of the radio continuum emission is thermal. Also, our Arecibo <TEX>$H_I$</TEX> data reveal an <TEX>$H_I$</TEX> shell, expanding with an expansion velocity of <TEX>$20-60km\;s^{-1}$</TEX>, that has an excellent morphological correlation with the radio continuum emission. The estimated physical parameters of the <TEX>$H_I$</TEX> shell and the possible association of non-thermal radio continuum emission with it suggest G192.8-1.1 to be an~0.3 Myr-old SNR. However, the presence of thermal radio continuum implies the presence of early-type stars in the same region. One possibility is that a massive star is ionizing the interior of an old SNR. If it is the case, the electron distribution assumed by the centrally-peaked surface brightness of thermal emission implies that G192.8-1.1 is a 'thermal-composite' SNR, rather than a typical shell-type SNR, where the central hot gas that used to be bright in X-rays has cooled down. Therefore, we propose that G192.8-1.1 is an old evolved thermal-composite SNR showing recurring emission in the radio continuum due to a nearby massive star. The infrared image supports that the <TEX>$H_I$</TEX> shell of G192.8-1.1 is currently encountering a nearby star forming region that possibly contains an early type star(s).