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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 26/122
251
- 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).
252
- Bach, Kiehunn
- Journal of the Korean astronomical society = 천문학회지
- 47, n.5
- pp.187-193
- 2014
- 원문 바로보기
Recent cosmological observations indicate that the reionized universe may have started at around z = 6, where a significant suppression around <TEX>$Ly{\alpha}$</TEX> has been observed from the neutral intergalactic medium. The associated neutral hydrogen column density is expected to exceed <TEX>$10^{21}cm^{-2}$</TEX>, where it is very important to use the accurate scattering cross section known as the Kramers-Heisenberg formula that is obtained from the fully quantum mechanical time-dependent second order perturbation theory. We present the Kramers-Heisenberg formula and compare it with the formula introduced in a heuristic way by Peebles (1993) considering the hydrogen atom as a two-level atom, from which we find a deviation by a factor of two in the red wing region far from the line center. Adopting a representative set of cosmological parameters, we compute the Gunn-Peterson optical depths and absorption profiles. Our results are quantitatively compared with previous work by Madau & Rees (2000), who adopted the Peebles approximation in their radiative transfer problems. We find deviations up to 5 per cent in the Gunn-Peterson transmission coefficient for an accelerated expanding universe in the red off-resonance wing part with the rest wavelength <TEX>${\Delta}{\lambda}{\sim}10{\AA}$</TEX>.
253
OPTICAL MULTI-CHANNEL INTENSITY INTERFEROMETRY - OR: HOW TO RESOLVE O-STARS IN THE MAGELLANIC CLOUDS
- Trippe, Sascha
- Journal of the Korean astronomical society = 천문학회지
- 47, n.6
- pp.235-253
- 2014
- 원문 바로보기
Intensity interferometry, based on the Hanbury Brown-Twiss effect, is a simple and inexpensive method for optical interferometry at microarcsecond angular resolutions; its use in astronomy was abandoned in the 1970s because of low sensitivity. Motivated by recent technical developments, we argue that the sensitivity of large modern intensity interferometers can be improved by factors up to approximately 25 000, corresponding to 11 photometric magnitudes, compared to the pioneering Narrabri Stellar Interferometer. This is made possible by (i) using avalanche photodiodes (APD) as light detectors, (ii) distributing the light received from the source over multiple independent spectral channels, and (iii) use of arrays composed of multiple large light collectors. Our approach permits the construction of large (with baselines ranging from few kilometers to intercontinental distances) optical interferometers at the cost of (very) long-baseline radio interferometers. Realistic intensity interferometer designs are able to achieve limiting R-band magnitudes as good as <TEX>$m_R{\approx}14$</TEX>, sufficient for spatially resolved observations of main-sequence O-type stars in the Magellanic Clouds. Multi-channel intensity interferometers can address a wide variety of science cases: (i) linear radii, effective temperatures, and luminosities of stars, via direct measurements of stellar angular sizes; (ii) mass-radius relationships of compact stellar remnants, via direct measurements of the angular sizes of white dwarfs; (iii) stellar rotation, via observations of rotation flattening and surface gravity darkening; (iv) stellar convection and the interaction of stellar photospheres and magnetic fields, via observations of dark and bright starspots; (v) the structure and evolution of multiple stars, via mapping of the companion stars and of accretion flows in interacting binaries; (vi) direct measurements of interstellar distances, derived from angular diameters of stars or via the interferometric Baade-Wesselink method; (vii) the physics of gas accretion onto supermassive black holes, via resolved observations of the central engines of luminous active galactic nuclei; and (viii) calibration of amplitude interferometers by providing a sample of calibrator stars.
254
- 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.
255
- 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.
256
- 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.
257
- 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.
258
- 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.
259
- 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.
260
- 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.