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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
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.
252
- 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.
253
- 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.
254
- 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.
255
- 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.
256
- 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>.
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
- Suh, Kyung-Won
- Journal of the Korean astronomical society = 천문학회지
- 47, n.6
- pp.219-233
- 2014
- 원문 바로보기
The main site of dust formation is believed to be the cool envelopes around AGB stars. Nearly all AGB stars can be identified as long-period variables (LPVs) with large amplitude pulsation. Shock waves produce by the strong pulsation and radiation pressure on newly formed dust grains drive dusty stellar winds with high mass-loss rates. IR observations of AGB stars identify various dust species in different physical conditions. Radio observations of gas phase materials are helpful to understand the overall properties of the stellar winds. In this paper, we review (i) classification of AGB stars; (ii) IR two-color diagrams of AGB stars; (iii) pulsation of AGB stars; (iv) dust around AGB stars including dusty stellar winds; (v) dust envelopes around AGB stars; (vi) mass-loss and evolution of AGB stars; and (vii) contribution of AGB dust to galactic environments. We discuss various observational evidences and their theoretical interpretations.
259
- Bong, Su-Chan
- Journal of the Korean astronomical society = 천문학회지
- 47, n.6
- pp.311-317
- 2014
- 원문 바로보기
Solar chromospheric surges are often reported to contain rotational motion. However, the details of the motion and driving mechanism of the surges are not yet fully understood. Recurrent surges with rotational motion at AR 10930 on the west limb are observed by Hinode Solar Optical Telescope (SOT) continuously from 11:21 UT on December 18 to 09:58 UT on December 19, 2006, using the <TEX>$Ca_{II}$</TEX> H broadband filter. We analyze details of the motion including number of turns from the rise of the surge to the fall, axial speed and acceleration. During the observation, rise and fall motion accompanying rotation appears recurrently. There occur a total of 14 surges at AR 10930 over 17 hours. The average duration is 45 minutes, and the average width, and length are 8 Mm, and 39 Mm, respectively. We speculate that the surges occurred by recurrent reconnections between the twisted prominence and large untwisted flux tube.
260
- Liu, Ying-Bo
- Journal of the Korean astronomical society = 천문학회지
- 47, n.3
- pp.115-122
- 2014
- 원문 바로보기
The New Vacuum Solar Telescope (NVST) is a 1-meter vacuum solar telescope that aims to observe the fine structures of active regions on the Sun. The main tasks of the NVST are high resolution imaging and spectral observations, including the measurements of the solar magnetic field. The NVST has been collecting more than 20 million FITS files since it began routine observations in 2012 and produces maximum observational records of 120 thousand files in a day. Given the large amount of files, the effective archiving and retrieval of files becomes a critical and urgent problem. In this study, we implement a new data archiving system for the NVST based on the Fastbit Not Only Structured Query Language (NoSQL) database. Comparing to the relational database (i.e., MySQL; My Structured Query Language), the Fastbit database manifests distinctive advantages on indexing and querying performance. In a large scale database of 40 million records, the multi-field combined query response time of Fastbit database is about 15 times faster and fully meets the requirements of the NVST. Our slestudy brings a new idea for massive astronomical data archiving and would contribute to the design of data management systems for other astronomical telescopes.