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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 26/123
251
- Hanna, Magdy
- Journal of the Korean astronomical society = 천문학회지
- 47, n.3
- pp.99-104
- 2014
- 원문 바로보기
We present the rst period variation study for the Algol eclipsing binary V346 Cyg by constructing the (O-C) residual diagram using all the available precise minima times. We conclude that the period variation can be explained by a sine-like variation due to the presence of a third body orbiting the binary in about <TEX>$68.89{\pm}4.69$</TEX> years, together with a long-term orbital period decrease (<TEX>$dP/dt=-1.23{\times}10^{-7}day/yr$</TEX>) that can be interpreted to be due to slow mass loss from the <TEX>${\delta}$</TEX>-Scuti primary component. The sinusoidal variation may also be explained by using the the Applegate (1992) mechanism involving cyclic magnetic activity due to star-spots on the secondary component. The present preliminary solution needs more precise photometric observations to be confirmed.
252
- Lee, Sang-Sung
- Journal of the Korean astronomical society = 천문학회지
- 47, n.6
- pp.303-309
- 2014
- 원문 바로보기
We present results of our investigation of the radio intrinsic brightness temperatures of compact radio jets. The intrinsic brightness temperatures of about 100 compact radio jets at 2, 5, 8, 15, and 86 GHz are estimated based on large VLBI surveys conducted in 2001-2003 (or in 1996 for the 5 GHz sample). The multi-frequency intrinsic brightness temperatures of the sample of jets are determined by a statistical method relating the observed brightness temperatures with the maximal apparent jet speeds, assuming one representative intrinsic brightness temperature for a sample of jets at each observing frequency. By investigating the observed brightness temperatures at 15 GHz in multiple epochs, we find that the determination of the intrinsic brightness temperature for our sample is affected by the flux density variability of individual jets at time scales of a few years. This implies that it is important to use contemporaneous VLBI observations for the multi-frequency analysis of intrinsic brightness temperatures. Since our analysis is based on the VLBI observations conducted in 2001-2003, the results are not strongly affected by the flux density variability. We find that the intrinsic brightness temperature <TEX>$T_0$</TEX> increases as <TEX>$T_0{\propto}{\nu}^{\xi}_{obs}$</TEX> with <TEX>${\xi}=0.7$</TEX> below a critical frequency <TEX>${\nu}_c{\approx}9GHz$</TEX> where the energy loss begins to dominate the emission. Above <TEX>${\nu}_c$</TEX>, <TEX>$T_0$</TEX> decreases with <TEX>${\xi}=-1.2$</TEX>, supporting the decelerating jet model or particle cascade model. We also find that the peak value of <TEX>$T_0{\approx}3.4{\times}10^{10}$</TEX> K is close to the equipartition temperature, implying that the VLBI cores observable at 2-86 GHz may be representing jet regions where the magnetic field energy dominates the total energy in jets.
253
- Lee, Youngung
- Journal of the Korean astronomical society = 천문학회지
- 47, n.6
- pp.319-325
- 2014
- 원문 바로보기
We map two molecular clouds located in the exact anticenter region emitting in the (J = 1-0) transition of <TEX>$^{12}CO$</TEX> and <TEX>$^{13}CO$</TEX> using the 3-mm SIS mixer receiver on the 14-m radio telescope at Taeduk Radio Astronomy Observatory. The target clouds with anomalous velocities of <TEX>$V_{LSR}{\sim}-20km\;s^{-1}$</TEX> are distinguished from other clouds in this direction. In addition, they are located in the interarm region between the Orion Arm and the Perseus Arm. Sizes of the clouds are estimated to be about 8.6 and 10.8 pc, respectively. The total mass is estimated to be about <TEX>$4{\times}10^3$</TEX> <TEX>$M_{\odot}$</TEX> using CO luminosity of the clouds. Several cores are detected, but no sign of star formation is found according to the IRAS point sources. Their larger linewidths, anomalous velocities, and their location at the interarm region make these clouds more distinguished, though their physical properties are similar to the dark clouds in the solar neighborhood in terms of mass and size.
254
- 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.
255
- 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.
256
- 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.
257
- 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>.
258
- 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.
259
- 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.
260
- 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).