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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 21/122
201
- Ahn, Kyungjin
- Journal of the Korean astronomical society = 천문학회지
- 48, n.1
- pp.67-73
- 2015
- 원문 바로보기
We present a novel method to implement time-delayed propagation of radiation fields in cosmological radiative transfer simulations. Time-delayed propagation of radiation fields requires construction of retarded-time fields by tracking the location and lifetime of radiation sources along the corresponding light-cones. Cosmological radiative transfer simulations have, until now, ignored this 'light-cone effect' or implemented ray-tracing methods that are computationally demanding. We show that radiative transfer calculation of the time-delayed fields can be easily achieved in numerical simulations when periodic boundary conditions are used, by calculating the time-discretized retarded-time Green's function using the Fast Fourier Transform (FFT) method and convolving it with the source distribution. We also present a direct application of this method to the long-range radiation field of Lyman-Werner band photons, which is important in the high-redshift astrophysics with first stars.
202
- LEE, SANG-SUNG
- Journal of the Korean astronomical society = 천문학회지
- 48, n.5
- pp.229-236
- 2015
- 원문 바로보기
We report results of investigation of amplitude calibration for very long baseline interferometry (VLBI) observations with Korean VLBI Network (KVN). Amplitude correction factors are estimated based on comparison of KVN observations at 22 GHz correlated by Daejeon hardware correlator and DiFX software correlator in Korea Astronomy and Space Science Institute (KASI) with Very Long Baseline Array (VLBA) observations at 22 GHz by DiFX software correlator in National Radio Astronomy Observatory (NRAO). We used the observations for compact radio sources, 3C 454.3, NRAO 512, OJ 287, BL Lac, 3C 279, 1633+382, and 1510–089, which are almost unresolved for baselines in a range of 350-477 km. Visibility data of the sources obtained with similar baselines at KVN and VLBA are selected, fringe-fitted, calibrated, and compared for their amplitudes. We find that visibility amplitudes of KVN observations should be corrected by factors of 1.10 and 1.35 when correlated by DiFX and Daejeon correlators, respectively. These correction factors are attributed to the combination of two steps of 2-bit quantization in KVN observing systems and characteristics of Daejeon correlator.
203
- LEE, TAESEOK
- Journal of the Korean astronomical society = 천문학회지
- 48, n.5
- pp.313-323
- 2015
- 원문 바로보기
Active galactic nuclei (AGN) are known for irregular variability on all time scales, down to intra-day variability with relative variations of a few percent within minutes to hours. On such short timescales, unexplored territory, such as the possible existence of a shortest characteristic time scale of activity and the shape of the high frequency end of AGN power spectra, still exists. We present the results of AGN single-dish fast photometry performed with the Korean VLBI Network (KVN). Observations were done in a “anti-correlated” mode using two antennas, with always at least one antenna pointing at the target. This results in an effective time resolution of less than three minutes. We used all four KVN frequencies, 22, 43, 86, and 129 GHz, in order to trace spectral variability, if any. We were able to derive high-quality light curves for 3C 111, 3C 454.3, and BL Lacertae at 22 and 43 GHz, and for 3C 279 at 86 GHz, between May 2012 and April 2013. We performed a detailed statistical analysis in order to assess the levels of variability and the corresponding upper limits. We found upper limits on flux variability ranging from ~1.6% to ~7.6%. The upper limits on the derived brightness temperatures exceed the inverse Compton limit by three to six orders of magnitude. From our results, plus comparison with data obtained by the University of Michigan Radio Astronomy Observatory, we conclude that we have not detected source-intrinsic variability which would have to occur at sub-per cent levels.
204
- SOHN, JUBEE
- Journal of the Korean astronomical society = 천문학회지
- 48, n.6
- pp.381-398
- 2015
- 원문 바로보기
Dynamical analysis of compact groups provides important tests of models of compact group formation and evolution. By compiling 2066 redshifts from FLWO/FAST, from the literature, and from SDSS DR12 in the fields of compact groups in , we construct the largest sample of compact groups with complete spectroscopic redshifts in the redshift range 0.01 < z < 0.22. This large redshift sample shows that the interloper fraction in the compact group candidates is ~ 42%. A secure sample of 332 compact groups includes 192 groups with four or more member galaxies and 140 groups with three members. The fraction of early-type galaxies in these compact groups is 62%, higher than for the original Hickson compact groups. The velocity dispersions of early-and late-type galaxies in compact groups change little with groupcentric radius; the radii sampled are less than 100 h<sup>−1</sup> kpc, smaller than the radii typically sampled by members of massive clusters of galaxies. The physical properties of our sample compact groups include size, number density, velocity dispersion, and local environment; these properties slightly differ from those derived for the original Hickson compact groups and for the DPOSS II compact groups. Differences result from subtle differences in the way the group candidates were originally selected. The abundance of the compact groups changes little with redshift over the range covered by this sample. The approximate constancy of the abundance for this sample is a potential constraint on the evolution of compact groups on a few Gigayear timescale.
205
- TRIPPE, SASCHA
- Journal of the Korean astronomical society = 천문학회지
- 48, n.3
- pp.203-206
- 2015
- 원문 바로보기
The masses of supermassive black holes in active galactic nuclei (AGN) can be derived spectroscopically via virial mass estimators based on selected broad optical/ultraviolet emission lines. These estimates commonly use the line width as a proxy for the gas speed and the monochromatic continuum luminosity, λL<sub>λ</sub>, as a proxy for the radius of the broad line region. However, if the size of the broad line region scales with the bolometric AGN luminosity rather than λL<sub>λ</sub>, mass estimates based on different emission lines will show a systematic discrepancy which is a function of the color of the AGN continuum. This has actually been observed in mass estimates based on Hα/Hβ and C<sub>IV</sub> lines, indicating that AGN broad line regions indeed scale with bolometric luminosity. Given that this effect seems to have been overlooked as yet, currently used single-epoch mass estimates are likely to be biased.
206
- Cho, Wankee
- Journal of the Korean astronomical society = 천문학회지
- 48, n.2
- pp.139-154
- 2015
- 원문 바로보기
We carry out three-dimensional hydrodynamic simulations of the supernova remnants (SNRs) produced inside molecular clouds (MCs) near their surface using the HLL code (<xref>Harten et al. 1983</xref>). We explore the dynamical evolution and the X-ray morphology of SNRs after breaking through the MC surface for ranges of the explosion depths below the surface and the density ratios of the clouds to the intercloud media (ICM). We find that if an SNR breaks out through an MC surface in its Sedov stage, the outermost dense shell of the remnant is divided into several layers. The divided layers are subject to the Rayleigh-Taylor instability and fragmented. On the other hand, if an SNR breaks through an MC after the remnant enters the snowplow phase, the radiative shell is not divided to layers. We also compare the predictions of previous analytic solutions for the expansion of SNRs in stratified media with our onedimensional simulations. Moreover, we produce synthetic X-ray surface brightness in order to research the center-bright X-ray morphology shown in thermal composite SNRs. In the late stages, a breakout SNR shows the center-bright X-ray morphology inside an MC in our results. We apply our model to the observational results of the X-ray morphology of the thermal composite SNR 3C 391.
207
- KIM, JUHAN
- Journal of the Korean astronomical society = 천문학회지
- 48, n.4
- pp.213-228
- 2015
- 원문 바로보기
The Horizon Run 4 is a cosmological N-body simulation designed for the study of coupled evolution between galaxies and large-scale structures of the Universe, and for the test of galaxy formation models. Using 6300<sup>3</sup> gravitating particles in a cubic box of L<sub>box</sub> = 3150 h<sup>−1</sup>Mpc, we build a dense forest of halo merger trees to trace the halo merger history with a halo mass resolution scale down to M<sub>s</sub> = 2.7 × 10<sup>11</sup>h<sup>−1</sup>M<sub>⊙</sub>. We build a set of particle and halo data, which can serve as testbeds for comparison of cosmological models and gravitational theories with observations. We find that the FoF halo mass function shows a substantial deviation from the universal form with tangible redshift evolution of amplitude and shape. At higher redshifts, the amplitude of the mass function is lower, and the functional form is shifted toward larger values of ln(1/σ). We also find that the baryonic acoustic oscillation feature in the two-point correlation function of mock galaxies becomes broader with a peak position moving to smaller scales and the peak amplitude decreasing for increasing directional cosine μ compared to the linear predictions. From the halo merger trees built from halo data at 75 redshifts, we measure the half-mass epoch of halos and find that less massive halos tend to reach half of their current mass at higher redshifts. Simulation outputs including snapshot data, past lightcone space data, and halo merger data are available at <uri>http://sdss.kias.re.kr/astro/Horizon-Run4</uri>.
208
- OH, JUNGHWAN
- Journal of the Korean astronomical society = 천문학회지
- 48, n.5
- pp.299-311
- 2015
- 원문 바로보기
We report first results from KVN and VERA Array (KaVA) VLBI observations obtained in the frame of our Plasma-physics of Active Galactic Nuclei (PAGaN) project. We observed eight selected AGN at 22 and 43 GHz in single polarization (LCP) between March 2014 and April 2015. Each source was observed for 6 to 8 hours per observing run to maximize the uv coverage. We obtained a total of 15 deep high-resolution images permitting the identification of individual circular Gaussian jet components and three spectral index maps of BL Lac, 3C 111 and 3C 345 from simultaneous dual-frequency observations. The spectral index maps show trends in agreement with general expectations – flat core and steep jets – while the actual value of the spectral index for jets shows indications for a dependence on AGN type. We analyzed the kinematics of jet components of BL Lac and 3C 111, detecting superluminal proper motions with maximum apparent speeds of about 5c. This constrains the lower limits of the intrinsic component velocities to ~ 0.98c and the upper limits of the angle between jet and line of sight to ~20°. In agreement with global jet expansion, jet components show systematically larger diameters d at larger core distances r, following the global relation d ≈ 0.2r, albeit within substantial scatter.
209
- Gould, Andrew
- Journal of the Korean astronomical society = 천문학회지
- 48, n.2
- pp.93-104
- 2015
- 원문 바로보기
WFIRST microlensing observations will return high-precision parallaxes, σ(π) <graphic></graphic>. 0.3 µas, for the roughly 1 million stars with H < 14 in its 2.8 deg<sup>2</sup> field toward the Galactic bulge. Combined with its 40,000 epochs of high precision photometry (∼ 0.7 mmag at H<sub>vega</sub> = 14 and ∼ 0.1 mmag at H = 8), this will yield a wealth of asteroseismic data of giant stars, primarily in the Galactic bulge but includindvvvvvg a substantial fraction of disk stars at all Galactocentric radii interior to the Sun. For brighter stars, the astrometric data will yield an external check on the radii derived from the two asteroseismic parameters, the large-frequency separation <∆ν<sub>nl</sub>> and the frequency of maximum oscillation power ν<sub>max</sub>, while for the fainter ones, it will enable a mass measurement from the single measurable asteroseismic parameter ν<sub>max</sub>. Simulations based on Kepler data indicate that WFIRST will be capable of detecting oscillations in stars from slightly less luminous than the red clump to the tip of the red giant branch, yielding roughly 1 million detections.
210
- LIM, BEOMDU
- Journal of the Korean astronomical society = 천문학회지
- 48, n.6
- pp.343-355
- 2015
- 원문 바로보기
There is much observational evidence that active star formation is taking place in the Hii regions Sh 2-255 – 257. We present a photometric study of this star forming region (SFR) using imaging data obtained in passbands from the optical to the mid-infrared, in order to study the star formation process. A total of 218 members were identified using various selection criteria based on their observational properties. The SFR is reddened by at least E(B −V ) = 0.8 mag, and the reddening law toward the region is normal (R<sub>V</sub> = 3.1). From the zero-age main sequence fitting method it is confirmed that the SFR is 2.1 ± 0.3 kpc from the Sun. The median age of the identified members is estimated to be about 1.3 Myr from a comparison of the Hertzsprung-Russell diagram (HRD) with stellar evolutionary models. The initial mass function (IMF) is derived from the HRD and the near-infrared (J, J −H) color-magnitude diagram. The slope of the IMF is about Γ = −1.6 ± 0.1, which is slightly steeper than that of the Salpeter/Kroupa IMF. It implies that low-mass star formation is dominant in the SFR. The sum of the masses of all the identified members provides the lower limit of the cluster mass (169M<sub>⊙</sub>). We also analyzed the spectral energy distribution (SED) of pre-main sequence stars using the SED fitting tool of Robitaille et al., and confirm that there is a significant discrepancy between stellar mass and age obtained from two different methods based on the SED fitting tool and the HRD.