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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 17/122
161
- Park, Songyoun
- Journal of the Korean astronomical society = 천문학회지
- 50, n.5
- pp.151-155
- 2017
- 원문 바로보기
We investigate the radio properties of the dwarf galaxy SDSS J133245.62+263449.3 which shows optical signatures of black hole activity. Dwarf galaxies are known to host intermediate mass black holes (IMBHs) with masses <TEX>$M_{BH}{\sim}10^{4-6}M_{\odot}$</TEX>, some of them being radio loud. Recently, Reines et al. (2013) found dwarf galaxy candidates which show signatures of being black hole hosts based on optical spectral lines. SDSS J133245.62+263449.3 is one of them; it shows a flux density of ~ 20 mJy at 1.4 GHz, which corresponds to <TEX>$L_{1.4GHz}{\sim}10^{23}W\;Hz^{-1}$</TEX>. This is much brighter than other black hole host dwarf galaxies. However, star formation activity can contribute to radio continuum emission as well. To understand the nature of the radio emission from SDSS J133245.62+263449.3, we imaged this radio loud dwarf galaxy at low frequencies (325 MHz and 610 MHz) using the Giant Metrewave Radio Telescope (GMRT). We present here the high resolution images from our GMRT observations. While we detect no obvious extended emission from radio jets from the central AGN, we do find the emission to be moderately extended and unlikely to be dominated by disk star formation. VLBI observations using the Korean VLBI Network (KVN) are now being planned to understand the emission morphology and radiation mechanism.
162
- Kang, Hyesung
- Journal of the Korean astronomical society = 천문학회지
- 50, n.4
- pp.93-103
- 2017
- 원문 바로보기
We explore the shock acceleration model for giant radio relics, in which relativistic electrons are accelerated via diffusive shock acceleration (DSA) by merger-driven shocks in the outskirts of galaxy clusters. In addition to DSA, turbulent acceleration by compressive MHD modes downstream of the shock are included as well as energy losses of postshock electrons due to Coulomb scattering, synchrotron emission, and inverse Compton scattering off the cosmic background radiation. Considering that only a small fraction of merging clusters host radio relics, we favor a reacceleration scenario in which radio relics are generated preferentially by shocks encountering the regions containing low-energy (<TEX>${\gamma}_e{\leq}300$</TEX>) cosmic ray electrons (CRe). We perform time-dependent DSA simulations of spherically expanding shocks with physical parameters relevant for the Sausage radio relic, and calculate the radio synchrotron emission from the accelerated CRe. We find that significant level of postshock turbulent acceleration is required in order to reproduce broad profiles of the observed radio flux densities of the Sausage relic. Moreover, the spectral curvature in the observed integrated radio spectrum can be explained, if the putative shock should have swept up and exited out of the preshock region of fossil CRe about 10 Myr ago.
163
- Sudou, Hiroshi
- Journal of the Korean astronomical society = 천문학회지
- 50, n.6
- pp.157-165
- 2017
- 원문 바로보기
<TEX>$H_2O$</TEX> maser emission at 22 GHz in the circumstellar envelope is one of the good tracers of detailed physics and kinematics in the mass loss process of asymptotic giant branch stars. Long-term monitoring of an <TEX>$H_2O$</TEX> maser spectrum with high time resolution enables us to clarify acceleration processes of the expanding shell in the stellar atmosphere. We monitored the <TEX>$H_2O$</TEX> maser emission of the semi-regular variable R Crt with the Kagoshima 6-m telescope, and obtained a large data set of over 180 maser spectra over a period of 1.3 years with an observational span of a few days. Using an automatic peak detection method based on least-squares fitting, we exhaustively detected peaks as significant velocity components with the radial velocity on a <TEX>$0.1kms^{-1}$</TEX> scale. This analysis result shows that the radial velocity of red-shifted and blue-shifted components exhibits a change between acceleration and deceleration on the time scale of a few hundred days. These velocity variations are likely to correlate with intensity variations, in particular during flaring state of <TEX>$H_2O$</TEX> masers. It seems reasonable to consider that the velocity variation of the maser source is caused by shock propagation in the envelope due to stellar pulsation. However, it is difficult to explain the relationship between the velocity variation and the intensity variation only from shock propagation effects. We found that a time delay of the integrated maser intensity with respect to the optical light curve is about 150 days.
164
- Gould, Andrew
- Journal of the Korean astronomical society = 천문학회지
- 50, n.1
- pp.1-5
- 2017
- 원문 바로보기
Like Hipparcos, Gaia is designed to give absolute parallaxes, independent of any astrophysical reference system. And indeed, Gaia's internal zero-point error for parallaxes is likely to be smaller than any individual parallax error. Nevertheless, due in part to mechanical issues of unknown origin, there are many astrophysical questions for which the parallax zero-point error <TEX>${\sigma}({\pi}_0)$</TEX> will be the fundamentally limiting constraint. These include the distance to the Large Magellanic Cloud and the Galactic Center. We show that by using the photometric parallax estimates for RR Lyrae stars (RRL) within 8kpc, via the ultra-precise infrared period-luminosity relation, one can independently determine a hyper-precise value for <TEX>${\pi}_0$</TEX>. Despite their paucity relative to bright quasars, we show that RRL are competitive due to their order-of-magnitude improved parallax precision for each individual object relative to bright quasars. We show that this method is mathematically robust and well-approximated by analytic formulae over a wide range of relevant distances.
165
- Lee, Jae-Woo
- Journal of the Korean astronomical society = 천문학회지
- 49, n.6
- pp.289-293
- 2016
- 원문 바로보기
We investigate the long-term spatial drift of the center and the temporal variation of the shutter delay time map of Y4KCam mounted on the CTIO 1.0 m telescope. We have collected shutter delay time maps over eight years as a part of our long-term survey program. We find that the center of the shutter delay time map can drift up to <TEX>$450{\mu}m$</TEX>, equivalent to <TEX>${\approx}30pixels$</TEX>, on the CCD. This effect can result in a small amount of error in integration time without the proper shutter delay time correction, but it does not appear to cause any significant problems in photometric measurements. We obtain a mean shutter delay time of <TEX>$69.1{\pm}0.8$</TEX> ms and find no temporal variation of the shutter delay time of Y4KCam over eight years, indicative of the mechanical stability of the shutter. We suggest that using a master shutter delay time correction frame would be sufficient to achieve high precision photometry, which does not exceed photometric errors <TEX>${\approx}1.7mmag$</TEX> across the CCD frame for exposure times longer than 1 s.
166
- SUH, KYUNG-WON
- Journal of the Korean astronomical society = 천문학회지
- 49, n.4
- pp.127-136
- 2016
- 원문 바로보기
We investigate optical properties of amorphous alumina (Al2O3) dust grains in the envelopes around O-rich asymptotic giant branch (AGB) stars using laboratory measured optical data. We derive the optical constants of amorphous alumina over a wide wavelength range that satisfy the Kramers-Kronig relation and reproduce the laboratory data. Using the amorphous alumina and silicate dust, we compare the radiative transfer model results with the observed spectral energy distributions. Comparing the theoretical models with observations on various IR two-color diagrams for a large sample of O-rich AGB stars, we find that the amorphous alumina dust (about 10-40%) mixed with amorphous silicate better models the observed points for the O-rich AGB stars with thin dust envelopes.
167
- Lee, Yongung
- Journal of the Korean astronomical society = 천문학회지
- 49, n.6
- pp.255-259
- 2016
- 원문 바로보기
We estimate the fractal dimension of the <TEX>${\rho}$</TEX> Ophiuchus Molecular Cloud Complex, associated with star forming regions. We selected a cube (<TEX>${\upsilon}$</TEX>, l, b) database, obtained with J = 1-0 transition lines of <TEX>$^{12}CO$</TEX> and <TEX>$^{13}CO$</TEX> at a resolution of 22' using a multibeam receiver system on the 14-m telescope of the Five College Radio Astronomy Observatory. Using a code developed within IRAF, we identified slice-clouds with two threshold temperatures to estimate the fractal dimension. With threshold temperatures of 2.25 K (<TEX>$3{\sigma}$</TEX>) and 3.75 K (<TEX>$5{\sigma}$</TEX>), the fractal dimension of the target cloud is estimated to be D = 1.52-1.54, where <TEX>$P{\propto}A^{D/2}$</TEX>, which is larger than previous results. We suggest that the sampling rate (spatial resolution) of observed data must be an important parameter when estimating the fractal dimension, and that narrower or wider dispersion around an arbitrary fit line and the intercepts at NP = 100 should be checked whether they relate to firms noise level or characteristic structure of the target cloud. This issue could be investigated by analysing several high resolution databases with different quality (low or moderate sensitivity).
168
- KANG, HYESUNG
- Journal of the Korean astronomical society = 천문학회지
- 49, n.3
- pp.83-92
- 2016
- 원문 바로보기
The Toothbrush radio relic associated with the merging cluster 1RXS J060303.3 is presumed to be produced by relativistic electrons accelerated at merger-driven shocks. Since the shock Mach number inferred from the observed radio spectral index, M<sub>radio</sub> ≈ 2.8, is larger than that estimated from X-ray observations, M<sub>X</sub> ≲ 1.5, we consider the re-acceleration model in which a weak shock of M<sub>s</sub> ≈ 1.2 - 1.5 sweeps through the intracluster plasma with a preshock population of relativistic electrons. We find the models with a power-law momentum spectrum with the slope, s ≈ 4.6, and the cutoff Lorentz factor, γ<sub>e,c</sub> ≈ 7-8×10<sup>4</sup> can reproduce reasonably well the observed profiles of radio uxes and integrated radio spectrum of the head portion of the Toothbrush relic. This study confirms the strong connection between the ubiquitous presence of fossil relativistic plasma originated from AGNs and the shock-acceleration model of radio relics in the intracluster medium.
169
- KIM, SEUNG-LEE
- Journal of the Korean astronomical society = 천문학회지
- 49, n.5
- pp.199-208
- 2016
- 원문 바로보기
We present photometric results of the δ Sct star V1162 Ori, which is extensively monitored for a total of 49 nights from mid-December 2014 to early-March 2015. The observations are made with three KMTNet (Korea Microlensing Telescope Network) 1.6 m telescopes installed in Chile, South Africa, and Australia. Multiple frequency analysis is applied to the data and resulted in clear detection of seven frequencies without an alias problem: five known frequencies and two new ones with small amplitudes of 1.2-1.7 mmag. The amplitudes of all but one frequency are significantly different from previous results, confirming the existence of long-term amplitude changes. We examine the variations in pulsation timings of V1162 Ori for about 30 years by using the times of maximum light obtained from our data and collected from the literatures. The O − C (Observed minus Calculated) timing diagram shows a combination of a downward parabolic variation with a period decreasing rate of (1/P)dP/dt = −4.22 × 10<sup>−6</sup> year<sup>−1</sup> and a cyclic change with a period of about 2780 days. The most probable explanation for this cyclic variation is the light-travel-time effect caused by an unknown binary companion, which has a minimum mass of 0.69 M<sub>⊙</sub>. V1162 Ori is the first δ Sct-type pulsating star of which the observed fast period decrease can be interpreted as an evolutionary effect of a pre-main sequence star, considering its membership of the Orion OB 1c association.
170
- Ann, Hong Bae
- Journal of the Korean astronomical society = 천문학회지
- 49, n.6
- pp.239-253
- 2016
- 원문 바로보기
We determined the warp parameters of 192 warped galaxies which are selected from 340 edge-on galaxies using color images as well as r-band isophotal maps. We derive the local background density (<TEX>${\Sigma}_n$</TEX>) to examine the dependence of the warp amplitudes on the galaxy environment. We find a clear trend that strongly warped galaxies are likely to be found in high density regions where tidal interactions are supposed to be frequent. However, the correlation between <TEX>${\alpha}_{\omega}$</TEX> and <TEX>${\Sigma}_n$</TEX> is too weak for weakly warped galaxies (<TEX>${\alpha}_{\omega}$</TEX> < <TEX>$4^{\circ}$</TEX>) and the cumulative distributions of weakly warped galaxies are not significantly different from those of galaxies with no detectable warps. This suggests that tidal interactions do not play a decisive role in the formation of weak warps.