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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 53/122
521
- Bong, Su-Chan
- Journal of the Korean astronomical society = 천문학회지
- 38, n.4
- pp.445-462
- 2005
- 원문 바로보기
In a companion paper, we have presented so-called Spatio-Spectral Maximum Entropy Method (SSMEM) particularly designed for Fourier-Transform imaging over a wide spectral range. The SSMEM allows simultaneous acquisition of both spectral and spatial information and we consider it most suitable for imaging spectroscopy of solar microwave emission. In this paper, we run the SSMEM for a realistic model of solar microwave radiation and a model array resembling the Owens Valley Solar Array in order to identify and resolve possible issues in the application of the SSMEM to solar microwave imaging spectroscopy. We mainly concern ourselves with issues as to how the frequency dependent noise in the data and frequency-dependent variations of source size and background flux will affect the result of imaging spectroscopy under the SSMEM. We also test the capability of the SSMEM against other conventional techniques, CLEAN and MEM.
522
- HIRANO NAOMI
- Journal of the Korean astronomical society = 천문학회지
- 38, n.2
- pp.219-222
- 2005
- 원문 바로보기
We present the high-resolution (2'-4') images of the molecular envelopes surrounding the evolved stars, V Hya, VY CMa, and <TEX>${\pi}^1$</TEX> Gru observed with the Submillimeter Array. The CO J=2-1 and 3-2 images of the carbon star V Hya show that the circumstellar structure of this star consists of three kinematic components; there is a flattened disk-like envelope that is expanding with a velocity of <TEX>${\~}16 km\;s^{-1}$</TEX>, the second component is the medium-velocity wind having a deprojected velocity of 40-120 km <TEX>$s^{-l}$</TEX> moving along the disk plane, and the third one is the bipolar molecular jet having an extreme velocity of 70-185 km <TEX>$s^{-l}$</TEX>. The axis of this high velocity jet is perpendicular to the plane of the disk-like envelope. We found that the circumstellar structure of the S-star <TEX>${\pi}^1$</TEX> Gru traced by the CO J =2-1 resembles that of V Hya quite closely; the star is surrounded by the expanding disk-like envelope and is driving the medium-velocity wind along the disk plane. We also obtained the excellent images of VY CMa with the CO and <TEX>$^{13}CO$</TEX> J=2-1 and <TEX>$SO\;6_5-5_4$</TEX> lines. The maps of three molecular lines show that the envelope has a significant velocity gradient in the east-west direction, suggesting that the envelope surrounding VY CMa is also flattened and expanding along its radial direction. The high-resolution images obtained with the SMA show that some AGB stars are associated with the asymmetric mass loss including the equatorial wind and bipolar jet.
523
- SATO BUN'EI
- Journal of the Korean astronomical society = 천문학회지
- 38, n.2
- pp.315-318
- 2005
- 원문 바로보기
We have carried out a precise Doppler survey of G-type giants aiming to unveil the properties of planetary systems in intermediate-mass stars (<TEX>$1.5-5M_{\bigodot}$</TEX>). G-type giants are promising targets for Doppler planet searches around massive stars, because they are slow-rotators and have many sharp absorption lines in their spectra and their surface activities are relatively low in contrast to their younger counterparts on the main-sequence (B-A stars). We are now monitoring radial velocities of about 300 late G-type (including early K-type) giants using HIgh Dispersion Echelle Spectrograph (HIDES) at Okayama Astrophysical Observatory. We have achieved a Doppler precision of about 6-7 m/s over a time span of 3 years using an iodine absorption cell. We found that most of the targets have radial velocity scatters of <TEX>${\sigma}{\~} 10-20 m\;s^{-1}$</TEX> over 1-3 years, with the most stable reaching levels of 6-8 m <TEX>$s^{-1}$</TEX>. Up to now, we have succeeded in discovering the first extrasolar planet around a G-type giant star HD 104985, and also found several candidates showing significant radial velocity variations, suggesting the existence of stellar and substellar companions. Observations have continued to establish their variability.
524
- SHEN ZHI-QIANG
- Journal of the Korean astronomical society = 천문학회지
- 38, n.2
- pp.261-266
- 2005
- 원문 바로보기
This paper reviews the progress in the VLBI (Very Long Baseline Interferometry) studies of Sgr A<TEX>$\ast$</TEX>, the best known supermassive black hole candidates with a dark mass concentration of <TEX>$4 {\times} 10^6 M_{\bigodot}$</TEX> at the center of the Milky Way. The emphasis is on the importance of the millimeter and sub-millimeter VLBI observations in the detection of Sgr A<TEX>$\ast$</TEX>'s intrinsic structure and search for the structural variation.
525
- SOHN J,
- Journal of the Korean astronomical society = 천문학회지
- 37, n.4
- pp.261-263
- 2004
- 원문 바로보기
We present a progress report on HCN(1-0) line observations toward starless cores to probe inward motions. We have made a single pointing survey toward the central regions of 85 starless cores and performed mapping observations of 6 infall candidate starless cores. The distributions of the velocity difference between HCN(1-0) hyperfine lines and the optically thin tracer <TEX>$N_2H^+$</TEX>(1-0) are significantly skewed to the blue, meaning that HCN(1-0) frequently detects inward motions. Their skewness to the blue is even greater than that of CS(2-1) Lee et al., possibly implying more infall occurrence than CS(1-0). We identify 19 infall candidates by using several characteristics illustrating spectral infall asymmetry seen in HCN(1-0) hyperfine lines, CS(3-2), CS(2-1), <TEX>$DCO^+(2-1)$</TEX> and <TEX>$N_2H^+$</TEX> observations. The HCN(1-0) F(O-l) with the least optical depth usually shows a similar intensity distribution to that of <TEX>$N_2H^+$</TEX> which closely traces the density distribution of the cores, indicating that HCN(1-0) is less chemically affected and so believed to reflect kinematics occurring in rather inner regions of the cores. Detailed radiative transfer model fits of the spectra are underway to analyze central infall kinematics in starless cores.
526
- FUJITA YUTAKA
- Journal of the Korean astronomical society = 천문학회지
- 37, n.5
- pp.571-574
- 2004
- 원문 바로보기
Clusters of galaxies are filled with X-ray emitted hot gas with the temperature of T <TEX>${\~}$</TEX>2-10 keV. Recent X-ray observations have been revealing unexpectedly that many cluster cores have complicated, peculiar X-ray structures, which imply dynamical motion of the hot gas. Moreover, X-ray spectra indicate that radiative cooling of the cool gas is suppressed by unknown heating mechanisms (the 'cooling flow problem'). Here we propose a novel mechanism reproducing both the inhomogeneous structures and dynamics of the hot gas in the cluster cores, based on state-of-the-art hydrodynamic simulations. We showed that acoustic-gravity waves, which are naturally expected during the process of hierarchical structure formation of the universe, surge in the X-ray hot gas, causing a serous impact on the core. This reminds us of tsunamis on the ocean surging into an distant island. We found that the waves create fully-developed, stable turbulence, which reproduces the complicated structures in the core. Moreover, if the wave amplitude is large enough, they can suppress the cooling of the core. The turbulence could be detected in near-future space X-ray missions such as ASTRO-E2.
527
- KURTZ S.
- Journal of the Korean astronomical society = 천문학회지
- 37, n.4
- pp.265-268
- 2004
- 원문 바로보기
Molecular clouds present many levels of structure, including clumps and cores of varying size and density. We present a brief summary of these cores, describing their observed physical properties and their place in the star formation process. We conclude with some speculation about pre-proto-stellar stages of molecular cores and the observational challenges in their observation.
528
- KANG HYESUNG
- Journal of the Korean astronomical society = 천문학회지
- 37, n.5
- pp.405-412
- 2004
- 원문 바로보기
Cosmological shocks form as an inevitable consequence of gravitational collapse during the large scale structure formation and cosmic-rays (CRs) are known to be accelerated at collisionless shocks via diffusive shock acceleration (DSA). We have calculated the evolution of CR modified shocks for a wide range of shock Mach numbers and shock speeds through numerical simulations of DSA in 1D quasi-parallel plane shocks. The simulations include thermal leakage injection of seed CRs, as well as pre-existing, upstream CR populations. Bohm-like diffusion is assumed. We show that CR modified shocks evolve to time-asymptotic states by the time injected particles are accelerated to moderately relativistic energies (p/mc <TEX>$\ge$</TEX> 1), and that two shocks with the same Mach number, but with different shock speeds, evolve qualitatively similarly when the results are presented in terms of a characteristic diffusion length and diffusion time. We find that <TEX>$10^{-4} - 10^{-3}$</TEX> of the particles passed through the shock are accelerated to form the CR population, and the injection rate is higher for shocks with higher Mach number. The CR acceleration efficiency increases with shock Mach number, but it asymptotes to <TEX>${\~}50\%$</TEX> in high Mach number shocks, regardless of the injection rate and upstream CR pressure. On the other hand, in moderate strength shocks (<TEX>$M_s {\le} 5$</TEX>), the pre-existing CRs increase the overall CR energy. We conclude that the CR acceleration at cosmological shocks is efficient enough to lead to significant nonlinear modifications to the shock structures.
529
- MINIATI FRANCESCO
- Journal of the Korean astronomical society = 천문학회지
- 37, n.5
- pp.465-470
- 2004
- 원문 바로보기
I address the issue of nonthermal processes in the large scale structure of the universe. After reviewing the properties of cosmic shocks and their role as particle accelerators, I discuss the main observational results, from radio to <TEX>$\gamma$</TEX>-ray and describe the processes that are thought be responsible for the observed nonthermal emissions. Finally, I emphasize the important role of <TEX>$\gamma$</TEX>-ray astronomy for the progress in the field. Non detections at these photon energies have already allowed us important conclusions. Future observations will tell us more about the physics of the intracluster medium, shocks dissipation and CR acceleration.
530
- RYU DONGSU
- Journal of the Korean astronomical society = 천문학회지
- 37, n.5
- pp.477-482
- 2004
- 원문 바로보기
Shock waves form in the intergalactic space as an ubiquitous consequence of cosmic structure formation. Using N-body/hydrodynamic simulation data of a ACDM universe, we examined the properties of cosmological shock waves including their morphological distribution. Adopting a diffusive shock acceleration model, we then calculated the amount of cosmic ray energy as well as that of gas thermal energy dissipated at the shocks. Finally, the dynamical consequence of those cosmic rays on cluster properties is discussed.