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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 8/122
71
- Magara, Tetsuya
- Journal of the Korean astronomical society = 천문학회지
- 54, n.5
- pp.157-170
- 2021
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We investigate flow and magnetic structure of a solar prominence with a focus on how the magnetic field originally determined by subsurface dynamics gives rise to the structure. We perform a magnetohydrodynamic simulation that reproduces the self-consistent evolution of a flow and the magnetic field passing freely through the solar surface. By analyzing Lagrangian displacements of magnetized plasma elements, we demonstrate the flow structure that is naturally incorporated to the magnetic structure of the prominence formed via dynamic interaction between the flow and the magnetic field. Our results explain a diverging flow on a U-loop, a counterclockwise downdraft along a rotating field line, acceleration and deceleration of a downflow along an S-loop, and partial emergence of a W-loop, which may play key roles in determining structural properties of the prominence.
72
- Kim, Ki-Beom
- Journal of the Korean astronomical society = 천문학회지
- 54, n.4
- pp.129-137
- 2021
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It has been established that the acoustic mode parameters of the Sun and Sun-like stars vary over activity cycles. Since the observed variations are not consistent with an activity-related origin, even Sun-like stars showing out-of-phase changes of mode frequencies and amplitudes need to be carefully studied using other observational quantities. In order to test whether the presumed relations between the global seismic parameters are a signature of the stellar activity cycle, we analyze the photometric light curve of HD 49933 for which the first direct detection of an asteroseismic signature for activity-induced variations in a Sun-like star was made, using observations by the CoRoT space telescope. We find that the amplitude of the envelope significantly anti-correlates with both the maximum frequency of the envelope and the width of the envelope unless superflare-like events completely contaminate the light curve. However, even though the photometric proxy for stellar magnetic activity appears to show relations with the global asteroseismic parameters, they are statistically insignificant. Therefore, we conclude that the global asteroseismic parameters can be utilized in cross-checking asteroseismic detections of activity-related variations in Sun-like stars, and that it is probably less secure and effective to construct a photometric magnetic activity proxy to indirectly correlate the global asteroseismic parameters. Finally, we seismically estimate the mass of HD 49933 based on our determination of the large separation of HD 49933 with evolutionary tracks computed by the MESA code and find a value of about 1.2M<sub>☉</sub> and a sub-solar metallicity of Z = 0.008, which agrees with the current consensus and with asteroseismic and non-asteroseismic data.
73
- Do, Thi Hoai
- Journal of the Korean astronomical society = 천문학회지
- 54, n.6
- pp.171-182
- 2021
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The lack of short baselines, referred to as the short-spacing problem (SSP), is a well-known limitation of the performance of radio interferometers, causing a reduction of the flux detected from source structure on large angular scales. The very large number of antennas operated in the Atacama Large Millimeter/sub-millimeter Array (ALMA) generates situations for which the impact of the SSP takes a complex form, not simply measurable by a single number, such as the maximal recoverable scale. In particular, extended antenna configurations, complemented by a small group of closeby antennas at the centre of the array, may result in a double-humped baseline distribution with a significant gap between the two groups. In such cases one should adopt as the effective maximal recoverable scale the one associated with the extended array and use only the central array to recover missing flux, as one would do with single dish or ACA (Atacama Compact Array) observations. The impact of the missing baselines can be very important and may easily be underestimated, or even overlooked. The present study uses ALMA archival data of the <sup>29</sup>SiO(8-7) line emission of the AGB star W Hydrae for a demonstration. A critical discussion of the reliability of the observations away from the star is presented together with comments of a broader scope. Properties of the circumstellar envelope of W Hya within ~15 au from the star, many of which are not mentioned in the published literature, are briefly described and compared with R Doradus, an AGB star having properties very similar to W Hya.
74
- Kang, Hyesung
- Journal of the Korean astronomical society = 천문학회지
- 54, n.3
- pp.103-112
- 2021
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The intracluster medium (ICM) is expected to experience on average about three passages of weak shocks with low sonic Mach numbers, M ≲ 3, during the formation of galaxy clusters. Both protons and electrons could be accelerated to become high energy cosmic rays (CRs) at such ICM shocks via diffusive shock acceleration (DSA). We examine the effects of DSA by multiple shocks on the spectrum of accelerated CRs by including in situ injection/acceleration at each shock, followed by repeated re-acceleration at successive shocks in the test-particle regime. For simplicity, the accelerated particles are assumed to undergo adiabatic decompression without energy loss and escape from the system, before they encounter subsequent shocks. We show that in general the CR spectrum is flattened by multiple shock passages, compared to a single episode of DSA, and that the acceleration efficiency increases with successive shock passages. However, the decompression due to the expansion of shocks into the cluster outskirts may reduce the amplification and flattening of the CR spectrum by multiple shock passages. The final CR spectrum behind the last shock is determined by the accumulated effects of repeated re-acceleration by all previous shocks, but it is relatively insensitive to the ordering of the shock Mach numbers. Thus multiple passages of shocks may cause the slope of the CR spectrum to deviate from the canonical DSA power-law slope of the current shock.
75
- Mun, Jae Yeon
- Journal of the Korean astronomical society = 천문학회지
- 54, n.1
- pp.17-35
- 2021
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We study galaxies undergoing ram pressure stripping in the Virgo cluster to examine whether we can identify any discernible trend in their star formation activity. We first use 48 galaxies undergoing different stages of stripping based on H i morphology, H i deficiency, and relative extent to the stellar disk, from the VIVA survey. We then employ a new scheme for galaxy classification which combines H i mass fractions and locations in projected phase space, resulting in a new sample of 365 galaxies. We utilize a variety of star formation tracers, which include g - r, WISE [3.4]-[12] colors, and starburstiness that are defined by stellar mass and star formation rates to compare the star formation activity of galaxies at different stripping stages. We find no clear evidence for enhancement in the integrated star formation activity of galaxies undergoing early to active stripping. We are instead able to capture the overall quenching of star formation activity with increasing degree of ram pressure stripping, in agreement with previous studies. Our results suggest that if there is any ram pressure stripping induced enhancement, it is at best locally modest, and galaxies undergoing enhancement make up a small fraction of the total sample. Our results also indicate that it is possible to trace galaxies at different stages of stripping with the combination of H i gas content and location in projected phase space, which can be extended to other galaxy clusters that lack high-resolution H i imaging.
76
- Kim, Minjun
- Journal of the Korean astronomical society = 천문학회지
- 54, n.1
- pp.1-8
- 2021
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We report X-ray timing and spectral properties of the pulsar PSR J0205+6449 measured using NuSTAR and Chandra observatories. We measure the pulsar's rotation frequency ν = 15.20102357(9) s-1 and its derivative <TEX>$\dot{\nu}=-4.5(1){\times}10^{-11}\;s^{-2}$</TEX> during the observation period, and model the 2-30 keV on-pulse spectrum of the pulsar with a power law having a photon index Γpsr = 1.07 ± 0.16 and a 2-30 keV flux F2-30 keV = 7.3±0.6 × 10-13 erg cm-2 s-1. The Chandra 0.5-10 keV data are analyzed for an investigation of the pulsar's thermal emission properties. We use thermal and non-thermal emission models to fit the Chandra spectra and infer the surface temperature T∞ and luminosity Lth of the neutron star to be T∞ = 0.5 - 0.8 MK and Lth = 1 - 5 × 1032 erg s-1. This agrees with previous results which indicated that PSR J0205+6449 has a low surface temperature and luminosity for its age of 800-5600 yrs.
77
- Park, Yong-Sun
- Journal of the Korean astronomical society = 천문학회지
- 54, n.6
- pp.183-190
- 2021
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We have revisited Monte Carlo radiative transfer calculations for clumpy molecular clouds. Instead of introducing a three-dimensional geometry to implement clumpy structure, we have made use of its stochastic properties in a one-dimensional geometry. Taking into account the reduction of spontaneous emission and optical depth due to clumpiness, we have derived the excitation conditions of clumpy clouds and compared them with those of three-dimensional calculations. We found that the proposed approach reproduces the excitation conditions in a way compatible to those from three-dimensional models, and reveals the dependencies of the excitation conditions on the size of clumps. When bulk motions are involved, the applicability of the approach is rather vague, but the one-dimensional approach can be an excellent proxy for more rigorous three-dimensional calculations.
78
- Kim, Minjin
- Journal of the Korean astronomical society = 천문학회지
- 54, n.2
- pp.37-47
- 2021
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Reverberation mapping (RM) is an efficient method to investigate the physical sizes of the broad line region (BLR) and dusty torus in an active galactic nucleus (AGN). The Spectro-Photometer for the History of the Universe, Epoch of Reionization and Ices Explorer (SPHEREx) mission will provide multi-epoch spectroscopic data at optical and near-infrared wavelengths. These data can be used for RM experiments with bright AGNs. We present results of a feasibility test using SPHEREx data in the SPHEREx deep regions for torus RM measurements. We investigate the physical properties of bright AGNs in the SPHEREx deep field. Based on this information, we compute the efficiency of detecting torus time lags in simulated light curves. We demonstrate that, in combination with complementary optical data with a depth of ~ 20 mag in B-band, lags of ≤ 750 days for tori can be measured for more than ~ 200 bright AGNs. If high signal-to-noise ratio photometric data with a depth of ~ 21-22 mag are available, RM measurements are possible for up to ~ 900 objects. When complemented by well-designed early optical observations, SPHEREx can provide a unique dataset for studies of the physical properties of dusty tori in bright AGNs.
79
- Lee, Chung-Uk
- Journal of the Korean astronomical society = 천문학회지
- 54, n.4
- pp.113-119
- 2021
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It is difficult for observers to conduct an optical alignment at an observatory without the assistance of an optical engineer if optomechanical parts are to be replaced at night. We present a practical tilt correction method to obtain the optimal optical alignment condition using the symmetricity of optical aberrations of a wide-field on-axis telescope at night. We conducted coarse tilt correction by visually examining the symmetry of two representative star shapes obtained at two guide chips facing each other, such as east-west or north-south pairs. After coarse correction, we observed four sets of small stamp images using four guide cameras located at each cardinal position by changing the focus positions in 10-㎛ increments and passing through the optimum focus position in the range of ±200 ㎛. The standard deviation of each image, as a function of the focus position, was fitted with a second-order polynomial function to derive the optimal focus position at each cardinal edge. We derived the tilt angles from the slopes converted by the distance and the focus position difference between two paired guide chip combinations such as east-west and north-south. We used this method to collimate the on-axis wide-field telescope KMTNet in Chile after replacing two old focus actuators. The total optical alignment time was less than 30 min. Our method is practical and straightforward for maintaining the optical performance of wide-field telescopes such as KMTNet.
80
- Lee, Jae-Ok
- Journal of the Korean astronomical society = 천문학회지
- 54, n.2
- pp.61-70
- 2021
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We investigate 20 post-coronal mass ejection (CME) blobs formed in the post-CME current sheet (CS) that were observed by K-Cor on 2017 September 10. By visual inspection of the trajectories and projected speed variations of each blob, we find that all blobs except one show irregular 'zigzag' trajectories resembling transverse oscillatory motions along the CS, and have at least one oscillatory pattern in their instantaneous radial speeds. Their oscillation periods are ranging from 30 to 91 s and their speed amplitudes from 128 to 902 km s-1. Among 19 blobs, 10 blobs have experienced at least two cycles of radial speed oscillations with different speed amplitudes and periods, while 9 blobs undergo one oscillation cycle. To examine whether or not the apparent speed oscillations can be explained by vortex shedding, we estimate the quantitative parameter of vortex shedding, the Strouhal number, by using the observed lateral widths, linear speeds, and oscillation periods of the blobs. We then compare our estimates with theoretical and experimental results from MHD simulations and fluid dynamic experiments. We find that the observed Strouhal numbers range from 0.2 to 2.1, consistent with those (0.15-3.0) from fluid dynamic experiments of bluff spheres, while they are higher than those (0.15-0.25) from MHD simulations of cylindrical shapes. We thus find that blobs formed in a post-CME CS undergo kinematic oscillations caused by fluid dynamic vortex shedding. The vortex shedding is driven by the interaction of the outward-moving blob having a bluff spherical shape with the background plasma in the post-CME CS.