• Publications of The Korean Astronomical Society
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• v.30 no.2
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• pp.229-230
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• 2015

The results of spectral studies of the CTTS type young star RY Tau with spectrograms of the ultraviolet and the visual ranges are presented. We show the first detection of periodic variability of the emission line intensities in UV and visual ranges with a period of 23 days.

• Publications of The Korean Astronomical Society
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• v.32 no.1
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• pp.67-71
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• 2017

Debris disks are circumstellar dust disks around main-sequence stars. They are important observational clues to understanding the planetary system formation. The zodiacal light is the thermal emission from the dust disk in our Solar system. For a comprehensive understanding of the nature and the evolution of dust disks around main-sequence stars, we try a comparative study of debris disks and the zodiacal light. We search for debris disks using the AKARI mid-infrared all-sky point source catalog. By applying accurate flux estimate of the photospheric emission based on the follow-up near-infrared observations with IRSF, we have improved the detection rate of debris disks. For a detailed study of the structure and grain properties in the zodiacal dust cloud, as an example of dust disks around main-sequence stars, we analyze the AKARI mid-infrared all-sky diffuse maps. As a result of the debris disks search, we found old (>1 Gyr) debris disks which have large excess emission compared to their age, which cannot be explained simply by the conventional steady-state evolution model. From the zodiacal light analysis, we find the possibility that the dust grains trapped in the Earth's resonance orbits have increased by a factor of ~3 in the past ~20 years. Combining these results, we discuss the non-steady processes in debris disks and the zodiacal light.

• Journal of The Korean Astronomical Society
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• v.43 no.4
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• pp.123-133
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• 2010

In order to study properties of the pulsation in the infrared emission for long period variables, we collect and analyze the infrared observational data at L band for 12 OH/IR. The observation data cover about three decades including recent data from the ISO and Spitzer. We use the Marquardt-Levenberg algorithm to determine the pulsation period and amplitude for each star and compare them with results of previous investigations at infrared and radio bands. We obtain the relationship between the pulsation periods and the amplitudes at L band. Contrary to the results at K band, there is no difference of the trends in the short and long period regions of the period-luminosity relation at L band. This may be due to the molecular absorption effect at K band. The correlations among the L band parameters, IRAS [12-25] colors, and K band parameters may be explained as results of the dust shell parameters affected by the stellar pulsation. The large scatter of the correlation could be due to the existence of a distribution of central stars with various masses and pulsation modes.

• Journal of The Korean Astronomical Society
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• v.54 no.6
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• pp.171-182
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• 2021

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 ²⁹SiO(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.

• Journal of Astronomy and Space Sciences
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• v.38 no.3
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• pp.175-183
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• 2021

The dependencies of the chemical element abundances in stellar atmospheres with respect to solar abundances on the second ionization potentials of the same elements were investigated using the published stellar abundance patterns for 1,149 G and K giants in the Local Region of the Galaxy. The correlations between the relative abundances of chemical elements and their second ionization potentials were calculated for groups of stars with effective temperatures between 3,764 and 7,725 K. Correlations were identified for chemical elements with second ionization potentials of 12.5 eV to 20 eV and for elements with second ionization potentials higher than 20 eV. For the first group of elements, the correlation coefficients were positive for stars with effective temperatures lower than 5,300 K and negative for stars with effective temperatures from 5,300 K to 7,725 K. The results of this study and the comparison with earlier results for hotter stars confirm the variations in these correlations with the effective temperature. A possible explanation for the observed effects is the accretion of hydrogen and helium atoms from the interstellar medium.

• Publications of The Korean Astronomical Society
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• v.32 no.1
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• pp.73-75
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• 2017

Debris disks are important observational clues to understanding on-going planetary system formation. They are usually identified by significant mid-infrared excess on top of the photospheric emission of a central star on the basis of prediction from J-, H-, and Ks-band fluxes and the stellar model spectra. For bright stars, 2MASS near-infrared fluxes suffer large uncertainties due to the near-infrared camera saturation. Therefore we have performed follow-up observations with the IRSF 1.4 m near-infrared telescope located in South Africa to obtain accurate J-, H-, and Ks-band fluxes of the central stars. Among 754 main-sequence stars which are detected in the AKARI $18{\mu}m$ band, we have performed photometry for 325 stars with IRSF. As a result, we have successfully improved the flux accuracy of the central stars from 9.2 % to 0.5 % on average. Using this dataset, we have detected $18{\mu}m$ excess emission from 57 stars in our samples with a $3{\sigma}$ level. We find that some of them have high ratios of the excess to the photospheric emission even around very old stars, which cannot be explained by the current planet-formation theories.

• Journal of The Korean Astronomical Society
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• v.40 no.4
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• pp.113-118
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• 2007

Recently far infra-red (FIR) polarization of the $850{\mu}m$ continuum emission from T Tauri disks has been detected. The observed degree of polarization is around 3 %. Since thermal emission from dust grains dominates the spectral energy distribution at the FIR regime, dust grains might be the cause of the polarization. We explore alignment of dust grains by radiative torque in T Tauri disks and provide predictions for polarized emission for disks viewed at different wavelengths and viewing angles. In the presence of magnetic field, these aligned grains produce polarized emission in infrared wavelengths. When we take a Mathis-Rumpl-Nordsieck-type distribution with maximum grain size of $500-1000{\mu}m$, the degree of polarization is around 2-3 % level at wavelengths larger than ${\sim}100{\mu}m$. Our study indicates that multifrequency infrared polarimetric studies of protostellar disks can provide good insights into the details of their magnetic structure.

• Journal of The Korean Astronomical Society
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• v.38 no.2
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• pp.237-240
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• 2005

We present images of L1551 IRS5 at angular resolutions as high as ${\~}$30 mas, corresponding to a spatial resolution of ${\~}$5 AU, made at 7 mm with the VLA. Previously known to be a binary protostellar system, we show that L1551 IRS5 is likely a triple protostellar system. The primary and secondary components have a projected separation of ${\~}$46 AU, whereas the tertiary component has a projected separation of ${\~}$11 AU from the primary component. The circumstellar dust disks of the primary and secondary components have dimensions of ${\~}$15 AU, whereas that of the tertiary component has a dimension of ${\~}$10 AU. Their major axes are closely, but not perfectly, aligned with each other, as well as the major axis of the surrounding flattened, rotating, and contracting molecular condensation (pseudodisk). Furthermore, the orbital motion of the primary and secondary components is in the same direction as the rotational motion of this pseudodisk. We suggest that all three protostellar components formed as a result of the fragmentation of the central region of the molecular pseudo disk. The primary and secondary components, but apparently not the tertiary component, each exhibits a bipolar ionized jet that is centered on and which emergers perpendicular to its associated dust disk. Neither jets are resolved along their base, implying that they are driven within a radial distance of ${\~}$2.5 AU from their central protostars. Finally, we show evidence for what may be dusty matter streams feeding the two main protostellar components.

• Bulletin of the Korean Space Science Society
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• 1992.10a
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• pp.15-15
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• 1992

AQ Cassiopeiae (BD+61`0242, uv=10, Sp=B3+Bg) is a totally eclipsing binary system with the obital period of about 12 days. 71 was observed for 15 nights in 1985 with the1.8-m telescope at the DAO, employing a Reticon and a three-stage image tube attached to the spectrograph. And also, photometric observations of AQ Cas had been made inUBv for six years from 1982 to 1988 at Yonsei University Observatory(YUO). This work includes UBV observations obtained at YUO as a part of The Ten-Year Observing Program(1982-1992). Double lined radial, velocity curves and Ufv light curves of AQC as are constructed. The light curves and radial velocity curves show a strong evidence of circumstellar matter or mass stream. It is clear at the phases of just outside externaleclipse contacts, particularly at phase 0.8-0.9, shown in Figures 1 and 2. A solution by combining the radial velocity and photometric curves of the binary was obtained with the Wilson-Devinney Code. We found that the system is semi-detached with the coolcomponent filling its Roche lobe. The absolute dimensions of AQ Cas are calculated. The result shows that this system consists of two massive and subgiant stars.

• Journal of Astronomy and Space Sciences
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• v.36 no.3
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• pp.105-113
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• 2019

Spectroscopic observations of barium star ${\zeta}$ Capricornus (HD204075) obtained at the 8.2 m telescope of the European Southern Observatory, with a spectral resolving power R = 80,000 and signal to noise ratio greater than 300, were used to refine the atmospheric parameters. We found new values for effective temperature ($T_{eff}=5,300{\pm}50K$), surface gravity ($log\;g=1.82{\pm}0.15$), micro-turbulent velocity ($v_{micro}=2.52{\pm}0.10km/s$), and iron abundance ($log\;N(Fe)=7.32{\pm}0.06$). Previously published abundances of chemical elements in the atmosphere of HD204075 were analyzed and no correlations of these abundances with the second ionization potentials of these elements were found. This excludes the possible influence of accretion of hydrogen and helium atoms from the interstellar or circumstellar environment to the atmosphere of this star. The accretion of nuclear processed matter from the evolved binary companion was primary cause of the abundance anomalies. The young age of HD204075 allows an estimation of the time-scale for the creation of the abundance anomalies arising from accretion of interstellar hydrogen and helium as is the case of stars with low magnetic fields; which we estimate should exceed $10^8$ years.