• Journal of The Korean Astronomical Society
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• v.34 no.2
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• pp.81-97
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• 2001

Since the first proposal by Paczynski, great efforts to detect Galactic dark matter by detecting light variations of stars located in the Magellanic Clouds and Galactic bulge caused by gravitational microlensing have been and are being carried out and more than 1,000 events have been successfully detected. In this paper, we review the progress in the theoretical and experimental progresses in microlensing. We begin with basics of microlensing and summarize the results obtained from the last 8 year observations along with the implications of the results. We also discuss the usefulness of microlensing in other fields of astronomy such as the stellar atmosphere, Galactic binary systems, and extra-solar planets. We finally discuss the problems of the current experiments and the new types of observations that can overcome these problems.

• Journal of The Korean Astronomical Society
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• v.47 no.6
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• pp.293-302
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• 2014

Simultaneous time monitoring observations of $H_2O$ $6_{16}-5_{23}$, SiO J = 1-0, 2-1, 3-2, and $^{29}SiO$ ${\upsilon}=0$, J = 1-0 lines are carried out in the direction of the Mira variable star TX Cam with the Korean VLBI Network single dish radio telescopes. For the first time, the $H_2O$ maser emission from TX Cam is detected near the stellar velocity at five epochs from April 10, 2013 (${\phi}=3.13$) to June 4, 2014 (${\phi}=3.89$) including minimum optical phases. The intensities of $H_2O$ masers are very weak compared to SiO masers. The variation of peak antenna temperature ratios among SiO ${\upsilon}=1$, J = 1-0, J = 2-1, and J = 3-2 masers is investigated according to their phases. The shift of peak velocities of $H_2O$ and SiO masers with respect to the stellar velocity is also investigated according to observed optical phases. The $H_2O$ maser emission occurs around the stellar velocity during our monitoring interval. On the other hand, the peak velocities of SiO masers show a spread compared to the stellar velocity. The peak velocities of SiO J = 2-1, and J = 3-2 masers show a smaller spread with respect to the stellar velocity than those of SiO J = 1-0 masers. These simultaneous observations of multi-frequencies will provide a good constraint for maser pumping models and a good probe for investigating the stellar atmosphere and envelope according to their different excitation conditions.

• Journal of The Korean Astronomical Society
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• v.42 no.6
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• pp.161-174
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• 2009

Observation of standard stars is of crucial importance in stellar photometry. We have studied the standard transformation relations of the UBV RI CCD photometric system at the Maidanak Astronomical Observatory in Uzbekistan. All observations were made with the AZT-22 1.5m telescope, SITe 2k CCD or Fairchild 486 CCD, and standard Bessell UBV RI filters from 2003 August to 2007 September. We observed many standard stars around the celestial equator observed by SAAO astronomers. The atmospheric extinction coefficients, photometric zero points, and time variation of photometric zero points of each night were determined. Secondary extinction coefficients and photometric zero points were very stable, while primary extinction coefficients showed a distinct seasonal variation. We also determined the transformation coefficients for each filter. For B, V, R, and I filters, the transformation to the SAAO standard system could be achieved with a straight line or a combination of two straight lines. However, in the case of the U filter and Fairchild 486 CCD combination, a significant non-linear correction term - related to the size of Balmer jump or the strength of the Balmer lines - of up to 0:08 mags was required. We found that our data matched well the SAAO photometry in V, B - V, V - I, and R - I. But in U - B, the difference in zero point was about 3.6 mmag and the scatter was about 0.02 mag. We attribute the relatively large scatter in U -B to the larger error in U of the SAAO photometry. We confirm the mostly small differences between the SAAO standard UBV RI system and the Landolt standard system. We also attempted to interpret the seasonal variation of the atmospheric extinction coefficients in the context of scattering sources in the earth's atmosphere.

• The Bulletin of The Korean Astronomical Society
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• v.41 no.2
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• pp.43.3-43.3
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• 2016

We present a library of high-resolution (R~45,000) and high signal-to-noise ratio ($S/N{\geq}200$) near-infrared spectra of 147 standard stars. High quality spectra were obtained with Immersion GRating INfrared Spectrograph (IGRINS) covering the full range of H ($1.496-1.795{\mu}m$) and K ($2.080-2.460{\mu}m$) bands. The targets are mainly selected as MK standard stars which have well-defined spectral types and luminosity classes, and cover a wide range of effective temperatures and surface gravities. The spectra were corrected for telluric absorption lines and absolute flux calibrated using Two Micron All Sky Survey (2MASS) photometry. We find new spectral indices in H and K bands and provide their EWs. We describe empirical relations between the measured EWs and stellar atmosphere parameters such as effective temperature and surface gravity.

• Journal of Astronomy and Space Sciences
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• v.15 no.1
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• pp.91-100
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• 1998

Color indexes and effective temperatures for 25 standard stars have been determined as a pilot project which show a relationship between color index and effective temperature in ultraviolet region. The effective temperature was determined by comparing energy distribution curves derived from the IUE low dispersion spectra with Kurucz atmosphere model. The UV color index was deduced by integrating fluxes in $300{\AA}$ interval of the IUE low disperion spectra. The relation between color index and effective temperature in ultraviolet is similar with that of optical region.

• The Bulletin of The Korean Astronomical Society
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• v.36 no.2
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• pp.152.1-152.1
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• 2011

We report preliminary results of long-slit near-infrared (NIR) spectroscopy of Luminous Blue Variables (LBVs) with moderate resolution of R ~ 2400. We obtained Jshort (1.04-1.26 micron) and Ks (2.02-2.31 micron) band spectra of 4 LBVs and 3 LBV candidates in Southern hemisphere using IRIS2, infrared imager and spectrograph, mounted on the 4-m Anglo-Australian Telescope. All targets are fairly bright in NIR so that we can obtain high signal-to-noise ratio for clear line detection and modeling. They are also widely distributed in the HR diagram so that we can compare the spectral properties of LBVs in different temperature and luminosity ranges. Among them, we present the results of two well-known LBVs AG Car and HR Car. Their spectra show similar properties with hydrogen, He I, and metallic lines such as Fe II and Mg II, most of them in emission. We discuss, in particular, the He I 1.083 micron lines formed in stellar wind because these two LBVs show large variation in their He I line intensities, compared to previous studies. Since the He I 1.083 line is known to be anticorrelated with the photometric variation of LBVs, strong line intensities with P-Cygni profiles in both stars indicate that they are now near the visual minimum phase. We model the obtained spectra using non-LTE atmosphere code CMFGEN of Hillier (1998) to derive stellar parameters such as wind velocity and mass loss rate, and discuss the long-term variability of stellar parameters of these LBVs. deduced from our otometric solution.