| Summary: | 碩士 === 國立清華大學 === 天文研究所 === 95 === Taiwan Earthshine and AsteroSeismology Telescopes (EAST) System is to study long-term variation of the earthshine and asteroseismology; Taiwan EAST System therefore will provide a huge amount of data in the future. In this thesis, we develop a method which could be applied to the data of EAST to survey unidentified variable stars and occultation events.
The brightness of a variable star changes with time; the variation is caused by stellar oscillation. For δ scuti variable stars, the period rang from several minute to several days.
The causes of occultation events are: (1) a planet passes through the line of sight between the host star and the observer, therefore caused a sudden drop of the host star’s brightness. (2) meteorites pass through the line of sight between the observer and a star, and blocking the star light from the observer.
Usually, we analyze variation stars or occultation events by applying aperture photometry. However, aperture photometry could only be applied to one target star each time, and is less effective to search unidentified variable stars or occultation events from mass EAST data. In this research, we hope to develop a more effective method to survey EAST data and identify possible variable star candidates. Of course, further analysis should be made after identifying variable star candidates.
The brightness of variable stars and occultation events vary with time, although the mechanism remains different. Therefore, brightness variation could be identified from calculating the standard variation of star’s brightness with respect to time. If the standard variation of a star is bigger than other stars, it could be a variable star.
There are two important steps to calculate the standard variation of star’s brightness. First, align images of an observation night. Second, calculate the 2-D spatial running mean from images. [If the area of running mean is smaller than a star in the image, we get a smoother image; however, this deviates from our research purpose. In our case, we choose the area of running mean bigger than a star. It turns out that the value at star’s center could be regard as the brightness of the whole star, for the reason that this center value is the summation of the star’s brightness and nearby background. In other words, it is like a simple aperture photometry.]
We apply this method to several days’ observation of EAST, and the results show that several known variable stars could be detected perfectly.
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