Using the Lomb-Scargle Algorithm to analyze the light curve of the variable star AZ Capricorni

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题名:	Using the Lomb-Scargle Algorithm to analyze the light curve of the variable star AZ Capricorni
作者:	藍聖朋;Lan, Sheng-Peng
贡献者:	天文研究所
关键词:	變星;星斑;variable star;Lomb-Scargle;starspots
日期:	2020-12-23
上传时间:	2021-03-18 16:46:35 (UTC+8)
出版者:	國立中央大學
摘要:	AZ Capriconi (BD-17 6128) 是顆光譜型態為 K6 的 BY Draconi 型態的變星，具有表面星斑與色球活動，因為星球自轉而造成亮度變化，已知有個3.4天的週期。我們從ASAS巡天計畫有關 AZ Cap長達18年的光變曲線，證實文獻中發表3.4天的週期，同時觀察到光變震幅隨年度而不同，試圖分析週期是否隨年度有變化。我們主要使用Lomb-Scargle的方法分析週期，先以模擬數據瞭解 Lomb-Scargle分析的參數，例如取樣頻率如何設定，以及如何排除偽頻率。對於頻譜圖我們以高斯曲線選取適當的頻率數目，從Nyquist frequency決定出頻譜圖中的頻率上限。以及藉由計算確認混疊如何影響頻譜圖而產生虛假峰值。最後我們將AZ Cap的光變曲線分年分析，得到平均亮度、震幅，以及週期的年度變化。我們發現AZ Cap亮度有越來越亮的趨勢，震幅與週期都隨年份有明顯變化，但皆無明顯的規律。我們推斷出AZ Cap的自轉傾角不小於32度，以2015年的數據推測其星斑有0.38星球半徑大小。 ;We present light curve analysis of the variable star, AZ Capriconi (BD-17 6128). A member of the Beta Pictoris moving star group, the star, of a spectral type of K6, is a known BY Draconi variable, i.e., with surface starspots and chromospheric activity to modulate the periodical brightness changes. We made use of the 18-year long (2001 to 2019) photometric observations taken by the All Sky Automated Survey (ASAS) to diagnose the long-term behavior in the light curve. Using the Lomb-Scargle periodicity algorithm, we confirmed the 3.4 day period previously known, but found no systematic period drift, as would be expected when starspots appear at different latitudes during a magnetic cycle, as observed in the Sun (aka, the Maunder diagram). AZ Cap continued to brighten in the 18 years covered by the observations. There were year-to-year amplitude variations, but no correlation was found with the light curve period. The sinusoidal amplitude of the data taken in 2015 indicated an effective spot radius of 0.38 of that of the star. It is likely that the X-ray emission and the starspot activity are caused by an unseen close-in, thereby synchronously rotating, companion to anchor the magnetic field. If so, the spot could extend to high latitudes or even to the pole. We infer that the AZ Cap has its rotation axis inclined no less than 32 degrees.
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