在疏散星團NGC663的兩個星野中搜尋短時間內能看到光度變化的變星

A search for variable stars with small variations in two selected fields toward the open cluster NGC 663

邵慶宇(Ching-Yu Shao)

指導教授 ： 傅學海

國立臺灣師範大學/文學院/地球科學系(暑期)教學碩士學位班/碩士(2011年)

摘要

疏散星團中的變星數量統計，可以用來認識與瞭解星團的演化狀態。本研究針對疏散星團NGC663中的兩個星野，使用短時間曝光、持續一小時的觀測，搜尋在數分鐘至數小時尺度內能看到光度變化的變星，作為初步統計之用。觀測在2003年8月9日晚上進行，使用國立中央大學鹿林山天文台1公尺望遠鏡配置V光度濾鏡，每幅影像曝光1分鐘，連續拍攝，持續約一小時，星野一得到57幅影像及星野二得到63幅影像。
每幅影像經過扣除暗電流、平場修正等標準程序，再以美國國家光學天文台所發展之天文影像處理軟體IRAF之DAOFIND、PHOT、點瀰散函數(PSF)進行擬合，獲得每顆恆星的座標與儀器星等。每幅影像中的恆星儀器星等，以參考星等(van den Bergh and de Roux, 1978)進行線性回歸擬合，轉換成V星等。兩個區域中可供分析的V星等範圍分別是12.5～19以及13.5～18.5。完成所有57幅與63幅影像之星等校準後，繪製每顆恆星的光度變化曲線(light curve，簡稱光曲線)，以肉眼檢視每顆恆星之光曲線，找出亮度有變化的恆星。
本研究在疏散星團NGC 663及其附近區域中的兩個星野（星野一與星野二），一共發現9顆疑似變星，其中4顆恆星亮度呈現持續上升或下降的趨勢，其他5顆的亮度則屬於不規則變化。依據文獻，在NGC 663中已經發現了一些變星，在本研究的兩個區域中有16顆已知變星。其中一顆的光度在本次一小時的觀測中呈現變化。由統計的角度來看，依據文獻(Pigulski et al. 2001)，在觀測得到的715顆恆星中確認了24顆變星，比例為3.36 %。而本研究中，兩個星野可供分析的共有1192顆恆星，已知變星佔的比例為1.34%，將疑似變星也列入統計，那麼變星的比例為2.10%，略低於3.36%。所以在疏散星團中搜尋短時間有變化的變星的可行性是相當高的。

關鍵字

變星 ； 疏散星團 ； 光曲線

並列摘要

The state of evolution of a star cluster could be studied by the percentage of variable stars in the cluster. In this work, two regions toward an open cluster NGC 663 are observed with V filter using one meter telescope (LOT) at Lulin observatory of National Central University on the night, August 9, 2003. The goal was trying to find the variable stars with variation in minutes to hours. There are 57 and 63 frames obtained for Region 1 and Region 2 respectively.
The magnitude of a star in each frame was derived by data reduction and magnitude calibration. The completeness of V magnitude reveals the useful magnitude range, 12.5 ~ 19 and 13.5 ~ 18.5, for searching variable stars in region 1 and region 2, respectively. Light curves of the stars were plotted and used to find out the brightness change in the interval of about an hour.
There are 9 suspected variable stars found in both two regions of NGC 663. The brightnesses of four stars show a continuous rise or fall, other stars are irregular. There are several variable stars in NGC 663 presented in other’s studies. 16 known variable stars are located in the two regions. In the observations, one of the known stars shows a change in the light curve but the others do not.
The percentage of variable stars (16) to total stars (1192) is about 1.34%. If the suspected variables (9) are included, the percentage will be 2.10 %. The value is slightly less than the percentage, 3.36 %, derived from the number of variables (24) and detected stars (715) given by Pigulski et al. (2001). This result implies that it is possible to use the short-term light variation for variable stars survey in the stellar clusters.

並列關鍵字

variable star ； open cluster ； light curve

參考文獻

McCusky, S. W., & Houk, N., 1964, Astron. J., 69, 412.

van den Bergh, S., & de Roux, J., 1978, Astron. J., 83, 1075.

Dias, W. S., Alessi, B. S., Moitinho, A., & Lépine, J. R. D., 2002, A&A, 389, 871. Fabregat, J., & Capilla, G., 2005, MNRAS, 358, 66.

Hoag, A. A., Johnson, H. L., Iriarte, B., Mitchell, R. I., Hallam, K. L., & Sharpless, S., 1961, Publ. U.S. Naval Obs. Second Serie, 17, 343.

Li, C. S., 2009, The photometry of three variable stars with Short Period: CY Aqr, GP And, and BP Peg, NTNU, Taipei.