| 摘要: | 我們使用鹿林一米望遠鏡觀測疏散星團， NGC 2324， NGC 2374 ， NGC 2420 以及 NGC 381 ，對每個星團， 主要的目標是找尋系外行星，同時利用類似的觀測方法及資料處理分析，可以找尋變星候選星，此為我們的第二目標。 在兩次的觀測與資料處理後，第一次觀測的三個星團，沒有找到掩星現象，在 NGC 2324 中找到兩個變星候選星， NGC 2374 中找到四個， NGC 2420 沒有找到變星候選星。第二次觀測的 NGC 381 ，發現了兩個疑似掩星現象的光度變化圖，以及兩個變星候選星。兩次的觀測資料點都不足，無論掩星現象或所發現的變星候選星，都需要進一步的資料做確認。 We present results of the search for exoplanets and variable stars in open cluster fields,NGC 2324,NGC 2374,NGC 2420,and NGC 381 with the Lulin One-meter Telescope (LOT).Our main scienti fic goal is to use time series CCD photometry survey to detect exoplanets via transit e ffects.Such detection technique demands high-precision photometry now being implemented and gradually improved at Lulin. The secondary scienti fic goal is to discover and study variable stars with the same data set and similar data analysis methods. Observations were performed with the LOT from January 28 to February 1,2003 for the NGC 2324,NGC 2374 and NGC 2420 star fields.The total photometric data acquired were about 700 frames in two filters (R and V )for the three open clusters, NGC 2324,NGC 2374 and NGC 2420.The inclusion of three open clusters into the first observation program was mainly to increase the detection probability (i.e., more target stars).But one side e ffect was that the typical time interval ( ∼30 min) between successive exposures was too large and the lengths of data coverage were too short to allow the identi fication of transit events.For this reason,we have changed the observational strategy -in the next observational opportunity from October 19 to 21,2003 -by using one filter (e.g.,R-band)and focusing only on one single open cluster,NGC 381.We have obtained 120 frames in total over three nights.While the cadence ( ∼3 min)was small enough this time,the search for transit events was handicapped by the short time interval ( ∼2 hours)available at each night. The pre-processing of the CCD frames and data reduction were performed with the standard routines in the IRAF CCDPROC and DAOPHOT II (Stetson 1987). The WCSTools were used to obtain the world coordinate system in the header of fit files.Positions of stars in di fferent images were then aligned by matching the relative distances from their neighboring stars.This procedure would permit the determination of light curves of the target stars.To search for exoplanets,the Box- fitting Least Squares (BLS)method described in Kovacs et al.(2002)could be used to analyze the light curves.The high e fficiency of BLS in the discovery of transit events has been demonstrated by the OGLE III (Optical Gravitational Lensing Experiment)team (Udalski et al.,2002a,2002b).However,because the testable light curves are not too many (<1000 stars),we have opted by visual inspection in our preliminary study.For variable star candidates,we determined the possible periods with the Phase Dispersion Minimization (PDM)method (Stellingerg 1978) and Lomb Normalization Periodogram (LNP)method (Lomb 1976 &Scargle 1982). We have found two variable candidates in the NGC 2324 star field and four variable candidates in the NGC 2374 star field.In the NGC 2420,no varable has been found.In addition,we have discovered some interesting light curves in NGC 381:two variable candidates and two possible transit events.However,much caution should be given to the results of NGC 381 because of the limitation of the data coverage (i.e.,only about two hours for each of the three nights).After this feasibility study,we intend to pursue the Lulin exoplanet search program with better de fined procedure and improved techniques. |
