| 摘要: | 研究造父變星 (Cepheids) 主要以週期作分類依據之一。從前人研究可知，透過造父變星的光度和週期之關係，可分類出第一 (type I) 與第二 (type II) 類型。但在本銀河系上各處造父變星的距離通常難以確定，連帶影響光度的不確定性。本研究嘗試使用另一種方式，觀測本銀河系的第一與第二型造父變星群，分別將觀測所得近紅外J、H波段的光變曲線，經傅立葉分析研究後，找出替代光度的方法。
我們使用SSP-4近紅外光度計 (Near-IR Photometer) 取得標準星和變星的近紅外J、H波段光度的數值。本研究觀測地點在鹿林天文台 (高山) 與中大科一館天文台 (平地) 進行觀測，除比較兩地的觀測數值差以外，也可利用中大科一館天文台觀測結果彌補觀測資料上的不足。
因SSP-4觀測資料量不足夠分析兩類型造父變星，所以引用其他本銀河星系一型造父變星觀測資料 (Monson and Pierce, 2011)，以及本銀河系二型造父變星觀測資料 (Matsunaga et al, 2006)；另加入大麥哲倫星系的一型和二型造父變星觀測資料 (Bharwaj et al, 2016)，以比較兩星系間之結果差異，並參考Fernie和Ethers (1999) 所提之方法。作者使用可見光B、V波段的觀測資料，經傅立葉分析後所得之參數，可區分出造父變星第一與第二兩類型。本研究欲探討利用上述相同方法，於近紅外J、H波段所得觀測資料上進行分析，是否會得出相同結果，並用此分析結果比較本銀河星系(Milky Way, MW) 與大麥哲倫星系 (Large Magellanic Cloud, LMC) 兩者分布之差異。
本研究使用Plotypus套件軟體對觀測資料點進行擬合，在篩選出理想的光變曲線後，進而得出傅利葉相位差 φ_21 和振幅比 R_21 等傅立葉參數。並用已知傅立葉參數為Y軸對週期作圖，初步發現LMC的二型造父變星資料分佈與Fernie和Ethers (1999) 結果近似。然而改換 φ_31、φ_41、R_31、R_41等參數後，則因非零資料點過少，皆無顯著界線可區分一型和二型造父變星。即使再換J減去J-H色指數 (color index) 後所得的最大值之相位差Δφ_tot，仍無明顯有區分兩類型造父變星界線存在。但使用Warren-Harvey test方法後，卻可將LMC和MW星系中兩類型的造父變星成功分離出來。;In previous research, the classical and Type II Cepheids can be distinguished from their optical light curves. Our research is about exploring the possibility to separate these two groups of variable stars in near-infrared bands. I use the SSP-4 photometer to observe a number of bright Galactic Cepheids in J, H band. This photometer will output the counts of standard stars and variable stars. Because the output counts is linearly proportion to the flux, we can calibrate the apparent magnitude by the method of linear regression.
In order to fit the light curve of our targets, we need to get about 20 data points per light curves. We choose the two sites for our observations on Cepheids: one is at Lulin Observatory with LOT (Lulin One-meter Telescope), the other one is NCUO (National Central University Observatory) with the 24-inch telescope. Because the weather condition is very different on these two sites, we can compare the data of observations between them. On the other hand, the data from NCUO can be use to complement the data taken with LOT.
Fernie & Ethers (1999) used the data of optical B, V band to distinguish the Cepheids of type I & II by Fourier decomposition. We use the same method in J, H band to the type I and II Cepheids in the Milky way (MW) and Large Magellanic cloud (LMC) galaxy. The type I data for MW Cepheids are taken from Monson and Pierce (2011), while the type II are taken from Matsunaga et al (2006). The LMC data for both types are taken from Bharwaj et al (2016). We use the software of Plotypus to fit the J, H band light curves. After choosing the ideal light curves, we can get the parameter like Fourier phase difference: φ_21 and the ratio of the amplitude: R_21. We have found the distribution of LMC ’s data are similar to Fernie and Ethers (1999). When we change to use Fourier parameters of φ_31、φ_41、R_31、R_41. we found that these parameters are not suitable to separate two types of Cepheids. We also tried the phase difference between J and J-H (Δφtot), but this parameter still cannot separate the two types of Cepheids as in the optical bands. The only good result is Warren-Harvey test. This method can separate type I and type II Cepheids successfully. |
