年輕疏散星團 NGC6910 與 Berkeley87 之光度、運動與動力

Abstract

本研究針對兩個年輕疏散星團NGC 6910與 Berkeley 87，以中央大學鹿林前山天文台一米望遠鏡進行光度觀測，獲得星團光度、星際吸收與紅化、距離與年齡等參數。以美國海軍天文台星表B1.0（USNO-B1.0）之數據，進行恆星計數統計，以及星團運動與動力方面的分析，以推算星團的半徑、密集度等參數。

在對NCC6910與 Berkeley 87中心區域進行CCD Johnson UBV三波段光度觀測，UBV三波段之光度誤差在 NGC 6910 分別為0.056, 0.028 與 0.021星等；在 Berkeley 87分別為0.039, 0.036 與 0.037星等。NGC 6910紅化為E(B-V) = 1.11±0.16，Berkeley 87紅化E(B-V) = 1.53±0.25。將修正紅化後的星團星色─星等圖（CMD）與恆星演化等時模型〈isochrones〉擬合的結果顯示，NGC 6910的年齡約為3.98百萬年至一千萬年之間，距離模式為10.88；Berkeley 87的年齡則在一千萬年左右，距離模式為 9.6。

根據USNO-B1.0 中POSS-I 103a-O blue sensitive plates 測定星等，星團1°x1°範圍內的座標作為資料庫，進行恆星計數統計分析。結果顯示NGC 6910的表面恆星數量密度分佈符合金氏模式(King 1962)，當距離為1500pc時，核心半徑(rc)2.48 pc，潮汐半徑(rt)26.13 pc，k=12.50 stars/pc2。Oort常數採用A = 14.4±0.8 km s-1 kpc-1 、B = -12.4±0.6 km s-1 kpc-1 (Feast and Whitelock 1997)，則星團總質量約為6400 M⊙。由於USNO B1的平均星等誤差為0.3 等 (Monet et al., 2003)，所以星數之完備性誤差範圍過大，Berkeley 87視野中的恆星密度分佈無法明顯呈現星團結構，有待未來以較大視野（30角分見方）進行光度觀測。

We present the CCD photometry observation of two young open clusters NGC 6910 and Berkeley 87 with National Central University Lulin Observatory 1.0-m Telescope. Based on the observation data, the magnitude, extinction, reddening, distance and age of clusters are derived. Star counting based on the USNO-B1.0 catalogue, to derive the radius and concentration of clusters.

In the Johnson U, B and V bands photometry calibration, the standard error are 0.056, 0.028 and 0.021 magnitudes for NGC 6910, respectively, and that are 0.039, 0.036 and 0.037 magnitudes for Berkeley 87, respectively. The reddening E(B-V) of NGC 6910 is determined to be 1.11±0.16 mag. Berkeley 87 is a more reddened cluster, its E(B-V) is 1.53±0.25. The age of NGC 6910 estimated with well isochrones fitting in 4~10 Myrs, distance modulus is 10.88. For Berkeley 87, the age is about 10 Myrs, and distance modulus is 9.0.

Base on the magnitude derived from POSS-I 103a-O sensitive plates listed in the USNO-B1.0 catalogue. Both clusters are analysis with the radial distribution of the stellar number density within the area 1°x1°. It indicates that NGC 6910 is consistent with King′s model. The fitted values of King model of NGC 6910 base on distance at 1500 pc are: core radius (rc) 2.48 pc, tidal radius (rt) 26.13 pc and k=12.50 stars/pc2. Base on these parameters and the Oort’s constants A = 14.4±0.8 km s-1 kpc-1 、B = -12.4±0.6 km s-1 kpc-1 (Feast and Whitelock 1997), the mass of NGC 6910 is estimated about 6400 M⊙. However, Berkeley 87 can not be compared with King model because of the larger errors of magnitude.