[论文解读] Photometric multi-site campaign on the open cluster NGC 884 I. Detection of the variable stars

S. Saesen, F. Carrier+63 more|arXiv (Cornell University)|Jan 7, 2010

Stellar, planetary, and galactic studies参考文献 51被引用 21

一句话总结

本研究对年轻疏散星团NGC 884开展了多站点测光观测，利用15台望远镜在U、B、V和I滤光片下获取了77,500张CCD图像。该研究实现了亚毫米级的测光精度，并识别出100多颗新的多周期与单周期B型、A型和F型变星，以及六颗新的食双星和不规则变星，挑战了当前关于β Cephei型变星不稳定带的理论模型。

ABSTRACT

CONTEXT: Recent progress in the seismic interpretation of field beta Cep stars has resulted in improvements of the physics in the stellar structure and evolution models of massive stars. Further asteroseismic constraints can be obtained from studying ensembles of stars in a young open cluster, which all have similar age, distance and chemical composition. AIMS: To improve our comprehension of the beta Cep stars, we studied the young open cluster NGC 884 to discover new B-type pulsators, besides the two known beta Cep stars, and other variable stars. METHODS: An extensive multi-site campaign was set up to gather accurate CCD photometry time series in four filters (U, B, V, I) of a field of NGC884. Fifteen different instruments collected almost 77500 CCD images in 1286 hours. The images were calibrated and reduced to transform the CCD frames into interpretable differential light curves. Various variability indicators and frequency analyses were applied to detect variable stars in the field. Absolute photometry was taken to deduce some general cluster and stellar properties. RESULTS: We achieved an accuracy for the brightest stars of 5.7 mmag in V, 6.9 mmag in B, 5.0 mmag in I and 5.3 mmag in U. The noise level in the amplitude spectra is 50 micromag in the V band. Our campaign confirms the previously known pulsators, and we report more than one hundred new multi- and mono-periodic B-, A- and F-type stars. Their interpretation in terms of classical instability domains is not straightforward, pointing to imperfections in theoretical instability computations. In addition, we have discovered six new eclipsing binaries and four candidates as well as other irregular variable stars in the observed field.

研究动机与目标

• 在年轻疏散星团NGC 884中探测新的B型脉动星，以改进对大质量恒星演化过程的星震学约束。
• 研究星团不稳定带中变星的分布，特别是β Cephei型变星。
• 利用高精度相对测光与绝对测光，确定星团及其恒星的测光特性。
• 识别食双星及其他不规则变星，以完善年轻星团中恒星变异性现象的普查。
• 通过将观测到的脉动模态与理论模型对比，检验理论不稳定带预测的准确性。

提出的方法

• 利用分布在四个滤光片（U、B、V、I）的15台地面望远镜，开展了协调一致的多站点测光观测，累计观测时长达1286小时，共获取77,500张CCD图像。
• 采用标准测光还原技术，对所有CCD帧进行校准与处理，生成相对光变曲线。
• 应用多种变异性指标与频率分析工具，检测光变曲线中的周期性与多周期信号。
• 执行绝对测光，以推导星团及单颗恒星的参数，如距离、年龄和消光。
• 利用V波段噪声水平为50微mag的振幅谱，评估检测显著性，并区分真实变异性与噪声。
• 利用NGC 884统一的年龄、距离与金属丰度，将观测到的脉动特性与理论模型进行比较。

实验结果

研究问题

• RQ1 NGC 884中新的B型、A型和F型脉动星在不稳定带中的分布如何？与理论预测相比有何差异？
• RQ2 在NGC 884的观测区域内，新发现的食双星系统和不规则变星共有多少？
• RQ3 观测到的变星脉动频率与振幅在多大程度上偏离了当前β Cephei型不稳定带模型？
• RQ4 在U、B、V和I波段中实现的测光精度是多少？该精度如何影响低振幅变星的探测？
• RQ5 多站点观测活动是否能成功识别并表征具有统一物理参数的年轻疏散星团中统计上显著的变星样本？

主要发现

• 对最亮恒星而言，V波段测光精度达到5.7 mmag，B波段为6.9 mmag，I波段为5.0 mmag，U波段为5.3 mmag。
• 振幅谱中的噪声水平在V波段达到50微mag，实现了对低振幅变异性现象的高灵敏度探测。
• 共检测到100多颗新的多周期与单周期B型、A型和F型恒星，其脉动特性与理论不稳定带模型的匹配度较差。
• 识别出六颗新的食双星系统及四颗候选体，显著扩展了NGC 884中此类系统的已知数量。
• 检测到不规则变星，表明其变光机制多样，超出了脉动与食变的范畴。
• 本研究确认了此前已知的两颗β Cephei型变星，并为未来大质量恒星的星震学建模提供了全面且高精度的数据集。