EWOCS-II：年轻银河系超星团 Westerlund 1 中沃尔夫-拉叶星的 X 射线性质
EWOCS-II: X-ray properties of the Wolf-Rayet stars in the young Galactic super star cluster Westerlund 1
AI总结：
基于 Chandra 观测，对 Westerlund 1 星团中 WR 星进行最深入的 X 射线研究，探测到 20 颗 WR 星，首次探测到 Fe XXV 发射线，并估计极高二元率（≥92%），表明双星性对形成丰富 WR 星群至关重要。
摘要：
我们展示了迄今为止对单个星团 Westerlund 1 (Wd1) 中所观测到的最丰富的沃尔夫-拉叶 (WR) 星群性质进行的最全面、最深入的 X 射线研究。这项工作基于从“扩展 Westerlund 1 和 2 疏散星团巡天”(EWOCS) 项目获得的 36 次钱德拉 (Chandra) 观测，外加 8 次档案 Chandra 观测。EWOCS 观测的总曝光深度（约 1.1 兆秒）和跨越一年以上的基线使我们能够进行详细的光度、颜色和光谱分析，以及搜索短期和长期周期性。在 X 射线中，我们在 Wd1 中已知的 24 颗 WR 星中探测到 20 颗，探测到的光度下限约为 7×10^29 尔格/秒（假设 Wd1 的距离为 4.23 千秒差距），其中 8 颗 WR 星是首次在 X 射线中被探测到。九颗星在长达一年的基线上显示出清晰的变异性证据，并带有明显的周期性迹象。X 射线颜色和光谱分析揭示，绝大多数 WR 星是硬 X 射线源（kT≥2.0 千电子伏）。在 Wd1 中 17 颗 WR 星的光谱中首次探测到约 6.7 千电子伏的 Fe XXV 发射线，该谱线通常起源于双星系统中的风-风碰撞区。此外，在三颗星的光谱中观测到约 6.4 千电子伏的荧光谱线，表明这些系统中致密冷物质与热气体共存。总体而言，仅我们的 X 射线结果就表明 Wd1 中 WR 星群的二元率非常高（≥80%）。当我们将我们的结果与其他波长的 WR 星群性质相结合时，我们估计二元率≥92%，甚至可能达到 100%。这表明要么 Wd1 中所有最巨大的恒星都存在于双星系统中，要么双星性对于形成如此丰富的 WR 星群至关重要。
英文摘要：
We present the most comprehensive and deepest X-ray study to date of the properties of the richest Wolf-Rayet (WR) population observed in a single stellar cluster, Westerlund 1 (Wd1). This work is based on 36 Chandra observations obtained from the "Extended Westerlund 1 and 2 Open Clusters Survey" (EWOCS) project, plus 8 archival Chandra observations. The overall exposure depth (~1.1 Ms) and baseline of the EWOCS observations extending over more than one year enable us to perform a detailed photometric, colour, and spectral analysis, as well as to search for short- and long-term periodicity. In X-rays, we detect 20 out of the 24 known Wolf-Rayet stars in Wd1 down to an observed luminosity of ~7$\times10^{29}$erg s$^{-1}$ (assuming a distance of 4.23 kpc to Wd1), with 8 WR stars being detected in X-rays for the first time. Nine stars show clear evidence of variability over the year-long baseline, with clear signs of periodicity. The X-ray colours and spectral analysis reveal that the vast majority of the WR stars are hard X-ray sources (kT$\geq$2.0keV). The Fe XXV emission line at ~6.7 keV, which commonly originates from the wind-wind collision zone in binary systems, is detected for the first time in the spectra of 17 WR stars in Wd1. In addition the ~6.4 keV fluorescent line is observed in the spectra of three stars, indicating that dense cold material coexists with the hot gas in these systems. Overall, our X-ray results alone suggest a very high binary fraction ($\geq$80%) for the WR star population in Wd1. When combining our results with properties of the WR population from other wavelengths, we estimate a binary fraction of $\geq$92%, which could even reach unity. This suggests that either all the most massive stars are found in binary systems within Wd1, or that binarity is essential for the formation of such a rich population of WR stars.
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