Title: Strikingly high fraction of fast rotators in Magellanic Cloud star clusters Show Chinese title

Title: 麦哲伦云星团中快速旋转恒星的极高比例

Abstract: There has been growing evidence that the rich star clusters in the Magellanic Clouds contain significant fractions of rapidly rotating stars. In this work, we aim to constrain these fractions by studying the colour-magnitude diagrams of four star clusters, selected among those with the most striking signatures of fast rotators. Using isochrones derived from PARSEC v2.0 stellar tracks, we generate distinct stellar populations, each covering a limited interval of initial rotation rates $\omega_\mathrm{i}$, referred to as 'Partial Models' (PMs). Using optimization algorithms and Monte Carlo Markov Chains, PMs are combined to create the final best-fitting model. In our analysis, we adopt two key assumptions: a uniform age and an isotropic distribution of stellar spin axes within each cluster. The solutions are allowed to explore the entire range of $\omega_\mathrm{i}$, and different values of age, metallicity, distance and foreground extinction. We find that the rotational velocity distributions in all four clusters reveal a high fraction of stars with $\omega_\mathrm{i}$ close to the break-up value, in all cases. Specifically, the fraction of stars with $\omega_\mathrm{i}>0.7$ exceeds $80\%$ in the clusters NGC 419 of the Small Magellanic Cloud (SMC) and NGC 1831 and NGC 1866 of the Large Magellanic Cloud (LMC). For NGC 2203 of the LMC, this fraction is smaller, although it still exceeds $50\%$, confirming that also this cluster is mainly populated by fast-rotating stars. Show Chinese abstract

Abstract: 有越来越多的证据表明，麦哲伦云中的富星团包含大量快速旋转的恒星。在这项工作中，我们通过研究四个恒星团的颜色-亮度图（这些恒星团是从那些具有最显著快速自转特征标志的恒星团中选出的）来限制这些比例。使用来自帕杰克（PARSEC）v2.0恒星轨迹推导出的等年龄线，我们生成了不同的恒星群体，每个群体覆盖一个有限的初始旋转率区间$\omega_\mathrm{i}$，称为“部分模型”（PMs）。利用优化算法和蒙特卡洛马尔可夫链，PMs 被组合起来创建最终的最佳拟合模型。在我们的分析中，我们采用两个关键假设：每个星团内的恒星自转轴具有均匀年龄和各向同性分布。解决方案被允许在整个$\omega_\mathrm{i}$范围内以及不同年龄、金属丰度、距离和前景消光值下探索。我们发现，所有四个星团中的自转速度分布都显示出高比例的接近破裂值$\omega_\mathrm{i}$的恒星。具体而言，在小麦哲伦云（SMC）的NGC 419星团以及大麦哲伦云（LMC）的NGC 1831和NGC 1866星团中，具有$\omega_\mathrm{i}>0.7$的恒星比例超过$80\%$。对于LMC的NGC 2203星团，这一比例较小，但仍然超过$50\%$，证实这个星团也主要由快速旋转的恒星组成。