发表机构
University of Vienna（维也纳大学）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
通过对猎户座和天蝎座-半人马座的普查，发现年轻星团呈自由膨胀，半成员半径随年龄线性增长，速度弥散无演化，星团未束缚，密度对比度定义成员会遗漏快星致表观膨胀慢。
AI 中文摘要
我们测量了两个最近的恒星形成区域中年轻星团随时间的扩散情况。我们的样本包含猎户座中44个由\texttt{SigMA}-\textit{Gaia}定义的星团，以及天蝎座-半人马座（Sco--Cen）中的31个星团，它们的等时年龄为2至25百万年（Myr）。我们通过\textit{eROSITA}选择的年轻恒星来扩展每个星团的普查范围，这些恒星与星团具有相同的视差和自行，且位于星团诞生后可能弹道运动的距离范围内。到10至30百万年时，约三分之一的前星团恒星位于星团编目边界之外。在两个区域和整个年龄范围内，我们发现：（1）半成员半径与年龄呈线性增长，即$\rfifty \backsimeq \boldsymbol{\text{birth}} t$；（2）速度弥散没有可测量的演化；（3）所有研究的星团均为未束缚的。这些结果支持自由膨胀理论。猎户座和天蝎座-半人马座的年轻星团从诞生的分子云中形成时就是未束缚的，以诞生时的速度弥散滑行，没有显著的弛豫过程或特征扩散时标。我们进一步表明，由密度对比度定义的成员身份会优先遗漏速度最快的恒星，导致相同星团看起来膨胀得更慢（与$t^{0.6}$成正比）、随年龄冷却并趋近于平衡态。
英文摘要
We measure how young star clusters disperse over time in the two nearest star-forming complexes. Our sample comprises 44 \texttt{SigMA}-\textit{Gaia}-defined clusters in Orion and 31 in Sco--Cen, with isochronal ages of 2--25\,Myr. We extend each cluster's census with \textit{eROSITA}-selected young stars that share its parallax and motion and lie within the distance they could have traveled ballistically since birth. By 10--30\,Myr, about one in three former cluster stars lies outside the cluster's cataloged boundary. Across both complexes and the full age range, we find that (1) the half-member radius increases linearly with age, $\rfifty \simeq \sigbirth t$, (2) the velocity dispersion shows no measurable evolution, and (3) all studied clusters are unbound. These results support free expansion. Young clusters in Orion and Sco-Cen emerge unbound from their birth clouds and coast at their birth velocity dispersion without significant relaxation or a characteristic dispersal timescale. We further show that memberships defined by density contrast preferentially omit the fastest stars, making the same clusters appear to expand more slowly ($\propto t^{0.6}$), cool with age, and approach equilibrium.
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