We investigate the time evolution of dense cores identified in molecular cloud simulations using dendrograms, which are a common tool to identify hierarchical structure in simulations and observations of star formation. We develop an algorithm to link dendrogram structures through time using the three-dimensional density field from magnetohydrodynamical simulations, thus creating histories for all dense cores in the domain. We find that the population-wide distributions of core properties are relatively invariant in time, and quantities like the core mass function match with observations. Despite this consistency, an individual core may undergo large (>40%), stochastic variations due to the redefinition of the dendrogram structure between timesteps. This variation occurs independent of environment and stellar content. We identify a population of short-lived (<200 kyr) overdensities masquerading as dense cores that may comprise ~20% of any time snapshot. Finally, we note the importance of considering the full history of cores when interpreting the origin of the initial mass function; we find that, especially for systems containing multiple stars, the core mass defined by a dendrogram leaf in a snapshot is typically less than the final system stellar mass. This work reinforces that there is no time-stable density contour that defines a star-forming core. The dendrogram itself can induce significant structure variation between timesteps due to small changes in the density field. Thus, one must use caution when comparing dendrograms of regions with different ages or environment properties because differences in dendrogram structure may not come solely from the physical evolution of dense cores.

中文翻译：

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用树状图识别的恒星形成核心的高度可变的时间演化

我们使用树状图研究在分子云模拟中识别的致密核心的时间演化，树状图是在模拟和观测恒星形成中识别层次结构的常用工具。我们开发了一种算法，使用来自磁流体动力学模拟的三维密度场将树状图结构与时间联系起来，从而为域中的所有密集核心创建历史。我们发现核心属性的人口范围分布在时间上相对不变，并且核心质量函数等数量与观察结果相匹配。尽管具有这种一致性，但由于时间步长之间树状图结构的重新定义，单个核心可能会发生较大的（> 40%）随机变化。这种变化与环境和恒星含量无关。我们确定了一群短暂（<200 kyr）的过度密度伪装成密集的核心，可能占任何时间快照的约 20%。最后，我们注意到在解释初始质量函数的起源时考虑核的完整历史的重要性；我们发现，特别是对于包含多颗恒星的系统，快照中由树状图叶子定义的核心质量通常小于最终系统的恒星质量。这项工作强调了没有时间稳定的密度轮廓来定义恒星形成的核心。由于密度场的微小变化，树状图本身可以在时间步长之间引起显着的结构变化。因此，