One of the computational challenges of cluster formation simulations is resolving individual stars and simulating massive clusters with masses of more than 104 M⊙ without gravitational softening. Combining a direct N-body code with smoothed-particle hydrodynamics (SPH) code, we have developed a new code, ASURA+BRIDGE, in which we can integrate stellar particles without softening. We add a feedback model for H ii regions into this code, in which thermal and momentum feedback is given within the Strömgren radius. We perform N-body/SPH simulations of star cluster formation. Without softening, a portion of massive stars are ejected from the forming clusters. As a result, the stellar feedback works outside the clusters. This enhances/suppresses the star formation in initially sub-virial/super-virial clouds. We find that the formed star clusters are denser than currently observed open clusters, but the mass–density relation is consistent with or even higher than that which is estimated as an initial cluster density. We also find that some clusters have multiple peaks in their stellar age distribution as a consequence of their hierarchical formation. Irrespective of the virial ratio of molecular clouds, approximately one-third of stars remain in the star clusters after gas expulsion.

中文翻译：

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天狼星项目。三、使用具有恒星反馈的直接 N 体积分器逐星模拟星团形成

星团形成模拟的计算挑战之一是解析单个恒星并模拟质量超过 104 M⊙ 且没有引力软化的大质量星团。将直接 N 体代码与平滑粒子流体动力学 (SPH) 代码相结合，我们开发了一个新代码 ASURA+BRIDGE，我们可以在其中整合恒星粒子而不进行软化。我们在此代码中添加了 H ii 区域的反馈模型，其中在 Strömgren 半径内给出了热和动量反馈。我们对星团形成进行 N 体/SPH 模拟。在没有软化的情况下，一部分大质量恒星会从正在形成的星团中弹出。结果，恒星反馈在星团之外起作用。这增强/抑制了最初亚维里/超维里云中的恒星形成。我们发现形成的星团比目前观察到的疏散星团密度更大，但质量-密度关系与估计的初始星团密度一致甚至更高。我们还发现，由于它们的层次结构，一些星团的恒星年龄分布有多个峰值。无论分子云的维里比率如何，大约有三分之一的恒星在气体排出后仍留在星团中。