arXiv 1302.21522013-02astro-ph.GA

球状星团的能量不平衡

No Energy Equipartition in Globular Clusters

M. Trenti, R. van der Marel

备注：: Minor revision, conclusions unchanged: 12 pages, 8 figures, MNRAS accepted
DOI：: 10.1093/mnras/stt1521

摘要：

人们普遍认为，球状星团经过许多两体弛豫时间后会演化到能量均分的状态，因此速度色散与恒星质量的关系为σ~ m^{-η}，其中η= 0.5。我们表明，这是不正确的，使用直接N体模拟与各种现实的IMF和初始条件。没有一个模拟系统能达到接近均分的状态。在中心附近，亮主序星的最大值η_{max}达到0.15 \pm 0.03。在大的时间，所有径向箱收敛于渐近值η_{ \infty } ~ 0.08 \pm 0.02。这种“部分均分”的发展是惊人的相似，在我们的模拟，尽管采用的初始条件的范围。致密残留物的η值往往比主序星高（但仍< 0.5），这是因为它们的质量函数更陡峭（演化）。中等质量黑洞（IMBH）的存在降低了η，这与我们以前在这些条件下发现的质量分离淬灭一致。所有这些结果都可以理解为双组分系统斯皮策不稳定性的结果，并由维什尼亚克扩展到连续质谱。质量分离（较重的恒星向核心下沉的趋势）经常被观测研究，但能量均分还没有。由于来自哈勃太空望远镜的高质量自行数据集的出现，现在可以测量η。详细的数据模型比较为球状星团动力学和演化开辟了一个新的观测窗口。比较我们的模拟欧米茄Cen观测产生良好的协议，确认球状星团一般不是在能量均分。假设按构造均分的建模技术（例如，多质量的M-K模型）最多是近似的。

英文摘要：

It is widely believed that globular clusters evolve over many two-body relaxation times toward a state of energy equipartition, so that velocity dispersion scales with stellar mass as σ~ m^{-η} with η= 0.5. We show that this is incorrect, using direct N-body simulations with a variety of realistic IMFs and initial conditions. No simulated system ever reaches a state close to equipartition. Near the center, the luminous main-sequence stars reach a maximum η_{max} ~ 0.15 \pm 0.03. At large times, all radial bins convergence on an asymptotic value η_{\infty} ~ 0.08 \pm 0.02. The development of this "partial equipartition" is strikingly similar across our simulations, despite the range of initial conditions employed. Compact remnants tend to have higher ηthan main-sequence stars (but still η< 0.5), due to their steeper (evolved) mass function. The presence of an intermediate-mass black hole (IMBH) decreases η, consistent with our previous findings of a quenching of mass segregation under these conditions. All these results can be understood as a consequence of the Spitzer instability for two-component systems, extended by Vishniac to a continuous mass spectrum. Mass segregation (the tendency of heavier stars to sink toward the core) has often been studied observationally, but energy equipartition has not. Due to the advent of high-quality proper motion datasets from the Hubble Space Telescope, it is now possible to measure η. Detailed data-model comparisons open up a new observational window on globular cluster dynamics and evolution. Comparison of our simulations to Omega Cen observations yields good agreement, confirming that globular clusters are not generally in energy equipartition. Modeling techniques that assume equipartition by construction (e.g., multi-mass Michie-King models) are approximate at best.

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