Why Do Stars Form In Clusters? An Analytic Model for Stellar Correlation Functions
Why Do Stars Form In Clusters? An Analytic Model for Stellar Correlation Functions
复制标题
为什么恒星会形成星团？
DOI：
10.1093/mnras/sts147
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发表时间：
2012
影响因子：
4.8
通讯作者：
P. Hopkins
中科院分区：
文献类型：
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作者：
P. Hopkins
最近，我们已经证明，如果 ISM 由超音速湍流控制，则可以使用偏移集形式来计算大范围尺度上自引力物体的统计数据。在最大的自引力尺度（“第一次穿越”）上，这些对应于GMC，在最小的非碎片自引力尺度（“最后一次穿越”）上，它们对应于原恒星核心。在这里，我们将这种形式主义扩展为严格计算核心（以及年轻恒星）的自相关函数和互相关函数作为空间分离和质量的函数，类似于光晕聚类的宇宙学计算。我们表明，这通常预示着恒星的形成在小尺度上非常强烈地聚集：恒星以星团形式形成，它们本身位于 GMC 内部。在双星体系之外，预测的相关函数按照弱幂律下降，直到达到与 GMC 的特征质量尺度相对应的特征尺度。在更大的尺度上，星团会减少，因此相对于实际的致密气体分布，恒星的形成在银河尺度上并没有强烈的偏差。精确的相关函数形状取决于湍流谱的属性，但其定性行为非常普遍。这些预测与对半径超过 4 dex 的年轻恒星和核心自相关函数的观测结果非常吻合。如果速度场中的大部分能量（因此对密度涨落的贡献）来自大尺度，则星团形成是超音速湍流的一般结果。然后，声波长度附近的自引力质量的分布会受到更大尺度上的波动的影响。我们同样表明，以“孤立”模式形成的恒星比例应该很小（\lesssim10%）。
Recently, we have shown that if the ISM is governed by super-sonic turbulent flows, the excursion-set formalism can be used to calculate the statistics of self-gravitating objects over a wide range of scales. On the largest self-gravitating scales ('first crossing'), these correspond to GMCs, and on the smallest non-fragmenting self-gravitating scales ('last crossing'), to protostellar cores. Here, we extend this formalism to rigorously calculate the auto and cross-correlation functions of cores (and by extension, young stars) as a function of spatial separation and mass, in analogy to the cosmological calculation of halo clustering. We show that this generically predicts that star formation is very strongly clustered on small scales: stars form in clusters, themselves inside GMCs. Outside the binary-star regime, the projected correlation function declines as a weak power-law, until a characteristic scale which corresponds to the characteristic mass scale of GMCs. On much larger scales the clustering declines such that star formation is not strongly biased on galactic scales, relative to the actual dense gas distribution. The precise correlation function shape depends on properties of the turbulent spectrum, but its qualitative behavior is quite general. The predictions agree well with observations of young star and core autocorrelation functions over ~4 dex in radius. Clustered star formation is a generic consequence of supersonic turbulence if most of the power in the velocity field, hence the contribution to density fluctuations, comes from large scales. The distribution of self-gravitating masses near the sonic length is then imprinted by fluctuations on larger scales. We similarly show that the fraction of stars formed in 'isolated' modes should be small (\lesssim10%).