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低金属量恒星形成IV。冷初始条件下的碎裂不依赖于金属度

Star Formation at Very Low Metallicity. IV. Fragmentation Does Not Depend on Metallicity for Cold Initial Conditions

Anne-Katharina Jappsen, Ralf S. Klessen, Simon C. O. Glover, Mordecai-Mark Mac Low

备注：
41 pages, 4 figures, 3 tables, accepted for publication in ApJ
发表信息：
Astrophys.J.696:1065-1074,2009
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摘要：

原始恒星的形成似乎导致恒星的质量至少比现代恒星的形成大一个数量级。有人提出，从原始到现代的初始质量函数的转变是由于金属丰度Z/Z_sun = 10^{-3.5}时有效金属线冷却的开始。然而，这些模拟忽略了分子氢冷却。我们使用相同的初始条件进行模拟，但包括分子冷却，使用一个复杂的网络，遵循分子氢形成，也直接遵循一氧化碳和水。我们发现，分子氢冷却允许大致相当的碎片进行，即使在零金属这些初始条件。表观转变只是代表了金属线冷却变得比分子冷却更重要的点。在所有情况下，碎片的质量都足以与原始恒星质量的模型相一致，这表明向现代初始质量函数的转变可能是由其他物理学如尘埃形成决定的。我们的结论是，这种额外的冷却机制，结合宇宙学坍缩产生的确切的初始条件，可能比金属线冷却在确定初始质量函数方面更重要，因此，在Z/Z_sun = 10^{-3.5}时，初始质量函数不太可能有急剧的转变。

英文摘要：

Primordial star formation appears to result in stars at least an order of magnitude more massive than modern star formation. It has been proposed that the transition from primordial to modern initial mass functions occurs due to the onset of effective metal line cooling at a metallicity Z/Z_sun = 10^{-3.5}. However, these simulations neglected molecular hydrogen cooling. We perform simulations using the same initial conditions, but including molecular cooling, using a complex network that follows molecular hydrogen formation and also directly follows carbon monoxide and water. We find that molecular hydrogen cooling allows roughly equivalent fragmentation to proceed even at zero metallicity for these initial conditions. The apparent transition just represents the point where metal line cooling becomes more important than molecular cooling. In all cases, the fragments are massive enough to be consistent with models of primordial stellar masses, suggesting that the transition to the modern initial mass function may be determined by other physics such as dust formation. We conclude that such additional cooling mechanisms, combined with the exact initial conditions produced by cosmological collapse are likely more important than metal line cooling in determining the initial mass function, and thus that there is unlikely to be a sharp transition in the initial mass function at Z/Z_sun = 10^{-3.5}.

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