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arXiv 0903.0346astro-ph.CO
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WIMP湮灭对原始恒星形成的影响

WIMP annihilation effects on primordial star formation

E. Ripamonti, F. Iocco, A. Bressan, R. Schneider, A. Ferrara, P. Marigo

摘要:

我们研究了WIMP暗物质(DM)湮灭对宇宙中第一代恒星形成的气体云的热演化和化学演化的影响。我们遵循一个典型的晕内的气体在非常高的红移维里化的崩溃,从维里化之前,直到一个阶段,从DM湮灭加热超过气体冷却速率。DM能量输入估计插入DM湮灭(如由原始DM配置文件的绝热收缩预测)在一个球对称的辐射转移计划释放的能量。除了吸收的能量的加热效应,我们还包括它对气体化学性质的反馈,这对确定晕中的冷却速率至关重要,因此也是第一批恒星的碎片尺度和金斯质量。我们发现,对于我们的基准参数值,当气体密度n>~ 10^4 cm^-3时,DM湮灭确实改变了自由电子,尤其是H2部分。然而,即使气体的H2丰度和冷却效率的变化大(有时超过100倍),对坍缩气体温度的影响要小得多(减少了<~1.5倍),因为气体冷却速率非常强烈地依赖于温度:然后,碎片质量尺度仅略微减小,暗示第一批恒星的初始质量函数没有显著变化。

英文摘要:

We study the effects of WIMP dark matter (DM) annihilations on the thermal and chemical evolution of the gaseous clouds where the first generation of stars in the Universe is formed. We follow the collapse of the gas inside a typical halo virializing at very high redshift, from well before virialization until a stage where the heating from DM annihilations exceeds the gas cooling rate. The DM energy input is estimated by inserting the energy released by DM annihilations (as predicted by an adiabatic contraction of the original DM profile) in a spherically symmetric radiative transfer scheme. In addition to the heating effects of the energy absorbed, we include its feedback upon the chemical properties of the gas, which is critical to determine the cooling rate in the halo, and hence the fragmentation scale and Jeans mass of the first stars. We find that DM annihilation does alter the free electron and especially the H2 fraction when the gas density is n>~ 10^4 cm^-3, for our fiducial parameter values. However, even if the change in the H2 abundance and the cooling efficiency of the gas is large (sometimes exceeding a factor 100), the effects on the temperature of the collapsing gas are far smaller (a reduction by a factor <~1.5), since the gas cooling rate depends very strongly on temperature: then, the fragmentation mass scale is reduced only slightly, hinting towards no dramatic change in the initial mass function of the first stars.

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