Cosmological simulations of Population III star formation suggest an initial mass function (IMF) biased toward very massive stars (M 100 M ☉) formed in minihalos at redshift z 20, when the cooling is driven by molecular hydrogen. However, this result conflicts with observations of extremely metal-poor (EMP) stars in the Milky Way (MW) halo, whose r-process elemental abundances appear to be incompatible with those expected from very massive Population III progenitors. We propose a new solution to the problem in which the IMF of second-generation stars formed at z 10, before reionization, is deficient in sub-solar mass stars, owing to the high cosmic microwave background temperature floor. The observed EMP stars are formed preferentially at z 10 in pockets of gas enriched to metallicity Z 10–3.5 Z ☉ by winds from Population II stars. Our cosmological simulations of dark matter halos like the MW show that current samples of EMP stars can only constrain the IMF of late-time Population III stars, formed at z 13 in halos with virial temperature T vir ~ 104?K. This suggests that pair instability supernovae were not produced primarily by this population. To begin probing the IMF of Population III stars formed at higher redshift will require a large survey, with at least 500 and probably several thousand EMP stars of metallicities Z ≈ 10–3.5 Z ☉.
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机译：人口III恒星形成的宇宙学模拟表明，当冷却是由分子氢驱动时，初始质量函数（IMF）偏向在红移z 20的微晕中形成的非常大质量的恒星（M 100 M☉）。但是，该结果与银河系（MW）晕中极贫金属（EMP）恒星的观测结果相矛盾，该恒星的r进程元素丰度似乎与非常庞大的人口III祖先预期的不相容。我们提出了一个新的解决方案，该问题是由于宇宙微波本底温度很高，因此在电离之前在z 10处形成的第二代恒星的IMF缺乏亚太阳质量的恒星。观测到的EMP恒星优先在z 10处形成，由居群II恒星的风将富集到金属化Z 10–3.5 Z gas的气体形成。我们对诸如MW的暗物质晕的宇宙学模拟表明，当前EMP恒星的样本只能限制在病毒温度T vir〜104？K的晕中z 13处形成的晚期人口III恒星的IMF。这表明配对不稳定的超新星并非主要由该种群产生。要开始探测更高红移形成的III号人口IMF的IMF，将需要进行大范围的调查，至少要有500颗甚至可能几千颗EMP金属的Z≈10–3.5 Z stars。
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