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arXiv 2103.09834astro-ph.GAastro-ph.CO
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超微弱的矮星系：揭开第一批恒星的最小质量

Ultra-faint dwarf galaxies: unveiling the minimum mass of the first stars

Martina Rossi, Stefania Salvadori, Ása Skúladóttir

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Accepted by MNRAS
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摘要：

在超暗矮星系（UFD）中没有检测到零金属丰度恒星，可以通过现有数据与化学演化模型预测之间的统计比较来约束第一批（PopIII）恒星的初始质量函数（IMF）。为此，我们开发了一个模型，该模型遵循孤立UFD的形成，并进行了校准，以最好地再现最佳研究系统的可用数据：Bootes I。我们的统计方法表明，UFD是研究PopIII IMF上持续未检测到零金属丰度恒星的影响的最佳合适系统，即其形状、最小质量（$m_{min}$）和特征质量（$m_{ch}$）。我们表明，占IMF的不完整采样是必不可少的计算预期数量的长寿的PopIII星低效恒星形成的UFD。通过模拟牧夫座I的色星等图，并考虑到观测恒星的质量范围，我们可以得到对$m_{min}$更严格的约束。通过对96颗已测金属丰度（$\rm i < 19$）的恒星在我们模型所代表的UFD中的应用，我们证明了：在$99\%$置信水平下，$m_{ch} > 1 \rm M_{\odot}$或$m_{min} > 0.8 \rm M_{\odot}$。这意味着我们的模型排除了PopIII恒星的当前IMF，并且强烈支持头重脚轻的PopIII IMF。我们可以限制$m_{min} > 0.8 \rm M_{\odot}$独立于PopIII IMF的形状，通过使用下一代仪器，如ELT/MOSAIC（$\rm i < 25$），瞄准四个UFD牧夫座I，武仙座，狮子座IV和波江座II，它可以提供超过10，000颗恒星的样本。

英文摘要：

The non-detection of zero-metallicity stars in ultra-faint dwarf galaxies (UFDs) can be used to constrain the Initial Mass Function (IMF) of the first (PopIII) stars by means of a statistical comparison between available data and predictions from chemical evolution models. To this end we develop a model that follows the formation of isolated UFDs, calibrated to best reproduce the available data for the best studied system: Boötes I. Our statistical approach shows that UFDs are the best suitable systems to study the implications of the persisting non-detection of zero-metallicity stars on the PopIII IMF, i.e. its shape, the minimum mass ($m_{min}$), and the characteristic mass ($m_{ch}$). We show that accounting for the incomplete sampling of the IMF is essential to compute the expected number of long-lived PopIII stars in inefficiently star-forming UFDs. By simulating the Color Magnitude Diagram of Boötes I, and thus take into account the mass-range of the observed stars, we can obtain even tighter constrains on $m_{min}$. By exploiting the 96 stars with measured metallicities ($\rm i < 19$) in the UFDs represented by our model, we demonstrate that: $m_{ch} > 1 \rm M_{\odot}$ or $m_{min} > 0.8 \rm M_{\odot}$ at $99\%$ confidence level. This means that a present day IMF for PopIII stars is excluded by our model, and a top-heavy PopIII IMF is strongly favoured. We can limit $m_{min} > 0.8 \rm M_{\odot}$ independent of the PopIII IMF shape by targeting the four UFDs Boötes I, Hercules, Leo IV and Eridanus II with future generation instruments, such as ELT/MOSAIC ($\rm i < 25$), which can provide samples of >10\,000 stars.

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